Motor for arm: what should be used when?

[Ether]

Originally Posted by JamesCH95

We've got two RS775s running through a a ~380:1 reduction on our arm (64:1 gearbox, 6:1 sprocket).

Can you give a ballpark estimate of the torque load on the arm?

(I use an inexpensive fish scale - available in the sporting goods section of any discount store - and a tape measure to get an estimate)

[JamesCH95]

The robot has been shipped to another regional, but I can make a reasonable guess of around 25lb-ft

[Ether]

Originally Posted by JamesCH95

The robot has been shipped to another regional, but I can make a reasonable guess of around 25lb-ft

25 ft-lb = 4800 ozin at arm => (4800/380)/2 = 6.32 ozin at each motor

From the motor curves for RS775:

RS775 @ 12.0 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     6.3   0.045   12272    94.4     6.6        57.3        78.6    72.9

12,272 rpm at motor = 194 degrees/sec at the arm.

A bit fast for the top end, but motors running nice and cool.



Throttle it down to 90 degrees/sec by reducing the motor voltage:

90 degrees/sec = 15 rpm at arm => 15*380 = 5700 rpm at motor

RS775 @  5.9 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     6.3   0.045    5700    88.7     5.6        26.6        33.5    79.5

Still running cool, and the voltage is in a reasonable range for control.


How about stall?

RS775 @  0.7 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out    watts in    eff%
     6.3   0.045       0     0.0     4.9         0.0         3.3     0.0

Very reasonable 3.3 watts dissipation.

[JamesCH95]

Very close to my "napkin" design

I aimed for about 180deg/sec at full speed, but couldn't quite get there with the finite available sprocket ratios.

We limit the 775s to 60% of full command speed, and the PID loop still does a great job of controlling them.

I hadn't done a stall calculation, we had been planning on using a DeWalt transmission with anti-back-drive pins, but found that the pins didn't like it when the arm was powered down, so we removed them. This also has the side benefit of being able to move the arm without powering up the robot or disassembling anything.

[Ether]

Originally Posted by JamesCH95

Very close to my "napkin" design

I aimed for about 180deg/sec at full speed, but couldn't quite get there with the finite available sprocket ratios.

I didn't account for gearbox inefficiency and manufacturing tolerances, so the numbers aren't anywhere as exact as the displayed values might suggest. So it might actually be closer to your "napkin" 180. I left the displayed values as-is in case anyone wanted to play around with the calculations and compare them.