Chief Delphi - fine control of robot with mecanum wheels

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- - fine control of robot with mecanum wheels (http://www.chiefdelphi.com/forums/showthread.php?t=84344)

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Jared341 (Post 938710)

Using the gyro was very simple - the algorithm was just:

Code:

for each motor

  motorCommand = <result of holonomic drive calculations>;

  if we are trying to go straight (e.g. yaw command is zero)

    motorCommand += Kp * getGyroRate();

  end if

  Tell motor to go "motorCommand"

end for

Am I reading the above code correctly: you use the gyro only to make the robot drive straight when so commanded? You don't use it to control turning or strafing?

~

Re: fine control of robot with mecanum wheels

I'll echo Jared341 on all points. We don't have so much experience with the holonomic platform as they do, but our experience matches exactly with respect to what has to be considered, and our solution ended up remarkably similar.

We understood the importance of keeping uniform load on all wheels at all times and chose the simplest way to do that: use three wheels and keep the CG in the middle. Low-tech, and very effective.

Unless all your drivetrains are perfectly linear the thing won't drive straight, since the motors are almost always running different speeds, and hence at different spots on the input vs speed curve.. In principle, it should be possible to get linearity with individual feedback loops on each wheel, but we weren't able to make that work. It helped, but it wasn't sufficient. Possible or not, it isn't easy. We ended up taking it out entirely for other reasons.

Gyro-based correction, in the other hand, is simple. Our feedback code looks almost exactly like the posted snippet. For driver control, rotation feedback isn't terribly important except when trying to go straight,, since the driver can't distinguish, for instance, between rotating 47 degrees per second and rotating 52 d/s. But there's a big difference between driving straight and drifting 5 d/s.

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Ether (Post 938741)

Am I reading the above code correctly: you use the gyro only to make the robot drive straight when so commanded? You don't use it to control turning or strafing?

~

Correct - for those two games (Rack N Roll and Overdrive), pointing our scoring mechanism in a consistent direction was the biggest obstacle to overcome in order to be effective. For strafing or turning, we let things run open loop - with a little practice, the driver is able to compensate for translation errors without even thinking about it.

There is no reason why you couldn't use the gyro to control your turns, but in general, 0 degrees per second is by far the most important turn rate setpoint. As buchanan pointed out, 47 deg/s and 52 deg/s doesn't make a lot of difference to a driver, but 0 deg/s and 5 deg/s does.

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Ether (Post 938382)

...with the lack of fine control of robot movements...

~

Team 241 has 8" mecanum, direct drive toughboxes,
voltage control CAN interface

----
We are considering adding and tuning a speed control based on encoders that are already mounted.
----

There have been many factors that we have found seem to affect "the lack of fine control of robot movements..."

+ consistent CIM free run speed - (some CIMs are outliers)
+ consistent gearbox efficiency -
+ consistency of mecanum roller turn resistance- keeping these even have been a constant maintenance task for our mechanics
+ the designed lack of roundness in the 8" mecanums - at very low power, this non-linearity is noticeable.
+ weight distribution - get 4 scales one for each wheel.
+ suspension to keep all wheels firmly on the ground (we added this early based on academic research- so we really don't know how important it is to this game/surface... Are some teams with mecanums but without suspensions satisfied with the handling for the Breakaway arena?)

and probably most important-
+++ giving the driver enough practice time to truly have a chance of learning to predict the behavior of the robot based on the commands sent to it.
we locked in on using a single driver almost exclusively due to the lack of enough practice time available.

Our original optimistic plans were to have speed control (not voltage control) completed at least two week before the robot had to ship so the drivers would have substantial time to learn the machine.

We have portions of a second bot but we did not get to a cloned drivetrain (no mecanums, only 2 CIMs on it).

The teams with fields to practice on and have cloned second bots to practice with are strongly advantaged both in getting the control right and more importantly, in training the drivers to play the game well and making the most of the control that they have.

I am bit amazed on how well some drivers have done with so little driving practice especially with new mecanum drive systems.

IMO, the most important control system to tune is the one between the drivers ears... ;)

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Jared341 (Post 938693)

...
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.

Can you adjust each motor's signal based on the ratio of normal forces? That is, if you know the percentages of robot weight borne by each of the corners, can you compensate for that by varying the command to each? This might also be useful for bump-climbing when the weight distribution on even a perfectly centered CG 'bot gets totally skewed.

Re: fine control of robot with mecanum wheels

http://www.chiefdelphi.com/forums/sh...94&postcount=1

We were able to improve driveability to an acceptable level by going back to tank drive (to take advantage of our driver's previous tank-drive experience) and using one joystick's X-axis (not used for standard tank drive) for strafe. We developed an algorithm to encode the 3 joystick commands (leftY rightY rightX) into commands for forward/reverse, turn, and strafe, which could be tuned independently (see link above). With a bit of tuning, it behaves just like the familiar tank drive, except it has the ability to strafe by using the joystick X-axis when necessary.

~

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Bill_B (Post 939753)

This would work in principle, but because of the dynamics of how a robot moves (e.g. the weight gets shifted towards the rear wheels when accelerating), you would need a very fast strain gauge or other type of force sensor to detect how these quantities vary over time. I'm not sure how much this would help in practice.

Re: fine control of robot with mecanum wheels

Quote:

Originally Posted by Bill_B (Post 939753)

As long as each wheel has enough normal force to provide enough friction so that it is not slipping, you want the same command applied to each wheel. The torque on each wheel gets transferred from the wheel to the ground, regardless of the normal force on the wheel, as long as the wheel is not slipping.

If a lightly loaded wheel is being given a command higher than the friction provided by the normal force on that wheel can support, and the wheel starts slipping, then what are your options? If you reduce the command to that wheel without reducing commands to the other wheels you will mess up the behavior of the bot (we're talking about a mecanum bot here). If you don't do anything, and let the one wheel slip, that will also mess up the behavior of the bot. Bottom line: you must have enough normal force on each wheel at all times in order for mecanum behavior not to be affected. That's why suspensions are used on mecanum bots: to distribute the normal load on each wheel more evenly.

~

Re: fine control of robot with mecanum wheels

As I said before, 868 is using mecanum wheels with no wheel feedback.
We are using 6" wheels. We've also geared our robot down significantly, I doubt our max speed is faster than 6 feet per second. This is probably why our robot is so controlled.

Sorry I couldn't help much; best of luck at your competitions.