fine control of robot with mecanum wheels

[Boommaster713]

Something to be aware of is that during testing my team found that in order to control the robot at low speeds the motors must be given 40% of maximum voltage (to overcome the robots moment of inertia) and then once the robot is moving the power can lower slightly.
Other issues that could impact fine control is the lag time between control system and robot.

My Team's robot uses a near identical setup, 8in andymark Mechanums wheels(with black mini wheels), andy mark toughboxes, chains connecting them with andymark wheel tensioners (Fiddly to the extreme to get right, but once right they havent failed through 13+ matches)
all done open loop with no feedback, with jaguars on PWMs, and joystick mapping for fine control

Robot control is a 3 axis joystick

[Ether]

Originally Posted by Boommaster713

My Team this year has also run a system identical to this setup and have not had any issues regarding the control of the robot(Once setup properly). So the basic premise of too much friction and high gear ratio is not the culprit.

The basic premise was too much friction and high gear ratio to be controlled open-loop. Is your bot's wheel speed controlled open-loop or closed-loop?

So a simple question would be what kind of control issues is your team experiencing?

"lack of fine control of robot movements, especially at low speeds"


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[TD912]

The joysticks return values between -1 and +1 for both the X and Y axes, And these values are then used to generate a signal for the speed controller. You can "curve" the output however you want. Normally, moving the joystick 50% forward would send a 0.5 signal to the contoller. Squaring it should give you finer control at lower speeds (moving the joystick 50% forward would result in a 0.25 signal being sent), while getting the square root would do the opposite (50% forward would end up sending a 0.71 signal instead!). In either case, you could still achieve full speed by moving the joysticks all the way forward. You should experiment to see what works best for you, and be sure to program in a way to keep your positive values positive, and negative values negative when squaring.

Just remember that the motors will still need to be able to produce enough force to move the robot, Plus there are things like deadband to deal with, so very low values like .1 might not appear to do anything.

Edit: By the time I typed this up, all these people replied already! Sorry if I repeated what was already said.

[TubaMorg]

We are using an open loop this year and we have used it in the past for our mecanum drive, though we use direct drive and a higher gear ratio than the KOP transmissions. We aren't using encoders to control wheel speed, but we ARE using a gyro to make sure the robot stays pointed in the right direction.

We aren't experiencing control issues perhaps because of our differences. You could fairly easily increase your gear ratio (in a way) without too much trouble by changing your sprocket sizes. A smaller output sprocket and a larger wheel sprocket will slow your wheels down.

Hope this helps...

[Dad1279]

As a point of reference, we used the AndyMark 8", directly driven with 12:1 transmissions. For control we used Logitech 3d, using the 'twist' of the joystick for rotation.

They were driven by PWM, w/o feedback. Rotation/twist was scaled down.

This worked fine on both our test-bot and the final robot.

[Ether]

Originally Posted by Dad1279

As a point of reference, we used the AndyMark 8", directly driven with 12:1 transmissions. For control we used Logitech 3d, using the 'twist' of the joystick for rotation.

They were driven by PWM, w/o feedback. Rotation/twist was scaled down.

This worked fine on both our test-bot and the final robot.

AndyMark 8"... check
3-axis Joystick ... check
"twist" for rotation ... check
PWM ... check
no wheelspeed feedback ... check
rotation "scaled down" ... check

only difference is wheels are connected to toughbox (12.75:1) transmissions via sprocket and chain.

What kind of curve-shaping did you do on your joystick outputs before feeding them into your wheel-command-calculation algorithm ?


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[boomergeek]

Originally Posted by Ether

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

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

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

[Ether]

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.


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