Control System Design Considerations
Contents
1. NEC publication NFPA 70 Further information can be found at http www nfpa org Another good reference from the N FPA is Electrical Standard Industrial M achinery publication N FPA 79 3 Additional UL information can be found at http www ul com controlequipment devices htm 4 Information for the N ational Electrical M anufacturer s Association N EM A can be found at http www nema org NEM A isalso being harmonized with the International Electrotechnical Commission IEC www iec ch and other European standards Additional information can be found at the Global Engineering Documents W eb site www global ihs com Global Engineering D ocuments is also the source for obtaining NEMA IEC and CE documents 5 Additional information can be found on OH SA s Web site http www osha gov SLT C controlhazardo amp 3 5 WAUTOMATIONDIRECT Technical Review WAUTOMATIONDIRECTi A Condensed Guide to Automation Control System Specification Design and Installation Part 4 Build Launch amp Maintain by Tom Elavsky AutomationDirect n Part 3 of System D esign we covered how to design our automated control system and the importance of documenting the design W e discussed the various types of documents that would be typical for an automated control system design why and how we would use these documents and finally the tools that can be used to create the documents In Part 4 we will cover the steps needed to2. entire schematic Asa first step we may want to apply power to the main circuit breaker or fused disconnect of our control system Then measure the voltage for proper values phase to phase and each phase to ground if the incoming power is three phase N ext we can turn on the main circuit protector and check the voltage at each device that is fed from the main source T hen start turning the circuit breakers on or replacethe fuses one circuit at a time and make additional voltage checks and test equipment operation that may be powered from the circuit Keep in mind that every control system will not be the same T herefore each system will require a different strategy to bring the equipment online safely Consider having motors uncoupled from their respective loads air pressure off disabling hydraulics and using Lockout Tagout LOTO procedures M easure voltages as you go If using a PLC connect aPC to it and monitor the ladder logic to make sure conditions States etc are responding correctly Maintain It is important to develop a routine maintenance schedule for your automated control system H aving aroutine schedule for checking critical components and devices in the system will increase the longevity of the system and more www automationnotebook com importantly it will help eliminate future problems Set up the schedule based on a monthly or quarterly time period depending on the item to be done T he followin
3. 162 valve SAFETY 3 163 O10 163 LS163 464 164A 164 d spare 164A i Figure 7 s Panel Layout Once we have our schematic finalized the next step is a panel layout drawing In most cases the actual panel is referred to as a subpanel We can mount all the components to a structure the subpanel wire all the components and do this before mounting the subpanel in the control system enclosure T he panel layout drawing should be done to scale and include dimensions for the panel builder to follow when laying out the components Special attention should be given to component location and Spacing We need to follow the manufacturer recommended mounting distances and clearances Figure 8 is a partial example of a panel layout drawing TRF211 3 X 3 WIREWAY 2 25 X 3 WIREWAY 252 X 3 WIREWAY 2 25 X 3 WIREWAY FU208 209 212 2 25 X 3 WIREWAY IN See the Figure 8 The higher voltage devices those that operate at 240 480 VAC should be mounted toward the top of the panel keeping as much distance as possible between the high voltage devices and any electronic devices such as PLCs DC power supplies elec tronic timers etc Keeping the high voltage devices toward the top allows us to cover all of the high voltage devices with a non conductive safety shield for personnel safety It keeps the lower voltage devices
4. Electrical Code NEC publication NFPA 70 Further information can be found at their W eb site at htto www nfpa org Another good reference from the N FPA is Electrical Standard Industrial M achinery publication NFPA 79 8 4 4
5. build start up and maintain our automated control system The build section will include tips on the use of a subpanel terminal blocks grounding shielded cable etc We will cover the steps to start up the system so that it is brought online in a safe and logical manner and also give some suggestions for developing a plan to maintain the control system As stated in the previous articles special expertise is generally required to design wire install and operate industrial automation control systems Persons without such expertise or guidance should not attempt control systems but should consider seeking the services of a qualified System Integrator Control systems can fail and may result in situations that can cause serious injury to personnel or damage to equipment The information provided in this series of articles is provided as is without a guarantee of any kind Build D uring the design of our control panel we pointed out the benefits of using a removable subpanel In building the subpanel it is best to secure the components from the front side This will make it easier to replace any failed device or component in the future Wecan also make Installation and maintenance easier by using terminal blocks mounted to the subpanel that will connect to all external devices This will allow the installing electrician to quickly dress and terminate the field wires Another terminating method that has added benefits is to d
6. control system At this point in our schematic we need to look at device wiring isolation strategies PLCs provide ideal isolation because its circuitry is divided into three main regions separated by isolation boundaries as shown in Figure 4 The PLC s main power supply includes a transformer that provides isolation and the input and output circuits that use opto couplers to provide additional isola tion When wiring a PLC it is extremely important to avoid making external connections that connect logic side circuits to any other Primary Side Secondary or Field Side Logic side Input i i Discrete inputs Circuit P Output Circuit Discrete outputs Isolation F igu re 4 Boundary lt Isolation Programming Device or Boundary Operator Interface Electrical isolation provides safety so that a fault in one area does not damage another Using Figure 5 as reference we see a transformer which provides magnetic isolation between its primary high voltage and secondary control voltage sides A powerline filter provides isolation between the control power source and the electronic devices 3 2 www automationnotebook com Technical Review WAUTOMATIONDIRECT Figure 5 also shows some general suggestions for device grounding and distributing the control power to various devices along with individually fusing these devices Proper grounding is one of the most important things in good automated c
7. grouped together allowing access to wiring termi nals that will aid in troubleshooting our control system In some cases a metal partition between the high voltage section of our control panel and any sensitive electronic devices can act as a shield from any EM I generated by the high voltage devices In our panel layout design we need to include wire duct between the various components T he wire duct simplifies the wire routing between components keeps the wires in place makes working with the wires easier and gives the panel a well organized look We should also make use of terminal blocks in our design www automationnotebook com Terminal blocks can be sized organized and even color coded to handle the different types of signals that enter and leave our control panel W e may choose to use black for high voltage red for inputs violet for outputs etc We should try to locate the terminal blocks so they provide the best wire routing from the components to the terminal blocks The terminal blocks also make it convenient for the electrician to terminate his field wiring when the control enclosure is installed Our design should include the selection of the enclosure that will house our control system We need to consider the envi ronment where the enclosure will be located O utdoors Indoors Wash down required Refer to the section on enclosures in Part 2 of this series of articles for references to N FPA sN ational Electrical Co
