Need to smooth out motor rotation to prevent changes in torque

Discussion in 'The Projects Forum' started by Cirrus, Mar 19, 2015.

  1. Cirrus

    Thread Starter New Member

    Mar 19, 2015
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    Hi there - I am extremely new to wiring and electronics and am actually a photographer. Nevertheless, I needed a motorized slider to take perfectly smooth video pans. To save money I tried making my own.

    After much trouble I have basically found a solution I think works pretty good. However, there are two central issues I am facing right now.

    Firstly, I invested in a PWM because I thought it would maintain more power at lower speeds (rather than a potentiometer). Unfortunately, when I try to dial down the PWM and go slow, the motor will start to jitter and even stall on and off. I have to increase the speed in order to maintain a more regular amount of movement.

    Secondly, even when moving at a higher speed, there seem to be fluctuations in the speed that I can only attribute to what I have read is "torque ripple", although it may just be fluctuations in other factors I'm not aware of.

    Basically, is there a way to wire the motor in a way that smooths out its speed, somehow making it perhaps slow-start but then maintain a constant voltage/speed, or at least minimize these issues?

    I read somewhere that people add capacitors to motors, but it seemed to be to reduce electrical noise. For some reason I thought it may provide a "buffer" so that the motor always has voltage to draw from, thereby maintaining a more constant torque.

    I have also read that servos somehow work in similar ways, but I don't know if getting one would solve this issue.

    Thanks.
     
  2. nsaspook

    AAC Fanatic!

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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You do not state the motor technology, DC, BLDC?
    For accurate rpm control you require some kind of feedback, the most simplest way is a drive that monitor current and makes the rpm adjustment based on this, the better is velocity loop control is by some kind of tachometer feedback, most decent drives have this option.
    One other simple method that does not require feedback is to use a stepper motor.
    What are the makes of the motor & drive you have?
    Max.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    brush type dc motors tend to loose torque at lower speeds, can you change some ratios in the drive mechanism to allow the motor to run at a higher speed the change in ratios should allow for the same slider speed with increased motor speed.
     
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  5. BobTPH

    Active Member

    Jun 5, 2013
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    Exactly. You should use a gear motor so that the max RPM is a little higher than the max you need. Then it will slow down much more smoothly.

    Bob
     
  6. MaxHeadRoom

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    There is a confusion of terms here, Actually DC motors have maximum torque at zero rpm. Torque and power is often the cause of confusion.
    For a DC motor, the output torque is proportional to the current going into the motor no matter what the motor speed.
    Just look at the spec sheets, the torque tapers off as it reaches the maximum rated rpm
    http://www.me.umn.edu/courses/me2011/arduino/technotes/dcmotors/motor-tutorial/
    Max.
     
    Last edited: Mar 19, 2015
  7. Cirrus

    Thread Starter New Member

    Mar 19, 2015
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    Hi there - thanks for the responses. I am in Australia so my timing may be off, but I'll try to answer what I can.

    It's a simple DC motor, geared down to 70 RPM. This gives me buffer so I can slow it down, but in this case when I go as low as I want it starts to stutter and stall.

    nsas: It's on a very expensive tripod head so I'm not sure how to dampen the movements between the camera and the slider economically, but this is a fantastic idea I had not thought of.

    alfacliff and bob: That is also a great idea, but as the motor is already geared I thought that was internally accomplished?

    This is the exact motor I have: http://www.jaycar.com.au/Electromec...C-Reversible-Gearhead-Motors---70RPM/p/YG2732

    I am considering the possibility that just getting a motor 5 times stronger may reduce this effect in the case that it will react less to the weight of the object being pulled?

    Max: can you explain how a stepper motor will be a better solution? And will a stepper be smooth?
     
  8. MaxHeadRoom

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    Jul 18, 2013
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    A stepper with gearing is quite smooth, there is a project out there and also one here where someone built a camera tracking mount for astronomy, an ex member here THE_RB has a site under Roman Black that was involved in the design, it may be on his site.
    I would not have expected any stutter with the gearing involved, but it depends what the nature of the load is?
    Is this one of the cheap PWM controllers off ebay?
    Max.
     
  9. Cirrus

    Thread Starter New Member

    Mar 19, 2015
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  10. Cirrus

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    Mar 19, 2015
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    Would getting a more powerful motor at a slightly lower RPM reduce any of the effects I've mentioned?
     
  11. MaxHeadRoom

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    Maybe first explore why you get the judder etc with the present set up, did you try a 12v lamp to see if anything shows visually?
    Max.
     
  12. Cirrus

    Thread Starter New Member

    Mar 19, 2015
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    I don't have a 12v lamp on hand.

    My reasoning is basically, let's say the timing belt that is connected to the pulley is tight, but not as tight as it could be. Maybe if I get a stronger motor and tighten the belt even more, it will prevent fluctuations in the resistance when the slider is actually moving.

    Like, right now, when I try really tightening the timing belt, the shuddering gets worse. So it feels like the motor seems to be struggling or something.
     
  13. MaxHeadRoom

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    To know the exact load would an advantage and also any backlash will aggravate it.
    Max.
     
  14. PlasmaT

    Member

    Feb 19, 2015
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    The smoothness of the rotation depends on the mass moment of inertia of the connected parts, i.e. just like a flywheel. Increasing the inertia may be not a solution for your case. So you got to stick with the existing inertia. Therefore you are left with manipulating the other parameters. That is the acceleration. In PWM, actually minor acceleration takes place as the pulses are applied. So to reduce the acceleration, use a higher frequency.

    Analogy: Smooth an alternative current with a large capacitor OR increase the frequency and smoothen with a small value capacitor.
     
    Last edited: Mar 20, 2015
  15. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the problem with using a bursh type motor and pwm are that the brushes and the pwm pulses can interact at low speeds, as the comutator segments move past the brushes, they can be out of time with the pwm, causing speed to be jerky. brush type dc motors were designed and optomised for dc not pulses.
     
  16. Cirrus

    Thread Starter New Member

    Mar 19, 2015
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    Is there a cost effective alternative to DC brush motor?

    Some people seem to use servos - is there a difference between the two in this regard?
     
  17. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, I think you need some feedback to control the speed. It would be easy enough to build, but often expensive to buy. But lets look.
     
  18. MaxHeadRoom

    Expert

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    A dc servo motor is a higher grade DC motor, usually has skewed windings on the rotor and more poles (more commutation bars), and often can be obtained with a dc tach.
    The thing that maybe causing your rough rpm is load, as mentioned, inertia, did you try the motor without load to see if it 'stutters'?
    Feedback, tach etc would probabally be the answer, a stepper motor does not require feedback as it moves a defined number of steps and gearing it down takes out the cogging effect otherwise seen at low rpm.
    Max.
     
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