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Choice of motor - application query

simonturner83
Hi

I am currently working on a design concept for a motion system - I would be very grateful for any advice that could be given on a suitable type of motor for the application (e.g. servo, stepper, DC motor etc.).
 The related requirements for the motion system are as follows:
  1. Rotate a cylindrical object placed on its side on friction rollers to allow inspection, where rotation occurs around the axial axis as per test tube rollers or pipe weld rollers.
    1. Details of the cylindrical object:
      1. It is unattached to the rollers, i.e. placed on for rotation / inspection then removed.

    2. Can weigh up to 20kg.



  • The cylindrical object is required to rotate at a very slow speed, typically 1rpm and lower, where the upper limit is likely to be in the region of 15rpm. The friction rollers on the motion system will be in the region of 1/10 to 1/20 of the circumference of the cylindrical object.

  • The rotation of the cylindrical object is required to be significantly consistent, i.e. smooth rotation with minimal stepping / juddering.

  • The motion  profile is likely to just be one full rotation of the cylindrical object at a fixed speed, but there may be a requirement to perform the rotation in increments with pauses in between (i.e. one full rotation split into 6x moves). The duty will be low, i.e. 5-10 such operations per day.

  • Allow some basic position/speed control, where closed-loop is preferable.

Many thanks.



  • 0
    davezawadi (David Stone):

    I think you have never tried to design a servo system Z, the problems very quickly can overwhelm the concept. Let's say an encoder with 100 positions around the shaft and your motor as advertised. How long do you have to stop the drum at the right position before it rotates past it and overshoots? You have some proportion of 3.6 degrees, which is not a lot given any inertia. If it does overshoot can it turn back? How do you stop it oscillating back and forth? A real feedback system will have several controls, the position (proportional to the rotation required), an integral term to make the servo get to the stop position if it "stops" a little distance away, and a differential term that controls the rate of speed change to be within the mechanical limits of the system. Each of these interacts, and getting optimum control from PID as it is called needs some measurements and a lot of maths, or a lot of luck!


    The reason I suggested a stepper motor is that the mechanical movement exactly follows the pulses applied to the coils, and then it stops even if a tiny bit out of position. It has a large braking force when stopped, and can be made to have very similar characteristics even when getting smooth rotation. It does not need any feedback for this application and is therefore simple to implement, and therefore will probably work first time it is powered up. All the drivers and components for software drive are readily available.


    Even mass-market products like laser and inkjet printers often use stepper motors, it is partly because there is little to go wrong, whereas a stiff bearing can make a full servo very badly behaved. I have had an experience of trying to find an electronic servo fault in a film camera with a duff bearing, the speed simply was not as stable as it should be,


    The O.P says, 
    1. The motion  profile is likely to just be one full rotation of the cylindrical object at a fixed speed, but there may be a requirement to perform the rotation in increments with pauses in between (i.e. one full rotation split into 6x moves). The duty will be low, i.e. 5-10 such operations per day.



    Well, why would I design such a simple thing when one can be bought off the shelf.  The cylinder is to be up to 20kg in weight and rotate at a snail's pace for "inspection" whatever that means. Inspection of the cylinder I presume, or the contents, we have not been told.


    It can be manually controlled as it rotates so slowly while it is being "INSPECTED".

    The drive system must be powerful enough and durable enough to provide steady rotation of the cylinder which has a varying unstable load as its contents.


    Just turn it by hand. Printer and camera motors are very tiny and will be unsuitable.


    Z.

  • 0
    Here is an example of a big stepper motor. Would it be powerful enough?

    Stepper Motors application High Torque MST51x up to 50Nm (jvl.dk)


    Z.
  • 0
    That one is far too big! A printer paper feed one geared down might be suitable. Remember you need the torque at 1 RPM and the motor will happily do several hundred, so you have several hundred times the motor torque available. Toothed belts are ideal for this kind of application. 50nM is quite a lot, MCBs 2.5 nM, direct from the motor shaft without gearing!  You wouldn't want to hang on to that motor shaft! Whilst a hand driven idea sounds OK, I am sure this actually wants a motor system for some reason, perhaps the whole lot is inside a nuclear plant away from the operator. It could be a training exercise. Many reasons for the question.
  • 0
    davezawadi (David Stone):

    ..... perhaps the whole lot is inside a nuclear plant away from the operator. 


     


    I reckon that it is to be used for gold panning.


    Z.


  • 0
    I reckon that a 50nM stepper motor would do the job as it will...


    1. Be powerful enough for the job.


    2. Be able to deal with the varying load as the contents of the drum shift.


    3. Keep its speed constant.


    4. Be powerful enough to be accurately controlled to fine degrees of movement.


    5. Be long lasting as it is robust and powerful.


    Examples of 50Nm torque. Approx.


    4D56 Mitsubishi Pajaro engine.


    Engine oil drain plug tightness 40Nm.


    Manual transmission check/fill plug. 59Nm.


    Oxygen sensor tightness. 40 to 50Nm.


    Wheel nuts 100 to 120Nm.


    The above gives an idea of torques in every day use.


    Z.




  • 0
    It will also do at least 500 RPM with that torque at the same time. I fear for the drum. The power available is about 2.6 kW, 3-ish horsepower, at 1 RPM would allow a rope around the drum to lift 100 of your cars at once! With a lesser gear ratio, it might make a good winch for your vehicle. A Stepper of about 10-20W maximum is indicated, geared to get perhaps 2RPM maximum, which would give a very small step size.
  • 0
    You need to select a stepper motor with an eye on the breakaway torque - you do not need enough excess to rip someone's arm off, especially with reduction gears. This is a job for a a few hundred watts only - not much more than electric drill power, not a crane motor.

    M.
  • 0
    It sounds to me as if the OP is asking for some bespoke kit to enable inspection and testing of a component via a ultrasonic/ crack detecting or other NDT nature. Some kind of metal cylinder involved in aerospace/nuclear or chemical processing apparatus or munition - shell casing?. It would assist enormously if the exact nature of the component and it's use were made known before everyone jumps in a any further solutions.

    If this is some kind of mission-critical kit, it needs to be right rather than Heath Robinson.
  • 0
    mapj1:

    This is a job for a a few hundred watts only - not much more than electric drill power, not a crane motor.

    M.


    A power drill geared down has a very high torque. A clutch mechanism with damping springs could be incorporated into the system to absorb shock at start up. Don't  forget the effect of the O.P's friction rollers.


    Z.


  • 0
    davezawadi (David Stone):

    It will also do at least 500 RPM with that torque at the same time. 


    Not necessarily. The speed of the stepper motor is determined by its controller not its power surely.


    Z.


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