8. interference from EM and ground noise Grounding refers to a connection to a grounding conductor to provide overload and interference protection As mentioned before grounding protects the instrumen tation devices or components from power surges and reduces the effect of EM and ground noise Figure 1 shows a typical method for grounding the subpanel to the enclosure cabinet to assure proper grounding Inside Rear of Enclosure Subpanel Hex Nut Mounting Stud Star Washer Scrape paint around subpanel mounting hole Figure 1 Note Please remember that bonding and grounding are important safety requirements that are mandatory by local codes and regulations T he installer must verify the local codes to determine what grounding and bonding methods are permitted Always make sure that power supplies are properly grounded to ensure elimination of electronic noise interference N ote When using ground lugs and installing more than one on the same stud make sure to install the first lug between two star washers and tieit with a nut Install the second lug over the nut of the first lug followed by a star washer and another tightened nut 4 2 Technical Review WAUTOMATIONDIRECT Shielded C ables A shielded cable isan insulated cable consisting of strands of copper or other material enclosed with a metallic shield underneath a jacketed sheath Shielded cables are used to reduc
9. their Web site at http www nfpa org Some of the more useful publications are the N ational Electrical Code N EC publication N FPA 70 and also as a good reference refer to the Electrical Standard for Industrial Machinery publication NFPA 79 1 4 Technical Review WAUTOMATIONDIRECTi A Condensed Guide to Automation Control System Specification Design and Installation Part 2 Control Device Specification by Tom Elavsky AutomationDirect aspects Involved in the use of automated control systems and discussed some tips used in identifying which manufacturing equipment and processes can be automated In Part 2 we will cover how to specify the various devices required for controlling the equipment in an auto mated system Your specifications need to include not only the controlling devices for your application but also items such as the housing or enclosure for the devices the type of wire required to meet the various codes agency approvals required for safety and insurance purposes environ mental conditions etc As stated in Part 1 special expertise is generally required to design wire install and operate industrial automation control systems Persons without such expertise or guidance should not attempt to design control systems but should consider seeking the services of a qualified system integrator Control systems can fail and cause serious injury to personnel or damage to equipment T he informat
10. 2 PUMP NO 1 2 LOW LEVEL HAND l AUTO CUTOUT i 132 PUMP NO 1 MOTOR STARTER 105 106 107 PUMP NO 1 RUNNING PUMP NO 2 OFF HAND 4 AUTO i PUMP NO 2 O MOTOR STARTER 109 110 111 PUMP NO 2 MOTOR START 15 PUMP NO 2 RUNNING This is a good point to mention surge suppression Surge Suppression devices are an important component in achieving a reliable power distribution system T hese devices protect the elec tronic components from sudden power surges that can cause considerable damage Inductive load devices devices with a coil generate transient voltages when de energized with a relay contact W hen a relay contact is closed it bounces which energizes and de energizes the coil until the bouncing stops The transient voltages generated are much larger in amplitude than the supply voltage especially with aDC supply If using a PLC the final section of our schematic will show the input and output modules Figure 7 is an example of the wiring for an input module We would make use of reference line numbering and in most cases we would show all of the input modules first then the output modules If we have analog I O we would want to show the analog inputs then the analog outputs and finally our discrete inputs and outputs G enerally we would use one sheet of our schematic to show each module i 156 157 SECURE Q Q BYPASS SS157 OVER TEMP 161 KJ TS161 CR307 162 O O
11. Technical Review WAUTOMATIONDIRECTi A Condensed Guide to Automation Control System Specification Design and Installation Part 1 System Identification and Safety by Tom Elavsky AutomationDirect PROGRAMMING SOFTWARE binary schematic servo ladder logic enn boolean ot gation IMTS min O NEM Bak RE CR ALi Y m 5 r ne cuca sr transformer upprepa fusing pou shielded cable IEEE circuit breaker SCADA h te power ast photoelectric sensor terminal block OKOLELE enclosure iowehart Ethernet zatian CONS Notor dri a operator interface MODBUS R4 485 HMI PC based control sinking sourcing ASCH PID PROORSS CONTROLLER ISA ENCODER Fhermocouple signal conditioner NIACIIR current loop If you have not been directly involved in the world of factory automation data acquisition process instrumentation or electrical controls in general then the above words and acronyms may be somewhat overwhelming But these words and many others are part of the language that s used in the industrial automation world For A Guide to Common Automation Terms refer to http support automationdi rect com docs glossary html The following is Part 1 of a four part series of articles on Control System D esign that can act as a general guide to the specification design and installation of automated control systems The information and references are presented in a logical order that will take you from the skills required to recogniz
12. ces U se these guidelines to prevent any possibility of interference with your equipment e Environmental specifications that cover the operating temperature humidity vibration noise immunity etc e Power requirements are specific to each piece of equipment W hen installing instrumentation always make sure to follow the manufacturer s power requirement guidelines for your specific piece of equipment e Use components with Agency Approvals such as UL CE etc e M ake enclosure selections based on component dimensions recommended mounting clearances heat dissipation and EM If installing a PLC base or chassis which consists mainly of mounting bonding and grounding it is very critical to the proper operation of the PLC and its related devices and components to closely follow the manufacture s recommenda tions There are many cases of a PLC experiencing noise problems when the problem is found to be that the base wasn t grounded to the subpanel I O Testing The last item to consider at the completion of building your control system isto do acompletel O checkout T his will assure that the point to point wiring between thel O module terminals and the field wiring terminal blocks has been done correctly To start create a list with each I O point shown and include any details of what criteria is being tested It is also helpful to include a check box that can be used to check off each point after it is tested N ormally t
13. coupling where low DC resistance is important and to a lesser extent Capacitive and inductive coupling Spiral Shield The spiral shield consists of wire usually copper wrapped in a spiral around the inner cable core T he spiral shield is used for functional shielding against diffusion and capacitive coupling at audio frequencies only Combination Shield T hese cables consist of more than one layer of shielding T he combination shield is used to shield against high frequency radiated emissions coupling and electrostatic discharge ESD It combines the low resistance of braid with 100 coverage of foil shields and is one of the more commonly used types of shielded cable in today s industry Figure2 shows atypical cross sectional area of a shielded cable that makes use of combination shields Mounting of Electronic Instrumentation Electronic instrumentation is typically installed inside an enclosure with other devices T herefore the installation of the 4 3 Shield Pairs 2 Foil Shield Filler as req d Braided Shield Jacket Figure 2 instrumentation must take into consideration that the panel layout accommodates all the necessary components In addition to the panel layout the following specifications should be considered Electronic instrumentation can be affected by interference from other electronic devices or EMI This interference causes static that may interrupt communications or signals from other devi
14. de NEC 2 the N ational Electrical M anufacturer s Association N EM A 4 O SH A and alist of items to consider when selecting an enclosure Bill of M aterials The Bill Of M aterials BO M should list each component in our automated control system the quantity of each component any designations or marks that allow us to easily identify the component on our schematic a description of the component and its part number W e also have comments or remarks about the component that will help the panel builder know what needs to be done when the control panel is being built Figure 9 is a short example of a bill of materials ITEM MARK DESCRIPTION PART NUMBER REMARKS 1 1 CB104 Circuit Breaker 3 pole 30 Amp 480 VAC G3P 030 2 1 TRF107 Control Power Transformer 230 460 VAC Pri 115 VAC Sec 250 VA CPT115 250 F Fuse Class CC Current Limiting Fast acting 600 VAC 1 AMP HCLR1 MEQ2 Fuse Time Delay 500 VAC 2 AMP Pushbutton 30mm Momentary Flush Head Green 1 NO Contact HT8AAGA Pushbutton 30mm Momentary Extended Head Red 1 NC Contact HT8ABRB Contactor 9 AMP 110 120 VAC Coil SC E02 110VAC Motor Starter A Overload Relay 6 9 AMPS Adjustable TK E02 900 Motor Starter A LS308 Limit Switch Side Rotary Lever 1 NO and 1 NC Contact ABP1H41211 Figure 9 TheBOM can bein theform of a table drawn on one of the sheets along with the schematic and panel layout It can also be done as a spreadsheet which wo
