VersaPlanetary Integrated Encoder - Where should it go?

[Chris Mounts]

We're using some of the new integrated encoders in a triple reduction setup. The total reduction will be 196:1 with a 775 motor.

My question: Is there an ideal location for them to go?

Should they go first in the stack? Just before the output? Does it matter other than from a programming perspective? Do encoders perform better at high RPM vs. low?

[Allthatslack]

Hi, I think it would be best for it to go right before the output shaft, but it is just a preference thing.

[MoistRobot]

The quadrature output is rated at 15000 cpr which is lower than the free speed of the 775pro. In my limited experience I would say you want the encoder near the output. It will give you a straight reading on the output shaft speed of the gearbox.

[Jean Tenca]

It's a question of what maximum RPM the encoder can handle and what resolution your application requires. If you look in their spec sheet, you'll see that depending on the mode it will work up to 15k RPM (quad) or up to 6600 RPM (PWM). Keep in mind the 775 runs at 13050 RPM as you plan what mode to use and where to place the encoder.

Also, with a 196:1 reduction, please be sure to take a look at the VP load ratings to make sure you don't have a failure. Let me know if you have any questions reading through that.

[kitare102]

Just checked, that reduction is supported in the load ratings. I would usually put the encoder just before the output shaft to make sure the slop in the reduction gears won't play into the reading. However, I'm fairly sure that the effect would be minimal.

[JesseK]

As another reference point:

The configuration on our turret is [BAG -> 10:1 -> 10:1 -> 4:1 -> VP Encoder -> 18:42 chain]. This is technically out of spec by a smidge but after careful consideration of the full system setup I believe it will work fine. The turret uses the really nice, sturdy Igus slew ring that prevents 'racking'. It doesn't require much torque to turn and at full motor power should only be about 4% of the maximum load rating for any of the failure conditions listed in the VP load rating guide (2.1 N-m at the hex output shaft + 100% safety margin). We went for higher precision in control since it only has a 75-90 degree FOV. In reality I don't believe the turret will ever receive full motor power - probably more like 75% while it resets to center. The thing seriously takes some oomph to turn by hand for manual alignment...

Conversely, the configuration on our tilt is [BAG -> 10:1 -> 10:1 -> Encoder -> 3:1] since the load for the tilt is a bit higher of a load, at about 50% of the load rating of the failure conditions at the worst point (including 100% safety margin). It may see full power when angling backwards, so we'll remain strictly within the BAG->VP "Green" zones in the guide.

[Jean Tenca]

Originally Posted by JesseK

As another reference point:

The configuration on our turret is [BAG -> 10:1 -> 10:1 -> 4:1 -> VP Encoder -> 18:42 chain]. This is technically out of spec by a smidge but after careful consideration of the full system setup I believe it will work fine. The turret uses the really nice, sturdy Igus slew ring that prevents 'racking'. It doesn't require much torque to turn and at full motor power should only be about 4% of the maximum load rating for any of the failure conditions listed in the VP load rating guide (2.1 N-m at the hex output shaft + 100% safety margin). We went for higher precision in control since it only has a 75-90 degree FOV. In reality I don't believe the turret will ever receive full motor power - probably more like 75% while it resets to center. The thing seriously takes some oomph to turn by hand for manual alignment...

Conversely, the configuration on our tilt is [BAG -> 10:1 -> 10:1 -> Encoder -> 3:1] since the load for the tilt is a bit higher of a load, at about 50% of the load rating of the failure conditions at the worst point (including 100% safety margin). It may see full power when angling backwards, so we'll remain strictly within the BAG->VP "Green" zones in the guide.

Two years ago there were multiple failures of the 10:1 stages due to a weak weld on the sun gear, which it appears they did correct. We are still a bit skeptical of their durability, especially of stacking 10:1's, so are only running one 10:1 stage per VP along with 7:1's and lower with an external gear reduction. I'd be concerned that with your reduction being in the red, if the motor is in brake mode and someone tries to rotate your turret you run the chance of having something fail. Personally, I think staying in the green and doing an external reduction would remove a lot of stress and risk.

Can't wait to see what everyone's building!