6/15/2009 11:35 AM | |
Joined: 3/21/2006 Last visit: 9/10/2024 Posts: 10228 Rating: (1181) |
Hi zaraza, For me your quesion is not so clear. But you could start with Tips and Tricks S7-200 Maybe you'll find there something similar like your case... Best regards Hristo Mihalev |
6/15/2009 11:50 AM | |
Joined: 9/27/2006 Last visit: 9/21/2024 Posts: 12282 Rating: (2684) |
Hello zaraza; This chocolate temperature controller acts on the system heating circuit to maintain the temperature of the chocolate as close as possible to the required temperature setpoint, no? In general, these are stand-alone PID controllers. Most PLCs, including the S7-200, offer integrated PID controllers that you can use (as in this case) to maintain an output signal close to the required setpoint. From the S7-200 System manual:http://support.automation.siemens.com/WW/view/en/1109582 Chapter 6, S7-200 Instruction Set. The PID Loop instruction (PID) executes a PID loop calculation on the referenced LOOP based on the input and configuration information in Table (TBL). Eight PID instructions can be used in a program. If two or more PID instructions are used with the same loop number (even if they have different table addresses), the PID calculations will interfere with one another and the output will be unpredictable. The loop table stores nine parameters used for controlling and monitoring the loop operation and includes the current and previous value of the process variable, the setpoint, output, gain, sample time, integral time (reset), derivative time (rate), and the integral sum (bias). To perform the PID calculation at the desired sample rate, the PID instruction must be executed either from within a timed interrupt routine or from within the main program at a rate controlled by a timer. The sample time must be supplied as an input to the PID instruction via the loop table. Auto-Tune capability has been incorporated into the PID instruction STEP 7-Micro/WIN offers the PID Wizard to guide you in defining a PID algorithm for a closed-loop control process. Select the Tools > Instruction Wizard menu command and then select PID from the Instruction Wizard window. You will use one S7-200 analog input to read the present temperature of the chocolate vat. This will be your Process Value (PV, in PID shorthand). You will use one of the analog outputs of the S7-200 to control the final element that heats/cools the chocolate vat, eihter a hot water valve, or a heating element. This is known as CV, the control value. You will have to enter the desired temperature (SP, setpoint) that you want to maintain in the chocolate vat, either through yiour S7-200 program, or through an HMI (operator interface) of some kind. You will also have to understand the fundamentals of PID control in order to "tune" (select the appropriate gain, integral and derivative times) so the process responds quickly and smoothly to changes to the setpoint or the process value.The idea is to maintain setpoint and process value almost equal at all times. . If you need basic information on this, I can strongly suggest you read the following posts from a gentleman named Ron Beaufort What Is D in PID?
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Last edited by: dchartier at: 6/15/2009 11:58 AM |
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This contribution was helpful to4 thankful Users |
6/18/2009 10:50 AM | |
Joined: 9/27/2006 Last visit: 9/21/2024 Posts: 12282 Rating: (2684) |
Hello zarzara; So the unit was an On/Off temperature controller. You should not need a PID controller to replace it then. Start by setting up your analog signal for the temperature signal. The type of signal you read (voltage, current, resistance (RTD) or Thermocouple) will depend on the sensor/transmitter you use in your circuit, and the analog module you decide to use in the S7-200. Once connected to the correct input on the CPU or an expansion module, you can read and scale properly (0-200 degrees) the value in your program. The following link will show you different S7-200 Tips and Tricks (sample applications and programs) that you can study for their different uses of analog temperature measurements: https://www.automation.siemens.com/microset/html_76/support/tipps/tt-messtechnik.htm Once you have captured and scaledthe temperature value, use simple compare instructions from the S7-200 instruction set (less than, greater than...) to control the heater circuit (you will have to determine how that works, a simple digital on/off signal hopefully...): -if the temperature value is greater than 200 degrees, stop the heater circuit. -if the temperature valueis less than190 degrees, start the heater circuit. -if the temperature value is greater than 210 degrees, set a High temperature alarm bit. -if the temperature is below 185 degrees, set a Low temperature alarm bit. The specific setpoints and alarm levels will best be decided by you. Remember to keep a deadband (a separation between the start and stop setpoints for the heater circuit) so the system does not continuously start and stop the heater when the temperatuire is almost exactly 200 degrees. This will lead to rapid deterioration of the heating circuit. And figure out how the alarm conditions will be shown to the operator: lights, sounders, HMI... Hope this helps, Daniel Chartier Hope this helps, Daniel Chartier |
Last edited by: dchartier at: 6/18/2009 12:52 PM |
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6/25/2009 9:46 AM | |
Posts: 5 Rating: (0) |
can you give me your mail if it possible to contact you and explain you in more details. |
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