Drivetrain Question
 

Hello all,

We have a cart in progress that we are going to make motorized. Our issue is that we want to be able to disengage the drive wheels from the motors to make it easy to push when not in use. We have yet to come up with an easy way to do this.

So, CD, do you have a solution to our problem?

Thanks,

Ryan

:cool:

Re: Drivetrain Question
 

Consider running a shifter with a neutral gear.

Re: Drivetrain Question
 

How about this? Use motorized rubber wheels in contact with the cart's wheels to spin them. Mount the driving assembly in a way that can be pivoted or lifted to break the contact.

Re: Drivetrain Question
 

We have, but the problem the problem with the neutral space is that it's had to get working since they are not really meant to stay in neutral. Besides, four new shifters are not exactly in our budget. :p

By the way, the drive wheels are mecanum. ;)

Re: Drivetrain Question
 

If this is a serious statement I would really reconsider. When moving around a mecanum wheels on anything but carpet at a quick pace you run into one of two issues
1) If it is slicker then carpet you get sliding and some minor issues with veering off to one side
2) Something rougher then carpet will damage the wheels quickly since they constantly are hitting it at an angle

(also when you push around unpowered mecanums they make a lot of noise)

Re: Drivetrain Question
 

Certain power wheelchairs (jpg link) have clutches built into the hubs to allow the wheels to spin freely if the chair is to be manually propelled.

Re: Drivetrain Question
 

Drive wheels with v-belt; tension it to engage the drive, loosen to coast.

Re: Drivetrain Question
 

This cart has gone through a season with non powered mecanums. They do make some noise, but they are not loud at all. If anything, they make people turn around and realize that there is a robot behind them. :eek: Keep in mind this was tested on tile floors with decent grout gaps.

And to Nate: This was our prior cart.
http://www.chiefdelphi.com/forums/at...5&d=1333563144
http://www.chiefdelphi.com/forums/at...6&d=1333563152

V-belts are a good option. We don't know if the belts would slip under the stress though.

Re: Drivetrain Question
 

This is the same way my lawn mower work's to spin the blades. Though it doesn't have a lot of torque needed so I can not say it would work or not. Prototyping is an option though ;)

Re: Drivetrain Question
 

The idea was shamelessly stolen from stuypulse's 2005 robot: http://www.chiefdelphi.com/media/photos/20224

Evidently worked for them.

Re: Drivetrain Question
 

OP, consider disabling dynamic braking before you attempt a mechanical solution. That may be enough to comfortably push the cart around.

Re: Drivetrain Question
 

The second goal of disconnecting the wheels for the motors is a matter of safety. Whatever way we end up doing it, there will be a deadman switch that works like one off of a treadmill. We like to keep our carts safe, and we're pretty well know for that. ::safety::

Re: Drivetrain Question
 

Ever heard of a clutch?

Re: Drivetrain Question
 

You could use the deadman switch to disconnect power to the motors directly, or through a relay, which would be similar to disconnecting them mechanically.

You could also get or mimic an OTS item like a wheelchair safety lock to prevent wheel rotation:

Re: Drivetrain Question
 

A couple of thoughts, If you are driving like typical robots, try using victor speed controls instead of Jags. Jags short the motor leads when off, so the motor is a brake. Victors do not do this, and you will not have this braking action when pushing.

Safety issue, put a electrical cut off switch by the controls.

Fix it electrically, of course, I'm electrical.

Re: Drivetrain Question
 

By "off", do you mean "unpowered"? If so, can someone please confirm this?

Re: Drivetrain Question
 

if the jag was in brake mode it would short the leads if it was in coast mode it would not.

Re: Drivetrain Question
 

The question on the table is this: Will an unpowered Jag short the motor leads, regardless of jumper position.

Re: Drivetrain Question
 

A thought for this...

Disconnecting the motor from the wheels with the deadman switch will lead to the cart coasting to a stop, potentially over a long distance carrying a lot or momentum (150 lb robot + 50 or more lb cart...) - in other words, it can defeat the purpose and still run over a little kid. Instead of going that route, use some Jaguars to control the drive train. Set the Brake/Coast header to Brake, so when it stops, the motor actually helps the cart slow down faster. Hook all the limit switch inputs up to a limit switch sitting behind a slot. Slide a card into the slot will close the circuit, allowing the cart to move. If the card is pulled out, the circuit will open, and the Jaguars will immediately stop, with no programming needed on your part!

