Then, feed that measurement into an Arduino.
Today I've taken some better pictures.
And I have had a lot of help -- thanks Bernd. Thanks everyone for their input. Thanks Unknown, for the suggestion of the light or magnetic sensor.
You said: I could put a sensor in the middle of the bed shaft.
It sounded a good idea, but when I opened the machine today, I noticed that it won't be possible, because the (rotating) bed shaft is too tighly nudged into a corner of the cast-metal frame of the machine. There is just no space around it. The flat band in the foreground is reasonably free, but it is not the shaft, it is part of the control mechanism for the feed dogs.
Thanks to Bernd who took a lot of time with me to evaluate different options.
We thought for a long time to attach a sensor that would sense the up-and-down movement of a lever somehwere in the actual vicinity of the needle movement.
Admittedly I have the feeling it's much too crowded there, and there is hardly any space to put anything in that wouldn't interfere with the existing mechanism.
But I have found out that not any old "up" position of the needle will do. The needle is high enough "up" over easily 1/4 of the whole rotation of the shaft, but over most of this, the upper needle thread is still caught somewhere within the guts of the bobbin mechanism, making a turn around the bobbin to wrap itself around the bobbin thread and create a stitch. In this position, if a seam has just been sewn, the fabric cannot be pulled out of the machine because the threads are still caught in the mechanism. That means that the motor has to stop at indeed one very particular point of the cycle, which appears to be the time when point A is highest.
Unfortunately, then, I can't see how we would be able to sense both the "up" and the "down" position of the needle -- because both are needed variously.
So far we mostly thought about two candidates for machine rotation sensing solutions:
Sensing Solution 1What Unknown suggested might actually work on the (top) arm shaft:
There is a little bit of space within the frame around this shaft.
- Fix a disk with one or more gaps in it around the arm shaft. It can probably not be slid on, but will probably have to be made in two parts and glued together around the shaft.
- Fix a light sensor to the frame that reaches around the disk and can sense the gap going past. This would probably be a photo interrupter like this or this.
- The radius of the disk would have to be small, and so, a relatively wide angle out of the whole rotation would be considered as "signal"/"up".
- The gap in these sensors is only 3 mm wide, so the disk would have to run very true, even though it has to be fixed in 2 parts around the shaft.
Sensing solution 2At the back end of the machine, where the big drive wheel picks up the rotation from the motor, there is a little bit of space that might be of interest:
- Attach some reflective material to the smaller drive wheel, best at the spot which is at the top when the needle is in the optimum "up" position. It is of advantage that the wheel is originally black.
- Fix a bracket to the frame above the top and attach a light sensor like this that senses reflected light.
- A second bracket with sensor could be fixed underneath the wheel and sense when the needle is in the "down" position.
Motor ControlDifferent to what I wrote before, the machine easily does 10 stitches per second, so 600 per minute, which would mean the motor then probably turns 8 times as fast, at 4500-4800 per minute.
I was told that it would be difficult to tell a conventional motor to stop that precisely at a certain point. I'll take my chances and try...
We had a look the foot control of the machine and found it contains a potentiometer that seems to range between 0 and 100 kΩ.
Thanks very very much to Bernd for all his help, assessments, and suggestions.