fine control of robot with mecanum wheels

[Ether]

Originally Posted by ajd

Most likely, you would want to decrease the sensitivity, which would give you more control, since the same change of the joystick position would give you a smaller change in motor speed. So squaring it seems like a better bet for more control.

Correct.

I'd like to hear from any teams that have implemented closed-loop wheel speed control via encoders connected directly to Jags, on a bot with mecanum wheels.

Also, are any teams using shiftable transmissions (high and low gears) ?


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[JamJam263]

Originally Posted by Ether

Correct.

I'd like to hear from any teams that have implemented closed-loop wheel speed control via encoders connected directly to Jags, on a bot with mecanum wheels.

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Team 263 is using exactly that. We found that the voltage control in an open loop was way too unreliable. Each wheel is directly hooked to a AM tougbox, with an encoder directly connected to the Jaguar that controls it. Using CAN speed control made everything more reliable, no drifting and a lot better traction control. The 1kHZ PID loop in the Jags helped use tremendously. Out of 60 Teams in NYC we where the only ones using CAN, and had quite a few comments from other teams using mecanum on how much better it preformed. The only issue that was not directly fixed with CAN and the built in PID was control at lower speeds. To fix this we simply used the throttle on the joystick to make an adjustable scale where setRPM=joystick*throttle*maxRPM. With adding the throttle everything worked without any issues. Using the throttle to make an adjustable scale works for voltage control, and a PID loop can easily be set up with the encoders hooked to the cRio in a short period of time. Actually switching our C++ code from an open system using pwm to CAN didn't take us more then an hour, neither did setting up the CAN network itself if you have the cables to do it.

[dtengineering]

Can't say this will help, but we did encoder based closed-loop on our Rack'n'roll mecanum (8" AM mecanums off 12:1 direct drive banebots)... using interrupts on the IFI controller. It did help... at least once we got the PID coefficients tuned. Until then it was wild!

Hopefully it will be easier with the new hardware...

Jason

[Ether]

Originally Posted by JamJam263

To fix this we simply used the throttle on the joystick to make an adjustable scale where setRPM=joystick*throttle*maxRPM. With adding the throttle everything worked without any issues. Using the throttle to make an adjustable scale works for voltage control...

Yeah, we also tried using the throttle* to scale the joystick (with open-loop PWM control) but it wasn't good enough. The driver wants more fine control, and I don't blame him.


*We used the throttle to control the parameter "a" mentioned in this post:
http://www.chiefdelphi.com/forums/sh...d.php?p=921992


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[efoote868]

My team is using mecanum wheels, I believe we are using the same setup or something similar.


One thing to keep in mind is that everything has a slight polarity, and a deadband. You can work to figure out where the motors start spinning, and then account for it in code.

Also, another thing my team has found useful is separating the rotation component from the XY joystick to another one - we found it was nearly impossible to spin without moving off axis until we did this.


After this season's competition I think I'll write a whitepaper on how I got the students to approach mecanum drive, our setup this year is very easy to control and I'd like them to be able to use it easily in future years (without having to worry about cheating).

[Ether]

Originally Posted by efoote868

My team is using mecanum wheels, I believe we are using the same setup or something similar.

Could I ask you to check with your lead programmer to find out if your bot using using closed-loop wheel speed control ?

Thank you.


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[efoote868]

There is no wheel feedback.

[Ether]

Hi Team 868,

Thanks for your reply. I have a bunch of questions, I hope you don't mind. Any answers you are willing to provide would be much appreciated:

My team is using mecanum wheels, I believe we are using the same setup or something similar.

Are you using 8" AndyMark mecanums? Are the wheels direct-driven, or chain-driven? If chain, what are the sprocket teeth counts? Are you using the KoP AndyMark 12.75:1 toughbox? If not, what is your gear ratio? Are you using Victor or Jag motor controllers?

One thing to keep in mind is that everything has a slight polarity, and a deadband. You can work to figure out where the motors start spinning, and then account for it in code.

Yes, we are aware of this. Nothing we have tried so far is able to achieve fine control at low speeds, given our gear ratio and open-loop control.


Also, another thing my team has found useful is separating the rotation component from the XY joystick to another one - we found it was nearly impossible to spin without moving off axis until we did this.

We have no trouble spinning in-place, with the rotation command on the "twist" axis of a 3-axis joystick.


