Speed control help

Discussion in 'The Projects Forum' started by missy_thompson, Sep 4, 2011.

  1. missy_thompson

    Thread Starter New Member

    Sep 4, 2011
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    Hello,

    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? I would like to keep this relatively inexpensive, but would definitely consider a complete redesign.

    Thank you for your help!
    Missy Thompson
    Department of Neuroscience
    University of Idaho
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    The easiest solution would be some kind of simple resistive voltage divider to allow for a smaller step per degree of the potentiometer. Could you draw a small diagram of your setup? You want to change the motor voltage from 1.5V to 3V, right. What's your power supply voltage? Did you just put the potentiometer in series with the motor?
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I think your little 3v motor will never give the repeatable speeds you need for your experiments. Its speed is already very unstable for small changes in control voltage and its speed will also be affected by load.

    You could switch to a closed loop DC motor with encoder and speed control circuit, which will be expensive.

    Probably the cheapest option would be a small stepper motor (like the ones in bubblejet printers) and a typical IC to drive the stepper like a ULN2004, with the "speed" pulse frequency controlled by a 555 timer.
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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  5. missy_thompson

    Thread Starter New Member

    Sep 4, 2011
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    Thanks for the feedback everyone. I've been playing around with the device more and I think that the potentiometer might provide the precision, but maybe I have it hooked up wrong.

    If I test the voltage across the potentiometer when it is not connected to the motor it divides the voltage equally across the 10 turns, which is the precision I would like. However, once I hook in the motor the voltage drops to 0V in approximately half a turn of the potentiometer (rather than the 10 turns). It makes me wonder if I don't have it wired incorrectly. I've attached a figure of the wiring configurations that I've tried (sorry I didn't want to incorrectly draw the schematic).

    Any thoughts?



    Missy Thompson, M.S.
    Human Performance Lab
    Department of Neuroscience
    University of Idaho
     
  6. nerdegutta

    Moderator

    Dec 15, 2009
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    Maybe I'm missing something, but why not try PWM?
     
  7. elec_mech

    Senior Member

    Nov 12, 2008
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    Hi Missy,

    As you have it hooked up, you're enabling a short to occur depending on the position of the potentiometer. I've attached a schematic showing you how you've currently got it wired and how to rewire it to do what you want to do.

    This is not the best way to control motor speed, but it may work for your application. There is no harm in trying this as long as the potentiometer is rated for the power draw of the motor. This is done by using a potentiometer that meets or exceeds the motor's power draw.

    The motor's power draw is determined by it's operating voltage and current. You said the motor is rated between 1.5-3V. To be safe, we'll assume 3V. I don't know how much current it consumes - if you have current draw information on the motor use it, otherwise, let's assume 0.5A or 500mA. The motor power is therefore 3 x 0.5 (V x I) = 1.5W. It's usually a good idea to double the rating of the potentiometer to be safe, as the pot rating is the absolute max. Therefore, the potentiometer should be rated for 1.5W x 2 = 3W or more in this example. If the pot wattage rating is too small, it'll get real hot when you power the motor and may ignite (probably not that bad, but possible).

    If this doesn't work, let us know and we can point you in the direction of a simple PWM circuit for better speed control, such as this one from SgtWookie:

    http://forum.allaboutcircuits.com/showthread.php?t=9016

    It might be a little more than you were planning to build, but this is easily put together quickly on a breadboard.

    Lastly, if you don't already have one or two, add a 0.1uF across the motor's + and - terminals to help eliminate spikes, especially if you end up using a PWM circuit.
     
    missy_thompson likes this.
  8. John P

    AAC Fanatic!

    Oct 14, 2008
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    I don't want this to come across as an insult, but I'm getting the impression that you don't know anything at all about electronics. Rather than getting anyone here to talk you through this design, I think the practical solution is to ask someone else at your university to build something for you; you want to have the thing to use it, not to become an electrical engineer!

    The use of a potentiometer in this case will fail because it only produces a voltage that's proportional to position as long as no current flows. Once you draw current from it (and a motor certainly will) the voltage will change, and may drop close to zero. The electrical buzzword is "a high impedance trying to drive a low impedance load".
     
  9. missy_thompson

    Thread Starter New Member

    Sep 4, 2011
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    elec-mech,

    Thanks so much for the help! I tried the configuration that you sent and while it is working a bit better, it's still not the precision that I need. The more that I've been reading it looks like PWM is the way to go. If you could point me in the direction of a simple design or kit that would be much appreciated!

    Thanks again!
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
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    Another configuration that might do the job would be to take the potentiometer's output through a resistor and then into the base of a transistor, which would drive the motor. It's definitely a quick-and-dirty solution but it's simple and easy. It's best for a small motor, otherwise the transistor needs heat sinking.
     
  11. elec_mech

    Senior Member

    Nov 12, 2008
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    Hi Missy,

    I would love to put together a kit for you or build and test a circuit myself, but I'd need a couple of weeks. I found a kit you can put together yourself. Here is the manual which does a great job of explaining how PWM (pulse-width modulation) works to control speed:

    http://www.ramseyelectronics.com/downloads/manuals/MSC1.pdf

    And here is the kit:

    http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=MSC1C

    It runs $35 and you'll need to solder the parts on, but it looks like it includes a case, so at least it looks professional. If you've never soldered, check with someone in the science or engineering department if you'd prefer not to do this yourself (but it is fairly easy). The company may offer a pre-built kit, I didn't have time to check as I need to run right now.

    Good luck!
     
  12. elec_mech

    Senior Member

    Nov 12, 2008
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