pic: FRC488's Octocanum Ver 2.0

[JohnGilb]

Ether, your follow wheel mounting assumption is correct. They were positioned as in your diagram, and they were omniwheels.

We did use them to calculate a rotation rate, let's call that Rotation_Follow. We also had rotation from the gyro, let's call that Rotation_Gyro.

We found that Rotation_Follow wasn't as good as the Rotation_Gyro, we suspect due to minute wheel scrub and small errors accumulating, but it did not suffer from drift. Consequently, we used Rotation_Gyro exclusively, but ignored any change in rotation while Rotation_Follow was 0 (typically at the start of the match before the robot was moving anywhere, or during testing when the robot spent a lot of time on a bench or disabled). This eliminated a large part of our gyro drift.

[Ether]

Originally Posted by JohnGilb

we used Rotation_Gyro exclusively, but ignored any change in rotation while Rotation_Follow was 0 (typically at the start of the match before the robot was moving anywhere, or during testing when the robot spent a lot of time on a bench or disabled). This eliminated a large part of our gyro drift.

I like it. Nicely done.

[Jared Russell]

Originally Posted by JohnGilb

Ether, your follow wheel mounting assumption is correct. They were positioned as in your diagram, and they were omniwheels.

We did use them to calculate a rotation rate, let's call that Rotation_Follow. We also had rotation from the gyro, let's call that Rotation_Gyro.

We found that Rotation_Follow wasn't as good as the Rotation_Gyro, we suspect due to minute wheel scrub and small errors accumulating, but it did not suffer from drift. Consequently, we used Rotation_Gyro exclusively, but ignored any change in rotation while Rotation_Follow was 0 (typically at the start of the match before the robot was moving anywhere, or during testing when the robot spent a lot of time on a bench or disabled). This eliminated a large part of our gyro drift.

This is a clever solution. A Kalman Filter would be another good way to combine accurate (but drift-prone) gyro measurements with less precise (but stable) odometry measurements.

[JohnGilb]

We tried a number of filters (Kalman was unfortunately beyond my reach, never had a strong grasp on linear algebra), but they turned out to be unnecessary - the gyro we used (don't have the model # on me) was incredibly accurate - it usually only drifted ~3 degrees over the course of each match, even through collisions.

The "stationary detection" we did was only necessary when the robot was put on the field but the start of the match was delayed several minutes (yeah, we've all been there).

[AdamHeard]

Originally Posted by JohnGilb

We tried a number of filters (Kalman was unfortunately beyond my reach, never had a strong grasp on linear algebra), but they turned out to be unnecessary - the gyro we used (don't have the model # on me) was incredibly accurate - it usually only drifted ~3 degrees over the course of each match, even through collisions.

The "stationary detection" we did was only necessary when the robot was put on the field but the start of the match was delayed several minutes (yeah, we've all been there).

Mind sharing the model number of that gyro?

Also, you didn't use the pair of follower wheels to try to determine rotation rate while translating, you just used it to detect the complete absence of velocity, correct?

[Ether]

The point midway between the translation follower wheels was said to be the center of rotation of the vehicle. To the extent that is true^*, you could extract vehicle rotation while translating.


^*The center of rotation likely shifts around especially if weight distribution changes as manipulators move.

[AdamHeard]

Originally Posted by Ether

The point midway between the translation follower wheels was said to be the center of rotation of the vehicle. To the extent that is true^*, you could extract vehicle rotation while translating.


^*The center of rotation likely shifts around especially if weight distribution changes as manipulators move.

Wouldn't that only be true while not translating? Translating would shift the center of rotation, would it not?

[theprgramerdude]

What's the maximum speed at which you can switch between Mecanums and high-traction wheels? Plus, about many switches can be performed before there becomes a serious air deficiency such that it is unreasonable to switch at all, assuming you have about 3-4 tanks at 120 PSI at the start.

[Dad1279]

Originally Posted by theprgramerdude

What's the maximum speed at which you can switch between Mecanums and high-traction wheels? Plus, about many switches can be performed before there becomes a serious air deficiency such that it is unreasonable to switch at all, assuming you have about 3-4 tanks at 120 PSI at the start.

YMMV, but we used 1 tank, (2) 1.5" bore, 2" stroke cylinders to switch between traction & mecanums, (2) .75" (6 or 8" stroke) to lift claw, and 2 small cylinders to deploy the minibot, and never had a want for more air.

[Ether]

Imagine a robot going around a circular track at constant speed such that it completes one lap every minute. The robot has a gyro mounted on it. The gyro rate will be 1 rpm, the same as if the robot were spinning in-place at 1 rpm.

I'll have to check the math, but I believe that with 3 followers mounted as shown, you can extract the vehicle's rotation rate (the same rate measure by a gyro) for any vehicle motion.

[AdamHeard]

Originally Posted by Ether

Imagine a robot going around a circular track at constant speed such that it completes one lap every minute. The robot has a gyro mounted on it. The gyro rate will be 1 rpm, the same as if the robot were spinning in-place at 1 rpm.

I'll have to check the math, but I believe that with 3 followers mounted as shown, you can extract the vehicle's rotation rate (the same rate measure by a gyro) for any vehicle motion.

Yes, but that's with 3 follower wheels. 488 only has two follower wheels measuring the floor, so I was clarifying as our swerve has two and there is no way we could infer rate from that (in fact we have to factor in the gyro rotation rate to even to get an accurate linear velocity).

EDIT: 488 actually has three follower wheels in that diagram, I was mistaken. Trivial calculation in that case.