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[Mr. Lim]

Ether,

It sounds like you're running a setup very similar to ours. I can't say we've "mastered" fine motor control with our mecanum wheels, but I'm happy to answer any questions you have. Here's a video of our practice robot, showing what we can/can't do:

http://www.youtube.com/watch?v=apcy5naz5TE

Are you using 8" AndyMark mecanums?

Yes.

Are the wheels direct-driven, or chain-driven? If chain, what are the sprocket teeth counts?

They are chain-driven. 19 tooth on the ToughBox output shaft, kit supplied 22 tooth on the Mecanum wheel hubs.

Are you using the KoP AndyMark 12.75:1 toughbox? If not, what is your gear ratio?

Yes we are using the KoP AndyMark ToughBoxes, unmodified.

Are you using Victor or Jag motor controllers?

We are using Jaguar motor controllers, using PWM control.

Our setup also uses the US Digital optical encoders, and the WPILib PIDController object. We are using closed loop I-only motor control, and we cube our joystick inputs before sending them to our control loop.

We also implemented a "slow-down" button, which scales both our translational and angular velocities to about 1/3 of max when held down. You can see it used in the video just prior to when our robot meets the ball - the robot seems to abruptly slow down.

[Ether]

Originally Posted by Mr. Lim

Our setup also uses the US Digital optical encoders, and the WPILib PIDController object. We are using closed loop I-only motor control

What made you decide to use closed-loop wheel speed control ? Was the performance less than satisfactory without it ?


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[Mr. Lim]

Originally Posted by Ether

What made you decide to use closed-loop wheel speed control ? Was the performance less than satisfactory without it ?


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The performance was far less than satisfactory without it IMHO.

The vector math behind mecanums isn't that complicated, but it depends on the ability to get a wheel turning at precisely the velocity you want it to.

[Ether]

Originally Posted by Mr. Lim

The performance was far less than satisfactory without it IMHO.

OK. I think that's our problem. We are using open loop.

The vector math behind mecanums isn't that complicated

Agreed. We did our own:
http://www.chiefdelphi.com/forums/sh...83&postcount=1

, but it depends on the ability to get a wheel turning at precisely the velocity you want it to.

OK. That's what I figured.

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[efoote868]

I think the only difference is the wheel size, 6 inch versus 8.
I understand that might provide a difference in the "low speed" handling.

For the drive, we're using Jaguars via PWM.


Sorry I don't have all the information... all of this is from memory.

[Ether]

Originally Posted by efoote868

I think the only difference is the wheel size, 6 inch versus 8.
I understand that might provide a difference in the "low speed" handling.

Yes. It could definitely make a difference, depending on other factors of course.

Sorry I don't have all the information... all of this is from memory.

I understand. But if I may ask without being too much of a pest, would you be willing to ask your lead engineer whether you have encoders on the wheels/gearboxes, and what the other details of your drivetrain are (gear ratio of transmission and tooth count on sprockets if using chain drive).

Thanks


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[Jared Russell]

The holonomic drivebases that we have used (in 2007 and 2008) had similar kinematics to mecanum drives, and we were also dissatisfied (at first) with the controllability of the drive at all speeds, and especially at low speeds.

The following solutions helped to get us under control, and each of the aforementioned robots managed to win a regional:

1. Make absolutely sure that all wheels are always touching the ground. Ideally, you should have some sort of suspension or at least a non-rigid frame that lets you deal with non-flat carpet.

2. Use a gyro to help the robot drive straight. We used a proportional-only controller with a feed forward component (meaning we did the holonomic drive kinematics to generate the command, and then added the output of a P controller with "yaw command" as setpoint and gyro rate as input). In general, we found that encoders on the wheels did help, but not nearly as much as the gyro (because of wheel slip, non-level surfaces, etc.) Also, this was back in the IFI era, so CAN speed control was not an option.

3. Limit each joystick to a maximum of two axes. We tried using a 3 axis (X,Y,Twist) joystick with our holonomic drive, but despite plenty of practice our drivers always seemed to do better with a two stick setup without a twist axis (one stick for X/Y and another that just uses the X axis for twist, or one stick for Y and twist - e.g. tank drive - and the other for strafing). Simply put, we found that too often the driver would accidentally use the twist axis when they didn't mean to.

4. Making sure that weight is distributed evenly! The vector math assumes all wheels have equal normal force - if your robot is far off from this assumption, your mileage will vary.