What can be the problems of using a stepping motor to drive your movie camera?

We considered and rejected the use of stepping motors some years ago. Here's why.

Stepping motors by definition turn with small but violent steps, instead of turning smoothly. This is easily detected by your ear as a buzzing noise. At most speeds, the stepping motor actually stops dead in its tracks many times per revolution. This may not be a problem when geared down and moving a cutting tool in machinery, for their original application back in the 1960s, but for a movie camera where the final speed is the same as the motor, or actually geared up, it has the following drawbacks:

1. Constant flutter in the driving speed can be hard on the camera's gears. This has been demonstrated in the Arri 16-BL cameras when using the old BL governor / reversing motor. Although it is not a stepping motor, it is similar in that it is constantly alternating between over and under speed at a high rate. It is known among repairmen that a 16-BL that has been used for a long time with the governor motor is no longer able to take flickerless footage even when using a new type motor, owing to wear on high inertia items, primarily on the mirror shutter gear. This is an expensive repair. Driving the stepping motor with a sine wave signal will reduce the drive flutter, but it is doubtful that it can be eliminated.

2. The vibration from the stepping motor can shake the camera and cause unsharp or unsteady pictures.

3. A stepping motor is an open-loop device instead of a closed loop servo. That is, the control electronics has no way of knowing if the stepping motor is actually doing what it is told to do. There is no simple means of providing an out of sync warning light. If the load is higher than anticipated, or the battery voltage is low, the stepping motor rotor will just give up, and sit there and quiver instead of rotating as expected. In TCS crystal motors on the other hand, the speed is actually checked dozens of times per film frame as part of the servo feedback, and the Sync Alarm lights up if there is a discrepancy.

Apart from this, usually a stepping motor is less efficient and draws more current than a conventional motor. This gives reduced filming time on a charge of the battery. The efficiency is further reduced if driving the motor with a sine wave signal.

Our smoothly turning Tobin model TTL timelapse and animation motor for Bolex can be seen here.

Our smoothly turning crystal motor for Bolex can be seen here.