Using Pots to Control Motor Speed

Discussion in 'The Projects Forum' started by koodz, Nov 3, 2012.

  1. koodz

    Thread Starter New Member

    Nov 3, 2012
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    My team is building a Sea Perch-style ROV to run an obstacle course and I'm trying to select appropriate potentiometers to control the speed of its motors.

    Our idea is to use one vertically-mounted motor to provide downward thrust to counteract the ROV's positive buoyancy. We were planning to just use a potentiometer to control the speed of that since it'll be constant.

    The forward thrust and turning would be provided by a motor on each side controlled by one DPDT switch each. One of our guys had the idea of wiring another pot between the power source and the DPDT switches so we could dial down the speed of the motors for fine maneuvering or dial it up for straightaways.

    The motors we're using are these:
    [​IMG]
    Voltage Range: 6V-12V. Nominal Voltage: 12V. Current: 0.14A. RPM: 12,500 max. Torque: 44.2g/cm. Terminal Type: Solder. Shaft Dia.: 2mm. Shaft Length: 10mm. Size: 24mm dia. x 32mm length.

    The power source is six AA batteries, so 9V total.

    We don't really want to wait for shipping, so I figured we'd end up just going to Radio Shack.

    Since this is such a low-powered setup should I just find the lowest maximum-resistance pot I can?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    See tht u buy the largest wire wound potentiometer available in the radio shack !

    Next decide whether u can use it.
     
  3. t06afre

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    May 11, 2009
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  4. ScottWang

    Moderator

    Aug 23, 2012
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    You may use NE555 PWM to adjust the speed.
    1. NE555 PWM + BJT.
    2. NE555 PWM + N ch MOSFET.

    You can search NE555 PWM on this web or google.
     
  5. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Or even better an MCU like a pic. It could then have some "smarts" about it.. I would think you would pretty much want in something like a rover anyway.
     
  6. koodz

    Thread Starter New Member

    Nov 3, 2012
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    Why decide whether I can use it?
     
  7. koodz

    Thread Starter New Member

    Nov 3, 2012
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    That looks like it would exceed the shoestring budget we're doing this on, and probably my competence as well. I'm a Civil Engineering student and not knowledgeable about electricity at all. I just want to pour concrete someday if I can get through building this submarine....:)
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Pots are expensive, especially if they are rated for enough current to drive a motor, albeit a small one. Many pots will go poof if you try to run a motor through one.

    The 555 timer based PWM approach is a good one and would not be as expensive in parts, although it would cost you time and effort.

    How about a voltage controller? You could use an LM317 voltage regulator and a cheap "normal" pot to control the voltage available to the motors. It's not very elegant and there are reasons people use PWM instead, but it would be quick, cheap and easy to set up for this. You could get everything you need a the Shack.
     
  9. koodz

    Thread Starter New Member

    Nov 3, 2012
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    Wow, you're right. The pots I'm seeing at Radio Shack are rated for like a half watt at most. I'm pretty much studying stuff from this site and wikipedia continuously today but with three motors of 2 ohms resistance each, I have what, 13.5W to deal with?

    That isn't going to work all right. I hate to abandon this idea entirely, but I can't spend much money at all on this project. I might have to live with jerky throttles.
     
  10. koodz

    Thread Starter New Member

    Nov 3, 2012
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    Given that I just now reviewed how to calculate wattage, if I want to keep the wattage in the circuit under one, and I have three motors each with 2ohms resistance wired in parallel, that's an equivalent resistance of 2/3ohm, right? So 9V/(2/3ohm)=13.5A, and 13.5A*9V=121.5Watts? Would I have to regulate the voltage down to 1/6 of a volt if I wanted to keep the current in the circuit under a watt?
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Whoa, each motor at 12v and 0.14A is at 12*0.14 = 1.68W. The motor looks like a resistive load of 12/0.14 = 86Ω. (It has inductive properties also, though.)
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    ROV= remote operated vehicle ?? What do you have for control, umbical cable, sound, RF ?? How many channels ?? ON- OFF, PWM, analog ??
    Who is going to flip your switches under water-- or do you mean relays ??, in which case H bridges should be investigated, will give power for motors with verry lo power signal requirements. Another thought: trying to push sub down sounds unstabls, sub may roll. This post sounds familiar- may be a similar one in files.
     
  13. koodz

    Thread Starter New Member

    Nov 3, 2012
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    Ha, you're right. I thought I heard in class we had resistance of 2Ω, but nope.

    We're only using 9V to drive these motors, so would that change the resistive load or is that just a specification of the motor?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    It's hard to say what the current draw at 9V might be, but probably less than the 140mA of the rating, and estimating it at 9/12 of that is going to give you a good idea. ie. same resistance
     
  15. koodz

    Thread Starter New Member

    Nov 3, 2012
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    Definitely switches, not relays. We're using Cat5 cable (8 strands) for the umbilical for this. We were given four of the motors I linked above, three DPDT switches, a 6-AA battery box and the cable and told to build the frame of the ROV from PVC. The only electrical components that will be under water will be the motors.

    The form factor is similar to this: http://www.seaperch.org/index but our frame is a different shape. I'm not worried about the stability. The vertical thruster is going to be centered on a bottom strut with the propeller aimed up through the frame of the ROV. I'm not worried about our frame design, just the electrical aspects of the controller.
     
  16. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    555 timers are like $2.00 from Radio Shack. Cheaper if you get that from China. That must be one thin shoestring

    http://www.radioshack.com/search/index.jsp?kwCatId=&kw=555 timer&origkw=555+timer&sr=1

    A Pic is only $3 or $4 but you will need a programmer.

    Of course you will need other associated components for Pic or 555 but could not be more than a few bucks more.
     
  17. koodz

    Thread Starter New Member

    Nov 3, 2012
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    So the general consensus seems to be I have to set up some kind of timing circuit if I want speed control rather than trying to smoothly adjust resistance?
     
  18. ScottWang

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    Aug 23, 2012
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  19. spinnaker

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    Oct 29, 2009
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  20. wayneh

    Expert

    Sep 9, 2010
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    PWM is sort of the state of the art for this chore, yes. But the right pot, if you could find one cheap enough, would work fine, as would the voltage regulator I mentioned. If you could live with just a "low" and "high" setting, you could even put a lightbulb or such in series with the motor and use a switch to short across that. Switch open would deliver less power to the motor, closing the switch would deliver full power.

    Lots of ways to skin the cat.
     
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