It is great that First included encoders and the ability to mount them to the gearbox. It’s too bad that combination will be effectively useless this year. With the limited traction, there will be so much wheel spinning, as well as lateral sliding, that gearbox encoders will not give any useful information about where the bot is, and what it’s doing.
<R06> talks a great deal about what wheels can be used for traction, and that no devices may be used to increase traction. What about a high traction wheel that touches the ground, but does not affect traction? Say, a wheel with an encoder attached? The way I interpret the rule, this would be acceptable.
The thing you would have to ensure is that the wheel can offer no resistance when pushed sideways (which is pretty hard). Even a caster design would offer some form of resistance until its direction was changed.
Actually, a caster was what I had in mind. I believe that any resistance from the caster will be at least an order of magnitude less than wheel friction, so in engineering terms is “negligible”, therefore can be ignored.
If the wheel was mounted so the axis of rotation was fixed, it would provide lateral traction. Even if the wheel was mounted so it could swivel, it would provide an instantaneous amount of traction while the wheel swiveled to its new position. The only way I can see this feasible is if you mounted a sphere in the middle, like the ball for an old mouse. Even then you would have to prove to FIRST that it provided no additional traction. Unfortunately, I don’t think they would accept this.
You could mount a wheel that was not powered. As long as you have good bearings that provide negligible friction in the axle, it won’t slip. An encoder could be attached to the axle. The issue with this is that you loose precious traction with each wheel that touches the ground that is not powered. If your willing to go this route however, you could mount a wheel in a perpendicular direction to receive encoder values for both the x and y axis. The wheels have such little traction, a wheel in the perpendicular direction won’t resist motion provided by the other wheels.
Why couldn’t you use encoders with slipping wheels? The sensors will certainly tell you actually how fast the wheel is spinning. Maybe you can use that information with an expected PWM signal vs. speed curve for traction control?
For example, if you observe that the wheels suddenly start spinning from a low speed to a higher speed, assume that slippage is occurring and reduce the drive signal.
Cars are really starting to take advantage of traction control. Maybe FIRST robots can learn from them?
Not quite correct
<R06>
ROBOTs must use ROVER WHEELS (as supplied in the 2009 Kit Of Parts and/or their equivalent as provided by the supplying vendor) to provide traction between the ROBOT and the ARENA. Any number of ROVER WHEELS may be used. The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub). No other forms of traction devices (wheels, tracks, legs, or other devices intended to provide traction) are permitted. The surface tread of the ROVER WHEELS may not be modified except through normal wear-and-tear. Specifically, the addition of cleats, studs, carved treads, alterations to the wheel profile, high-traction surface treatments, adhesive coatings, abrasive materials, and/or other attachments are prohibited. The intent of this rule is that the ROVER WHEELS be used in as close to their “out of the box” condition as possible, to provide the intended low-friction dynamic performance during the game.
R06 does not stop you from increasing traction, it just prevents you from using anything else to provide traction
Negligible traction is still traction.
Traction control and accelleration control will be key this year to winning alliances. IMHO
It looks like now is a great time to start programming! Traction control and robot position/velocity/acceleration information could really help a team control their robot.
Those of us who don’t have to drive on icy roads are probably at a disadvantage…oh well…
We’ve been exploring intelligent slip-prevention systems, and that would be quite the challege even without trying to use the camera to track and shoot balls at a target… we’re probably going to try but it will be tough.
As to the idea of providing another wheel which would be measured with the encoders, perhaps a trans-wheel? it would have extremely low friction when moving sideways, and trans-wheels are not “wheels, tracks, legs, or other devices intended to provide traction”
I don’t know how optically “interesting” this floor will be. namely, is there going to be a visible difference between one spot on the floor and another? it might need something as precise as a laser mouse. it’s an interesting idea.
I was part of a group that used optical mice as encoders. It was a bit of a pain in the neck to start, but worked well once it worked at all.
One idea I’m fiddling with is measuring the current to the motors to approximate torque. A 20% difference between slip and grip should be measurable. I wish we had CAN this year, since the Jag’s have that feature built in.
Your encoder wheel doesn’t have to rob (much) traction from the rest of your robot. If you mount a fifth rover wheel on a swing arm or something similar, it will use only its own weight for traction. Then it’s just a question of how good of a bearing do you need. ABEC 7 anyone?
How about simply using two omni wheels with encoders mounted at a 90 degree angle from each other. These would not generate resistance when changing direction and could easily be the x and y for movement.
you could just use a kit wheel and make it pivot that way your not breaking any rules because its not being powered it would not slip so the encoder would know where it was
years ago the railroads used quantity four 3,600 hp locomotives to pull a large unit train.
today they use at most three. the way they did that is by automated traction control.
a doppler radar is under the locomotive measuring the actual speed. the traction computers optimize the power delivered to the wheels so that there is optimal slippage driving wheel, about 3%
they got rid of a whole locomotive, plus they get greatly improved fuel economy and operating performance.
anyone game for trying this ? we certainly have the computer to do the job !!
put a dedicated speed controller, drive motor combo on each wheel, figure out where you are actually going, versus where you want to go and off you go.
