Holonomic + gyro/PID

[EricS-Team180]

Kris noted the simplest use of a yaw rate sensor and it's easiest implementation: driving straight. We have used this technique on most of our 'bots, holonomic, treaded & wheeled, since 2004. The algorithm is a simple multiplier on the yaw rate detected. We usually determine it empirically, This is added to the motor outputs to create an automatic, opposite spin to the measured yaw rate. When driving straight, the yaw rate is close to zero, so the effect is nil...easy and powerful. Add in a small deadband around zero yaw rate, if it wants to twitch when still. This also helps, if someone is trying push on a corner of your 'bot. Typically we give the driver a button or switch, to turn the effect on and off. We always want the driver to have the final decision on handling.

We've used the gyro for field-centric control, as well. Our results are mixed - a great implementation with our 2004 holonomic 'bot, and a rather disastrous implementation with our swerve drive, last year. Give yourself a lot of time for driver training and robot testing.

Eric

[Ether]

Originally Posted by EricS-Team180

and a rather disastrous implementation with our swerve drive, last year.

What was disastrous about it?

[Jogo]

Wow you guys are all really helpful! I think two separate issues are coming up here, mostly due to my ignorance.


1) Driver-centric vs. Robot-centric: It seems that this issue is not related to the type of drive train. For example, by turning the joystick into a polar coordinate, if I push the joystick to (0.5,pi/4), then I could see using a gyro to constantly PID to the angle even without holonomic wheels. Error in this case could be caused by the gyro, which if it mounts up might require the driver to zero the gyro midway through the match.

2) Holonomic vs….well, non-holonomic: Having never tried a holonomic drivetrain, I was wondering if error could mount up naturally. For example, pushing the joystick to (0.5, pi/4) might for some reason not move the robot at precisely this amount due to environmental factors. I was wondering if teams use a gyro or other sensors to deal with this, but maybe this isn’t a significant problem.

I'd appreciate any attempts to help clarify this stuff for me. Thanks

[Ether]

Originally Posted by Jogo

2) Holonomic vs….well, non-holonomic: Having never tried a holonomic drivetrain, I was wondering if error could mount up naturally. For example, pushing the joystick to (0.5, pi/4) might for some reason not move the robot at precisely this amount due to environmental factors. I was wondering if teams use a gyro or other sensors to deal with this, but maybe this isn’t a significant problem.

This isn't a holonomic vs non-holonomic question. You could ask the same question (or a similar question) about a simple tank drive: "If I push both joysticks forward by the same amount, will the robot go precisely straight ahead?". The answer is no, and it doesn't matter because the driver adjusts (within reason) the joystick position(s) to get the response he seeks.

The only reason holonomic is different is because it has three degrees of freedom, and it's annoying for the driver if, for example, he's got a Halo-style interface and he has to continuously hold a rotation command in order to keep the robot from rotating while he's busy working the fwd/rev and strafe joystick. That's when gyro feedback or a "calibrate" button on the joystick is nice to have.

[Jogo]

Alright, I'm starting to understand. In the case of holonomic, there is usually joystick 1 (fow/rev and strafe), and joystick 2 (rotation). Would it be advisable to use a P/PID function that controls the rate of rotation of the robot? For example, if joystick 2 is at (0,0) but the gyro senses 5deg/sec, make some corrections. No reason to use P/ID on fow/rev and strafe?

[Ether]

Originally Posted by Jogo

Alright, I'm starting to understand. In the case of holonomic, there is usually joystick 1 (fow/rev and strafe), and joystick 2 (rotation).

That is one of four popular ways to do it. It's called Halo-style. See this link for 2 other ways. A fourth way is to use a 3-axis joystick: X, Y, and Z (twist).

Would it be advisable to use a P/PID function that controls the rate of rotation of the robot? For example, if joystick 2 is at (0,0) but the gyro senses 5deg/sec, make some corrections.

If your drivetrain is well-designed and balanced, and your wheels are carefully aligned and the mec rollers spin freely, and you use PID to control the 4 wheel speeds, it shouldn't be necessary.

But you can if you want. Here's one way.

No reason to use P/ID on fow/rev and strafe?

Right. Besides, how are you going to sense fwd/rev and strafe speeds for the PID process variable input? With a couple of idler wheels?

[Jogo]

Cool, even if it isn't necessary during driver control, it would likely be for autonomous mode.

Thank you so much!

[EricS-Team180]

Originally Posted by Ether

What was disastrous about it?

...insufficient testing to uncover errors in the steering motor's output logic coupled with a bad Cypress board and a hardware issue with our CAN bus. Using the sensor to determine field orientation angle worked fine. We ditched the Cypress board, fixed the CAN issues and reverted back to robot-oriented control and what you are calling a halo-style driving scheme. We just did not have time to untangle the logic.

