actuating a brake or anti-backdrive

[RonnieS]

Originally Posted by loafdog

Hi,

We don't plan on using pneumatics on our robot so I was looking for ideas that used something else to control/move a mechanism that would brake/stop elevator.
I was wondering if servos can be used to control brakes or some other kind of anti-backdrive component. Poking around andymark I saw shifting gearboxes controlled by servos for example. I've seen people on CD discuss bike brakes, ratchets, etc..
But I haven't seen anyone discuss how you can implement these ideas. Anyone done this before and have details/pics they can share?

(note, I'm not a mechanical type.. i do software, just trying to get ideas to share with my team).

THanks.
-Maciej

Look at HOT's tech notes and see how they used a motor, I believe it was a window motor, to actuate their shifting. You can do a linkage that actuates the bike disc break on something like a cam.
I don't know what your gearbox ratio is to do your lifting or how much you wan't to lift but I would strongly suggest looking at your motors used, gear ratios, and power consumption to see if you can hold is strictly with programming which would be stalling the motor. For something like a cim, it wouldn't be as big of a deal to stall under the right loads compared to something like a 550 or 775 that relies on the turning of the shaft to cool it. There are some pretty good papers on Chief already talking about thermal mass and such.
-Ronnie

[Mike Marandola]

In addition to worm drive boxes, there are some drill gearboxes that have locking chucks which essentially prevent backdriving. The most popular one is the Dewalt XRP and there is a cheaper one from Harbor Freight.

[Jared]

It may just be simpler to use a transmission which cannot backdrive, like a worm gear. They provide large amounts of reduction in a small amount of space, and do reasonably well with large amount of torque. We plan on using a 10:1 worm reduction in our elevator for exactly these reasons.

If you'd like more details on the worm drive we're using, let me know and I can send you some specifics.

[adciv]

Provided your torque requirements aren't too high, and you're using a motor which can handle it, you can implement a brake in software. Short version, you can use a PID to maintain the position of the motor using a holding current. We've done this before using CIMs and Bag motors. This does require a sensor.

[Ether]

Originally Posted by adciv

Provided your torque requirements aren't too high, and you're using a motor which can handle it, you can implement a brake in software. Short version, you can use a PID to maintain the position of the motor using a holding current. We've done this before using CIMs and Bag motors. This does require a sensor.

... and if you're clever enough with the software, you can use the friction in the system to help hold the load when stationary.

[gpetilli]

Originally Posted by Ether

... and if you're clever enough with the software, you can use the friction in the system to help hold the load when stationary.

I hope our software does not add 60lb of friction to the system . Although that might explain why the robot does not drive as fast as we would like

[Ether]

Scenario: Because of friction in the system, any force between 20 and 30 pounds will hold the load stationary once it's not moving.

If your control algorithm is clever enough, it will use as close to 20 pounds as possible to hold the load.

[asid61]

Originally Posted by Ether

Scenario: Because of friction in the system, any force between 20 and 30 pounds will hold the load stationary once it's not moving.

If your control algorithm is clever enough, it will use as close to 20 pounds as possible to hold the load.

Where is that 20-30lb estimate from? We need a brake on ours, and it would be nice if we could just hold that little amount of weight.

[Ether]

Originally Posted by asid61

Where is that 20-30lb estimate from?

It's not an estimate. It's just a scenario to illustrate a concept.

Or were you asking why there's a range, rather than just a single number?

[MrForbes]

I'm drawing a blank...probably because I can't see what it is we're trying to figure out how to stop moving. Can you post some pictures or drawings or renderings of the robot or design?

[gpetilli]

Originally Posted by Jared

It may just be simpler to use a transmission which cannot backdrive, like a worm gear. They provide large amounts of reduction in a small amount of space, and do reasonably well with large amount of torque. We plan on using a 10:1 worm reduction in our elevator for exactly these reasons.

If you'd like more details on the worm drive we're using, let me know and I can send you some specifics.

We are also using a worm gear drive - AndyMark RAW. Many right angle drives are worm gears and can hold position without requiring the motor to be energized. Bevel gears are also right angle, but don't exhibit the anti-backdrive feature. Other legal worm drive motors include the window motors and the tailgate motor.

Just a warning that the window motor also has a pin that engages when not energized that causes issues with speed controllers - there are CD threads on how to remove the pin.

Also keep in mind that the ratcheted gearboxes from last year can not drive in both directions - by design. Depending on how you unload your elevator, you may need to drive the motor backwards.

We chose the RAW drive due to its simplicity to control. The PID loop does not have to do anything special (like engage a break) and can drive to a desired position in either direction. Given the relatively large load of up to 60lb, we were concerned with the holding current for traditional gearboxes.

AndyMark shows the RAW box as out of stock, and has removed the estimated restock time of Jan 26th. We did not want to wait till Jan 26th and ordered the individual part to build our own - which turn out to be more difficult than imagined. They do seem to have restocked the "worm box" and perhaps it is sturdy enough for your elevator. Other wise, your best bet may be the tailgate motor if you want a worm drive.

[notmattlythgoe]

We used a bike brake and caliper on our elevator prototype. You can see it here.

[cad321]

Originally Posted by notmattlythgoe

We used a bike brake and caliper on our elevator prototype. You can see it here.

Yes but how did you actuate the caliper?

[adciv]

Originally Posted by cad321

Yes but how did you actuate the caliper?

It's a pneumatic cylinder. You can see it in the video near the disk brake.