motorised pots

Mark Smart smart at nn.com
Wed Jul 15 23:42:26 CEST 1998


> JH said:
> If you find a good (analogue) control circuit, please tell me.
> An opamp that compares the set voltage with the momentary
> potentiometer voltage, and thus drives the motor forward and backward,
> would be all that's needed. But designing the right time constant, and
> probably a little tolerance window where the motor doesn't move,
> might be some amount of work.

Hey, after lurking on this list for a long time, I can contribute
something! I have been trying to figure out how to do this for a long time
using exactly the type of op-amp position regulator circuit which was
mentioned before. I wanted to incorporate this into my massive GR-300
guitar-synth-hacking project. The cheapo motorized pots which another
earlier post metioned are probably the same ones I am using. They're from
Mouser, they are rotary, and only $10 each. And you can still turn the knob
by hand without messing everything up, since there's a little clutch
inside.

I tried several circuits to get this to work. I actually got one that
worked (albeit VERY slowly) with an LF351 op-amp. But it was too slow, and
I'm sure the op-amp wouldn't survive for very long, it was not designed for
that kind of load! So I looked at power op-amps. I was thinking about
using an LM1877, but it has a limit of 0.7V limit on input voltage, which
made it useless in my circuit. I also tried an LM1876, and I couldn't get
that to work either. It behaved very erratically. I don't think these chips
were intended to do DC stuff.

Then this last week I was cleaning out my office at work and found a really
old SGS Thomson data book. There is a power op-amp in it called the L272,
and an example circuit for "automobile headlight positioning" which looks
like exactly what is needed. There are two selector switches, one of which
is mechanically connected to the motor. The selector switches are wired as
voltage dividers with resistors between their output terminals, so it
looks like they could easily be replaced with pots.

Another interesting part about this circuit is that it uses two op-amps
whose outputs go to the pins of the motor. Hence you get bidirectionality
with a single supply. I have ordered a couple of L272's and am going to try
this when they get here. 

I looked on the SGS thomson web site, and there are data sheets there for
this chip. However, the "headlight" circuit is not included in them
anymore. My databook is from like 1984 and is called "power linear
actuators". I suppose I could copy the schematic for people if they want,
or scan it and email it. I have no idea if it really works. It runs on
12volts, but I'm sure it can easily be adapted for use with these 5V
motorized pots.

I have looked around (including in amateur robotics books) for a servo
circuit which DIDN'T use PWM, and it seems to be a real rarity. I always
wanted to avoid PWM because of the potential noise problems on audio gear.


************************************************
*     Mark Smart                               *
*     Electronics Engineer                     *
*     NovaNET Learning, Inc.                   *
*     smart at medusa.nn.com                      *
************************************************



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