Help with Dagu Rover 5

I'm currently attempting to build my first robot so please bear with me if I come across a little slow!

I've connected the Rover 5 platform’s two motors to the Arduino Uno using an H-bridge chip (SN754410) for testing purposes. My problem is that the two motors are running at different speeds, so I've measured the voltage (my multimeter is broken and wont measure current) at each and got the following results:

PWM 25% 50% 75% 100% Motor 1 1.57v 3.76v 4.98v 5.50v Motor 2 0.34v 2.11v 3.77v 4.93v

Can anyone shed any light on this? I've switched the H-bridge chip for another and got the same results, could it be the motors are faulty? Or am I missing something?

Also, could I drive the motors using I2C? Perhaps with something like this http://proto-pic.co.uk/serial-controlled-motor-driver/

Many thanks!

Alex

The Rover 5's two motors may have different characteristics, and thus draw different amounts of current at a given voltage (this is often a result of friction in the gearbox, or other such mechanical issues). If you want them to have the same speed, you will need to use feedback on the speeds of the output shafts (possibly via encoder) as an input to a control loop.