This can separate your two issues nicely, which might let you use an easier solution for pushing the cart around than if the solution had to solve both problems at once.


Oh, another idea I just had... a similar limit switch/slot concept, except the limit switch would activate some pneumatic cylinders, which would push some wooden skids straight down into the floor. Set it up so they can lift the cart wheels + robot completely off the floor. Now, the deadman switch serves two purposes - it stops the cart immediately, and while in your pit it can give you a more stable working surface that won't roll around!

Re: Drivetrain Question
 

A dead man's brake doesn't need to actually be part of the drive assembly.

One can arrest the momentum of a wheeled cart just by creating friction between the surface on which the cart rolls say by dropping a rubber covered shoe down on that surface to create drag (might want to tapper the edges on that shoe so it doesn't stop quite so violently that the robot might come off the cart). It might be much easier to make a brake like this.

Re: Drivetrain Question
 

I very much agree with your first idea about disconnecting drive motors, very good observation.

I think your second idea could introduce a serious safety hazard. Pneumatic components that always return to one state when de-powered can actuate at un-intended times. If they are explicitly switched you might run into other issues. For example, the cart's battery could run low and the pistons could then actuate unexpectedly, causing the whole cart+robot to raise/lower without warning. If the skids default position were 'deployed' down, and there was an issue with the cart, it might come to stop in a hurry in a queue or pit lane, which would be most inconvenient.

The idea of using skids to stabilize the cart for working is good, but I would not rely on them as a safety mechanism, nor would I have them pneumatically actuated.

Re: Drivetrain Question
 

Clearly I didn't explain that last idea well enough :) The pneumatics wouldn't return to a specific state when de-powered, they would work the same as pneumatics on FRC robots - they remain in whatever state they were at when power was removed (air leakage might result in the pistons slowly losing their pressure). The switch simply indicates which direction they would be at. If you wanted to do this without needing a controller on board, see the attached rough diagram.

Re: Drivetrain Question
 

I see, that would be better that what I was assuming. However, it still requires the electrical system to function for this safety system to function, and is susceptible to an inadvertent flip of the switch, which isn't ideal in my mind.

Of course none of this detracts from a pneumatic jack's usefulness and definite cool-factor :cool:

FYI: plenty of FRC robots have one-way solenoid valves, i.e. return to one state when not powered. It would be dangerous to assume that all robots use 2-way solenoid valves.

Re: Drivetrain Question
 

True, there are 1 way solenoid valves... My team hasn't done much with pneumatics, and the teams I've inspected have all used 2-way valves, So I don't really have much experience with them. That must be the basis of my assumption :)

Re: Drivetrain Question
 

simplest and safest way to prevent unwanted actuation of the ram is to physically lock the ram in the extended or retracted position with a pin or something

Re: Drivetrain Question
 

Please explain how that would work procedurally.

Re: Drivetrain Question
 

I didn't mention that we would have dual disk brakes that would engage for a few seconds after an error to stop the drive.

My thought was to create a gearbox that only had one speed and a spring return cylinder that disengages the motor. Two of the wheels would have our disk braking system from above.

Re: Drivetrain Question
 

what i meant was something that works in the same manner as how the pin that holds the receiver and insert ends of a trailer hitch together except used to lock something in place and keep it from rotating.

another idea is you could put a piece of "c" channel that is smaller than the clevis over the ram's rod to physically jam it and keep it from retracting.

Re: Drivetrain Question
 

The issue with this is that it relies on a human-in-the-loop to function. My opinion is that safety equipment on a robot cart and similar devices should not rely on human involvement. If someone forgets to install the pin, or student A thinks "student B did it" and student B thinks "student A did it" then you have a situation that is exceedingly dangerous as those around it are assuming the piston is locked out when in fact it is not.

Re: Drivetrain Question
 

One could make a latch system that works like a automotive jack stand.

As you extend the piston to lift the cart it would ratchet it's way down and then prevent reverse movement without manual intervention. In fact they make small cheap automotive jack stands one could probably tweak for this purpose.

Re: Drivetrain Question
 

If you are going wth pneumatics or brakes, just use spring loaded cyclinder or mechanism that deploys the brakes in an unenergized state. Use a single solenoid, to retract/release the brake when under electric control. loss of power, loss of pneumatics will deploy brakes.

Look up rail cars, trailer brakes, they use air pressure to release the brakes. Lose the air pressure and the rail car stops, not continuing down the tracks...