Bot Steering Question

Hi, I bought a 4 wheeled robot kit http://www.robotshop.com/dfrobot-4wd-arduino-mobile-platform-3.html and have assembled it along with a motor shield from Adafruit. All 4 motors are fixed in place. I can get it to go forwards and backwards, but how do I steer it or make turns? Using the code below with the AFMotor library and motorLF as the left front wheel , how do I turn left? I've already tried various combinations like moving both left wheels forward, etc.... and is does go in that direction slightly by skidding. What is the correct way to steer a 4 wheeled robot when all 4 motors and wheels are fixed in place or does this only go forwards and backwards?

  motorLF.run(FORWARD);
  motorRF.run(FORWARD);
  motorLR.run(FORWARD);
  motorRR.run(FORWARD);
  delay(1000);
  motorLF.run(BACKWARD);
  motorRF.run(BACKWARD);
  motorLR.run(BACKWARD);
  motorRR.run(BACKWARD);
  delay(1000);
  motorLF.run(FORWARD);
  motorRF.run(RELEASE);
  motorLR.run(RELEASE);
  motorRR.run(RELEASE);

You would steer the same way you would a treaded vehicle like a tank. To turn left, you would go forward with the right 2 wheels and in reverse with the left 2 wheels. It is less of a turn and more like a rotation.

I would imagine that if you don't actively break the inside wheels they will free wheel and you won't get much steering.

Can you break the inside wheels? If not how abut reversing them, but be ready for a serious spin.


Rob

You would steer the same way you would a treaded vehicle like a tank. To turn left, you would go forward with the right 2 wheels and in reverse with the left 2 wheels. It is less of a turn and more like a rotation.

Pauly:

In addition, if you want to do a gradual turn with a differentially steered robot chassis, you vary the speed of the treads by different amounts while they are going in the same direction. This can be done using the PWM speed selection mode of your controller for each motor.

What is really interesting about this platform though, is the fact that you have in actuality four independent "wheels" in a differential arrangement, but that are independently controlled; this is unlike a standard tank-tread differential steered vehicle, since such a vehicle only has two "wheels" to control (each tread). Perhaps by varying the speed and direction of the independent wheels, some other control/steering mode may be available? Sounds like something to experiment with!

Also - if you can actively brake the motors, that would be best, but those gearmotors might not backdrive well, so braking may be unnecessary.

:)

Thanks for the help guys. I'll give your suggestions a try.

You will probably have to use "skid steering" Like bobcat loaders and such. When turning you might try giving a little rotation to the non turning side to make the non turning side slide a little easier.