15. derstand as much as possible about automation control system devices and their terminology so that your communications with the System Integrator go faster and more smoothly In most cases special expertise is required to design and install industrial automation control systems Persons without such expertise or guidance should not design and install automation control systems because they can fail and cause serious injury to personnel or damage to equipment The information provided in this series of articles is provided as is without a guarantee of any kind Wedo not guarantee that the information is suitable for your particular application nor do we assume any responsibility for its use in your application It is our intent to produce this series of articles as a usable guide with additional information including a typical real world application that can be followed from concept to completion It is not our intent for the guide to cover every possible topic dealing with automation control systems or to even suggest that the topics being covered are fully detailed Instead the topics are aimed at giving the reader a good starting reference for automated control systems In Part 1 we will cover the topics of Safety and Identifying an operation or process that could benefit from automation In upcoming issues we will cover control device specification control system design and construction control system installation and finally con
16. e an operation or process suited for automating to tips on setting up a program to maintaining the control system Whether you are an expert or a novice at electrical control devices and systems the information presented should give you a check list to use in the steps to implementing an automated control system Electrical control systems are used on everything from simple pump controls to car washes to complex chemical processing plants Automation of machine tools material handling conveyor systems mixing processes assembly machines metal processing textile processing and more has 1 1 increased productivity and reliability in all areas of manufac turing utilities and material processing You may have come to realize that an operation or process used to produce your end product is very laborious time consuming and produces inconsistent results You may have also visualized ways that would allow you to automate the operation Automating the process will reduce the amount of manual labor improve throughput and produce consistent results You may have the skills to develop the mechanical means and select the appropriate equipment to make this happen and although you have a basic understanding of elec trical control devices you may not have the experience to put it all together Your first option may be to enlist the help of a qualified System Integrator If you do decide to use a System Integrator it would be beneficial to un
17. e could drop low enough to cause inaccurate input data Also the use of an isolation transformer for example 115 VAC primary to 115 VAC 3 3 secondary can provide additional suppression of EM from other equipment Isolation transformers should be used near equipment that produces excessive electrical noise If DC power is required in our control system we need to calculate the worst case amperage draw load of all the devices that will be powered from the D C supply W e also need to look at the amount of ripple the devices being powered can tolerate and select a DC power supply that can meet the most stringent requirement Ripple is the amplitude of the AC component that rides on the DC voltage signal A typical rating for most applica tions involving DC powered sensors would be 100 mV peak to peak It is also a good Idea to double the calculated amperage capacity of the DC power supply T his is especially important if our control system needs to meet Underwriters Laboratories Inc UL 508A T he next section of our schematic will show the hard wired devices that are powered from our control voltage 115 VAC If our control logic is based on hard wired relays this is where we would show the hard wired connections along with the normal 115 VAC powered devices such as D C power supplies 115 VAC power to PLC power supplies auxiliary devices etc Figure 6 isa partial example of the hard wired section of our schematic
18. e other means to deal with it such as applying a brake to rotating equipment to stop it before personnel can come in contact with It Grounding To prevent electrical shock incorporate good grounding practices in the design construction and installation of your system U se protective devices for faulted conductors to prevent fire and also realize that good grounding practices can reduce electromagnetic and radiated noise interference to sensitive electronic devices Control Power Distribution Develop a power distribution scheme in the control system circuitry according to code that ensures all circuits are protected with fusing circuit breakers or other interrupting means coordinated such that only the faulted circuit will be opened de energized allowing other powered equipment and devices to continue to operate Unauthorized Access M ake sure all enclosures and cabinets that have energized circuits are secured to prevent unauthorized personnel from gaining access without the proper tool key or other authorized means 1 3 Finger Safe and D ead Fronts Another safety area to consider is the use of devices that have finger safe terminal connections which are surrounded by insulated guarding T he use of protective guards over live circuits should also be consid ered even on control panels that have limited access so it is Safer for maintenance electricians and authorized personne to troubleshoot or make adjustmen
19. e the interference from electrical noise Some instrumentation requires the use of shielded cables for specific connections When installing instrumentation verify whether any connection requires a shielded cable Failure to use the shielded cable will result in erratic readings or signals from the instrumentation If the product being installed requires shielded cables the grounding specifications provided by the manufacturer manual must be followed Improper installation of shielded cables may cause a ground loop that will cause failure on a processor or would allow noise into the logic circuit There are various types of shielded cable available for different uses The shielded cables listed below are the most commonly used for automation control systems and instrumentation e Foil Shield T hese cables consist of aluminum foil laminated to a polyester or polypropylene film T he film provides mechanical strength and additional insulation The foil shield provides 100 cable coverage for electrostatic shielded protection Foil shields are normally used for protection against Capacitive electric field coupling where shielded coverage Is more important than low DC resistance Braided Shield T hese cables consist of groups of tinned bare copper or aluminum strands O ne set is woven in a clockwise direction then interwoven with another set in a counter clockwise direction Braided shields provide superior performance against diffusion
20. ed down etc Photoelectric sensors are sensitive to the atmosphere in which they can efficiently work If there is a lot of dust dirt and or mist in the air then the optics can easily become dirty or coated reducing their sensitivity and oper ating distance Output devices Output devices are used to control actions such as motion start stop of equipment like conveyors and pumps on off control of valves operator alerts prompts status indica tions etc Typical output devices include relays motor starters pilot lights operator interface graphics and numeric display etc These output signals like input signals are also discrete either ON or OFF The signals can be wired from a PLC output module to control the devices such as starting and stopping motors energizing a valve to control water flow illu minating a pilot light to alert an operator to a condition such as Bin Full etc O utput signals can also be wired directly to a controlling device using hardwired relay logic Of importance to our specifications are the ratings of these output devices e What isthe operating voltage 24 120 or 240 Volt AC or DC e If they are rated for DC are they sinking or sourcing e What isthe current draw e What isthe duty cycle e What is the operating temperature range e What are the mounting dimensions For example if our process uses a solenoid valve to control water flow to a wash station we would need to know the ope
