Arduino or similar gurus, I need your help!

[Poita (Peter)]

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.

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

Any ideas?

[Poita (Peter)]

BTW, it is easy to fit say, a hall effect sensor on the wheel, I would like to have the brake operate to keep that sensor pulse synched to an external pulse generated by another system, or if the arduino was capable of the level of accuracy, from an internal timer on the Arduino.

[tonybarry (Tony)]

Hi Peter,

Your situation is fairly easily solved. Just a bit more info needed.

The system you are trying to govern has energy applied to it by some means, and some method (you mention a brake) to control excess speed.

The controller to work the brake might be a servo, if the force levels are not too high. Alternatively you could use a throttle on the power applied - again, the details would make it easier to decide on what to use.

Servos for this purpose can be obtained from hobby shops, for high force servos use the ones for sailing winches or steering servos for RC cars. They have substantial torque.

If you can work out a way to control the speed electronically, the Arduino bit is quite easy.

Regards,
Tony Barry
WSAAG

[julianh72 (Julian)]

Poita,

Do you need to control an existing motor, or would it be OK to use a new motor? And what sort of power / torque are you looking at? (Desktop / hobby motor, or a serious piece of machinery?)

In principle, it should be easy to control with an Arduino or similar, as long as you can find a suitable electrical interface between the Arduino and the motor. There are plenty of cheap Motor Controller "shields" that will handle a couple of amps at up to 20 volts DC or so (e.g. check out http://arduinoinaustralia.com.au/col...d-type-_-motor ), but you'll need an "industrial strength" motor controller if you're talking about a larger motor or an AC motor.

Two options spring to mind:

1. If you can use a new motor, use a stepper motor with micro-stepping control on the Arduino. A bit of trivial maths would enable you to work out how many micro-steps per second are required to get the required rotation speed, and it should then run at a very steady speed. This is basically how home-built rep-rap 3D printers work, and they need very precise control and synchronisation between the 4 main drive motors (x, y and z position, and extruder feeder).

2. Alternatively, you could use feedback to control a variable speed motor. Put some sort of position coder on the motor output shaft (Hall Effect, rotary encoder, etc) to generate a pulse for every rotation, and then use a feedback loop to sped up or slow down the motor as required using PWM to vary the motor speed.

[Poita (Peter)]

Originally Posted by julianh72

Poita,

Do you need to control an existing motor, or would it be OK to use a new motor? And what sort of power / torque are you looking at? (Desktop / hobby motor, or a serious piece of machinery?)

In principle, it should be easy to control with an Arduino or similar, as long as you can find a suitable electrical interface between the Arduino and the motor. There are plenty of cheap Motor Controller "shields" that will handle a couple of amps at up to 20 volts DC or so (e.g. check out http://arduinoinaustralia.com.au/col...d-type-_-motor ), but you'll need an "industrial strength" motor controller if you're talking about a larger motor or an AC motor.

Two options spring to mind:

1. If you can use a new motor, use a stepper motor with micro-stepping control on the Arduino. A bit of trivial maths would enable you to work out how many micro-steps per second are required to get the required rotation speed, and it should then run at a very steady speed. This is basically how home-built rep-rap 3D printers work, and they need very precise control and synchronisation between the 4 main drive motors (x, y and z position, and extruder feeder).

2. Alternatively, you could use feedback to control a variable speed motor. Put some sort of position coder on the motor output shaft (Hall Effect, rotary encoder, etc) to generate a pulse for every rotation, and then use a feedback loop to sped up or slow down the motor as required using PWM to vary the motor speed.

Can't swap the motors out, it is vintage projection equipment.
The existing motors range from large 100V Induction and Universal motors, to smaller 24-30v DC motors.

For the smaller projector with the 30V DC motor, I built a PWM speed controller, but the speed still varies a bit under load. I do have Hall effect sensor on that system, so could monitor the speed, but I don't really know how to go about that.
The motors spin at 1700RPM or so, but it is translated into 24 rotations per second at the film gate, and that is where the sensor is.

For the larger projectors I cannot access the motors, they run a bit faster than 24fps, so I have a physical brake to slow them down, a servo could probably handle it. I can attach a hall effect sensor to the gate on all of the projectors to monitor the 24fps, but not the actual motor rotational speed.
They are all belt driven, so have some slip, the gate however is the important 'speed' part, and it has no slip where the sensor is attached, if I am making sense?