I'm building a motor controller for a movie film camera motor. It needs to run at 1440 rpm %.001 and variable from 480 to 1920 rpm. The motor now is 7.5V DC rheostat controlled. I'll be adding an encoder to it but I'm unsure how to control it. (Maybe a PID circuit?)
So any help and info would be great.

Thanks,
Matt

 

The biggest single factor affecting the success of your control loop will be the speed and resolution of your speed feedback. Use a high-resolution incremental encoder and sample the speed as often as possible (perhaps once per revolution?). If you wait too long to get a speed reading, then it will be too late to make speed corrections and you will not be able to achieve the ultimate is tight speed control.

-Robert Scott
Real-Time Specialties
Embedded Systems Consulting

 

A stepper motor would work well. You would control it by frequency.

Bill

 

Matt:
The most important thing is to think of this as a positioning application, incrementing commanded position at the appropriate rate, and trying to drive the position error (the difference between this and the counted position from the encoder) to zero. If you do this, you will have no long-term steady-state error other than that of the clock crystal frequency you are ultimately dependent on (at 0.001% you are asking for 10 parts per million, which requires a good crystal).

Velocity accuracy specs are notoriously squirrelly things. What exactly do you mean by 0.001% accuracy? Long-term steady-state accuracy is one thing, but what about short-term variations? By the way, you will never get 0.001% short term. You'll never even be able to measure that fine. If you have short-term specs, my advice here is to get them turned into position error specs, which is probably what people really care about anyway. I've seen whole projects founder on exactly was meant by a velocity accuracy spec, how it was to be measured, what the reference was, etc.

 

I'm also really interested in this. I've sent a million questions to a million people, but no one has been able to point me to any consultant or firm that can help me design a PWM circuit that can speed control a brushless DC motor at some different speeds from off the shelf circuits. I'm also doing it for a motion picture camera. I mean, every CD player in the world has a speed controlled DC motor, but for some reason no one seems to know anything about this. It can't be that hard. Any tips?

 

Matt,

Interesting problem. I have no experience with movie camera but here's some comments for what it's worth:
1. The film is coiled up in a reel and a 2nd reel takes up the film as it unwinds. As the reel unwinds the film radius shrinks on one reel and increases on the other, so it seems that one doesn't want constant shaft speed (rotational) if one wants to maintain a linear speed. Similar problem with cassette tapes and video tapes. May be the reason for old movies pictures jump around? It seems to me that linear or translational speed is what you'd want to maintain. Somehow, you'd need to detect this.
2. Assuming you'd use two motors, one to coil in forward and one for reverse. You could control one in speed mode and the other in torque mode, this way you can maintain tension and speed. Or you let the other one coast but make sure that you clamp the bemf if it is going to "pump up" your dc link and exceed your component ratings.
3. PID algorithm is proven and fairly easy to implement. A lot is written on the subject.
4. Brushless or dc servo motor can do the job.
oj

 

Matt

Take a look a Honeywell 14 track tape deck, they solved this problem a long time ago.

Matt Hyatt

 

On October 5, 2003, oj wrote:
> Interesting problem. I have no experience with movie camera
> but here's some comments for what it's worth:
>
> 1. The film is coiled up in a reel and a 2nd reel takes up
> the film as it unwinds. As the reel unwinds the film radius
> shrinks on one reel and increases on the other, so it seems
> that one doesn't want constant shaft speed (rotational) if
> one wants to maintain a linear speed. Similar problem with
> cassette tapes and video tapes. May be the reason for old
> movies pictures jump around? It seems to me that linear or
> translational speed is what you'd want to maintain. Somehow,
> you'd need to detect this. <

The problem of the two different reel speeds are solved by the capstan, which regulates the speed of the media across the heads. The old movies used to "jump around" because in the projector because the frame is required to stop momentarily so the shutter can open and illuminate the screen, and occasionally the film gets off the sprockets because the tension isn't right.

> 2. Assuming you'd use two motors, one to coil in forward and
> one for reverse. You could control one in speed mode and the
> other in torque mode, this way you can maintain tension and
> speed. Or you let the other one coast but make sure that you
> clamp the bemf if it is going to "pump up" your dc link and
> exceed your component ratings. <

On old tape transport mechanisms and most movie projectors the two reels both use torque control. The supply reel has a very low torque in reverse, which the media feed mechanism - the capstan I mentioned above - overcomes, and the take up reel runs in the forward direction to maintain the tension. The capstan motor - i.e. a *THIRD* motor actually controls the media speed. I don't have any reason to believe that a camera doesn't operate the same way.

(On cheap system the "torque control" was actually a clutch plate which can slip.)

I'd strongly suggest you take apart an old tape deck and an old movie projector to get you ideas. These problems were solved 60 years ago. But you regulate your speed by the capstan, not the reels, or you'll run into all the same problems they did way back then.

> 3. PID algorithm is proven and fairly easy to implement. A
> lot is written on the subject. <

Unless you're expecting some disturbances in the system that you need a control mechanism to correct for, why not just map speeds to output in a table or mathematical function and let it go at that. PID will always have the slight overshoot to settle down.

> 4. Brushless or dc servo motor can do the job. <

Lot's of examples in old tape decks on this. DC Servos were very popular.

--
Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/

 

professional 35 mm film cameras use a little lamp and a photodiode to get a sample of the shutter speed as the transport mechanism moves the film. this is the speed you need to regulate. Do it by means of a PLL. one input is the desired frequency the other input to the phase comparator is the pulse signal from the photodiode, the error signal drives a power amplifier for the motor. Take up reel motor has a clutch and follows the film speed. inside the film cartridge there is a tensioning lever that also acts as a sensor for the amount of film left.

good luck!!
h.m.

 

The Cinema Products CP-l6 And CPR-l6 sound cameras have an excellent crystal control system and they published a complete schematic in their service manuals. The company is out of business, but Whitehouseav.com might send you a copy of the schematic. The CP16 is the older model and shows discreet parts.Also http:www. tobincinemasystems.com modifies camera motors. Been there and done it. Don't you try to reinvent the wheel on this project. Too many headaches!

 

You guys are complicating things. All I want is a precision regulation of a DC motor. That's it.

A film camera is driven through its movement. That speed is constant - you do not need to drive the film cores at fluctuating speeds. They're easily driven by slip clutch and unregulated DC motors.

I know Tobin Cinema Systems makes good replacement motors for old film cameras that are speed controlled, but I'm developing a new product and can't pay his prices - this must be solved through some consultant.

Is there anyone who can recommend a eletctronic consultant or design firm that could design a precision speed control for a brushless DC motor?

Please email me if you have any tips or info.

Thanks,
Adam
[email protected]

 

Hi, I finally have time to work on this project again. I going to base it on this: “http://www.interq.or.jp/japan/se-inoue/e_pic6_9.htm” which uses a PIC 16F873. I'll add an encoder to the motor, and need to modify the code. But it looks like a good route to go to control a movie camera.

 

Sorry for the bad link, This one should work http://www.interq.or.jp/japan/se-inoue/e_pic6_9.htm

 

Animatics make a credit card sized DC or brushless motor driver of 24V 3A that can be stand-alone programmed to do what you want. Could read a thumbwheel (or pot) for speeds, light an LED for on or off-speed, has PID tuning. Or you could just buy one of their motor/drive combos. Or use an IMS MDrive stepper/driver combo. Maybe too noisy?