21. esign our control panel with mating connectors so that the field wiring could be plugged into connectors mounted on the panel Wiring Recommendations T he following guidelines provide general information on how to wire most automation equipment For specific information on wiring a particular PLC or device refer to the 4 1 installation manual for that device or PLC e Each terminal connection can accept one 16 AWG or two 18 AWG size wires Do not exceed this recommended capacity e Always use a continuous length of wire Do not splice wires to attain a needed length Use the shortest possible wire length e Use wire trays for routing where possible e Avoid running control wires near high energy wiring e Avoid running input wiring close to output wiring where possible e To minimize voltage drops when wires must run along distance consider using multiple wires for the return line e Avoid running DC wiring in close proximity to AC wiring where possible e Avoid creating sharp bends in the wires Install a powerline filter to reduce power surges and EM I RFI noise Important W iring Safety Recommendations Warning Providing a safe operating environment A N for personnel and equipment is your responsibility and should be a primary goal during system plan ning and installation Automation systems can fail and may result in situations that can cause serious injury to personnel or damage to equipment Do not
22. g a control system that would fall into the category of a Supervisory Control And D ata Acquisition SCAD A sytem M ost of these control systems would be comprised of PLC type I O that interface to a PC with appropriate software Part 2 will cover Control Device Specification References For information on PLC Logic and Principles by D oug Bal of InterC onnecting Automation PLC training books and training through technical schools and organizations visit http support automationdirect com docs training pdf For Web sites with free tutorials that cover basic electricity principles visit any of the following www thelearningpit com elec bas theory etb menu html or http www kilowattclassroom com Index htm or http www mrplc com or http www plcs net You may also want to visit the AutomationD Irect Customer Forums at http forum1 automationdirect com cgi bin U Itimate cgi Information for the N ational Electrical M anufacturer s Association NEMA can be found at their Web site at http www nema org NEM A is also being harmonized with the International Electrotechnical Commission IEC Web site www iec ch and other European standards Additional information can be found at Global Engineering D ocuments Web site at www global ihs com Global Engineering D ocuments is also the source for obtaining NEMA IEC and CE documents Further information for the N ational Fire Protection Association NFPA can be found at
23. g are some of the items you may want to consider in your maintenance schedule e Check and record voltages at various circuits e Tighten all connections with power removed e Check backup batteries and or replace on a routine schedule e Check indicators and perform lamp tests e Visually inspect for loose or frayed wiring moisture in enclosure etc e Check to make sure plug in connectors are tight and secured e Test all alarm systems horns sirens etc e Check and record any configuration settings e Perform and record calibrations e Check all I O points on a yearly basis e Check and record power usage e Check equipment run times for determining maintenance or replacement e M easure device current to set a benchmark and compare for changes e Review any diagnostic history including events and alarms e Check diagnostics that may be programmed into the H MI operator interface Footnotes 1 Information for the N ational Electrical M anufacturer s Association NEMA can be found at their Web site at http www nema org NEM A is also being harmonized with the International Electrotechnical Commission IEC Web site www iec ch and other European standards Additional information can be found at Global Engineering D ocuments Web site at www global ins com Global Engineering Documents is also the source for obtaining NEMA IEC and CE documents 2 The National Fire Protection Association NFPA produces the National
24. hat have to be moved or jogged to the proper position Accidental Powering of Outputs Do not rely on the automation control system alone to provide a safe operating environment You should use external electromechanical devices such as relays or limit switches that are independent of any electronic controlling device such as a solid state relay or aPLC output module to provide protection for any part of the system that may cause personal injury or damage T hese devices should be installed in a manner that prevents any machine operations from occurring unexpectedly For example if the machine has a jammed part the controlling system or PLC program can turn off the motor rotating a saw blade H owever since the operator must open a guard to remove the part you should also include a bypass switch that disconnects all system power any time the guard is opened Orderly Equipment Shutdown Whether using a control system designed around relays and timers or aPLC an orderly system shutdown sequence should be included in your design If a fault is detected then any mechanical motion valve position etc needs to be returned to its fail safe position and the equipment process stopped T hese types of problems are usually things such as jammed parts broken cutting tools bin full etc that do not pose a risk of personal injury or equipment damage If a detected problem would result in risk of personal injury or equipment damage then us
25. hic results Another reason might be for insurance or compliance purposes O ne of the most specified and premier safety testing laboratories is Underwriters Laboratories UL The most applicable area of interest for control systems is U L s Standard for Safety 508A If your control system panel requires being built to UL508A then you will need to contract directly with UL to become a UL508A panel builder or use an existing UL508A pane builder Additional information can be found at http www ul com controlequipment devices html Compliance to UL508A for AutomationD irect products can be found on our Web site at http support automationdirect com compliance html T he following are other safety points to consider in the design of your automated control system Emergency Stop The control systen must provide a quick manual method of disconnecting all system power to the machinery equipment or process T he disconnect device or switch must be clearly labeled Emergency Stop After an Emergency shutdown or any other type of power interruption there may be requirements that must be met before the control sytem or PLC control program can be restarted gt gt 1 2 Technical Review WAUTOMATIONDIRECT For example there may be specific register values in the PLC memory that must be established or maintained from the state prior to the shutdown before operations can resume T here may also be mechanical positions of equipment t
26. his list can be created from an 1 0 list or tag name list that was created when designing your PLC ladder logic or H M I operator interface Include the test criteria for each point on the list As an example discrete input and output points would be listed as normally off and then checked for their on state Analog points both input and output could be checked at different values For example if using a current input module you may want to simulate 4 mA low value 12 mA middle value and 20 mA high value The actual testing normally requires a two person team O ne person usesaPC connected to the PLC to view the status of each point tested and to simulate outputs and the other person physically applies a signal to inputs and monitors outputs with the use of an indicator on discrete outputs and a meter on analog outputs Start U p The startup of our automated control system begins once we have installed our control system enclosure and auxiliary equipment terminated all field wiring and completed required testing This process is also called commissioning the automated control system and related egui pment process Asa starting point it is best to isolate the various sections of our control system power wiring by removing the fuses and or opening circuit breakers The best tool to use during commissioning isthe schematic diagrams We will want to start at the incoming power and basically work our way through the
