Bi-pot comparator circuit motor control

Discussion in 'The Projects Forum' started by jongscx, Sep 10, 2009.

  1. jongscx

    Thread Starter New Member

    Sep 10, 2009
    3
    0
    Here's my thing. I have two pots(R1 and R2) and an electric motor.

    I need to make it so that when R1 > R2, the motor turns on in one direction (CW) and when R1 < R2, the motor turns the other direction. But, once R1=R2, the motor stops.

    I don't want to involve a microcontroler/PIC, or (god-forbid) an arduino in something this simple. I'm guessing this can be done with literally a handful of discrete components on a breadboard, I just can't figure out the circuit.


    The motors are also slow enough that speed control won't be a problem. De-bouncing on the other hand, so it doesn't keep alternating to just too much to too little ad nauseum would be nice too...


    I'm using this to control a bed. Currently, it has 4 buttons, up/down for the head and up/down for the feet. I want to replace them with two knobs and place pots on the joints. That way, I'll set the angle and it will adjust automatically without having me hold the button until it's in the right spot.
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The basic circuit would be a differential amplifier driving a power (servo) amplifier.

    The differential amp 'sees' any difference between the outputs of the two pots and drives the servo/motor until the difference is zero.

    The design of the servo amp very much depends on the motor voltage and current, and whether a single or dual polarity power supply is available.


    The simplest parallel in a commercial device is probably an antenna rotator; you turn the dial to the direction you want and the antenna motors round until it matches.
     
  3. Gustav180

    Member

    Aug 25, 2009
    17
    0
    Hello jongscx

    Some questions, is the rotation full speed untill R1 = R2 or is it depending the difference between the value of R1 and R2?

    Is it low voltage DC-motors, i.e. 12 V or i.e. 117 / 230 V AC-motors?

    Gustav
     
  4. jongscx

    Thread Starter New Member

    Sep 10, 2009
    3
    0
    I actually want to interface with the existing push-button system and use relays to emulate button-presses, so really, I just need to have it output a voltage when there's a difference in one way, and flip the output the other way when it's the other. From there, I'll split it to two relays, an SPST that will turn on the circuit no matter which way, and a DPDT relay with a diode that will only activate when it's one direction... (does that make any sense?)

    It's on an electric hospital bed and I want to keep the original's functionality without redoing the entire circuitry. Also, they drive some beefy linear actuators (700 pound pushing force... really, they need to lift 700 lbs on a bed?), which run off the 120V wall voltage.
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    OK, still use a diferential amplifier to compare the two pots, but then feed that to two comparators with the other inputs connected to preset pots so you can adjust the thresholds at which they switch on/off.

    Have one set positive of the neutral position, the other negative.

    Connect the comparator outputs to sections of darlington driver or darlington transistors via resistors, then to the relays.
     
  6. jongscx

    Thread Starter New Member

    Sep 10, 2009
    3
    0
    OK, diff amp it is... now, how do I build one?

    I look up "differential amplifier circuit" on google, and all the pictures show is a sideways triangle with an A on it, two inputs on the left, and an output on the right, and a Vcc and a ground above and below.... kinda black-boxy.

    I did find an actual circuit-looking thing on wikipedia:

    http://en.wikipedia.org/wiki/Differential_amplifier

    Is this something I can print out, bring to radio shack and have them give me the right parts?
    If so, for the inputs, can I just mount my pots connecting inputs to the Vs+ and the "input 1" and "input 2" and my relay will connect to output and ground?

    I'm sorry, I failed out of univ before I got to take the circuits classes and I don't actually have any experience with transistors/resistors except for replacing them...

    thanks
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Hi,
    OK, for the differential amplifier, my first choice would be an INA126 (or similar).

    The data for this chip is here:
    http://focus.ti.com/lit/ds/symlink/ina126.pdf

    I'd follow that with two LM311 comparators, one to select each direction. Data here:
    http://www.national.com/ds/LM/LM311.pdf


    The differential amp tells you the difference between the two pot settings, ie if the target pot it higher or lower than the feedback pot (if the output is higher or lower than 0V or mid-supply, depending if you use dual or single power supplies).

    You then feed that signal to one input of each comparator, and feed the other input from a pot, one for positive movement and one for negative movement.
    eg. one comparator might compare to +1V and the other to -1V, so the motor runs until the difference in position between target and feedback is within that 'window'.

    The size of the window or dead zone is set by the comparator pots and the gain setting on the difference amp. (possibly start with a gain of 10).

    I'd get some bits and start experimenting on a plug-in breadboard, using two manually controlled pots for inputs and a couple of LEDs rather than the relays, with everything on a battery until you get it working at that stage.
     
Loading...