AutoTuning for PLC's


Thread Starter

Joe Malter

Is anyone familiar with auto loop tuning for PLCs? I'm working with an OEM who's machine requires several (4 cascaded) PID loops. These loops will have setpoint profiles (temperature) that are developed by the end user. It seems like auto tuning is the prefered method for handling the tuning given a control engineer with most likely not be around.

Any thoughts?

Steve Jamison

The loops in the 205 (automation direct) package don't always come up with ideal values. I found just sitting there and adjusting the terms is the best way to get it to run well. Once the terms are set, they seem to work on a wide range of set points. In the case where I used the 250 loops, I used the same terms for a ramp from room temp to 255 F and it follows quite nicely. You can initiate an auto-tune using from the ladder, but I'd want to check it out. You may be better off storing sets of PID terms in memory and auto loading them for various ranges of set points.
Steve, NJ Industrial Controls
Thanks for the reply.

I feel confident with the first time tuning but the subsequent adjustments are at issue. The process is part of an OEM machine and I don't think the end users (and for that matter the OEM technicians) are all that process savvy. Hence, the search for auto tuning.
Be careful with auto tuning. Depending on the approach, autotuing may only be optimized around 1 operating point. If the process is non-linear and the operating point changed significantly, the auto tuning may not be appropriate for a changed operating point.

Edgar F. Hilton

I have heard that fuzzy logic tuning algorithms for PIDs have some success in this area, but I personally would not trust it too much.

If your process is SISO, and you can model (or approximate) your system as:

m x''(t) + c x'(t) + k x(t) = f(t) + D(t)


m, c, and k are unknowns, yet are bounded, and quasi-static
f(t) is the control output from your controller
D(t) is an external disturbance, unknown yet bounded

then the best and perhaps the most robust technique would be to use an "Adaptive, Robust Controller". If you can design a PID, then you should have no trouble with one of these. Slotine's nonlinear book describes this scheme in detail.

I hope that this helps.


Edgar F. Hilton FSMLabs, Inc.
voice: 850.893.0300
fax: 206.350.4EFH

Michael Lloyd

The Siemens TI555 has an autotuning function. The Koyo PLC's sold by Automation Direct (some of them) have auto tuning available. I've not used either one as I prefer to tune the old fashioned way.

Tony R. Gunderman

The Siemens SIMATIC 555 PLC has auto-tuning built in (must use SoftShop instead of TISOFT). It can be setup for automatic operation.