Paper - Asee peer logo
Contents
1. 4 shows the hysteresis of the ON FF Controller operation as the Error signal varies between 2 F and 2 F The operation begins with a check on whether the Controller is ON or OFF This is accomplished with VI 2 AI Sample Channel vi and the comparator C1 The output of C1 is either TRUE or FALSE If TRUE then the Controller is OFF and if FALSE then the Controller is ON VI 2 takes its input from Channel 1 of Device 1 DAQ Board number As described earlier analog input Channel 1 is physically wired to DAC output Ch 0 which controls the operation of the fan Thus by testing the DAC output Ch 0 we can determine whether the Controller is ON or OFF This will place the Controller operating point either on the lower segment or the upper segment of the hysteresis loop in Fig 4 Vco 5V ON 2 OV OFF 2 Fig 4 The Operation of the ON OFF Controller At this time V1 M1 and S1 determine the value of the Error signal Vg V1 takes the temperature sample from the analog input Ch 0 to which the temperature sensor is wired MI multiplies the temperature sample by the scaling factor TCalibrate and S1 subtracts this value from the Front Panel digital control Set Point Vsp The result is the Error signal The Controller has to make a decision whether to turn the fan ON or OFF This decision making process is implemented with nested Boolean Case structures The reader should follow the hysteresis loop in Fig 4 and the2. accommodates data acquisition instrument control data processing and data presentation LabVIEW which can run on PC under Windows Sun SPARstations as well as on Apple Macintosh computers uses graphical programming language G language departing from the traditional high level languages such as the C language Basic or Pascal All LabVIEW graphical programs called Virtual Instruments or simply VIs consist of a Front Panel and a Block Diagram Front Panel contains various controls and indicators while the Block Diagram includes a variety of functions The functions icons are wired inside the Block Diagram where the wires represent the flow of data The execution of a VI is data dependant which means that a node inside the Block Diagram will execute only if the data is available at each input terminal of that node By contrast the execution of a traditional program such as the C language program follows the order in which the instructions are written L 9G y abed LabVIEW incorporates data acquisition analysis and presentation into one system For acquiring data and controlling instruments Lab VIEW supports IEEE 488 GPIB and RS 232 protocols as well as other D A and A D and digital I O interface boards The Analysis Library offers the user a comprehensive array of resources for signal processing filtering statistical analysis linear algebra operations and many others LabVIEW also supports the TCP IP protocol for exchanging data bet
3. system described in this article is a prototype that mimics the operation of a large air conditioning system Within the constraints of the design and the limits of the physical configuration the system performed within the design limits The dead band was set to 2 F which makes the Controller switch at 2 F at the upper end and 2 F at the lower end The rate of cooling achieved by this application was estimated to be approximately 1 minute to cool the air around the temperature sensor from 76 to 72 F Its accurate determination was not done because it depends on many factors such as the volume to be cooled enclosure and its insulating properties and other factors Bibliography Basic Concepts of LabVIEW 4 by L Sokoloff Prentice Hall 1997 Analog and Digital Control Systems by R Gayakwad and L Sokoloff Prentice Hall 1988 Graphical Programming by G W Johnson McGraw Hill 1994 LabVIEW Data Acquisition VI Reference Manual National Instruments LabVIEW for Windows User Manual National Instruments LabVIEW Function Reference Manual National Instruments LabVIEW for Windows Tutorial National Instruments LabVIEW Getting Started with LabVIEW for Windows National Instruments Industrial Control Electronics by J Webb and K Greshock 2 Ed Merrill 1993 Modern Industrial Electronics by T Maloney 3 Ed Prentice Hall 1996 Industrial Electronics by Humphries amp Sheets 2 Ed PWS Kent 1986 Biography Leonar
