I am using Blender 2.79 on Linux. I've been able to create a rigid body with two motors (angular) turning some gears (the motor velocities interact at a differential). This works as expected.
The problem is I want to change the motor speeds depending on frame. I don't see a way to make them animated. If I change the target velocity it changes for all frames, but I'd like to smoothly change velocity based on frame. How does one animate the speed of a rigid body angular motor? What kind of key frame do I need?