slightly OT, need some electronic circuit help

Shane Hathaway shane at hathawaymix.org
Mon Oct 8 15:18:52 MDT 2012


On 10/08/2012 02:03 PM, Michael Torrie wrote:
> On 10/08/2012 01:30 PM, Shane Hathaway wrote:
>> The large current requirement (10A) and the need for a middle position
>> make the circuit interesting and difficult to achieve using low cost
>> analog components.  An analog H bridge would work well if you only
>> needed 2 positions.  You can't dump that much current through an
>> ordinary op amp.  (Audio amplifiers use big, expensive op amps that
>> easily surpass the cost of an Arduino.)  PWM is the usual solution for
>> handling that much current.
>
>> However, can you achieve the middle position using a simple mechanical
>> spring?  That would allow you to use a very simple H bridge.
>>
>> It would help a lot to understand the application better.  What kind of
>> resting position do you want?  When the power is lost, do you want it to
>> revert somewhere, or hold its position firmly/loosely?
>
> Okay, so here's what I have.  I have a linear actuator, which is
> essentially a screw driven by a motor.  And the actuator has a built-in
> potentiometer to give you position.  The motor uses very little or lots
> of amps depending on the load.  I won't be having much load, so the amp
> draw is very low, like under 1 amp at 12 VDC, most likely.  The
> actuator, being a screw, cannot move when power is not on.  So I don't
> need to hold it actively.  It is also very slow (compared to a servo),
> so PWM is not required.  Full on voltage or no voltage is sufficient.
> As I said before, I don't need super-accurate positioning, so I don't
> need the actuator to seek back and forth around the target point.  When
> it gets there it can just stop, even if it's too far or too close by a
> small amount.
>
> This actuator is controlling a hydraulic valve assembly so it only needs
> three positions: extended, middle, and retracted.  It will always be in
> one of these positions; there's no resting position.
>
> Nick's idea of using limit switches is a good idea.  I am confident a
> simple circuit can do it all without that kind of extra mechanical stuff
> though.
>
> Really I'm not worried about amp draw because I can use the result of
> the op amps to drive a relay to switch the higher amps if I need to.

Ok, it seems like you want 2 op-amps wired as comparators.  One op amp 
decides whether to move the motor forward, the other decides whether to 
move it backward.  (You can get multiple op-amps on a chip, so this 
doesn't increase the parts count.)  You'll have to calibrate the circuit 
so that the op amps don't activate simultaneously.

To keep things simple, let's say each op amp drives a DPST (or DPDT) 
relay.  That solves your voltage issue: it isolates the control circuit 
from the motors, so you can apply reverse voltage to the motors.  You 
can also use a voltage regulator in case the supply is too noisy.

Each op amp should perform a slightly different comparison.  When you 
want the middle position, the op amp that controls forward movement 
should compare the motor sense voltage with a voltage slightly lower 
than the control voltage, while the other op amp compares the motor 
sense voltage with a voltage slightly higher than the control voltage. 
You can adjust the voltages using resistor-based voltage dividers.

Does that make sense?  The comparator circuit on this page is a good 
reference:

http://www.swarthmore.edu/NatSci/echeeve1/Class/e72/E72L2/Lab2%28OpAmp%29.html

Shane



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