Holonomic Drive System Issues, Robot Drives Diagonally...

[AllenGregoryIV]

Originally Posted by omalleyj

I think you need explain that more clearly. The robot will go at its maximum speed in the direction any pair is going in for the omni@45 arrangement. And .7071 of that speed in the regular front-back side-side directions. The ratio is 1.4, but you give the impression that it will go faster than its wheels are turning, which is impossible in any direction.

I have found that Ether is better at explaining this than me. You can look at the gif that he attached on this page

There are several research papers published on this (mostly for robocup).

Thus, the addition of angled omni-wheels can result in a net body velocity that is greater than the maximum radial velocity of the wheel (Ashmore and Barnes, pg. 7).

That paper goes on to say that by traveling with rotation and in an arch a holonomic robot can travel even faster. We have thought about creating a control scheme that takes advantage of this but it would be very difficult, in the context of a FIRST robot.

[Ether]

Originally Posted by omalleyj

I think you need explain that more clearly. The robot will go at its maximum speed in the direction any pair is going in for the omni@45 arrangement. And .7071 of that speed in the regular front-back side-side directions. The ratio is 1.4, but you give the impression that it will go faster than its wheels are turning, which is impossible in any direction.

Omni vehicle with 45 degree-mounted wheels goes faster in the forward direction than omega*R (R being wheel radius and omega being wheel angular velocity in rad/sec).

A chart comparing omni, mecanum, and standard wheels can be found here.

A force analysis of mec vs omni can be found here. A kinematic analysis can be done using a similar approach.

[omalleyj]

Originally Posted by Ether

Omni vehicle with 45 degree-mounted wheels goes faster in the forward direction than omega*R (R being wheel radius and omega being wheel angular velocity in rad/sec).

A chart comparing omni, mecanum, and standard wheels can be found here.

A force analysis of mec vs omni can be found here. A kinematic analysis can be done using a similar approach.

Ether, I have to go build a robot now, I'll take a look.

[Omgsar]

So, judging by the responses that I have seen so far, the consensus is that it is not a coding problem?

[AllenGregoryIV]

I would say that it is not a problem with the code that you posted here.

It may be a problem with cRIO or the electronics. (Have you updated your cRIO image to the one that was released this week?)

Also you may want to try using mecanumDrive_Cartesian(giving x and y axis instead of magnitude and direction) instead of polar and see if that works better for you.

Have you tested on blocks and seen which wheels spin when you move each joystick axis? When you move the joysticks, do all four wheels ever spin? If you apply full power forward do your Jaguars light up in the way they are supposed to? (two red and two green most likely)

If you are still having this problem at Alamo(I hope you resolve it by than), I'll try and stop by your pit and see what we can do.

[Andrew Schuetze]

Thanks for all the suggestions and looking over the code. Current and most recent "fix" was to broaden the deadbands. It is now driving much more as expected. Possibly the deadband was too narrow and minor off-center stick movements where exagerated when translated into robot movement.

I'd still like to compare robot performance between the Discobots and this omnidrive robots and continue the evolution of our design process.

[AllenGregoryIV]

Yes, hopefully at some point we will be on the same alliance at Alamo. I think we will definitely give the other alliance some problems. They will be trying to figure out which way we are going and only we will know

See you guys in 8 days.

[Andrew Schuetze]

I keep envisioning a pivot move aka some great NFL running back that is heading down field and spins around a defender or your favorite NBA point guard for a driving layout

What would it take to program such a spin around either the front right or front left wheel We currently have drive/strafe and then zero degree spin coded to two joysticks. Maybe it is just driver control with drive input coming from one joystick and a spin componant coming from the other....

[Ether]

Originally Posted by Andrew Schuetze

What would it take to program such a spin around either the front right or front left wheel

The math is not that hard. See attached image.

Calculate how fast you want to rotate. Call that omega. That's your rotation command. Multiply omega by |AC|. That's your translation magnitude. Theta is your translation direction angle. Translation velocity vector V is perpendicular to |AC|.

[AllenGregoryIV]

The way we do it is with driver practice. To be able to pivot you have to give the robot some rotation (in our set up the x axis of the drivers right hand) and move the robot in an arc which is to keep the y constant while moving the x from positive to negative (this happens on the left stick).

We were also trying to figure out if the driver could drive around the minibot pole while holding orientation to the pole. This is not easy in our current control scheme but possible you have to make a circle with left joystick and hold a constant rotational velocity with the right one.

This is all with field centric control. Without that the first move becomes much harder while the second is much easier.