[Ether]

Originally Posted by EricS-Team180

...insufficient testing to uncover errors in the turning motors' output logic.

Yeah, I was wondering if that's what you meant. I've been thinking about this problem for a while; it has many subtle nuances. For example, if a wheel is going in direction "theta", and the command says "go in direction theta+180", do you turn the wheel or reverse the wheel's rotation? What if the command is theta+175? Do you turn 175 degrees or do you reverse motor direction and turn 5 degrees? What about theta+100 or theta+95? How does the wheel's speed affect these decisions? What about wheel "wind up" (assuming you're not using slip rings), e.g., What if the wheel rotation is limited to +/- N turns? What if the wheel is limited to +/- 180 degrees, or +/- 90 degrees, or whatever... how does this affect the logic and where do you put the "discontinuity" for least impact on the driving? If the commands can change more rapidly than the motors can respond (both the turning motor and the driving motor), then it seems that every control iteration you would need to compare the process variables to the commands to determine the best course of action - your logic must have access to the process variables.

I'm not asking you to answer all those questions (unless you want to, which would be great). Those are just the questions I've been thinking about - if anybody wants to discuss.

[kamocat]

I know in 2009, team 1983 Skunkworks used swerve drive. They didn't have continuous-turn potentiometers, and so when they needed to turn past a certain point, they would flip the module 180 degrees and reverse the direction of their drive motors. It worked surprisingly smoothly, but that may be in part due to the low coefficient of friction.
Here's a drawing of their drive base:
http://www.chiefdelphi.com/media/photos/32444

[Ether]

Originally Posted by kamocat

I know in 2009, team 1983 Skunkworks used swerve drive. They didn't have continuous-turn potentiometers, and so when they needed to turn past a certain point, they would flip the module 180 degrees and reverse the direction of their drive motors. It worked surprisingly smoothly, but that may be in part due to the low coefficient of friction.

Do you know where they positioned the discontinuity?

I'm certainly not recommending this, but suppose for example they located it at the 12 o'clock (straight-ahead) position. I can imagine a lot of weird wheel movement going on when trying to drive straight ahead with small directional changes. So I wonder where's the best place to put it. Perhaps it depends on the game played played.

[kamocat]

Well, it was a 7/8ths turn potentiometer, so they definitely had some leeway.
I think they didn't switch it until they had to; instead of switching at 3 and 9, they switched at 5 when turning CW, and 7 when turning CCW.

[BEEKMAN]

Hi,

We used a field oriented drive system this year, it definitely made Breakaway SO much easier.

We originally used a compass to read the directions, but quickly realized that there was too much interference around the sensor. We then used a gyro to do it. The WPILib version of holonomic drive is NOT advised, we found many small errors in it, in addition to the fact that the code was labeled as "Experimental". The Error you are referring to is likely the walking of the gyro. We noticed little to no error after only the 2.5 mins of a match, even while crossing the bumps 3-5 times per match. Actually, using a holomonic drive system made this easier.

The field oriented drive system allows us to have many usefull features:
1) Easier control: Anyone can use it...point in a direction, and the bot goes that way!
2) Auto aligning features: Hit a button and line up for the bump automatically, while all you do it point in the direction of the bump.
3) Auto hang aligning: Again, hit a button and drive directly towards the tower; bot will align to it by itself
4) Drive in a direction: We refer to it as "the trigger". While we are driving towards a ball, regardless of if the bot is facing towards the ball, as we drive to it (using the shortest possible distance) the bot will turn in that direction, eliminating the need to stop, turn, then drive.

these are only a couple features that we had. This is a quick video we made while testing: http://www.youtube.com/watch?v=HT3WhTcQvw4 Not all features are noted here

I should also note we used a 3 axis joystick, so there was only one-easy to use-joystick for robot motion control. If you saw us in action at some later competitions (mostly off season) you can see us "having fun" on the field sometimes.

[Ether]

Originally Posted by BEEKMAN

2) Auto aligning features: Hit a button and line up for the bump automatically, while all you do it point in the direction of the bump.
4) Drive in a direction: We refer to it as "the trigger". While we are driving towards a ball, regardless of if the bot is facing towards the ball, as we drive to it (using the shortest possible distance) the bot will turn in that direction, eliminating the need to stop, turn, then drive.

These are very innovative ideas. Nice work!

What type of drivetrain did you use?

[Jogo]

Originally Posted by BEEKMAN

4) Drive in a direction: We refer to it as "the trigger". While we are driving towards a ball, regardless of if the bot is facing towards the ball, as we drive to it (using the shortest possible distance) the bot will turn in that direction, eliminating the need to stop, turn, then drive.

I'm curious as to the theory of how this is accomplished. For example, we've done the same thing without holonomic wheels (called it 'Vector Drive'). However, this was accomplished using "skid" (speeding up one side of the robot, slowing down another). Is there a different principle involved for holonomic?