speed control by optical encoder


Thread Starter


We are a science center that has built a Large Plexi-glass toilet. We have designed an exhibit allowing two kids to spin large wheels to pump water into the flush tank of the "Big Sh*tter" called the BS device. when the tank is full the unit flushes the water ( a closed system ) and a flushing sound is generated.

What we want to to is vary the volume of water being pumped into the tank by the speed
(revolutions per second) the kids are spinning the large wheels (otpical encoders). There are two spinners therefore two pumps. We would also like to display the volume of water each kid has pumped and keep a running total on the volume of water processed.

anyone with and quick and cheap solution for the above we would really enjoy your input

Rob M
Hey Rob Im not an expert on these types of systems I deal with mainly Materials handling.. But if you were to use a Allen Bradley 1300 VFD drive with a few flow switches I believe that would take care of anything you need.

Steve Myres, PE

Maybe instead of an encoder on the control wheels you should tachometer generators. ServoTek has some fairly inexpensive ones. This would give you an analog signal which you would then put into the drive for speed reference signal. Buy flow totalizers from Georg Fischer (Signet) or Burkert for display of pumped total.

The most cost effective would be to use a frequency to voltage convertor, such as sold by Red Lion or Action Instruments. The analog voltage could then provide a speed reference to a low cost AC inverter, such as sold by Dart Controls or Minarik.

Red Lion also sells reasonably priced encoders and pulse generators. Minarik or Dart may be able to supply everything that you need. You might try AutomationDirect.com also. They sell drives and encoders.

You might even be able to find a drive that will take a frequency input directly as it's setpoint. That would be the simplest.

If you have questions, let me know on or off of the list, I will try to help.

Bill Sturm

Bruce Durdle

Surely the most effective way would be to give them a direct drive to a positive displacement pump - why bother with the fancy bits? It will also impress on them the effort required to do such a simple task, and perhaps a bit of sympathy towards the need for energy conservation measures!

A rev counter on the driven shaft would give the required indications.