Selecting and sizing motors for gantry robot conversion


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Thread: Selecting and sizing motors for gantry robot conversion

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    Default Selecting and sizing motors for gantry robot conversion

    Hey gang,

    I built a prototype gantry robot for precise and fast hot melt adhesive application that was well received on our shop floor and I now have a budget to scale up and build a production machine.

    As fortune has it, I had an old folding carton sample maker (tangential controlled oscillating knife and scoring wheel, with no Z-axis) with a robust steel frame fall into my lap for next to nothing. The X-axis (gantry) is ~7' long and the Y-axis (table length) is ~4.5' (ideal for my application). The gantry is rack and pinion driven with slaved servo motors at each end and the carriage is rack and pinion driven with one servo motor.

    Selecting and sizing motors for gantry robot conversion-dt-jpg

    The electronics are circa mid-80s and the servo motors only have 175 oz-in stall torque, making the motion pretty darn slow. The gantry is stoutly made and weighs in at 160 lbs. And I will be adding to that a Z-axis and glue head, approximated at 40 lbs.

    I will be replacing the entire electronic package and want to significantly increase the speed and acceleration (hoping for at least 500 ipm). BUT, I am constrained by the existing motor size (NEMA 23), reduction gearing, and rack and pinion.

    Can any of you CNC Zoners help me select the appropriate motor type (stepper, servo, brushless servo) to max. out speed and acceleration given the existing gear train?

    Also, before I dump a small fortune into the electronics, I need to predict whether this set up will be able to obtain the speeds I'm hoping for.

    I've reviewed the Arvid's motor selection guide, but it's not geared (pun intended) for rack and pinion drive trains.

    Thanks in advance,

    Matt

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  2. #2
    Gold Member Jim Dawson's Avatar
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    Default Re: Selecting and sizing motors for gantry robot conversion

    The simplest and best option might be the ClearPath product line https://www.teknic.com/products/clea...sShowPopUp_100 One of the advantages is that they have the drive built into the motor, just supply power and a step & direction signal.


    I have used their NEMA 34 replacement motors on a larger machine than you are working with and rapid at over 1000 IPM with no problems. They may have a NEMA 23 that would be a drop in replacement for your current motors. Your existing motors have a torque curve that drops off rapidly with increasing RPM, where the ClearPath units have a nearly flat torque curve up to their rated RPM.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Selecting and sizing motors for gantry robot conversion

    That looks very interesting. Thanks for the link.

    I'll need some help selecting the power/torque requirements however.

    For the gantry motors I have a 22 tooth spur gear on the motor shaft, which drives the larger 176 tooth side of a compound gear for a 1:8 speed reduction. The smaller side of the compound gear is a 22 tooth pinion gear that engages the rack, which has a DP = 16 tpi.

    For the carriage motor, I have a 22 tooth spur gear on the motor shaft, which drives the larger 128 tooth sideof a compound gear for a 11:64 speed reduction. The smaller side of the compound gear is also a 22 tooth pinion gear that engages a rack, which has a DP = 20 tpi.

    So if I'm doing my math correctly, to push the gantry to 500 ipm, both motors have to turn 2,900 rpm, and to push the carriage to 500 ipm, the single motor has to turn 2,644 rpm.

    But in reality, the glue patterns tend to be line segments that are only a few inches long, so acceleration is key to getting any decent performance. We do run multiple parts in a fixture that correctly positions them on the table, and transit from part to part can be upwards of 20".

    'm calculating for a total weight (gantry + carriage + head) of ~200 lbs. The carriage + head will be ~40 lbs.

    There are zero cutting forces associated with the glue application.

    So far, all of the NEMA 23 stepper motors I'm looking at have pretty dramatic torque fall off (<150 oz-in) at speeds over 1,000 rpm, so I'm thinking that I may have to get into servo motors in order to get the performance I'm seeking. But I don't know how much torque I'll need to the kind of acceleration I'll need.



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    Gold Member Jim Dawson's Avatar
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    Default Re: Selecting and sizing motors for gantry robot conversion

    With that much gear reduction, I would guess that these motors would do what you want. https://www.teknic.com/model-info/CP...del_voltage=75

    A little over double the peak torque of your existing motors, and 3170 RPM.

    Or you could go with the NEMA 34 model

    https://www.teknic.com/model-info/CP..._voltage=75VDC

    With a nearly flat torque curve of 532 oz/in to 3000 RPM.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Selecting and sizing motors for gantry robot conversion

    Do you know any of the physical specs of the existing motors yet? Shaft diameter and type (D shape, keyed, etc.) mounting hole size and pattern, and boss diameter?
    If you can match those with your new motors it will make the job a lot easier. If not, there will be some fabrication to be done or new gears to be purchased.



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Selecting and sizing motors for gantry robot conversion

Selecting and sizing motors for gantry robot conversion