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Mechanical Design Assistance???

RWReynolds

Hey everyone,

 

I'm not a mechanical engineer by any stretch on anyone's imagination. I have one mechanical design issue that I'm not sure how to approach. I kind of know the theory, but I have no idea how to approach it practically.

 

Here's the problem...

 

I have a load, my cultivator unit, that will be about 3' - 4' tall, maybe 12" to 18" in diameter and weigh between 20 and 40 lbs. The whole thing will be mounted on a lazy susan style turntable. The plan is to have a gear or pulley at the bottom that will be used with a hight torque stepper motor, and chain or belt, to rotate the cultivator. I'm using a stepper motor because they are pretty efficient and can be controlled very easily at very slow speeds. I want the rotation speed to be maybe 3 to 4 revolutions per day. A stepper would work well for this I think.

 

I have no idea how to properly size the motor. I see them speced at anywhere from 10s of oz-in to 1000s of oz-inches of torque. I get the concept of torque being rotational force. And that required torque has to do with acceleration time up to the desired RPM. But I have no idea how to relate this to my requirements.

 

I'm a 'puter software with some dangerous electronics knowledge guy! HELP!!! And feel free to tell me this just a stupid idea and I should be using a washing machine motor or something if that's the case.

 

For reference, here's a drawing of the rig. Note the drive at the bottom right. Please excuse the misspelled labeling. I haven't gotten around to correcting it yet. lol...

 

image

  • in reply to jw0752

    Hi John,

     

    "The next time you have one of these great questions it would be best to start your own thread."

     

    Understood and will do (I have one about maintaining ptp communication over the usb cable data wires while switching the 5v usb wire (which can act as a trigger to start exposure on multiple cameras with extremely high synchronisation) that has been in the back of my mind for ages that somebody may be able to comment on).

     

    "I saw a person in a motorized wheel chair do a 360 degree pivot"

     

    Cheers - will dig into that.

     

    Thanks,

     

    Andrew.

  • in reply to Former Member

    niman

    Andrew,

    While I presume the maths is correct (beyond me I'm afraid) the actual torque is also going to be dictated by the speed.

    Most DC motors would have some form of curve (Torque v speed/rpm) and the shape of that curve would vary.

     

    I think you might be overthinking this by trying to plot 500 individual points.

     

    It seems simple to me, decide on the gearing that provides a suitable rotational speed that your subject will be comfortable with.

    This will be the speed that is 100% or 255 PWM value.

    It might be that the nearest you get is using a PWM value of 230. ie 230 = full-speed

     

    Do some experimenting with the acceleration time to go from something above zero to full-speed to prevent 'jerking the subject'.

    The de-acceleration time/rate is likely to be very similar.

    You can change the rate of PWM by changing the step value as it likely that changes at the lower end will have more effect than changes at the upper end.

     

    Storing 500 numbers is not a problem as they are all bytes (0-254)

     

     

    Mark

  • in reply to mcb1

    Appreciate your further comments Mark.

     

    Unfortunately the Trinamic youtube video I linked to previously does appear to show the easiest way to do this is by using the equations. It would be similarly straightforward to pre-calculate 500 points if a linear acceleration profile were desired (rather than sin2) - I believe ther Arduino library (Accelstepper) that I previously referred to does this linear acceleration "on the fly".

     

    "Most DC motors would have some form of curve (Torque v speed/rpm) and the shape of that curve would vary."

     

    For the record, here's the Torque/Speed profile for my 1 second move (Set1a) - the original model assumed zero friction (perhaps stiction in angular talk?) and a 14.5 gearhead reduction:

    image

     

    DC Motor problem?

    As I now understand it - the big problem with trying to use a dc motor with a pre-calculated time/motor current (via PWM) profile is, of course, that the calculation is only accurate for the single moment of inertia on which the motion model was built. I hadn't recognised the full significance of this in my previous quote, repeated here:

    image

    i.e. it was referring to a system with constant mass (or moment of inertia in my case) and therefore no requirement for encoder feedback.

     

    On further contemplation of such a system - while it may be theoretically possible to calculate (and position) a suitable extra mass on the turntable (in addition to the mass of the person to be scanned) to keep the complete system moment of inertia constant each time a scan occurs that seems totally impractical. Final position errors would be inevitable in practice.

     

    Further, at the minute, assuming no attempt were made to maintain a constant moment of inertia, I can't visualise how I could use an encoder with feedback to successfully inter-relate with the single pre-calculated PWM table in order to provide precise position/speed control.

     

    So, while the torque/power characteristics of steppers are far from ideal it does seem possible that the pre-calculated table of stepper step times referred to in my post here https://www.element14.com/community/message/177404/l/re-mechanical-design-assistance#177404 is possible of replicating the required motion profile even if the system moment of inertia should change & open loop. I agree, unfortunately, this is not a straightforward approach and may require multiple steppers to provide the required Torque/Power (by simple arithmetic addition of each) but, at the minute, it actually seems the most straightforward approach.

     

    I'd wish to be proven wrong though.

     

    Andrew.

     

  • in reply to Former Member

    Repeating Mark's comment from above (https://www.element14.com/community/message/157611/l/re-mechanical-design-assistance#157611 ) here:

    image

    In considering that in connection with multiple stepper motors:

    image

    If in fact 4 steppers were used it seems Torque & power of each (approx 2.83Nm / 27 W) may be readily available.

     

    Is there any benefit between having the pivot at point "A" or point "B" on the plate?

    Is any particular placement of the spring preferable and how might it's "strength" be calculated/selected?

    Should I assume the plate itself should, at least, be Aluminium?

     

    Any comment (other than it will be expensive image) appreciated.

     

    Andrew.

     

    PS: John, I checked out the wheelchair - it seems the motor is probably a Merits Health Products 24V 120Watt. I've been unable to find any details of the controller.