Chapter 11 - Automatic
Contents
1. e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic Chapter XI Control of an application with a frequency converter There are several ways how to control drives with a PLC The start and stop features are easily controlled with the DOs of a PLC but the speed control varies from application to application The simplest way is to use the built in DIs of the frequency converters For example with two DIs available four different preset frequencies can be called as shown in the example in Table 0 1 It is required to study the table in the drive user manual because variations exist between different drive manufacturers Combinations of three DIs on a drive already allow 8 discrete frequencies to be set Table 0 1 Preset frequency selection according to the state of inputs Digital Input 1 state Digital Input 2 state Preset frequency po Preset frequency 2 po Preset frequency 3 The preset frequency values are pre programmed using the drive menu The DIs of the drive are electrically connected to DOs in the S7 300 PLC SM All that has to be done in STEP 7 is to write a program that controls these output values in accordance with application specific input conditions determined by the system designer The drawback of using the digital signals is that only a limited number of previously determined frequencies can be used To overcome this most variable speed drives have also built in Als usually 0 10 V or 4 20 mA This allows the PL2. AutoMatic No Is it working time Is the working time timer signal canceled Have movements been detected on one floor or on none of the floors Have movements been detected on three or more floors No Ventilation pump works with the lowest speed Ventilation pump works with the highest speed Ventilation pump works with the medium speed Switch the ventilation pump off END Fig 0 4 Flowchart of ventilation pump control The variables used in the PLC programs are shown in Table 0 3 Speed and Speed3 are auxiliary variables to simplify the program writing Table 0 3 Variables of the project TIMER I 124 4 BOOL Signal is 1 during 8 am 5 pm Mon Fri All other times it is 0 e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic START Is it working time Is the working a time timer signal canceled Movement on floor 1 Movement on floor 2 Movement on floor 3 Movement on floor 4 Open the floor 4 air valve END Close the floor 4 air valve Fig 0 5 Flowchart of air valve control The PLC program is written in LAD language The program printout is represented below Also comments to every part of the program are added APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic Network 1 Movement sensor 1 Network 2 Movement sensor 2 L1 SEN L2_SEN S OFFDT Q Netw
3. a frequency converter FC four contactors K1 K2 K3 K4 four air valves with electrical motors LI_VAL L2_VAL L3_VAL L4_VAL and one Siemens S7 300 PLC with digital I O module s e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic L1 L2 L3 Li L2 L3 PE Frequency converter U1 V1 W3 PE Fig 0 2 Electric diagram for the power part PLC program The ventilator can ta work on three speeds The lowest speed is used when any of the movement sensors does not have an output signal or only one does The medium speed is used in case two motion sensors have an output signal and the fastest speed is used in the case of a simultaneous signal from three or four sensors These speed values are preprogrammed on the frequency converter and controlled via its DIs according to Table 0 1 Preset frequencies 1 2 and 4 from the table are used It is recommended to set 51 Siemens 57 300 PLC digital input output module ov ow orfoa 05 ons 6 7 F ofie Frequency converter JKL KLO Co OV O Fig 0 3 Electric diagram for control circuit low voltage part the preset frequency 3 to be the same as the preset frequency 2 It is not used in the control program but in this case if the wires connecting the outputs of the PLC to the inputs of the frequency converter are mixed up the program still works as intended e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic To pre
4. C to control the speed of the drive directly with an analog signal Usually the lowest analogue signal O V or 4 mA equals 0 Hz of the frequency converter and the highest analog signal 10 V or 20 mA to 50 Hz This analog signal controlling the drive speed can for example be the output of a PID control FB Another way to control a frequency converter is to use field bus communication like PROFIBUS ROFINET MODBUS etc Besides speed control it allows one to change other frequency converter parameter values like acceleration deceleration choose different speed control modes and monitor electrical drive parameter values like voltage current torque etc Communication possibilities of a frequency converter field bus are covered in detail in the user manual Application example In this application example a control system of the ventilation in a warehouse with 4 floors is given Fig 0 1 2 Every floor has an infrared motion sensor to detect if workers are present and it sends a signal to the PLC which controls the speed and air valves of the ventilation pump There is a press button S1 on floor that allows the ventilation system to work over time e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic RESET TIMER 51 Fig 0 1 Warehouse layout Electric diagram The electric part of the system consists of four infrared motion sensors L1_SEN L2_SEN L3_SEN L4_SEN one separate timer TIMER one pushbutton switch S1
