30.6Note to students

30.6.2Building a “Desktop Process” unit

A more sophisticated approach to gaining hands-on experience tuning PID controllers is to actually build a working “process” that the controller can regulate. A relatively simple way to do this for students is to build what I like to call Desktop Processes, where a loop controller is used to control the speed of a motor/generator set made from small DC “hobby” electric motors. An illustration of a “Desktop Process” is shown here:

Two-wire cable

Motor speed signal

You must build your own variable-speed drive (VSD) circuit to convert the controller’s 4-20 mA output signal into a DC voltage powerful enough to drive the motor. This same circuit should also contain components for “scaling” and filtering the tachogenerator’s DC voltage signal so it may be read by the controller’s input. Fortunately, the following circuit is a proven and simple design for doing just that:

Diodes included in this design protect against reverse-polarity power supply connections and inductive “kickback” resulting from de-energizing inductive loads.

Photographs showing a complete “Desktop Process” unit in operation, including close-ups of the motor/generator set and the variable speed drive circuit board, appear here:

As you can see from the photograph, the motor and generator are held in a short length of split

PVC pipe. This is a simple way to clamp and align both machines so their shafts turn on the same centerline. The coupling between the two shafts is nothing more than a piece of rubber tube (or wire insulation, or heat-shrink tubing, or even electrical tape!).

An optional accessory to add to a Desktop Process is a data acquisition unit capable of measuring the DC voltage motor speed and controller output signals, plotting them on a computer display for further analysis. This becomes very useful when fine-tuning PID response, allowing students to visually recognize oscillation, overshoot, windup, and other phenomena of closed-loop control.

The controller model shown in these photographs happens to be a Siemens 353, but any loop controller capable of receiving a 1-5 volt DC input signal and generating 4-20 mA DC output signal will work just fine. In fact, I’ve connected this very same VSD and motor/generator set to di erent controllers41 to compare operation.

Interesting experiments to perform with a Desktop Process – other than PID tuning practice – include the following:

•Introducing process loads by touching the spinning motor shaft (slowing it down using your finger) and compare the responses between the controller’s “manual” and “automatic” modes. This proves to be a very e ective way for students to comprehend the di erence between these two modes of operation. I have yet to encounter a student who does not immediately grasp the concept after doing this experiment for themselves, feeling the motor’s shaft speed respond to their finger load in both modes, also watching the controller’s output response.

•Switching the controller mode from reverse action to direct action to see how a process “runs away” when the loop feedback is positive rather than negative.

•Switching the VSD action from direct to reverse, then reconfiguring the controller’s action to complement it, maintaining negative feedback in the system.

•Try switching between auto and manual modes in the controller, comparing the response with and without the feature of setpoint tracking. Again, this is a concept many students struggle to grasp in theory, but immediately comprehend when they see it in action.

41Among these di erent controllers were a Distech ESP-410 building (HVAC) controller and a small PLC programmed with a custom PID control algorithm. In fact, a Desktop Process is ideal for courses where students create their own control algorithms in PLC or data acquisition hardware. The significance of controller scan rate becomes very easy to comprehend when controlling a process like this with such a short time constant. The contrast between a DDC controller with a 500 millisecond scan rate and a PLC with a 50 millisecond scan rate, for example, is marked.