4. the Front Panel includes the recessed box with two digital controls They are used by the operator to input the Set Point Vsp value of and the scaling factor TCalibrate which converts the temperature sensor output from millivolts to degrees F The thermometer indicator measures the current temperature and the Cooling indicator displays the Controller state ON orOFF The last object in the Front Panel is the Run Stop switch which is used to initiate and terminate the VI execution Front Panel Error Signal TCalibrate Set Point ane 200 ra otra 0 0 Samel Cooling Temperature 4 0 100 0 Q 877 0 100 200 300 400 OFF 80 0 Controller Out 60 0 6 0 RUN STOP 40 0 20 0 in 00 Y 20 4 0 0 0 200 300 400 500 600 Fig 3 The Front Panel of the Temperature Controller The Block Diagram The Block Diagram is the graphical program that shows the data flow of the temperature control operation Unlike a high level language program like the C language where instructions are executed in the order that they are written the execution of a LabVIEW 999 py abed VI depends solely upon the flow of data a particular object inside the Block Diagram will execute only if data is available or present at all its input terminals The execution continues at each node that has the data Fig 5 shows the details of the Block Diagram which can be used to describe the operation of the ON OFF controller while Fig
5. Session 3659 LabVIEW Implementation of ON OFF Controller Leonard Sokoloff DeVry Institute Abstract This paper describes an application of LabVIEW to system control which includes data acquisition data processing and the display of data The application described in this paper emphasizes the hardware and perhaps to a greater extent the software used to control a physical process The use of the computer in data processing and control applications is a trend that one sees in today s industrial environment This application is one of many that is offered to the students in the Industrial Controls laboratory at DeVry in order to provide them with hands on experience that they are likely to experience on the job Virtual Instrumentation is a current technology that is making a significant impact in today s industry education and research DeVry Institute selected LabVIEW as an good representative of this technology and is using LabVIEW in its curriculum at all DeVry campuses in the United States and Canada This article is a result of a research project for LabVIEW implementation into the Industrial Controls course LabVIEW is also used in the communication and physics courses LabVIEW is one of many skills that the student will need as he enters today s highly competitive job market I Introduction LabVIEW Laboratory Virtual Instrument Engineering Workbench a product of National Instruments is a powerful software system that
6. code in Boolean Cases 1 2 and 3 If the output from Comparator C1 is TRUE then the True frame of Boolean Case Iwill be executed The Controller must be OFF and its operating point is on the lower segment of the hysteresis loop in Fig 4 We must check next if the Error signal is greater than 2 F This is done inside the True frame of Boolean Case 1 If the Error signal is greater than 2 F then the True frame of Boolean Case 2 outputs OV keeping the fan OFF But 98 y a6ed if the error signal is equal to or less than 2 F then the False frame of Boolean Case 2 outputs 5v to turn the fan ON If C1 output is FALSE the Controller must be ON Comparator C3 inside the False frame of Boolean Case 1 checks the Error signal if it is less than 2 F If TRUE the True frame of Boolean Case 3 outputs 5 V to keep the fan ON And if FALSE then the False frame of Boolean Case 3 outputs Ov thus switching the fan OFF This operation is inside the While Loop which is enabled by the RUN STOP switch in the Front Panel As long as the switch is in the RUN position its terminal counterpart in the Block Diagram outputs a TRUE to the condition terminal keeping the While Loop enabled a FALSE disables the While Loop As long the While Loop is enabled the code inside the loop is repeatedly executed This results in acquiring a temperature sample once a second To stop the operation the user must click on the RUN STOP switch The two Waveform Chart
7. d Sokoloff was born in Russia and immigrated to the United States in 1950 and was awarded BSEE degree from Stevens Institute of Technology 1959 the MS Applied Science degree from Adelphi University 1964 and the PhDEE candidate from Stevens Institute of Technology Worked in industry as semiconductor application and circuit design engineer 1959 1970 For the past 28 years with DeVry Institute currently as senior professor teaching associate level and bachelor level courses in advanced mathematics and in electrical engineering Currently involved in curriculum development projects using Virtual Instrumentation PLC and PLD 0 9gE Y abed Page 4 356 11 Page 4 356 12 Page 4 356 13 Page 4 356 14 Page 4 356 15