5. d fastest speeds which were detected with networks 5 and 6 Preset frequency 2 is used therefore the first input signal of the frequency converter is set to 0 and the second input is set to 1 Speed1 Speed3 TIMER FC IN1 R _ EC IN2 4 s gt _ Network 8 F requency converter is switched on NR a Sa LL S 1 TIMER s FC WORK i m S 1 E Ae 9 F loor air valves are open is ae ee ee EN TEA D SEATA A AATA AAE TIMER TO Ll VAL gin T1 L2 VAL m O T2 L3 VAL m gt T3 L4 VAL m gt For example when during work hours a movement is detected on floor 3 then this floor sensor signal is extended over 5 minutes Network 3 The ventilation is set to work on the APPLICATION OF PLC IN INDUSTRIAL AUTOMATION m O AutoMatic lowest speed Network 5 and the valve on floor 3 is opened Network 9 After 5 minutes if no more movements were detected there floor 3 valve is closed Also warehouse lighting system can be fully automated so that the workers do not have to switch on and off the lights To update the application only four contactors are needed which switch each floor lights on and off and a simple program change is needed in network 9
6. ext to outgoing arrow lines In most cases the answers are Yes and No or True and False Delay Presents the waiting period or delay time that is normally part of a process Wait for 258 Data Presents data input and or output from the Show filled flowchart bottle number on the display e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION AutoMatic Name of Symbol Description meaning symbol Stored data Presents data storage in the flowchart Store the working hours of the machine Predefined Presents a process step or series of process process Bottle fill steps that are declared and defined elsewhere ila in different parts of the same document Connector Presents a jump from one point to another in the flowchart The connectors show jumps which occur on same page Connectors are usually labeled with capital letters A B AA to show matching jump points Off page Presents a jump from one page to another in connector the flowchart this means that the flowchart continues on the next page Off page connectors are usually labeled with numbers 1 2 11 Figs 0 4 and 0 5 present the control algorithms flowcharts of the warehouse ventilation system The first algorithm controls the ventilator and the other one controls the air valves Sample project programs composed according to the algorithms are executed separately and almost at the same time e APPLICATION OF PLC IN INDUSTRIAL AUTOMATION
7. ork 3 Movement sensor 3 Network 4 Movement sensor 4 T T3 L _SEN S5 OFFDT _ LA_SEN 5 OFFDT Qi S Q S5T 5M TV SST 5M TV BI j Networks 1 to 4 are used to program the life span of the signals from movement sensors If a movement is detected the timer output is activated When the signal from the sensor disappears the timer is started and the output is kept active during the time written in input TV When a new signal is detected from the sensor the timer is reset Bi only one movement sensor is active or none are active and it is working time then the ventilation pump works with the lowest speed Since this is the lowest speed the Preset frequency 1 is used and both inputs of the frequency converter are 0 TO T1 T2 TIMER FC INL R FC IN2 R Speedl H If at least ate movement sensors have signal 1 and it is working time then the ventilation pump works with the highest speed In this case Preset frequency 4 is used therefore both input signals of the frequency converter are set to 1 APPLICATION OF PLC IN INDUSTRIAL AUTOMATION Tl T2 TIMER FC IN1 Ss T1 T3 LLJ S 1 W FC I N 1 S T2 T3 __ H Speed3 T2 T3 If it is working time and the ventilation pump is not working with the lowest and highest speed 1 then the ventilation pump works with medium speed This is done by eliminating the lowest an
8. vent turning off the ventilation during short periods of no movement in the storage area timers TO T1 T2 T3 in the control program are used to increase the life span of the signals from motion sensors If a movement is detected during the signal delay period the corresponding timer value is reset An external timer is used to turn the ventilation off after the work day and to turn it back on again in the morning There is a switch to override the timer signal manually and turn the ventilation on during other times for example 1f working extra hours is needed A signal from the movement sensor causes opening of the air valve on the given floor to allow the air to enter this floor The timer TO T1 T2 T3 signals are also used here Flowchart diagram can be used to describe this sample application In Table 0 2 the symbols column shows the flowchart symbols mainly used 23 Table 0 2 Main symbols of the flowchart Name Symbol Description meaning Terminator Used to present the start and end of the flowchart diagram Flowchart diagram is drawn between these symbols Word START marks the flowchart start and word END the end Process Used to present action task or process which Filling a is written inside the rectangle bottle Decision Presents the condition by which the program moves on to the next task or process Each Is the bottle decision can have at least two or more filled answers which are written n
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