Backdrive

So for our climbing mech this year our team has decided to use an andymark motor (am-2971) connecting to a rod by chains (#35) and sprockets. However we are having troubles with the backdrive of the motor. We were wondering what were some different motors/techniques we could use that have a good anti-backdrive.

Paras,
There are very few motors that are available that would serve to hold the full robot weight by themselves. Worm-drive transmissions are known for their resistance to backdrive but they still will backdrive depending on the load. Everyone will recommend using some form of brake, latch or ratchet to keep your robot pressing the sensor pad at the end of the match.

I take it that this is something that is going in your 30lb Withholding Allowance?

First up I would try and steer clear of the Andymark PG’s for climbing tasks( exception being if it has a 9015 which is a good motor), something like an old Toughbox Mini with 1 or 2 CIM’s would be a better option( a PG will struggle to climb the rope in 30 seconds).

If you do opt for the AM-2971 make sure that you add enough reduction using sprockets( this depends on your spool and rope type/size).

In regards to preventing backdrive, any correctly geared motor isn’t going to have a lot of backdrive resistance( worm gearboxes can be backdrive resistant, but stay away from those for this).

Your best option is probably to integrate backdrive resistance into the mechanism, for example a 1/2" ratcheting spanner can be used on a 1/2" hex shaft or something like a ratchet and pawl.

Our climber spool is on a 1/2" hex shaft. We use a 1/2" ratchet wrench directly on the shaft to prevent back driving.

It does take practice to get the ratchet released at the end of the match. We need two people to hold the robot to take pressure off the ratchet and a 3rd person to release it.

Also, make sure your software only drives the motor in one direction.

Good luck with your season.

David

The easiest way to prevent backdriving is to add a flex head ratchet like the one pictured below. If you just add two bolts surrounding the wrench on both sides, it will stayed secured until the end of the match, then you can go out flip the ratchet and lower the robot.

Of course you could find it at a local hardware store but McMaster Carr does sell one

EDIT:

+1 on this if you try and run it against the ratchet you can stall the motor

Firstly, have you tested whether or not that motor will actually work on your climber? The math looks like it would be either too slow or too weak depending on how you gear it.

As far as backdriving goes, the simplest way I’ve seen is to run a 1/2" hex shaft through the center of your climber drum (most teams are doing this already anyways) and attach a 1/2" Ratcheting Wrench to one end and secure the non-ratcheting side to the robot in such a way that it cannot rotate (but make sure it’s removable so you can get your robot down after the match).

One thing to remember with any sort of ratcheting mechanism though is to make sure you only drive the motor in the direction that doesn’t fight the ratchet, else you could have a bad day.

*Just picked some random numbers to show how to do the computation:

150 lbs robot
3 feet lift height
5 seconds time to complete the lift

150*3/5 = 90 ft_lb/sec = 122 watts.

am-2161 motor max power is 37 watts.

Thanks everyone!
If we were to change our motor to something else do any of you have ideas what we can use instead? We could try the gearboxed CIMs but I was wondering if there are other motors that we can use. We would need something with a torque around 13.21 Nm (9.74 ft lb).

You need to figure out how much power you need.

Then pick a motor that has at least that much power, preferably more.

Then select your gear ratio to work with that motor.

False.

https://www.instagram.com/p/BQMyRUDBGGb/?taken-by=frcteam3072&hl=en

This was at 3/4 speed also.

We are looking for something at least 292.857 watts, we are basing off of:

360 lbs*3 ft/5sec = 216 ft_lb/sec –> 292.857 watts.

Our team is using 360 lbs because we decided to take the max weight of the robot x 3, 3 ft is the actual climbing height plus some, and 5 sec is the time climbing that we would like to achive

We’re using a CIM motor on a 16:1 VersaPlanetary (with a ~1" winch drum) and in our tests we have had no issues climbing in under 5 seconds. We might actually gear it down a bit more, it doesn’t lack power but it is a bit quick for the driver to control manually.

Ok, but how much did the robot weigh?
This also does not take into account the several seconds it will likely take to actually acquire the rope. In a real-world scenario you’d be cutting it pretty close. You also don’t want to stall that motor or it will fry very quickly.

Your desire for a safety factor of 3 is admirable but likely overconservative. There are only 2 FRC-legal motors with that much power.

A safety factor of 2 (rather than 3) opens up the field a bit.

You could use two motors instead of 1 to get the power you need, but that complicates things needlessly.

If we go for a factor of 2 do you know which motors can we use?

Just look at the motor spec list published by FIRST.

Try to find it yourself. It’s good to know how to do these things. If you hit a brick wall I’ll look it up for you.

I believe I found it, I have looked at these 2 sites:

http://team-paragon.org/wp-content/uploads/2013/01/2013_FIRST_Motors-571.pdf

Great work, this webpage: http://motors.vex.com/ has results of first hand practical testing, rather than the specifications given by the “manufacturers”, the testing found a major discrepancy in the performance of the RS-550, 9015 and RS-775-18 motors.

Could be a good idea to double check against this webpage once you have selected.

Good job. You’ve shown the CD community that you’re willing to do your homework.

Your second link is the 2015 motor specs. The FIRST website is a bit misleading because they link to that from a page which appears to be for 2017.

To find the 2017 FRC motor specs, go to http://www.firstinspires.org/ and then click on the links shown below.


link2.jpg
link3.jpg



link2.jpg
link3.jpg

Rather than use a ratchet, how about attach an encoder to the climber shaft and use software instead.:wink:

If it’s useful to you, you can use this spreadsheet which can help you match which gearboxes are usable with which motors. Note that the 775Pro motor is not listed here, but it has the same gearboxes mounting abilities as the other 775 motors.

On the “Gearbox” sheet there is a single colored dot by each gearbox name. If the motor name on the “Motor” sheet has a dot of the same color next to it then it means that the given gearbox is compatible with the given motor. It is so you can immediately see what the gearing options for a given motor are.