Mod-8 Film - Getting to know stepper motors.

This blog is active again after nearly 4 years. I have been focussed on the creative side of film-making. But a technical challenge appears and here goes with getting into microprocessors, robotic tech, and a new one for me: stepper motors.

Mod-8 is a project to modify and modernise Super-8 film and cameras:
Mod-8 Document One

Starting with: investigating stepper motors for film transport in cameras and digitisers. In previous robotics projects I was into servos for movement. The stepper motor is new to me. However the A4988 controller looks like a good helper so I have got to know that. I found helpful information in "How to Drive Stepper Motors with the A4988" a Youtube video from "Christopher's Factory":
https://www.youtube.com/watch?v=jsKug7eEzK8&t=49s

The sales websites describe having separate power supplies for control circuits (5V) and motor (8V or more). I had a 5V motor, VITECH 20BY, so I simply use one 5V power supply for everything and that is working just fine. 

For replacing motors in movie cameras, there are needs for speed that are pushing stepper motors to their limits. In the camera I am working on, Sankyo 620, it will need about 1500 rpm for the shutter motor and a step advance of 4.2mm in 0.02 seconds for the film drive motor.
The VITEC 20BY is a candidate shutter motor. How fast can it do continuous run? Answer: reliable up to 1800 rpm. On my test setup it stops rotating and buzzes unhappily at 2280 rpm. This means that it can work as a solution for the Sankyo 620 running at 18 frames per second, but not 24 frames per second. It happens to be easy to fit in as a replacement for the existing dead motor although it will need a change of gear wheel to get it down to about 1500 rpm. It delivers on precise speed control. It is however not giving me what I want which is a general replacement for many Super 8 movie cameras. That motor would need precise speed control over a range of 500 to 4000 rpm to fit the range of common camera speeds and camera to shutter gear ratios. The evaluation candidates are:

• Other stepper motors. I am waiting on delivery of a "Nema 8".
• DC motors with a speed measuring device attached. e.g "sensor module with photoelectric encoders".
• BLDC motors. These are used in drones. The info I am reading so far is all about good high power performance. It is unclear to me how well they can for control at lower speeds and power levels. I have ordered one.

Elsewhere in the camera, I propose a stepper motor for intermittent film transport.

My first idea for this, based on the Nema 11 specs:

Command the Nema 11 to move 54 steps. 
Use its 5mm diameter drive shaft as the transport capstan.

Can do by programming an “Arduino Nano” microprocessor.
Nema 11 moves 1.8 degrees per step.

54 x 1.8 = 97.2 degrees

Distance = (97.2/360) * 5* pi

= 4.24 mm

Doing some stepper motor practical work gives a reality check.

To move 54 steps in about 0.02 second, needs each step to happen in 400 microseconds

My measurement of the VITECH 20BY is that its fastest reliable step time is 1500 microseconds.

Can the Nema 11 do better? I am still waiting for delivery to find out.

In the meantime, the most helpful info I have found is “Christopher’s Factory”

https://www.youtube.com/watch?v=jsKug7eEzK8&t=49s

Christopher tests a Nema 17 with an A4988 driver board.

Christopher gives 350 microseconds as the fastest reliable high pulse. 

That suggests a fastest reliable step time of 700 microseconds which is not good enough.

Possible solution: a sleeve on the shaft to increase the diameter from 5mm to 10mm.

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Test setup