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Selecting an optimum sampling time of PID controllers

Created by: hdhosseini at: 4/18/2012 2:28 PM (27 Replies)

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5/7/2012 5:08 PM	Rate (1)
Lpmspa Advanced Member Joined: 2/8/2007 Last visit: 9/9/2024 Posts: 12 Rating: (10)	hello, I calculate the optimal cycle time with this formula: CT = ((T75-T0) - 1.25*(T75-T25))/10 CT = Cycle Time T75 = Time at 75% of change of PV T25 = Time at 25% of change of PV T0 = Time at 0% of change of PV the rules that use are: put the control in manual (example: output = 15%) wait for the stabilization increase of 10% (example: output = 25%). save T0 wait for the stabilization T25 and T75 calculated as the time to reach 25% and 75% of change I use also this formula form my on-line Tune

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5/7/2012 6:59 PM	Rate (0)
IBN-Service Posts: 8946 Rating: (999)	RangO hello, I calculate the optimal cycle time with this formula: CT = ((T75-T0) - 1.25*(T75-T25))/10 ... Usually I do "T75/10" for maximum scan time, working always perfect for me.
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5/7/2012 7:01 PM	Rate (1)
IBN-Service Posts: 8946 Rating: (999)	Aret Well, if you have 2 groups of PID-loops but only OB35 available (no other OB3x) such a solution can provide proper grouping Hi aret, I know that you already know, but for later readers just the hint that you can use a simple counter in OB35 and call PID blocks depending from counter value inside the OB35. So you do a lot of blocks very easy, just take care that the scantime now is (OB35 timex max. counter)
IBN-Service Posts: 8946 Rating: (999)
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5/7/2012 7:30 PM	Rate (0)
Aret Posts: 2348 Rating: (264)	Interesting formulas guys, thanks for sharing them IBN, yeah i thought of counters of course, but is times i need are not dividable (although it's hard to imagine in this particular case) then working with counters would be head ache. I know there's a lot of ways to work around - but it's interesting if my idea works
Aret Posts: 2348 Rating: (264)
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5/7/2012 8:07 PM	Rate (1)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365)	IBN-Service that you can use a simple counter in OB35 and call PID blocks depending from counter value inside the OB35. Just to share interesting experience while implementing PID controllers in Microcontrollers.It is not flexible like PLC with different cyclic interrupts (OB3X).You can create a cyclic interrupt by a timer and you should calculate for that.You select your base time (50ms or 100ms) and using counters for making the correct sampling time. Best regards, Hamid Hosseini
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5/7/2012 9:37 PM	Rate (0)
Aret Posts: 2348 Rating: (264)	Hamid, thats exactly IBN-Srevice is talking about. The good variant is when you need 100ms and 500ms time. But thing become worse if you need 450ms and 111ms time Please don't ask why 111ms It's just to show that in case 2 times have different dividers it can be quite complicated task to make them on counters - though in case of sampling time it's not important.
Aret Posts: 2348 Rating: (264)
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5/8/2012 8:44 AM	Rate (0)
IBN-Service Posts: 8946 Rating: (999)	Aret ... if you need 450ms and 111ms time ... Just as an example: Basic scan time 1ms, counter goes up to 450. Call the 111 thrad at 0, 111, 222, 333, 444 ms. But, we talk a little bit theoretical now...
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5/8/2012 9:04 AM	Rate (0)
Lpmspa Advanced Member Joined: 2/8/2007 Last visit: 9/9/2024 Posts: 12 Rating: (10)	i share, in my site, a implementation of FB41 with a sample time programmable and a parameters switch.... link to download: http://blog.ranghetto.com/?p=167 i use a "timer" maked withsystem tick (sfc64) also i make a cascade controller look inside my library ;)

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