Robot 'carter' can't walk in a straight line

I made a robot car based on “carter”, but I recently encountered a problem when using it. Robots don’t walk in straight lines.

Later, I used the official “carter_v1”, added it to the scene “full_warehouse.usd”, and added drive to “Drive>>Angular>>Target Velocity” of “carter_v1” (the speed of the two wheels is the same), and found the same problem. The route the robot takes is not a straight line. And the faster the speed, the more serious the slope of the path. If you follow a straight line, the Y-axis should have a constant value, but as time increases, the Y-axis value also increases.

I have tried many methods, including adding drive to the rear wheel and increasing resistance, but all have the same result, it still goes diagonally.

Hope to get some help to solve this problem, thank you very much.