I'm tossing around the idea of building a dolly rig from surplus parts. I think I have a workable concept, but there is an open question in my mind that troubles me: what's to keep the motor from spinning backwards under the weight of the dolly and camera when it's not energized? Does such an application require use of a stepper motor or servo motor? Or can a gearmotor work?

Well, when a motor is not energized, it has no natural braking capability of its own (this includes steppers, DC motors, and servos).

To "brake" a motor, you either have to provide power to it, use an external brake, use such a high gear ratio that the load cannot spin the motor (1), or use a gear train with a natural "braking tendancy" such as very highly reduced worm gearing (2).

(1) - remember that if a motor is geared 100:1, the rotational force applied to the output shaft has the inverse ratio - that is, while the force of the motor shaft is increased by 100x at the output shaft, the force applied from the output shaft to the motor shaft is reduce by 100x

(2) - at very high reduction ratios (> 100:1), worm gearing exhibits nearly infinite friction when the gear is driven instead of the wheel, due to the design and mesh of the two surfaces.


A servo is really no different than a DC motor in this sense, as all a servo is, is a DC motor with feedback and a closed-loop control circuit.

!c

Thanks. OK, so now I'm wondering... what the heck keeps the DP dolly from slipping backwards in move-shoot-move mode when it's vertical?

Gear ratio = 524:1 =)

Additionally: the L293B goes into "brake" mode when not being driven, so there is some power consumed - just not nearly as much.

!c

Thanks once again. No surprise, this, but... one question answered leads to more questions, if I may...

The gear ratio you stated implies use of the Dayton 2L007 (8.8 RPM)...
The Stage Zero video mentions a top speed of 30"/minute, which (assuming the pulley is ~2" diameter) implies use of the 2L006 (4.5 RPM)...
Yet, Milapse mentions here on the forum that he's using (or has used) the 2L004 (1.5 RPM)...

This wide range of possible motor speeds seems to indicate that any of them will work, depending on one's needs. Is this so?

Also... isn't the L293B for driving steppers? Is something needed beyond a simple PWM controller?

You got the pulley size wrong - and the listed 8.8 RPM is incorrect on some sites, it's 8.75 RPM on the badges of the motors we use. The pitch circumference of the pulley is 3.53", which makes the diameter ~ 1.12". (And, the actual top speed is 30.89"/min =)

You can use any motor up to about 1A, or use a higher rated motor by ensuring that you never reach full load capacity. (As Jay mentions using the 25RPM Dayton motor.)

The L293B is a 4-channel push-pull driver, it can be used to drive steppers with a different circuit, but in our design it is used to drive two DC motors with directional control and PWM input.

!c

Thanks, this is very helpful.