I thought about employing a speedometer and tach, plotting the relationships for comparison. I only needed it for first gear so that simplified things. If all else fails and you end up using your right foot for your traction control, try something I learned on street radials: Get the car moving, then lift for a split second. Once you lift you can floor it without wheelspin. John ----- Original Message ----- From: "Andrew Warren" To: "Microcontroller discussion list - Public." Sent: Monday, October 18, 2004 3:53 PM Subject: Re: [PIC] Traction Control / Wheelspeed sensing > Andy Meng wrote: > > > I am working on a wheel speed sensing and traction control system > > as part of our schools Formula SAE race car. Basically, it has more > > power than traction (about 80HP/500 pounds) and we don't have much > > time to practice driving it, so the more help we can get in the > > traction department, the better (especially in the > > acceleration/drag race event). > > Andy: > > If you can give him a dozen-or-so runs to find the right launch > RPM and shift point(s), I think your unassisted driver will > perform as well or better than your electronic traction-control > system. > > If you don't have enough practice time for him to learn how to > launch the car, you certainly don't have enough practice time to > tweak and tune a traction-control system. > > But, since you probably want to have something to do between now > and the end of May... > > > The basic principle is to compare the speeds of the front (non > > driven) and back (driven) wheels to determine how much slip/loss > > of traction there is. Apparently, about 10% slip is best. > > 10% is only a general rule-of-thumb; the optimum amount of slip > could vary between half and twice that value. Tire compound, > construction, temperature, and condition all affect it, as does > the surface you're driving on. > > Also... Comparing front-wheel RPM to rear-wheel RPM is easiest > if a) tire deformation doesn't significantly change the rolling > circumference of your rear tires, b) you're not cornering, c) > you have a locked differential. > > I wouldn't expect you to have a problem with tire deformation and > I assume that you're only planning to use the traction-control > system for the straight-line acceleration event, not for the > autocross... But since you probably have a limited-slip or (yuck) > open diff, you need to think about how you'll measure wheelspeed > when your rear wheels aren't spinning at the same rate. > > > If the back wheels are spinning too fast, the throttle is cut > > electronically (strict throttle by wire is prohibited for obvious > > reasons, but just cutting fuel/spark isn't) to limit the slip (and > > hopefully maintain it with decent control). > > Consider retarding the ignition timing instead. > > > I have designed a wheel speed sensor (there will be 4 copies of it) > > How many pulses per revolution? More pulses gives you the > potential for quicker response, but also could lower your timing > resolution and make your front-to-rear ratio calculation less > accurate. Have you done the math to find a good compromise? > > > it has a hall effect sensor, PIC10F or 12F (timing pulses), MCP2515 > > CAN controller, and a CAN transceiver IC that hangs on the bus. > > Hopefully the CAN transceiver will make it pretty easy - all it > > needs is SPI data from the PIC. > > > > The other end will be a bit more difficult. I still haven't worked > > out the algorithms yet. I think it will likely consist of a similar > > CAN interface feeding a 16F or 18F PIC (some of the 18's also have > > a built in CAN interface). This PIC will have 4 outputs so it can > > individually cut cylinders. I think this PIC should probably be > > done in C to make it easier to try new algorithms. > > Have you added up all the timing to make sure that you can do > everything (read the wheelspeed, transmit the data to your > central PIC, process it, reduce/increase power) fast enough? > > Four sensors with embedded microcontrollers sending data > asynchronously, plus a central PIC that has to act on that data, > is a complicated system. Think hard about ways to reduce the > complexity. > > > I would appreciate comments on the system outlined above. > > If it were me, I'd simplify the output side of the system by > choosing to reduce engine power by retarding the timing; it's > fast, effective, and less intrusive with fewer unwelcome > side-effects than cutting fuel or spark. It also requires only > one output instead of the four that you're planning on. > > I'd simplify the input side, too. The entire system can be made > ENORMOUSLY simpler if you find a way to sense slip without > measuring wheelspeed. Such a way exists, but I don't want to > ruin the surprise for you. > > Good luck... > > -Andy > > === Andrew Warren -- aiw@cypress.com > === Principal Design Engineer > === Cypress Semiconductor Corporation > === > === Opinions expressed above do not > === necessarily represent those of > === Cypress Semiconductor Corporation > > _______________________________________________ > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist