Hello Everyone,

I am designing a small Hydro Electric Power station. One of the features that the customer required is to have an adjustable power setting. The customer, in their infinite wisdom, wants a dial for their adjustment. Electrically, I was planning to use an analogue input card to the S7 PLC, which was connected a POT (connected to the dial) and using a voltage divider to give me the required 0-10 input to the analogue card. My question is, is there an industry standard device for this method? Any information would be great guys.

 

You can usually buy potentiometers and knobs separately from an electronics distributor. The pot mounts through a hole in the panel, and secures with a nut. The knob attaches to the pot with a set screw. Make sure to get a pot with a metal shaft, as the plastic ones will break too easily. You'll need to calibrate the pot somehow, for both zero and span.

A lot of the automation vendors have potentiometers in their push button and pilot light catalogue as well (e.g. AutomationDirect).

The other alternative is to use a rotary encoder with a counter input to the PLC. You might need to make your own knob and mounting hardware for this though.

 

If you do use a POT you will probably want one with a linear taper and not one with an audio taper.

Jerry

 

Sounds like a fun project.

If you decide on an encoder, you may want to consider opting for an absolute encoder as you will lose position with most incremental encoders if power is lost.

To give you an example of what is out there, I work as an Application Engineer for an encoder company and we offer a couple flavors of 8 bit absolute encoders (QD787, QD110) that have been used in potentiometer replacement applications.

If you already have the analog input, and proper power supply for it, a potentiometer set up as a voltage divider (with a current limiting resistor on one end) is definitely the cheapest way to go.

To answer your question, I don't know of any industry standard for a potentiometer in this type of application.

Jim Miller
Applications Engineer
Quantum Devices Inc.
jmiller at quantumdev. com
www.quantumdev.com

 

I suggest a wire wound 1,000 Ohm.

Perhaps a 10 Turn pot with the appropriate knob that will give you an accurate reference. I assume you are only controlling a portion of the output, e.g. 0V + 50 kW, 10 V - 100 kW.

Good Luck,
Roy