Tanto 2

[fwsorg]

Mario- God knows Ive tried to persuade him

Christian- the 140 x 140 is for something different.

Christian- Electronics

[fwsorg]

We are toying with traction control, but that doesnt really help keep the nose on the ground. In the speedos which are being developed, theres an accelerometer to measure tilt, and so apply power so as to keep the nose on the ground. we may however want to nose to rise a couple of inches up in the air, so we can make full use of the spikes on the front of the wedge, and the accelerometer will allow us to do that too. Its anaologous to adjusting the gain on your gyro- we should be able to adjust the tilt control.

[storm]

Fantastic idea guys

....so many bits to go wrong

[jamesb]

oh I love traction control, so much wasted power and grip......

anyone know what slip angles are?
nope.
I love traction control (on otherpeoples machines)

[jamesb]

ps... this is a hint to ask me for advice on making efficient traction control.

[fluppet]

Ill bite, James (especially since some of my more obscure designs would be a bit lacking in the physical traction department). Care to start a discussion/advice session in hints & tips? (Hmm, we dont have a section thats for nonspecific weights; unless admin are so inclined, I guess we default to heavyweight.)

--
Fluppet

[fwsorg]

Ed, Ive had this conversation with you before ...
... I do not design Robot wars electronics which rely on sensitive Chips- I do have a modicum of common sense. As I have explained to you before, should any of the modules (for example the accelerometer) stop working, the 2 processors will simply work without them. Should 1 of the processors fail, the other one will take over its job. The whole speedos are designed for things to go wrong! Short of an axe chopping the thing in half, weve tried to think of every eventuality!

[fwsorg]

Andrew- Basics of traction control (and this will include slip angles :-

Contrary to popular beleif, a robot is not generating the most force if its wheels are spinning- the most traction from a wheel is obtained when the wheel is only just on the verge of slipping. So if you want to most traction when pushing another machine, you want to reduce the power of the motor (assuming the wheels have been slipping) untill the wheels are only just about to spin.

A slip angle is the angle at which the surface of the wheel will just break contact with the surface that it is on. The slip angle therefore depends on the surface youre running on, the amount of pressure on the wheel etc. For most traction, you must apply power to your wheels so that they only just begin to slip- this is the slip angle. I invite james to explain this far better than I because Im not very good at explaining such concepts, and hes had a lot more experiance with this kind of thing that I have.

[jamesb]

yeah, here is the secret, Ive been sitting on it for a few years, but as Im not going to do traction control, I might as well let the cat out of the bag now.

To achieve efficient traction control you need the following.

a realtime feedback loop, measuring wheelspeed (or motorspeed), lateral g, groundspeed.

you also need data about your wheels on the required surface, this can be simulated on a rig, but finetunin must be built in.

Right, the way this works.....

in a straight line, the robot will accellerate and decelerate when the tyres are operating within their maximum effective slip angle.

to find out the value of this (this also explains what a slip angle is) you need to run the wheel on a rig. this rig is set to measure the torque of the motor, and the torque on a slave shaft which simulates the floor of the arena. This way you can measure grip of the tyres.

accellerating the motor should accellerate the slave shaft at the same rate. If the slave shaft accelerates slower than the motor, you have wheelspin. A torque comparrison gives you a graph of how much wheelspin you get.

you will find that, from when the speeds first vary (ie, wheelspin first occurs) to a noticable difference in torque, there is a gentle squeeling. This is a tell tale that the tyre is working at its hardest, and is within its slip angle (the theory being, the wheel turns, say 367 degrees for the slave shafts 360, so it is an angle of 7 degrees different caused by the slipping.

so what does this mean?

if you look at the graph, you will see that the accelleration did not drop for this area of slip. Beyond about 7 degrees (this is an example for a go kart tyre) the torque on the slave drops suddenly. this is where you need the traction control. Regualr traction control cuts in as soon as the wheel speed is different. this ignores this area of slip, and so automatically looses it.

by having groundspeed, and motor speed, you can program the traction control to allow for the slip angle, as derived from your rig tests, and so maximize your available traction. This works in deceleration to, as for example F1 abs was working within slipangles, unlike car ABS.

to maximize lateral grip, you need sensitive g-sensors, but best avoid thse, as they will get confused when storm 2 hits you.

there we go, a nice description of why previos traction control didnt provide enough of an advantage to make it viable, it cut off the bit where most grip was produced.

oh, before everyone starts looking at this, think about one more thing.
Tornado pushing on the side of... um... Smidsy, for example.
Cant move him eh, wheels are not gripping? traction control would not help here, what would help is spinning the wheels up for a second, which covers the floor in tyre rubber, then push again and voila, more grip.

traction control, Ill never use it.