Ian H. Gibson
Process, Control and Safety Systems
Q: Could you please explain the basis on which to select PID output (between on-off PID with pulse-width modulation and continuous analog output) for operating a control loop?
K V Ratnakar
A: The type of final actuator determines whether the controller output needs to be continuous, on-off, or three-state. A PID controller can be used to drive any of these, with the proper interface such as pulse-duration module. For information on how to drive on-off actuators from a PID controller, see my book, Process Control Systems, 4th ed., pp. 167-174.
F. G. Shinskey
A: The output of your control algorithm is dictated by your final control element actuator (valve, drive, stepping motor, power electronic device, relay, etc.). It’s up to the equipment supplier to choose it; then you choose what control law is suitable for your process.
Senior Process Control Consultant
Invensys Systems Canada Inc.
A: My experience has been the choice is mostly a matter of cost and convenience, in regard to the device driven by the controller. Both output types will work for most applications. But it's usually easier (less costly) to control things like motorized valves and heating elements with on/off operation than in a continuous (non-discrete) manner. As the pulse-width resolution approaches that of the controlled device’s operational range, there's simply not much practical difference between the two anyway.
Al Pawlowski, PE
Baton Rouge, La.
Q: I am looking at some software that will run OPC with my Honeywell Experion that will give me some type of “health,” if you will, on the loop. I know there are several companies out there that sell this type of software. Can you give me any advice on this?
E&I Reliability /Planner
Graphic Packaging International
A: We work regularly with Experion and use
- OPC from Honeywell
- OPC from Matrikon
We have installed Control Performance Monitor from Matrikon many times with great success.
Michel Ruel PE
TOP Control Inc.
A: You can extract a fair bit of health information directly from OPC on Honeywell. I generally have shapes that provide that general status information native to Experion, so the operators can see what's going on. Are you wanting to display information to operators or use for troubleshooting as an engineer?
Bridget A. Fitzpatrick
Senior Principal Technical Consultant