27. i sion making going on in between so that the outputs are controlled based on the status of the inputs This brings us to our third cate gory the decision making element This element can be performed by a PLC where we have inputs outputs and a central processing unit CPU that uses ladder logic programming to make deci sions based on input status and the logical conditions in the program see figure 3 A similar device that can be looked at in the same manner is a personal computer PC The keyboard mouse scanner etc would serve as input devices and the monitor printer soeakers etc would be the output devices T he microprocessor used on the motherboard along with its memory the operating system and the application program would serve as the decision making element As a matter of fact PC s are used In some automated control systems as the decision making element together with industrial input and output 1 0 modules T hese PC based systems rely on the communication ports or Ethernet connections to monitor and control the I O The application software typically allows a programmer to develop agraphical interface that gives an oper ator interaction with the equipment or process With some research and experience you will learn how to determine how much decision making ability your control system requires Cost restraints may require you to compare implementing the control system with either aPLC PC based c
28. ing isa must to protect your automation equipment and devices from serious damage failures and even potential risk to users Grounding is the foundation of achieving a reliable power distribution system During the panel and control system build it is important that a reliable grounding system be implemented Poor grounding or improper or defective wiring may be the cause of most problems affecting power quality T he following is a list of existing grounding standards that may be used for reference e EEE Green Book Standard 142 e EEE Emerald Book Standard 1100 e ULOGA Installation Requirements for Lightning Protection Systems e AEA 1996 International Association of Electrical Inspectors Soars Book on Grounding e EC amp M Practical Guide to Quality Power for Electronic Equipment Military H andbook Grounding Bonding and www automationnotebook com Shielding of Electronic Equipment Once all the important considerations mentioned above have been determined the mounting bonding and grounding of the chassis may be started T he following list provides a brief explanation of each of these terms e M ounting refers to the actual physical installation of each device instrument or component to either the Subpane or other connected equipment e Bonding refers to the joining of metallic parts of a chassis such as frames shields assemblies and enclosures Joining or bonding these components properly reduces the
29. ion in this series of arti cles is provided as is without a guarantee of any kind With that said the first skill we need to develop in this effort will be the gathering of all the equipment parameters and specifications needed to specify the devices required to control the equipment We need to be the proverbial detective who would ask questions such as n Part 1 of Control Systen Design we covered safety e What is the operating voltage e What isthe power rating e How much current does it draw e What is the operating temperature range e What isthe relative humidity range e What arethe mounting dimensions e What arethe minimum mounting clearances e What isthe duty cycle e How will the system be used e Who will be using the control system 2 1 Control System D evices T he devices you need to specify in your control system will generally fall into one of three categories input devices output devices and the processing unit Input devices Input devices are used to sense a condition detect move ment or position indicate a limit or set point has been reached sense intervention by an operator detect an alarm etc Typical input devices may includelimit switches photoelectric sensors pushbuttons proximity sensors an operator interface etc T hese input signals are generally in an ON or OFF state We can look at an input from a device such as a photoelectric sensor used to detect an obstruction and
30. ission IEC Web site www iec ch and the European H azardous Location Ratings For more information check the Instrument Society of America s ISA Web site at www isa org Additional resources on the subject can be found at www ul com hazloc Another area of safety that needs to be considered for automated control systems is lockout tagout procedures as specified by Occupational Safety and H ealth Administration OSHA Lockout tagout refers to specific practices and procedures to safeguard operators and maintenance personnel from the unexpected energization or startup of machinery and equipment or the release of hazardous energy during service or maintenance activities In order to have your control system make use of a lockout tagout procedure the design should include the ability to shut off neutralize or isolate any energy source such as the main electrical feed but also any pneumatic hydraulic or mechanical energy storage device The means to do this should be considered in the initial design of the automated control system Additional information can be found on O SH A s Web site at http www osha gov SLT C controlhazardousenergy There are many reasons why the electrical devices that you will use in the design of your automated control system should be listed approved or registered with a testing labora tory One reason is to ensure that the device meets standards that will prevent failure that could lead to catastrop
31. nema org N EM A is also being harmonized with the International Electrotechnical Commission IEC Web site www lec ch and other European standards Additional information can be found at Global Engineering D ocuments W eb site at www global ihs com Global Engineering D ocuments is also the source for obtaining NEMA IEC and CE documents 5 Additional information can be found on OSHA s W eb site at http www osha gov SLT C controlhazardousenergy 6 Additional UL information can be found at http www ul com controlequipment devices html 3 1 WAUTOMATIONDIRECTi Technical Review A Condensed Guide to Automation Control System Specification Design and Installation Part 3 Design by Tom Elavsky AutomationDirect n Part 2 of Control System Design we covered specifying the various devices required for interfacing and controlling the electrical equipment in an automated control system In Part 3 we will cover the steps needed to design our auto mated control system The design topics will include planning by defining our sequence of operation creating a schematic with the devices shown in ahigh voltage to low voltage order input to output design layout panel layout wiring diagrams bill of materials software tools to document our design choices between using hard wired relays versus a PLC with programming etc As stated in the previous articles special expertise is gener ally required to design wi
32. o have your control system UL 508 approved and will allow the power supply to operate at a lower temperature thus increasing its life gt gt 2 3 WAUTOMATIONDIRECT T he following table is an example of NEM A s common environmental specifications that generally apply to automation equipment IEC also has a list of common environmental specification designations for enclosures and equipment COMPARISON OF SPECIFIC NON HAZARDOUS APPLICATIONS INDOOR LOCATIONS PROVIDES A DEGREE OF PROTEC TYPE OF ENCLOSURE TION AGAINST THE FOLLOWING ENVIRONMENTAL CONDITIONS 5 6 11 Incidental contact w encl equip e e Falling dirt e e Falling liquids and light splashing Dust lint fibers and flyings Hose down and splashing water Oil and coolant seepage Oil or coolant spray and splashing Corrosive agents Occasional temporary submersion Occasional prolonged submersion Static Electricity Most equipment and devices will operate down to 5 relative humidity H owever static electricity problems occur much more frequently at humidity levels below 30 M ake sure you take adequate precautions when you touch the equipment Consider using ground straps anti static floor coverings etc if you use the equipment in low humidity environments Agency Approvals Some applications require agency approvals for particular components Some of these required appro
33. ontrol or simple hardwired relay logic But don t forget the fact that a PLC gt gt Sinking Output Typical wiring for sinking and sourcing output devices Figure 2 Decision Making Figure 3 2 2 or PC based control system allows easier changes and future expansion O ther device types Another area of inputs and outputs involve the use of analog signals in a control systen Analog signals are variable and can reoresent a range of values AS a quick example we may want to monitor the level of a liquid in a tank that is 100 feet tall We can use a sensor that will produce a signal that is reoresented by a voltage range of 0 to 10 volts DC with 0 feet being equal to 0 VDC and 100 feet being equal to 10 VDC Analog signals are typically linear so a5 VDC signal would tell us the tank level is at 50 feet The analog signal could be wired into aPLC analog input module and in the ladder program we could compare the actual level to a set point and produce a discrete signal that would cause an output point to start a pump to raise or lower the level Inductive devices W hen selecting a device to control a prime mover such as an industrial motor to power a conveyor or a valve to control a hydraulic cylinder you will need to determine the ratings of the equipment e What is the operating voltage e What isthe maximum current draw e What type of environment is it being used in An industrial induction mot
