Thanks zk....

If anyone's interested here's why I want to control the speed. I set myself a rather academic exercise, to move a laser pointer along a wall at constant linear speed. Of course with constant angular speed, the linear speed increases at the sides. The incremental angle dtheta subtends a smaller distance along the wall when it's normal than it does at the sides.

If my calculus is right, for constant linear speed I get that

dtheta/dt = k cos^{2}thetaWith theta = 0 (beam normal to wall), cos theta is 1 and dtheta/dt is maxed, and it decreases towards the side.

(Even if the maths* is wrong I know qualitatively that the angular speed needs to be maxed when the beam is normal to the wall and decreases towards the sides

)

* or math singular depending on where you live...