Coaxial crab stalling

[Tytus Gerrish]

one of our engineers pointed out to me tonight that if a wheel stalls on my coaxial crab drive desing. the torque will be exherted on the module and chalange the control of the globe and stear the entire system. ive only seen few other coaxial crab drives I would like to know if they have had this problem and if so, how was it dealt with, Or even if the torque of the globes can be contested and i have anything to wory about in the first place

[Rob Colatutto]

If you stall the Globe motors they only have a stall current draw of 18.5A so don't worry about drawing too much current from them. However stalling a motor is never a very good thing for it. Now, you mean a wheel stalls while you are rotating the modules or if a wheel stalls while your pushing someone? If the wheels are stalling when your pushing then you might either want to lower your coef. of static friction that way you will spin the wheels if you get stuck, or just increase the gear reduction to overcome the friction. If the modules have problems turning you could try using both Globes to turn them with one continuous wrap around all 4 wheels similar to the method Wildstang used last year on thier competition bot.

[Tytus Gerrish]

coaxial crab...
Im woried about backdriving the globes and throwing my stteing out because my Drive motors wont be transmiting torque thru the wheels when theyre jammed, it will be into the module witch is rotated by the Globe

[JVN]

Originally Posted by Tytus Gerrish

coaxial crab...
Im woried about backdriving the globes and throwing my stteing out because my Drive motors wont be transmiting torque thru the wheels when theyre jammed, it will be into the module witch is rotated by the Globe

Tytus,
You should always design your "low gear" so it has enough torque to slip your wheels (no stall!) This means, you don't have to worry about stalling motors/modules when you're "fighting".

In high gear... don't get into pushing matches. Especially not for extended periods of time. Experiment with your wheel traction, see what you can get away with. This will greatly depend on how many motors each of your coax gearboxes use, and what ratio they are running at.

Make sure the coax-swerve steering/programming are such that you don't end up having wheels out of allignment. (there is nothing worse than watching a swerve with 3 wheels going 1 direction, and the 4th going somewhere else.)

Also... try to protect your gearboxes/wheels from objects getting inserted and jamming them up. (add shields, etc).

Another way to deal with this would be to just gear your swerving motors (globes) down enough so they can easily handle a little extra "bleed off" torque from the semi-stalled, slipping wheels.


John

PS - Finding good ratios for your drive is the most important thing. You want to find a good balance for high, and low. (or are you trying for a CVT?) There are piles of threads discussing this design process. For the application you're talking about (with 2 motors per side: drill & chip or similar power outputs) I'd recommend probably ~2-3 fps low gear, and ~11-12 fps high gear (max).

Remember, it is always better to err on the "slow" side. Especially with a swerve drive. (for reference, Wildstang was ~6-7fps and even that seemed fast!)

[Raul]

Originally Posted by Rob Colatutto

If the modules have problems turning you could try using both Globes to turn them with one continuous wrap around all 4 wheels similar to the method Wildstang used last year on thier competition bot.

A clarification:
Wildstang has never driven all 4 wheel steering with one continuous chain. I know Chief Delphi did this in 1999. We always have 2&2 steering on 4-wheel crab drive.

Tytus, please clarify - are the Globe motors driving the wheels or doing the steering?

Raul

[Tytus Gerrish]

Globes are stterig

CIM's and Boshe's in 2 gearboxes [linked to 2 modules each by either a kevlar belt a V-belt or a chain(yet undecided)] in either side are powering

[Swampdude]

I don't think his question was understood. The word stalling is causing everyone to focus on the motors I think.
What I tried to point out to him is the rotational force being applied to the drive shaft at each module, will travel through the least resistance. It can either rotate the wheel, or turn (steer) the entire module. So the resistance is the wheel friction vs. steering torque / globe. You have to look at the assembly to make sense of this.

Crab module picture

Imagine where the 2 (green) bevel gears mesh, if the vertical gear (on the horizontal red shaft) couldn't rotate, the horizontal gear (on the vertical red shaft) would then try to make the vertical gear orbit around it. Then the only thing stopping it would be the steering system torque. Therefore the torque output of the steering system must be greater than the output of the drive system to counteract this imposed rotational force.

This drive force would be constant against the steering system, causing a range of misalignment depending on the wheel resistance. If the bot were pinned, and the throttle was pegged, the 4 wheel modules would be spinning around like tops. The only way I saw to overcome this would be to build a low ratio globe gearbox. Which will have a very low rpm = slow steering.

He sure did an excellent job on this thing!

[Madison]

As John said, if you design your gear ratios so that the wheels slip instead of stalling the motors, the situation you've described won't happen.

If the motor is driving near stall and the wheels begin to spin, that offers less resistance than fighting against the steering controls, and thus, the wheels will spin before the drive motors start fighting against the steering motors.

[Tytus Gerrish]

The slipahe is somthing i considered, i would go into it more but right now im in a hotel in clearwater for a legoleague competion tomarro and i need to help my kids get theyre programs done, So,

Thanks Dan, But im still working on it i have the laptop with autocad on it with me and i gace coppies of autocad and the .DWG's to 3 students other than myself and theyre all working on it now too.

[Raul]

[quote]

Originally Posted by Swampdude

Therefore the torque output of the steering system must be greater than the output of the drive system to counteract this imposed rotational force.

This drive force would be constant against the steering system, causing a range of misalignment depending on the wheel resistance. If the bot were pinned, and the throttle was pegged, the 4 wheel modules would be spinning around like tops. The only way I saw to overcome this would be to build a low ratio globe gearbox. Which will have a very low rpm = slow steering.

I see what you mean now. But even if you gear the globe motors down to overcome the drive torque, every time you accelerate, it will put a variable torque on the steering and cause oscillations on the steering feedback system.

Raul

[Tytus Gerrish]

the problem is insted of the motors stalling they will backdrive the Globes and steer the modules instead , OR the wheel might slip a little
Mabye thee drawing will help yall Understand...

[camtunkpa]

Tytus,

If I am reading you right you are concernced about the motors losing index if they are back driven. I would be assuming that you are depending on the motor to only move when it get an input from the operator via a joystick or whatever....the thing you have forgotten to make your crab work flawlessly is a feedback control system.

A feedback control system would use a potentiometer to monitor the travel of your belting or chain or whatever. This potentiometer gets tied into an analog input of the control system on the machine itself.....then you can ask your programmers to make a nice little control loop to allow for a tolerance. This programming will allow the position to be monitored and the robot will adjust itself. If you are worriied about the belt slipping, monitor the shaft that allows each wheel to pivot.

Yeah and I'm normally brief about things, but I just got done taking a final on control systems and everything is really fresh in my mind. Good luck and hope to see the crab up and working!!!!!

Cliff
222 Alum.

[Tytus Gerrish]

the feedback system is in the desing in my mind but not in my drawings because Firstcadlibrary dosent have a 10 turn pot .DWG

im trying to solve the problem mechanicaly so i wont have to rely on the software- Of witch I know Nothing

[Jim Meyer]

One way to reduce the problem would be to do most of the gear reduction in the modules so that your input shafts are spinning at high speeds with relative low torque.

If you designed them so that the input shafts rotated in opposite directions when going forward for a pair of modules that are steered together the torques would cancel. This could create problems is one of the modules has more traction than the other though.

Neither of these options are easy to implement but I thought I'd throw them out there anyway.