Driving a SSR with a PID Loop ???

brettshelley · Jan 2, 2008

Hi guys, I am having sleepless nights over this problem!

I am reading a temperature value and using a PID routine to make sure that the temperature stays at the process value. My problem is this:

The PID loop gives me a variable from 0 - 4000 depending on where the temperature is.

How can i manipulate this so that as the variable changes, the pulse of the output is driven directly proportional to the value???

Eg, 4000 = ON
2000 = pulse 50% ie. 10ms
etc

????
Cheers,

Brett

jsatkanpak · Jan 2, 2008

Most of the PID controllers (hardware and/or software) have a setting for On/Off control. This is commonly used for heating applications to drive an SSR-controlled heating element. By using On/Off control, the PID will automatically scale the cycling of the output the way you described.

TConnolly · Jan 2, 2008

Duplicate thread alert.

http://www.plctalk.net/qanda/showthread.php?t=35852

Electrically_Bound · Jan 2, 2008

jsatkanpak said:
Most of the PID controllers (hardware and/or software) have a setting for On/Off control. This is commonly used for heating applications to drive an SSR-controlled heating element. By using On/Off control, the PID will automatically scale the cycling of the output the way you described.

jsatkanpak's idea sounds the best route to go - if it is an option. Is this a PID block in a plc? I'm sure that there are several ways to accomplish this. Off the top of my head I'm thinking that you scale the output of PID function and the move it into a timer and combine this with a one-shot to effectively form a programmable-length one shot.

Some plc's also of pulse width modulated function built into them that may be of use.

Beware, I have not tried these methods, but I'm just trying to come up with some ideas.

CroCop · Jan 2, 2008

Electrically_Bound said:
jsatkanpak's idea sounds the best route to go - if it is an option. Is this a PID block in a plc? I'm sure that there are several ways to accomplish this. Off the top of my head I'm thinking that you scale the output of PID function and the move it into a timer and combine this with a one-shot to effectively form a programmable-length one shot.

Some plc's also of pulse width modulated function built into them that may be of use.

Beware, I have not tried these methods, but I'm just trying to come up with some ideas.

This is quite common.
In slower acting loops, everyone uses a free running self resetting timer, & proportions the signal to be on for a % of that free running timer.

TConnolly · Jan 2, 2008

Based on a post in another forum I'm assuming that Brett is using a Mitsubishi PLC. My experience with Mitsubishi PLCs is limited to reading Mitsubishi programs for conversion to another PLC (nothing against Mitsubishi, its an issue of standardization).

In an AB PLC I would use a self resetting timer and an SCP instruction where the PID output is scaled to the timer preset. This scales the PID output value to the time range. Then while the accumulated time is less than the time scaled PID output value the heater is on.

brettshelley · Jan 2, 2008

Hi Guys, well, tried the ON/OFF control which worked to some extent. When I wanted a constant value of 1000, the ON/OFF control worked the following way.

760 - ON
940 - OFF

No good! I autotuned, same problem.

Maybe the timer system would work? By the way, the PLC is a LG.

New to me as I normally program Mitsubishi!

Customer wanted LG!!!

Any other ideas would help!

Cheers,

Brett

TConnolly · Jan 2, 2008

brettshelley said:
By the way, the PLC is a LG.

New to me as I normally program Mitsubishi!

Customer wanted LG!!!

That's the problem with assuming.

danw · Jan 2, 2008

What model LG and where is the programming manual on the web?

panic mode · Jan 2, 2008

as Alaric mentioned, there was a mitsubishi sample in your other thread...

760/940 ratio would be spot on if 1000 represented 44.7% but since you mention
range 0-4000, value of 1000 is just 25%...

also what do you mean by ON/OFF control (you mean solution posted by Peter?)
if so, how did you configure interrupt (update interval)?
can you post screenshot of your code?

brettshelley · Jan 3, 2008

The PLC type is a GM6-B. I ordered the type B as it had the PID built in to the cpu.

This is driving me crazy still. All i want to do is PID control a flippin SSR to keep the temperature at a constant!!!!

I have uploaded the PID section of the manual, maybe someone outhere can make more sense of it than I can....!

Cheers,

Brett

brettshelley · Jan 3, 2008

Alaric,

Based on your example and a little tweaking of timers and scaling, BOOM.

The whole thing worked like a charm.

Had to change the AT samlpe rate to 2s, the PID sample to 2s, the Pulse timer to 2s and it works very very well.

Thanks to all who helped me regarding this matter. I will make a PDF of the programme and upload it so that you can see.

Best regards,

Brett

brettshelley · Jan 3, 2008

Just to add, changed ALL the scan times to 1 second and the pulse behaves much better now...!

Even when I adjust the SV of the PID loop, it adjusts accordingly. Great!!!

Now all I have to do is copy the programme 16 times as I have to control 16 heaters via PID. Hope that the scan time will fair OK!!!

Wish me luck and again many many thanks.

brettshelley · Jan 3, 2008

As promised the first zone routine in PDF.

Hope that this helps anyone else who have a similar problem.

Thanks go out to all the guys who gave me ideas...

Regards,

Brett

Driving a SSR with a PID Loop ???

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