closed loop DC motor speed control

piclt

Member
You need to try the motor out with the speed control first. If it does not work then the sync does not matter because you have nothing to connect it to...?? You had no sync control on your old ac motor
 

AllyCat

Senior Member
Hi,
Why do you need it..???....
Because, as I estimated in #24, the desired Open Loop speed accuracy could be around 10 ppm or 0.001%. :( It's necessary to maintain the phase of the camera shutter opening at the same time as the frame is stable (and illuminated) in the film gate, over a sequence of (say) 50 * 60 * 20 = 60,000 frames (i.e. 20 minutes).

Cheers, Alan.
 

inglewoodpete

Senior Member
I'm not sure why a stepper motor has not been mentioned in this thread yet. Surely with just a PICAXE at the helm, it would be easy to synchronise the motor speed with just about anything.
 
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piclt

Member
Yes, I know a stepper motor would be better, but a DC motor AND Tacho generator had already been bought and fitted and even at this late stage of 43 posts later #43 the DC motor has not even been tested to see will it drive the projector at any sort of stable speed from the Laptop PSU with mosfet/picaxe acting even as a simple on/off switch to start with. The speed of a DC motor will vary wildly due to variations of load and supply voltage and even with closed loop speed control using the tacho feedback if the system is not stable the Picaxe will continuously be chasing its tail. Which is why we dont really need the sync pulses at this stage until we see that it works at a stable speed, the sync pulse will only be a nudge to keep the film in the right place at the right time.

And back at #5 papaof2 suggested using the original ac motor with an inverter. The original problem was only that it drifted out of sync after long periods of use, but at least the motor drove the system with reasonable stability which could have been tweaked.

Was the original ac motor a series motor or a synchronous/shaded pole motor...??
 

rq3

Senior Member
I'm not sure why a stepper motor has not been mentioned in this thread yet. Surely with just a PICAXE at the helm, it would be easy to synchronise the motor speed with just about anything.
This is a superlative idea! And should cost about $20 to implement. The projector mechanism should take the exact same number of motor steps to get from frame to frame, at which point the picaxe would output a pulse to trigger the video camera. Heck, you could probably crank the thing up so that it was dubbing film at 100 frames per second, or whatever the projector could physically tolerate.
 

AllyCat

Senior Member
Hi,

In principle, there's little difference between a Stepper and a Synchronous motor, but both generally give much lower (maximum) torque and need at least one "H-bridge" (bi-directional) "High Power" driver, compared with a simple, single PWM/FET driver to a dc motor, which the OP has already planned to use, but apparently not implemented yet?

IMHO the issue is not driving the motor, but synchronising the movement of the film in the gate to each instant when the camera "shutter" closes. The OP insists that he cannot obtain this from the camera, so an (inferior) alternative is to assume that it occurs exactly every 20 ms, and to derive a "sync" pulse from a crystal oscillator, or from the 50Hz mains for an initial test. Note that "flywheel" sync is basically just an early version of PLL technology, which is now found at the heart of almost all modern electronic devices (including the PICaxe ! ).

Alternatively, I see that the camera has a "Time Lapse Capture" mode, but it's not clear from the manual if that can use an external trigger (e.g. each time the film is stationary in the gate), or if it uses only an (asynchronous) internal LF pulse generator.

Cheers, Alan.
 
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