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04/21/05 11:47
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#92030 - Simple PID controller schematic
There have been many postings on PID controllers and I thought I will just share a recent experience :

I recently did a pump validation test rig in which the pump had to be driven by a 30kW AC (open loop vector control) drive. Each cycle comprised of varied speed settings that had to be sequentially called up. The whole rig was controlled by a MCU set up running with a P89V51rd2. Initial trials of commanding the speeds using a look up table using the calibration data and an 8bit DAC proved just OK. But the repeatability of speeds achieved was far from satisfactory.

Closed loop control seemed the only way out.Searching the market for a simple stand alone PID or PI controller yielded nothing suitable - most of them had either a single adjustable set-point ( like temp. controllers) or via RS-232 RS-485 etc. What I needed was a simple 0-10V remote input for setpoint.

I had earlier designed one such controller for a position control application and improved with ideas from our Kai Klaas (Thanks to him ). It worked very well for a servo valve position control. So I decided to add a relay for remote reset of the I-term and convert it into a PI control.

I have tried this in the drive control with feedback from a speed sensor based on LM331 (F to V ). The cost of the whole modification ( LM331 + 2 nos LM324 and some handful of components) was nothing compared to the stable rpm that we achieved.

Just to share the idea, I have posted the circuit here: http://www.8052.com/users/RRAGHU/PID_V3.GIF

Its a PID from first principles and hence no description is needed. Some significant points are

* No identified tuning procedure. What generally applies for any PID loop goes well.
* Since the circuit is made of basic Op-Amp building blocks, modifying to suit your need is easy.
* HOME indication comes on when |SP-PV|< 0.1V. Can be adjusted even finer.
* Each term (P,I,D) gain adjustable and switchable.
* Output stage can drive both current input or voltage input amplifiers.
* External Reset for I_Term.

Can be successfully applied for simple position control loops alongwith hydraulic pneumatic proportional valves or speed control applications with reasonably stable loads.

PS: May not be suitable for Auto piloting aircrafts or closed loop guidance of rockets ;-)

Raghu

List of 11 messages in thread
TopicAuthorDate
Simple PID controller schematic            01/01/70 00:00      
   Why Analog Whynot Direct Digital Control            01/01/70 00:00      
      Why not DDC ?            01/01/70 00:00      
         RE:            01/01/70 00:00      
            Unique loads            01/01/70 00:00      
               Nothing as good as Analog.            01/01/70 00:00      
                  Copy of drawing            01/01/70 00:00      
               HOT( hydraulik OT), Happily WEOT too            01/01/70 00:00      
                  Pump develops flow and ...            01/01/70 00:00      
   Well Done            01/01/70 00:00      
      The next step..            01/01/70 00:00      

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