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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
    yes but if you import like that you become responsible for making it meet all the machinery directive etc. requirements, emergency stops, alarms guards and so on, as delivered, stuff off Alibaba generally does not.
  • 0
    O.K. well let's make one. Please see from about 19.30 onwards.

    How to make a welding rotator - YouTube


    Z.
  • 0
    davezawadi (David Stone):

    How about the load inertia Simon?


    I'm afraid I am not in a position to quantify the load inertia at present, however the cylindrical object has an approximate diameter of 150mm and the 20kg load is always concentrated at the lowest place in the cylinder during motion (i.e. the majority of the load comes from loose material within). Hopefully this will give some indication, I imagine the load inertia could be regarded as 'not-inconsiderable' but this will definitely be fleshed out during the design. I would be grateful if you could advise whether this information would favor a stepper or DC servo?


    This is for an industrial application that will certainly require a bespoke automated solution, my apologies if there was any uncertainty about this.


    Our currently favored solution is for a geared DC servo with simple PWM control, with the option of incorporating encoder feedback / positional control if deemed to be necessary.   


    Many thanks for all your input on this matter, it is very much appreciated.



     


  • 0
    davezawadi (David Stone):

    How about the load inertia Simon?


    It only has to rotate at 1rpm.  This isn't a spin dryer.


  • 0
    But you need to go back via the gearing and if you get it wrong the servo won't be stable. As it is a cylinder with loose contents this is going to be a bit tricky. I still suggest a geared down stepper as you are in one-off land. Many fewer surprises to give problems.
  • 0
    well if we assumed a 6 inch diameter tube of thin wall thickness and half load, that puts an upper limit on the inertia, (essentially integrate ove a uniform semi-circular weight) but adds a dissipative aspect from energy lost as the liquid or granular material within is agitated - this works in your favour.

    The only remaining question is the one about overshoot - if we ask for a dead stop from full speed, how accurate do we need it to be, and is it OK if it rolls back a bit - which it will always of course, but if we can accept +/-1 degree it is easier spec than 0.1 degree, in turn easier than 0.01 degree or 0.0001 -  if we were reading a 300 DPI printed label for example you might accept 1/300 of an inch of movement  within that ~ 18inches of circumference , so one part in  ~6000 of 360 degrees, so 0.06 deg.   I suspect however something quite a bit looser will be fine.

    With a bound for that that we have all the info  we need.

    M.
  • 0
    Simon Barker:

    A geared DC motor would do the job.  They can be bought off-the-shelf.  An optical rotary encoder attached to one of the drums would tell you how far it has turned.  Use a little microcontroller to read the encoder, and turn the motor on and off.  Use PWM if you need to control the speed.


    Like this perhaps.....with suitable gearing........

    Parvalux PM5 Geared Motor 50Vdc, 100 rpm, 160w 4.2A 6.3Nm H7FL PM5-0023/CONT | eBay


    Z.


  • 0
    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, 




     


     "(i.e. the majority of the load comes from loose material within)."                                                                                                                                                      Just what do you mean by "inspection?" I assumed inspection of the cylinder. Do you mean inspection of the cylinder contents now that you have mentioned more details? I think we need to know.


    Z.


  • 0
    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, extra friction was not an expected fault!
  • 0
    Worm drive?

       - Andy.

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