Speed control help

[missy_thompson]

I am conducting a research experiment where I need to control overground running speed. To do this individuals will follow a marker on a string that is pulled across the laboratory. I am using a small electric motor (1.5-3V) connected to a tamiya gear box (too get the rpm low enough) that spins a pulley pulling a string across the lab. I am using an Enercel switchable ac power adapter with a 1k ohm multi-turn potentionmeter to control the voltage. However, this set up is not sensitive enough, i.e. I need to be able to have ~10rpm changes in motor speed and currently I'm getting ~100rpm per 0.1V and it is very difficult to control 0.1V increments.

I have also used a 25 ohm single turn potentiometer and had the same problem. I also have a large knob on the potentiometer to try to make voltage adjustments easier.

Does anyone have any possible suggestions on how to better control voltage/speed? I would like to keep this relatively inexpensive, but would definitely consider a complete redesign.

[Blustar2]

A simple SCR speed control would be ideal here, but I'm not sure that one would work satisfactorily with such low voltages.

[I_Daniel]

You could try a variable transistor power supply with a volt meter across the output which would give you a better read-out. A linear potentiometer should also give a better control than a log. potentiometer.

The 100 RPM per 0.1 volt step seems to be a problem associated with the motor, (Strength of field coils perhaps ?) but if it is a constant 100RPM per 0.1volt then would it not be possible to recalibrate the speed by dividing it by 10 ?