8. he desired value of the Process Variable temperature in this application The two signals Vpy and Vsp are applied to the difference amplifier whose output is the Error signal Ve Vsp Vrv The Controller block in Fig la is the heart of a control system It accepts the Error signal Vz and produces an appropriate output In practice a control may be one of several types ON OFF Proportional Proportional plus Integral or Proportional plus Integral plus Derivative PID These controllers differ in the manner in which they operate or process the Error signal PID controller is much more sophisticated than an ON OFF controller and harder to design This application uses a simple ON OFF controller to illustrate the temperature control process Diff Amp Signal Conditioning Signal Conditioning lt a Process Actuator Vco 5V ON Ve min OV OFF Ve max b Fig 1 Closed Loop Control System e 9G e r7 bed The Controller Output is often subjected to signal conditioning in order to provide the proper signal level or power as required by the Actuator If the Actuator is a motor then the signal conditioning block may be a power amplifier that generates the appropriate power to drive the motor The use of negative feedback is the key to the proper operation of a control system Consider the operation of the ON OFF control system depicted in Fig 1b The object of the temperature control system descr
9. ibed in this paper is to provide air condition cooling control Suppose that the Controller is OFF Vco OV providing no cooling The operating point is now on the bottom part of the hysteresis curve in Fig 1b This results in increasing temperature and also in increasing Vpy The Error signal Vg Vsp Vpy is decreasing since Vsp does not change Vg continues to decrease until Vg Vemm At this point the controller switches ON Vco 5V and drives the actuator fan in this experiment which provides cooling The Error signal now begins to increase because Vpy is dropping It continues to increase until Vg Vemax At this point the Controller switches OFF shutting OFF the fan and the cycle repeats The difference Vemax Vemm is called the dead band It is the range of the Error signal in which the controller is either ON or OFF No regulation of the Process Variable occurs inside this range The dead band is necessary because without it the system will oscillate constantly between ON and OFF operating states This article describes a prototype that mimics the operation of a large temperature control system A fan is used as the actuator that provides the cooling The graphical language of LabVIEW to be described in the following sections is used to implement the function of the controller III System Hardware The data acquisition board DAQ board serves as the interface between the computer and the real world as shown by a bloc
10. ically used with this option makes this process somewhat slow Waveform input data acquisition is buffered and hardware timed The timing is provided by the hardware clock that is activated to guide the acquired data points quickly and accurately The acquired data is stored temporarily in the memory buffer until it is retrieved by the data acquiring VI The temperature control application described in this article uses two Easy VIs The AI Sample Channel vi is used to acquire data from Analog Input Channels 0 and 1 while AO Update Channel vi outputs 0 V or 5 V to D A channel 0 to control the operation of the fan Additional software required to run the temperature control VI described here is LabVIEW and the DAQ board driver NI DAQ These are shown in Fig 2 G9G r a ed V On Off Controller Software The Front Panel All programs which are written inside the LabVIEW environment are called VIs Each VI consists of a Front Panel and a Block Diagram The Front Panel includes various controls and indicators while the Block Diagram contains various functions and other VIs that are interwired among themselves Shown in Fig 3 is the Front Panel of the temperature control VI As shown the Front Panel includes two Waveform Charts and other objects The top Waveform Chart displays the error signal the difference between the set point and the process variable and the bottom chart displays Vco the Controller status Other objects inside
11. k diagram in Fig 2 It is installed in the PC that operates under Windows 95 In this application MIO 16E 10 board was used Ch 0 one of the analog input channels is wired to the external temperature sensor Ch 1 is wired to the D A Ch 0 one of the DAC output ports and also to the fan Thus the current temperature data is coming into computer via analog input Ch 0 and the control signal that controls the operation of the fan comes from the computer via D A output Ch 0 In addition analog input Ch 1 monitors the operation of the fan as it receives the same signal from the computer as does the fan r 9gE 7 abe Fan DAQ Board D A Out Ch 0 1 COMPUTER PC Fig 2 System Hardware Block Diagram IV System Software LabVIEW offers three categories of data acquisition VIs Easy VIs Intermediate VIs and Advanced VIs Easy VIs perform most common VI operations They are simple to use because the configuration complexity is designed into the VI icon These icons usually include some of the Intermediate VIs which in turn are made up of the Advanced VIs Advanced VIs are the fundamental building blocks for all data acquisition Vis and have the most programming power and flexibility Analog input data acquisition options include immediate single point input and waveform input In using the immediate single point input option data is acquired one point at a time Software time delay to time the acquisition of the data points which is typ