34. ontrol system design T he more details we can show on the schematic to reflect all points that need to be grounded the better chance we have of a properly grounded control system that provides both safety and functionality 3 Phase Electrical Power Distribution Li L2 L3 ke Figure 5 3 Phase Devices Why is grounding important Electronic instrumentation such as PLCs and field I O are typically surrounded by various types of electronic devices and wires T hese electronic devices may include power supplies input output signals from other instru mentation and even devices that are near the instrumentation enclosure All these may present a risk of Electromagnetic Interference EM or transient interference T histype of interfer ence may cause failure or erratic operation of the device We should consider using a second transformer to source AC power to DC power supplies Input circuits should be utilized to isolate the output circuits and prevent voltage from the output transients spikes from being induced into the input circuits In some cases we may need to use a constant voltage transformer to stabilize the incoming AC power source supplying the PLC to minimize shutdowns due to power surges voltage dips and brownouts W hen using a constant voltage transformer to power a PLC the sensors connected to the PLC Inputs should use the Same power source O therwise the AC source voltag
35. or may have ratings such as 230 460 VAC 3 phase 1725 RPM a FLA full load ampere of 10 5 amps at 460 VAC etc This information can be obtained from the manufacturer s catalog or directly from the motor nameplate In the case of a motor you will need the ratings to choose the motor starter or possibly a variable frequency drive for either start stop control or speed control of the motor O ther considerations T here are other points to consider in the specification of devices being used in your automated control system duty cycle surge suppression enclosure type heating and cooling power environment static electricity and agency approvals Duty Cycle When using a solenoid valve you will want to know its operating voltage nominal current draw and current inrush to help select the type of output device required to control its operation It is also important to have an understanding of the solenoid valve s duty cycle time on vs time off We would not want to operate a solenoid valve rated at 50 duty cyclein a continuous mode with an on time of 10 seconds and an off time of only two seconds T he short off time would not allow for the solenoid to properly cool down Surge Suppression Solenoid valves motor starters etc make use of an inductive coil for their operation and the coil can produce high voltage spikes that can damage output devices and nearby electronic equipment It is always recommended to use some form of su
36. our product onto a conveyor The cylinder fully extended signal is used to de energize the solenoid valve that provided the air pressure to the pneumatic cylinder We also need a limit switch to indicate when the cylinder has fully retracted and provide a signal to the start stop control of the conveyor that the product push cylinder is out of the way for the next product Another appli cation for a sensor is to indicate when the product has reached the conveyor T he sensor can bea limit switch with a roller arm that comes in contact with the product or a photoelectric sensor that can detect the product by using an infrared beam of light The photoelectric approach may be the better choice because the position of the product on the conveyor belt may vary Refer to our Web site at www automationdirect com static specs peselection pdf for information on selecting photoelectric sensors We would continue with this analysis looking at each piece of equipment or component in our system and select a device that could control or sense it Some examples include an electrical solenoid valve to control water used to wash residue from a product or a pneumatic valve to control air pressure to a cylinder operating a gate that diverts product on a conveyor or energizing a control relay to signal that a product is in position on a scale In some instances we may need to vary the speed rate or position of our controlling device such as varying the s
37. peed of a conveyor changing the amount a valve opens to control a flow rate or remotely changing the setpoint level for a tank This could be accomplished by using an analog output signal An analog output signal is a varying signal that corresponds to the real value we have determined and calibrated into the device For example a0 to 10 VDC signal could represent a conveyor speed of 0 to 500 feet per minute An analog signal to the speed controlling device for the conveyor motor of 5 VDC would result in aconveyor speed of 250 feet per minute Identifying devices to control motion flow events etc and sensing completion is basically identifying thel O inputs and outputs of our control system Once these devices are identified they can be used as the field devices in a PLC based www automationnotebook com system or they can be hard wired for simpler applications You will also want to determine if your automated control system will benefit from the use of an operator interface also referred to as a Human M achine Interface H M1 If your process requires making changes to setpoint values process time flow rates etc then the useof an HM I is the best way to proceed In these situations you will most likely need a PLC that can easily communicate with the H M I device If your application requires keeping data records for reference traceability history trending meeting regulations etc then you should look at usin
38. program such as M icroSoft Visio M icroSoft s W ord software program has a built in drawing tool that contains flowchart symbols In some cases the application may be better suited to using a timing chart in which each condition and event is graphed in a time relationship to each other as shown in Figure 2 Once we have a sequence of operation developed and a list of our input and output devices we can deter mineif our automated control system is best suited for hard wired relay logic or can benefit from a PLC 1 A PLC can be cost effec tive when used in place of only ahalf dozen industrial relays and a couple electronic timers It adds the flexibility of making future logic changes without the labor of making wiring changes Homing Operation Figure 1 Pusher Return Sensor Cleared Sensor Actuated Part in Position Dwell Timer 2 seconds Part Pusher Figure 2 Time Schematic T he next step in our design is to develop a schematic M ost electrical designers and engineers define a schematic as a drawing that shows the logical wiring of an automated control system A control schematic is normally drawn in the form of a ladder showing the various wiring conditions T his analogy of a ladder is what PLC ladder logic was based upon It made the transition to PLC ladder logic easier for engineers and electricians because they were accustomed to trouble shooting hard wired relay control system
39. rating voltage of the valve and how much current it draws We also need to know not only the on state current draw of a valve but also the inrush current so that we can properly specify a PLC output module or a control relay Although a valve may be rated to draw 250 mA continuous current it may havean inrush of 800 mA when first energized If an output module has eight output points and each point is rated for 1 amp continuous duty after thermal considerations the entire output module has a total rating of 6 amps and therefore has a common fuse rated at 6 amps If we had solenoid valves connected to all eight output points and our program called for them all to energize at the same time the total inrush current would be 8 times 800mA or 6 4 amps and most likely would blow the fuse The solution could be to select an output module with a higher current rating or to use the ladder program to sequence the valves preventing them all from being energized at the same time Another option is to split up the valves between several output modules using the remaining points to power smaller loads such as pilot lights www automationnotebook com Certain output types may have derating curves depending on the ambient temperature and the number of outputs energized Keep in mind that DC output modules can be sinking or sourcing type The processing unit All control systems can typically be defined as having inputs outputs and some form of dec
