PWM Signal Measurement

Dear Friends,

A motor is supplied by a PWM signal so that speed is proportional to the average value of the PWM signal. I need to measure the profile of the average value of the signal. Is a typical analogue input channel can do this or need pre-conditioning circuit.

Thank you for any help.

Sounds like a DC motor but you didn't say. Is it?

Dear DickDV,

Yes, it is a DC motor.

LO-LO

what is frequency of the PWM?
what is sampling speed of your analog card?
what is range of current you want to measure?
what is voltage of the motor circuit?
what is type (and range) of the analog input you plan on using?
do you need isolation?

About 1 kHz, peak volt 140 V (I can divide it down). Motor current about 2 A. Need isolation. No idea on analogue card yet. Just to find any card can perform such measurement without need of signal pre-conditioning (low pass filter etc) first.

I personally don't know of any plc analog input that will allow you to use this without filtering. If you know your PWM period you may be able to use a counter card to measure your pulse width. In a DC system the pulse width should be roughly proportional to speed. This is very PLC dependent though. The easiest and cheapest method will most likely be using a passive first order low pass filter and an isolator module prior to a PLC input.

If this is just for fun or personaly use you may be able to build up a circuit using a clock source, an opto-isolator, a gate, a counter and a D/A converter to get an analog value equal to the pulse width. But I wouldn't use this in an industrial environment.

Keith