Heading as a function of wheel rotations

[fishingspree2]

Hello, I have a three robots which consists of two motorized wheels and a skid wheel. The two motorized wheels are on the front of the robot. I am trying to find the change in heading of the robot based on the previous state.

I have found out that ΔHeading = [(ΔD_L-ΔD_R)*radius of wheels]/wheelbase. where ΔD is the difference in wheel rotations between t_i and t_i-1. I have a feeling that this is not correct, since the axis of rotation changes depending on the angular velocity of each wheel. For example, if one of the wheels is at rest while the other rotates, then the axis of rotation of the robot is the wheel at rest. If the two wheels are rotating in opposite direction, then the axis of rotation of the robot is halfway between the two wheels. The formula I have derived does not take this into account.

Can anyone point me in the right direction?
Thank you

 

[Baluncore]

Can anyone point me in the right direction?

The effect you are concerned about is not a problem because it actually cancels.

When the radius of the turn is in the midway between the wheels, one wheel moves forward half the distance, while the other moves backwards half the distance. The difference is still the true heading.