12. s in the Front Panel show the error signal and the Controller output The Controller switches between OV and 5V as shown by the hysteresis loop in Fig 4 The Wait Until Next ms Multiple function provides 1 s time delay between the data points The Waveform Charts shown in Fig 3 are set to display 600 data samples 10 minutes of data 899 y abe Device Set Point Controller OFF u lfWe gt 2 keep fan OFF Error Signal TCalibrate Device ay Sample Eee Controller OFF TRUE YES FALSE NO Controller Out Wait Until Next ms Multiple 1 sec delay z Controller ON If Ve lt 2 keep fan ON C3 E Output Y Boolean 2 keep fan ON Case 3 Device AO Update Channel vi Out to Fan falsan Las 1 1 Output av J Keep Fan OFF Device AO Update Channel vi Out to Fan RUN STOP x Boolean E amp tum fan OFF Case 3 E Devicee A0 Update Channel vi p He Channel Dut to Fan Output 5 Turn Fan ON amp Device Out to Fan Fig 5 The Block Diagram of ON OFF Controller 6 99 p ebed VI Conclusion This article focuses more on the control software than the hardware The prime objective is to provide the student with a practical application that uses a graphical language as a design tool Although only the cooling task is considered in this application students are assigned a project to complete the design and implement the heating control using the G language The
13. sts of components and circuits that work together to maintain the process at a desired operating point Every home or an industrial plant has a temperature control that maintains the temperature at the thermostat setting In industry a control system may be used to regulate some aspect of production of parts or to maintain the speed of a motor at a desired level Although a control system can be of open loop type it is more common to use negative feedback The block diagram shown in Fig la illustrates the basic structure of a typical closed loop control system The Process represents any physical characteristic that must be maintained at the desired operating point In this paper it is the temperature that is to be maintained at the desired value The purpose of feedback is to provide the actual or the current value of process variable In this application a solid state temperature sensor is used to monitor the temperature It outputs a voltage that is too small for practical purpose typically in the millivolt range 799 7 abed The signal conditioning block that follows amplifies this signal to a useful level The signal conditioning block may also be used for calibration purposes by scaling the voltage from the sensor to the corresponding temperature The output from the signal conditioning block is designated in Fig la as Vpy the current value of the Process Variable The Set Point designated as Vsp represents the user input It is t
14. ween the server and the client LabVIEW v 5 also supports Active X Control allowing the user to control a Web Browser object This paper describes an application of LabVIEW to system control which includes data acquisition data processing and the display of data In order to perform data acquisition LabVIEW software latest version is 5 1 and the DAQ board driver software NI DAQ must be installed The DAQ data acquisition board must also be installed inside the computer The extender board that gives the user access to various pins on the DAQ board is connected to the data acquisition board by a flexible cable DAQ board contains many components that are necessary for data acquisition A typical board has 8 analog input data channels that are multiplexed and applied to the instrumentation amplifier The A D converter digitizes the analog input data The onboard FIFO First In First Out memory provides a temporary storage of data in buffered data acquisition applications There are also two D A converters that convert digital data to analog form and pass it to the analog output ports for use by external devices The DAQ board used in this application is MIO 16E 10 This is a multipurpose I O data acquisition board with 16 analog input ports and two analog output ports Its settling time of 10 us determines its maximum sampling rate of 100 kHz It has a 12 bit resolution and a 10 V dynamic range II System Overview A control system consi
Download Pdf Manuals
Related Search