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PID cycle time/sampling time

Created by: VIC at: 10/6/2011 9:03 AM (2 Replies)

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10/6/2011 9:03 AM	Rate (0)
VIC Regular Member Joined: 5/14/2007 Last visit: 10/14/2020 Posts: 2 Rating: (0)	Dear all, I have some questions regarding standard PI control functionality: 1. Now i have object with big response time (lag time). According to documentation (standard PID control, Controlling with SIMATIC Practice book) sample time should be more or equal 5000ms. For my object may be more. Also PID function (FB1) should be called in time interrupt OB. Does it mean that cycle for OB should be also 5000ms (for example), that is equal sample time? If I right understand, cycle parameter for FB1 should be equal time between two calls of last one? 2. If will be necessary to increase sample time for 10s, what should I do - change time parameters for OBx block or other way – use for example SW puls 10s to enable or jump FB1 for each control loop in my project? 3. I'd like to implement LP_SCHED function. According to documentation - "If more than one control loop is to be processed in one run of a watchdog interrupt OB, the ”LP_SCHED” FC may also be called several times. All calls of this FC must be carried out before the call of the control loop FBs. You must then enter the time base of the watchdog interrupt OB divided by the number of FC calls at the input parameter TM_BASE of the ”LP_SCHED” FC." In this case-should I make many DB_LOOP for each call of FB and set number of control loop to 1 in each DB? Now I have ten control loop for heating zone in big kiln. If I will set 5000ms for OBx, so should I set 500ms for TM_BASE parameters? What theoretically result should I get from LP_SCHED in cycle parameters for control loop?

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10/6/2011 7:20 PM	Rate (1)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 9/14/2025 Posts: 12408 Rating: (2717)	VIC Dear all, I have some questions regarding standard PI control functionality: 1. Now i have object with big response time (lag time). According to documentation (standard PID control, Controlling with SIMATIC Practice book) sample time should be more or equal 5000ms. For my object may be more. Also PID function (FB1) should be called in time interrupt OB. Does it mean that cycle for OB should be also 5000ms (for example), that is equal sample time? If I right understand, cycle parameter for FB1 should be equal time between two calls of last one? 2. If will be necessary to increase sample time for 10s, what should I do - change time parameters for OBx block or other way – use for example SW puls 10s to enable or jump FB1 for each control loop in my project? 3. I'd like to implement LP_SCHED function. According to documentation - "If more than one control loop is to be processed in one run of a watchdog interrupt OB, the ”LP_SCHED” FC may also be called several times. All calls of this FC must be carried out before the call of the control loop FBs. You must then enter the time base of the watchdog interrupt OB divided by the number of FC calls at the input parameter TM_BASE of the ”LP_SCHED” FC." In this case-should I make many DB_LOOP for each call of FB and set number of control loop to 1 in each DB? Now I have ten control loop for heating zone in big kiln. If I will set 5000ms for OBx, so should I set 500ms for TM_BASE parameters? What theoretically result should I get from LP_SCHED in cycle parameters for control loop? Hello Vic; I can't help you with your last question, as I have not worked with the LP_Sched function Hopwever, I would like to clear up an issue that is frequent with newbies on the Siemens PID parameters: "Cycle" and the scan time of the function block. Whenever you use a PID block, in order for the calculations to be reliable, you must make sure that the parameter"Cycle"is the same (or at least a multiple of) the scantime of the block calling the PID. Or, as explained in the following FAQ: What should you watch out for when calling and assigning parameters to the controller functions in function blocks (S)FB41, (S)FB42 and (S)FB43? The "CYCLE" parameter corresponds to the scan time. The time between block calls must be configured here (e.g. time base of the OB35). At worst, use a multiple of the OB35 scantime as a parameter Cycle. But I have always had better success with the same values for both, generally 100msby default. Do not worry about the integral time, even though it is very long, it will be used in the PIDcalculations many times before the PID block reacts, but that is of nop consequence. Hope this helps, Daniel Chartier

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This contribution was helpful to 1 thankful Users VIC

10/6/2011 8:44 PM	Rate (0)
VIC Regular Member Joined: 5/14/2007 Last visit: 10/14/2020 Posts: 2 Rating: (0)	Thanks for detailed answer. What is not so clear for me – from my first question “…According to documentation (standard PID control, Controlling with SIMATIC Practice book) sample time should be more or equal 5000ms…” - does it mean scan time of function block? Or other meaning? I understand your answer regarding Integral time and scan cycle. But in documentation there are many information about different sample time for different process(fast, slow etc.). Result of these questions – my second one. Because, if this scan time – I don’t know what is the right way to realize it? Or I no need to care about it? With best regards
	Last edited by: VIC at: 10/6/2011 8:50 PM
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