40. re install and operate industrial automation control systems Persons without such expertise or guidance should not attempt control systems but should consider seeking the services of a qualified System Integrator Control systems can fail and may result in situations that can cause serious injury to personnel or damage to equipment The information provided in this series of articles is provided as is without a guarantee Design T he design for our control system will be in the form of a documenting task The challenge will be to get our design specifics down on paper so that it can be easily understood It isimportant that anyone can look at our documents In the future and be able to interpret the information U seful to us at this step will be any notes and lists that were developed during the Identifying and Specifying phases of our automated control system Sequence of O peration In most cases the first tep in designing our control system will be to define the process or actions to take place by way of a sequence of operations description T he sequence should show or list each operational step in our process Our particular application may be better suited to using a flowchart that shows the sequence of operation by means of deci sion making steps and actions that need to take place Figure 1 shows a partial example of a flowchart A flowchart can be developed with graph paper and a pencil or an application software
41. rely on the automation system alone to provide a safe operating environment U se external electro mechanical devices such as relays or limit switches that are independent of the automation equipment to provide protection for any part of the system that may Cause personal injury or damage W arning Every automation application is different Therefore there may be special requirements for your particular application Be sure to follow all N ational State and local government requirements for the proper installation and use of your equipment Plan for Safety As we have stressed in previous articles the best way to provide a safe operating environment isto make personnel and equipment safety part of the planning process Examine every aspect of the system to determine which areas are critical to operator or machine safety If you are not familiar with system installation practices or your company does not have established installation guide lines you should obtain additional information from the following sources NEMA The National Electrical Manufacturers Association located in Washington D C publishes many different documents that discuss standards for industrial control systems You can order these publications directly from NEM A Some of these include e ICS 1 General Standards for Industrial Control and Systems ICS 3 Industrial Systems e ICS 6 Enclosures for Industrial Control Systems NEC The Na
42. rge suppression to eliminate these voltage spikes Enclosures Selecting a proper enclosure is important to ensure safe and proper operation of your equipment The minimum considerations for enclosures should include e Conformance to electrical standards Reference NEC 4 e Protection from the elements in an industrial environment Reference N EM A 4 e Common ground reference Reference NEC e Access to the equipment Reference O SH A e Security or restricted access R eference O SH A e Sufficient space for proper installation and maintenance of equipment H eating Coolin Ensure that the devices used in your control system aren t Subject to overheating or if installed in a colder climate the devices aren t being used below the listed low temperature Operating range Your control system because of its physical location may require you to have both a cooling system such as an A C unit and a small heating unit as part of the same enclosure This will ensure the devices are always operating within their temperature specifications Basic thermal manage ment is not difficult for most automated control systems Investing alittle thought during the specification stage can save you a great deal of redesign down the road DC Power If using DC voltage from a power supply in your control system consider using a power supply rated for at least twice the calculated load T his should satisfy one of the requirements if you need t
43. rient parts sense product in position detect when an Operation is complete etc As a simple example let s say we have a conveyor that moves our product from point A to point B The conveyor is powered by a 3 phase AC motor which is turned off and on by a manually controlled motor starter and includes for fire protection both short circuit and overload protection The system requires an operator standing at the motor starter to watch as the product reaches the entrance to the conveyor and to turn the conveyor on to move the product T he operator must also turn the conveyor off oncethe product has reached the discharge end To automate the conveyor we will need to replace the manually controlled motor starter with an electrically controlled motor starter including short circuit and overload protection We will need to size the motor starter to work with the existing conveyor motor Refer to our Web site at http www automationdirect com static specs fujimcselection odf for information on specifying and sizing motor starters W ewill also need to identify where to locate sensors such as limit switches photoelectric sensors proximity sensors etc that will indicate when an operation is completed This is required so our control system knows when to proceed to the next sep in our operation As an example we usually need a limit switch to detect when a cylinder is fully extended as in the case when the cylinder is used to push
44. s shown in aladder fashion It is normal practice as shown in Figure 3 to show input type devices on the left hand side of drawings and output devices on the right hand side For example the symbols for protective devices fuses contacts and overload relay elements are shown to the left while the symbol for the motor is shown to the right The schematic should start with the incoming power including protective devices such as circuit breakers and or fuses Our design should show the distribution of the AC power and include all circuitry and required devices for conformance to the National Electrical Code NEC 4 and any local codes that might apply in our area 480 VAC 60 Hz Li L2 L3 10 AWG BLK rao PUMP MOTOR 5 HP 7 FLA LT114 PANEL LIGHTING DOOR ACTIVATED SWITCH FLOURESCENT LIGHT 114A It isnormal practice to show any high voltage devices such as 3 phase motors 480 or 240 VAC auxiliary equipment etc in this first section of the schematic N ext we will show a control power transformer used to step the higher incoming voltage down to our system control voltage 115 VAC O ur control voltage can be something other than 115 VAC for example we could have a control voltage of 24 VDC which is common for many electrical control devices The control transformer needs to be sized VA rating based on our known or calculated load of devices that will be powered from the transformer in our automated
45. se the work for future electrical control system designs The drafting software can also be used to create our sequence of operation flowchart or timing diagram Add ins for the various drafting software packages are geared toward electrical control system design These add ins contain pre constructed elements of different manufacturers electrical devices This may include schematics of PLC 1 0 modules power supplies communication devices etc T hese pre constructed elements also Include scaled outlines of relays motor starters terminal blocks etc that can be dropped into your panel layout design One such add in package that works with AutoCAD and AutoCAD LT for electrical control design and includes some pre constructed elements for various PLC manufacturers is ECT s promis draw software More information on the promise draw can be found at http www automationdirect com static specs pcdrawspecs pdf T his software has the ability to act as a database for components that would be used in our control system design and can aid in coordinating the components between our schematic panel layout and bill of materials Discussion of automation control systems will continuein Part 4 Build Launch and M aintain Footnotes 1 For Considerations for Choosing a PLC refer to http support automationdirect com docs worksheet_guidelines htm 2 TheNational Fire Protection Association N FPA produces the N ational Electrical Code
46. state that when the sensor sees the obstruction the sensor is ON in other words we have a true condition W hen the sensor is not obstructed then the input is OFF or we can say the condition is false T hese types of signals are called discrete signals meaning they are always one of two states ON or OFF They can be wired into a PLC input module and the PLC can be programmed to use the status of the signals to execute the logic to control the automated system Or these same signals can be used in a relay logic system where control relays are hardwired to create the system logic Of importance to our specifications are the ratings of these input devices e What isthe operating voltage 24 120 or 240 Volt AC or DC e If they are rated for DC are they sinking or sourcing e What distances can they sense e How much force can be applied to the actuator e How much current do they require Asanote most PLC DC input modules can be configured as sinking or sourcing Sinking Input Also keep in mind that when selecting a sensor device such as to detect the presence of product or sense the end of travel for a mechanism it is very important to consider the environment in which the sensor will operate T his should Typical wiring for sinking Figure 1 not only include and sourcing input temperature and devices humidity ranges but in some cases Indoor or outdoor use altitude ability for the sensor to be wash
