Which dc motor should I opt for?

Discussion in 'The Projects Forum' started by Knigh7, May 7, 2015.

  1. Knigh7

    Thread Starter New Member

    Feb 25, 2014
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    I'm doing a project on light source tracker (1D only). I want to connect a plate (500g-1kg approx ) to the motor such that the plate is always facing the light source. As I haven't worked much with dc motors much I am not sure which dc motor to use? gear, stepper? What should I look for while selecting the motor?

    As the speed of tracking is important I think it will depend on the motor speed and for rotating the plate we will need torque as well... Can someone guide me?
     
  2. SpiderElectron

    New Member

    May 7, 2015
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    Given the need for speed and torque, I think I would go with a stepper motor. That will give you predictable step distance, speed and plenty of torque - at the expense of a more complicated drive circuit, but you can pick up stepper drive modules from ebay for not very much money, and the only signals you need to supply are direction and step.
    I can't comment on a particular make/model of stepper, but something like this might work well.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I agree the stepper would be the simpler way to go, a stepper is a servo that does not require feedback or PID loop as a DC brushed does in this kind of positioning application, which makes things simpler.

    There have been a few past similar solar tracking projects here and elswhere, I believe there is one on Roman Blacks site, he used to be a member here under The_RB.
    Max.
     
  4. Knigh7

    Thread Starter New Member

    Feb 25, 2014
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    Thanks for the help guys but I'm still a little confused... why not a simple dc motor?
     
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I would use a geared DC motor for this application. Simpler and more efficient than a stepper, IMO.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    response speed is important in your selection. you will be moving a fairly large mass and velocity and inertia can destroy the 'snap' action response time, requiring a very large motor to overcome the inertia of the mass in motion. more info on how fast you need to go between point A and point B would aid in our understanding your needs. Gear motors have large torque capabilities but cannot move quickly. verses steppers which can move very fast but could then be overloaded trying to quickly halt a massive load in motion at a high speed
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    We don't know what the minimum tracking speed should be or what the application is; the TS only stated that "the speed of tracking is important", nor do we know the required accuracy.
     
    Last edited: May 8, 2015
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Well the reason is because a simple DC motor does not listen very well. If you want it to move by exactly 17.3 degrees, and you send it the voltage that you think it needs to do the job, for the length of time that you anticipate it will take to do the job, it may travel 17.3 degrees, or it may travel 9.6 degrees or 37.9 degrees. It depends on the gearing, the variable load on the motor (the angle of your plate), temperature, and other things. With a stepper, if you want it to travel 17.3 degrees, and you know it's a 48step/rev motor with 10:1 gearing, and you send it 23 pulses [(17.3/360)*48*10], it will go exactly 17.3 degrees and stop there. It's the cheapest option for accurate positioning. That's what's used in your printer, scanner, etc.

    If you want to get the same positioning accuracy out of a plain 'ol DC motor, then you have to attach some sort of position feedback mechanism to it (encoder, potentiometer) and you have to run it from an intelligent motor driver which monitors the position feedback and runs in closed loop, continually adjusting output power to keep the DC motor aligned to the desired position. This is called a "servo." This servo method is more costly and more involved. It has better performance (faster, more powerful, etc) but is generally reserved for more demanding applications.

    But if your positioning requirements are not very tight, then a cheap DC motor with worm gear reduction could be an option. A worm gear does not allow inertia to force the motor to spin. So for example, if you wanted the motor to go from present position @ 10 degrees, to 30 degrees, and an error of plus/minus 10 degrees is acceptable, you could just apply power for however long it takes to move 20 degrees, and you should have a reasonable chance of stopping somewhere in the acceptable range and staying there. You would need to size your motor much larger than what's required to do the job. Large enough so that it can't tell the difference between minimum and maximum load; that way it's speed won't be affected by varying load as the plate is aligned to different angles. On the small scale you're talking about, that shouldn't need to be an expensive problem.
     
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