where are all the REAL engineers in this thread?
doenst anyone use feedback to control their robot?
yes when you put castors on a bot the steering gets squirrely - thats why you also use a yaw rate sensor that measures how much the bot is turning, and you fly-by-wire!
you close the loop on steering by having the SW look at what the driver is commanding the bot to do (how hard driver wants to turn) and looking at the yaw rate sensor to see how fast the bot actually IS turning
and use a PID closed loop algorythm to MAKE the bot do what its told!
why would you do anything else? this works with 2 wheel skid steering, 2 wheel castor steering - four wheel steering - 6 wheel.... whatever the gearheads throw at you, the sparkies can turn the machine into a PID controlled nice and tight, highly responsive servo like machine
the beauty of this is not only does it make the robot go straight when you want it to go straight, it also allows very precise slow turns. If you want the bot to turn just a tiny bit, the SW will put the necessary power to each motor to make that happen, adjusting the levels 40 times a second.
Try it on ANY bot - feedback is the most powerful tool an engineer has at their disposal - see them F111s flying around, they would be completely unstable, unable to fly AT ALL without fly-by-wire closed loop feedback controlling its motion. And guess what? the same type of yaw rate sensors they use are available to you in the suppliers catalogs.