Comments on PID-controller circuit? - Page 1
Forum section: electronics / projects

author: awallin
author_rank: Frequent Contributor
timestamp: October 07, 2014, 04:13:19 pm
content: Hi all, I'm building a general purpose PID-controller for various use in the lab, mostly temperature control at first. Desired specs are:+/- 10V input, set-point, and outputconstriction on 2-sided 'eurocard' 100x160 mm, plugs into 3U rack-case"slow", i.e. will be used for signals <<1 MHzwidely tunable P/I/D gains, coarsely by jumpers, fine-tuning with potentiometers at the front panelnot a huge emphasis on low noise, DC-accuracy, or linearity - standard performance will do.Here's what I came up with:http://imagebin.ca/v/1cvUcm4vubp6orhttp://ibin.co/1cvUcm4vubp6AD8221 instrumentation amplifier at the input, then standard op-amp (what opamp? tl071 is just a generic/default choice in the schematic) circuits for P/I/D, with jumper-selectable gains, and potentiometers for fine-tuning the gain, then a summing amplifier, and a jumper to select inverted or non-inverted polarity for the output. A DG418 analog switch resets the integrator, via a control switch on the front panel. Also an overall enable-switch DG418 that grounds the output to disable it.Comments, suggestions, improvements!? Hopefully I haven't missed something obvious...anyone got the schematics for SRS SIM960 ?http://www.thinksrs.com/products/SIM960.htmthanks,Anders

author: Spikee
author_rank: Frequent Contributor
timestamp: October 07, 2014, 05:39:06 pm
content: This is just silly, use a micro / fpga.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 07, 2014, 06:03:48 pm
content: I've done nearly identical circuits in a commercial product decades ago and they can be fun and educational to play with, but have to echo the above comment. In this day and age, why bother?

author: Jeroen3
author_rank: Super Contributor
timestamp: October 08, 2014, 01:20:51 pm
content: It may be bad choice for new designs other than educational or retrofits.But sometimes analogs are still better for the job than digitals. Digitals have a limited resolution and they economically end when you're going above a few thousand PID calculations per second.

author: Kremmen
author_rank: Super Contributor
timestamp: October 08, 2014, 03:56:51 pm
content: If i guess correctly awallin is a fellow countryman so he would know of Stromberg (ABB) where i used to design analog controllers decades ago.Recalling those days - nothing basically wrong in this circuit; maybe a bit simpler would do equally well (you can actually combine all terms into one opamp pretty easily. The opamp won't mind). One thing that is not so easy to handle in this kind of analog circuit; integrator windup limiting. Basically when the command output hits a rail, it is antiproductive to continue the error integration because the controller cannot do anything more than it is already doing. Once the plant starts to respond it takes considerable time to bring the error integral back to linear range, meanwhile overshooting the setpoint in the other direction. You can create elaborate arrangements to cut the signal to the integrator when the output hits a limit but that does not work nearly as well in the analog domain as it does in the digital.Other things to consider are (maybe) joltless switching from manual to automatic control and back - always depending on exactly what is being controlled, of course.One more vote for a MCU, perhaps a small inexpensive PIC, AVR or ARM. Any of those will do nicely and by coincidence i just recently wrote a simple PID procedure for an Atmel Sam4S (Cortex M4), though it will of course run on almost any hardware, there being nothing specific to just this chip. This one however will run at 120 MHz so it can manage reasonably quick processes although in this case it will be a very slow one. An FPGA of course can manage seriously fast loops but at the cost of much harder implementation.