# Calculating backdrive gear ratio needed

We were wondering if friction would even make it harder to back drive, so if we calculated a ratio that worked in a frictionless mechanism, it would only work better with friction. This may be backwards thinking.

That is surprising. How much did your robot weigh and what was your lifting mechanism? If you don’t mind.

The only reason it works at all is because of friction. The trick is figuring out just how much friction there is, both in the motor, and in the geartrain. And keep in mind that dynamic friction is usually lower than static friction.

If you really don’t want it to move, you need to have some positive method to keep it from moving. Such as adding a lot of friction (such as a brake?), or a positive mechanical stop.

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Brake mode on a motor can only generate a braking force if the motor is turning. Brake mode works by feeding the current made by the motor being back-driven into a current resisting that motion. There is always inefficiency in that conversion, and that current is only made when the motor’s speed is above 0. Thus brake mode cannot hold a mechanism at 0 speed.

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If you want to assure your motor won’t back drive, worm gearboxes are a solid option. They have a lot of inefficiency that makes it nearly impossible to back drive. On the other hand, they are probably harder to implement then a simple versaplanetary with a large reduction. I don’t recommend them for an elevator.

That’s what I thought, too. Somehow, this isn’t actually true. Shorted motors provide greater resistance to initial turning than open ones. Or at least the force increases far faster than linear with speed at very low speeds.

@Ambrose, you could measure the resistance torque of the bare motor in brake mode with a torque wrench or a string on a pulley or similar, and calculate a gear ratio from there. That would likely be rather more reduction than you’d need due to inefficiencies in the gearing.

Do you have any data on this?

Anecdotal and sensory.

• A robot which would roll off the batter in 2016 in coast mode but stay consistently in brake mode. (Even though I told the kids it wouldn’t work, they tried it and it did.)
• Putting about a 2" diameter pulley on a CIM and trying to turn it by hand from a stop with the wires open vs with the wires shorted.

I’ll see if I can work up something more solid this weekend.

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Interesting.

I’d love to see some quantitative data (ideally a braking torque vs. rpm correlation while braked) on this, especially for the higher power motors. Manufacturer and other (internet) literature implies and states braking should provide zero torque at zero RPM:

Brake mode does not have any effect when the motor is not rotating but can make a large difference in robot behavior when used on a motor attached to a high reduction gearbox. (CTRE, p39)

I’ve felt the apparent change in zero rpm torque with a lot of mechanisms in brake mode. I wonder if there is some significant nonlinearity to the braking torque as a function of RPM, or if it’s simply down to human perception (for things like spinning pulleys by hand) and mechanical losses in powertrains.

~135lb on 1” diameter spool for cabling. Motor was on a versaplanetary with a 1:1 chain drive to the spool shaft

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Any reason a ratchet couldn’t do the same job?

You want to be able to run the motors backwards as well to lower the elevator. If you could disengage the ratchet during a match, this might work.

If the intention is to prevent the mechanism from sliding down after the electricity is cut, then brake mode should not be a part of the discussion

I would encourage you to read this electronics stack exchange answer on braking a brushed DC motor: https://electronics.stackexchange.com/a/16666

Brake mode works as long as the robot is powered on, even if it’s disabled after the match end.

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Sorry I guess there was a misunderstanding. By electricity being cut I thought the original author meant power off not disabling. My bad.

Yep, not sure if they need it to come down though so it could work

OK, I would not get any difference exactly at zero speed. I put my little demo robot* on a 34" long ramp and the angle where static friction broke was the same (~8.5" rise, ~14°) whether the motors were shorted or not.

When I reduced the slope to 10° (6" rise) and gave it a manual push at about 8-10 ips, the robot ground to a halt when shorted, but continued rolling when open.

There does appear to be a minimal benefit of putting the motors in brake mode. They won’t provide any more holding force at static friction, but perhaps they can help get you to the static state more than coast mode will.

* I did not install a battery. I did brake or coast mode by physically connecting the motor leads together or not.

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