47. tc will go a long way in helping you understand the various codes and standards There are many good publications and articles on the subject of basic electricity and some local technical colleges offer courses covering subjects dealing with basic electricity Some even offer courses in Programmable Logic Controllers PLCs which can be very useful when dealing with automated control systems Also many Web sites offer free tutorials covering basic electricity and PLCs It would be beneficial to have some understanding of electronic devices such as the operation of a transistor and other solid state devices as well as understanding of the use and operation of electrical test and measurement instruments such as voltmeters current loop meters clamp on amp meters etc At aminimum you should follow all applicable sections of the N ational Fire Protection Association N FPA fire code and the codes of the National Electrical M anufacturer s Association NEMA There may also be local regulatory or government offices that can help determine which codes and Standards are necessary for the safe installation and operation of electrical control equipment and systems Please keep in mind that if the automated control system you are developing needs to be accepted in the international market the N ational Electrical Code N EC as a publication www automationnotebook com of NFPA is being harmonized with the International Electrotechnical Comm
48. tional Electrical Code provides regulations concerning the installation and use of various types of electrical equipment Copies of the NEC Handbook can often be obtained from your local electrical equipment distributor or your local library Local and State Agencies M any local governments and state governments have additional requirements above and beyond those described in the NEC Handbook Check with your local Electrical Inspector or Fire M arshall office for information Grounding Why is grounding important Electronic instrumenta tion such as PLCs and field I O are typically surrounded by various types of electronic devices and wires T hese electronic devices may include power supplies input output signals from other instrumentation and even devices that are near the instrumentation enclosure All these may present a risk of Electromagnetic Interference EM I or transient interference This type of interference may cause erratic operation of components and cause failures In addition to device interference automation equipment and devices could be damaged by powerful line surges T hese line surges may come from common voltage fluctuations from a power supply lightning or unintentional contact with a high voltage line A power surge will cause a temporary failure fuse burn up or even very serious damage to the equipment Grounding provides a low impedance path that limits these voltages and stabilizes interference Ground
49. trol system maintenance T he topics will be broken down as follows Part 1 SAFETY and IDENTIFICATION Part 2 SPECIFICATION Part 3 DESIGN and BUILD Part 4 INSTALL and MAINTAIN The first and most important item to consider before attempting an automated control system or even a simple on off control for a pump is safety both for personnel who may be working with or near the automated equipment as well as to prevent damage to the equipment To minimize the risk of potential safety problems you should follow all applicable local state and national codes that regulate the installation and operation of your control system along with the equipment or process it is designed to control T hese codes vary by area and usually change over time It will be your responsibility to determine which codes should be followed and to verify that the equipment installation and operation is in compliance with the latest revision of these codes M ost likely your control system will be dealing with electrical energy so your first goal will be to eliminate the risk of fire and electrical shock to personnel T he top organizations that provide applicable standards and codes are listed below but even before you get to this area of safety it would be wise to educate yourself as much as possible about electricity and electrical equipment in general A good understanding of basic electricity including DC and AC theory and practice Ohm s Law e
50. ts to electrical control devices D ead fronts should be used on control system enclosures where the operator needs to make adjustments to devices such as selector switches thumbwheels potentiometers etc and the controls need to be inside the enclosure so as to protect them from outside weather conditions T he dead front is normally an interior door that is mounted in front of the main control panel T he outside enclosure door may still require key entry by the operator but the dead front interior door with adjustable devices is interlocked so that it requires a switch to open it disconnecting power to the electrical devices mounted on the main control pana Closed loop Control It is your responsibility in any type of closed loop control system to ensure that if the feed back signal is lost the system shuts down so as not to cause injury to personnel or damage to the equipment Identifying Process for Automation T he first step in configuring an auto mated control system is to identify what can be automated You need to havea good understanding of basic electricity and safety It is TE mooie that you have an understanding of basic hydraulics pneumatics mechanical operating mechanisms electronics control sequences etc and a solid knowledge of the operation or process that you are going to automate You should understand how to control motion and movement regulate the flow of fluids dispense granular mate rials o
51. uld allow easy indexing and future referencing Wiring Diagram A wiring diagram sometimes referred to as an intercon necting diagram is used mainly for installation by the electrician for routing and terminating the wiring between the various devices and enclosures in the control system Figure 10 is a good example of a wiring diagram Main Control Cabinet A1 WELL HIGH LIMIT 1 2 SL Flex PUMP MOTOR Wire No s 3 129 HP 7 FLA 1 2 St Flex Wire No s 171 Wire No s 3 129 1T2 1T3 GND 1 2 St Flex Wire No s 3 129 Figure 10 3 4 Technical Review It should include all control enclosures or cabinets any external devices that are wired into control enclosures junction boxes conduits wireways etc The wiring diagram usually includes conduit sizes distances number of conductors between devices wire sizes colors wire numbers terminal blocks etc The wiring diagram is also useful for system startup and later for locating wire routing and devices during troubleshooting Design Tools Although all of the tasks related to documenting the design can be performed with nothing morethan a pencil paper and a ruler it is normally more efficient to use a software drafting utility such as AutoD esk s AutoCAD or AutoCAD LT software T he biggest advantage in using a software drafting program to create schematics panel layouts bill of materials and wiring diagrams is the ability to re u
52. vals are e UL Underwriters Laboratories Inc e CUL Canadian Underwriters Laboratories Inc e CE European Economic U nion T he requirements for any of these agency approvals need to be part of your specification and will determine the selection of most of your controlling devices Enclosure Lighting and Convenience Receptacle It is always a good idea to include interior lighting for your control system enclosure or cabinet to help during routine maintenance to the control system Provide a convenience receptacle inside the control system enclosure to supply power to test equipment calibration equipment etc Product Selection Suppliers literature and Web sites are an excellent resource for evalulating product specifications For industrial control product selection information for AutomationD irect products refer to http www automationdirect com static specs productselec tion html 2 4 Netdepis Part 3 Control System D esign Footnotes 1 For Sinking and Sourcing Concepts refer to www automationdirect com static specs sinksource pdf 2 For Considerations for Choosing a PLC refer to http support automationdirect com docs worksheet_guidelines html 3 For more information on surge suppression for outputs see chapter 2 of any PLC user manual available on our W eb site 4 Information for the N ational Electrical M anufacturer s Association N EM A can be found at their W eb site at http www
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