Controlling a manual brake on a motor.

Projects General Guidance
1

I have a little project where I need to keep a system rotating at a fixed rpm.

We are talking around 20 rotations per second, nothing too fast.

The system has a manually operated brake, and we use this currently to adjust the speed, with a camera trained on a mark on one of the wheels, the camera refresh is set at 25 frames per second, and we just sit there for hours adjusting the brake to keep the mark in place. Could also use a strobe light I guess in place of the camera.

The device is a film projector, I am using the brake to keep it at 25fps.

I'd like to automate this brake process using something like an Arduino, but not sure where to start.

Any ideas?

2

Sounds like a simple enough project, but surely, rather than operating this "brake" it would be easier to have control over the motor driving the thing in the first place? Could this be arranged?

3

KenF:
Sounds like a simple enough project, but surely, rather than operating this "brake" it would be easier to have control over the motor driving the thing in the first place? Could this be arranged?

I've had no luck with this approach, some projectors use universal motors, others use induction or DC motors, so it gets a bit tricky.

The smaller projectors sometimes have a pot to control the speed, I guess that could be manually driven, but the larger projectors run 100V AC induction motors typically.

4

dokworm:
I've had no luck with this approach, some projectors use universal motors,

Are you looking for a solution that can apply to a range of different projectors, or do you just want a solution for the single projector that you own? (Presumably you know what sort of motor is in your own projector?)

You have told us nothing about how the brake is applied?
What would an Arduno have to do to apply or release the brake?

...R

5

Robin2:
Are you looking for a solution that can apply to a range of different projectors, or do you just want a solution for the single projector that you own? (Presumably you know what sort of motor is in your own projector?)

You have told us nothing about how the brake is applied?
What would an Arduno have to do to apply or release the brake?

...R

I own 4 different projectors, Regualr8, Super8, 16mm and 35mm, and would like to control them all the same way, well at least conceptually the same way.
Is it possible to upload pictures here, I can include an image of the breaking mechanism, but a servo should work.
My main question is would the Arduino be fast enough and accurate enough to be able to do this 'on the fly'.

6

Greensprings:
if it is a projector, you could use the film as a source to sync the speed
could use a VFD to sync a large AC motor

I'm not sure what you mean about using the film as the source to sync the speed?
Also, not sure what a VFD is? Variable Fequency Drive?

7

I'd imagine that all of your projectors have DC motors. If this is the case you could simply have an n channel mosfet that you put in series with your motor to ground. This could be a simple 2 pin socket on your controller. Assuming you pick an appropriate mosfet, this would be able to cope with ANY of the motors it is likely to encounter.

If you could setup an opto slot detector in line with the sprocket holes, this could be an ideal input for your speed sensing. This could be connected via another simple socket (maybe using a screened cable). Having many pulses per frame would also make the speed monitoring much more accurate.

You would need a separate method of configuring the number of holes per frame (I daresay they vary dependent on the film format). This could be a simple multi position switch or maybe a menu option that you select through a menu system.

I'm sure this would be quite doable. Do some research on your motor voltages. I'm sure you'll find them quite usable.

8

dokworm:
Is it possible to upload pictures here,

Yes. If you use the full editor (click Reply) you can add attachments.
Please keep the pictures below 1281 pixels.

...R

9

KenF:
I'd imagine that all of your projectors have DC motors. If this is the case you could simply have an n channel mosfet that you put in series with your motor to ground. This could be a simple 2 pin socket on your controller. Assuming you pick an appropriate mosfet, this would be able to cope with ANY of the motors it is likely to encounter.

If you could setup an opto slot detector in line with the sprocket holes, this could be an ideal input for your speed sensing. This could be connected via another simple socket (maybe using a screened cable). Having many pulses per frame would also make the speed monitoring much more accurate.

You would need a separate method of configuring the number of holes per frame (I daresay they vary dependent on the film format). This could be a simple multi position switch or maybe a menu option that you select through a menu system.

I'm sure this would be quite doable. Do some research on your motor voltages. I'm sure you'll find them quite usable.

Thanks Ken,
Actually only one of the four projectors is a DC motor, the others are 100V AC motors, one is an induction motor, the other a universal motor.

The DC motor is 30V 3Amps, I replaced the projector motor controller with a PWM controller that has a speed control via a potentiometer.

The film sprockets cannot be used for the speed, as projectors drive the film intermittently. The film is pulled down into the gate, sits there for a bit less than a 24th of a second, and then is pulled down again so that the next frame is in the gate. The film does not move at a constant, linear speed, it stops and starts.

The driveshaft that operates the film pull down does rotate at a constant speed (24 rotations per second, in a perfect world) and I have fitted a hall effect sensor to the shaft, and get a nice clean 5V pulse from that.
If I could adjust the speed electronically on the DC motor using the hall effect pulses as feedback, that would solve the problem for the super8 projector. I'd be quite happy to have 2 separate solutions, one for the DC and the brake system for the AC ones.

10

Even the AC motors can be controlled. Check this out.

11

If the projectors already have a brake specifically for controlling speed then I can see the attraction of using an Arduino to control that rather than delving into the innards of the motor control system.

But until you show us a diagram or photo of the brake control lever (or whatever it is) it is impossible to give advice.

...R