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position tracker with op amp

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Simon:
Right, here is the deal. I have a water valve with an incorporated potentiometer to feedback the valves position (open/close or partly)

Now I've been asked to design a circuit to control the valve with a potentiometer, so if I turn the control pot up the valve will open more and the circuit will determine when to stop driving the motor when the feedback pot is basically sending back the same voltage as the voltage set by the control pot.

So I assumed this was a comparator job. I started with two comparators both taking the same signal on the inverting and non inverting inputs but with the inputs switched. So theoretically when one is has a H output the other is L. I'm using an LM339 and naturally our good friend offset came along and there is a state where both outputs are on. This means that as I'm driving the motor that opens and closes the valve with a H-bridge one almighty BANG !

So I experimented and put a fixed potential divider on the output of the control pot so that there is another control output at 99% of the value set by the control pot. So now each op amp gets a slightly different control signal and I get some window where both outputs are off.

Now I'm feeling really pleased with myself until I realized that my gap between the control pot output and the secondary output off the fixed divider at 99% has to be at least the offset voltage or twice it maybe and that the lower i go the smaller the actual voltage "window", so with a 2% value on the control pot I'm back to square one ! but the position pot only goes down as far as 7.7% anyway and a trial at 4.6% seems reassuring.

Have i found the simplest method of doing this or am I missing something obvious. ? Are there comparators with very good offset specs that don't cost the planet ?

I could use a micro controller and program in as much hysteresis as I want, but I'd rather not or at least know i have an analogue solution up my sleeve.

ejeffrey:
How about using a single comparator as a direction controller, and an absolute value circuit followed by a comparator as an enable control for dead-band control.  The first comparator always says 'open' if the valve is too closed and vice versa, but the second comparator disabled the bridge completely if the absolute value of the error is less than 0.1 volt or something.  That should let you separately control the setpoint and the deadband

gregariz:
Hmm... double ganged potentiometers... maybe a second channel into a second comparitor with a known offset when they are 'close' that you can use as a 'close' indicator.

Simon:

ejeffry

that's an interesting possibility, so basically I'd have to work out the difference between the two pot outputs and if over a certain value let the valve operate. The only thing is it would have to work with both polarities, I'd need a differential amplifier to get the difference and then compare that to my absolute "offset"

Simon:

--- Quote from: gregariz on July 21, 2011, 09:25:40 pm ---Hmm... double ganged potentiometers... maybe a second channel into a second comparitor with a known offset when they are 'close' that you can use as a 'close' indicator.

--- End quote ---

i did think of that but assumed that the cost of a dual pot might be prohibitive but then I have not looked, might be the most robust solution

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