Sunday, 8 July 2012

First Success: Needle Position Sensed!

In my post on the 30th of June I said: The most interesting position of the entire machine cycle we want to know about is when the needle is up and the upper thread is drawn out of the fabric as much as possible. This would be when the thread take-up lever is highest. Again, let's have a look at the arm that guides this lever:
Thread take-up lever up -- optimal position to sense (and later stop the machine in) The needle shaft is up but the thread take-up lever is down -- so even if we could sense this position, it wouldn't help us much.
As it turns out, all metal parts of the machine are connected, including the metal frame and the lever. We used a recycled piece of scrap PCB board just above the lever mounted on a bolt with a few nuts and washers to make the entire machine a simple electric switch.We soldered some of the exposed contacts of the board together to make a larger contact area.
For the firs experiment, we just connected an LED that will light up whenever the lever is "up".

A little bit of scrap PCB, a bit of soldering lead, an LED, a resistor, a power source....
The lever is down, it doesn't touch the board, and the LED is off.
The lever is up and touches the board, closes a circuit and... voilĂ  ! The LED is on!

And this is the entire set-up in motion:

Simple circuit for the very first
machine rotation sensing solution.
The machine itself constitutes the switch.
 We also connected an oscilloscope and noticed two things:
  • At maximum speed, the signals occur 75 ms apart. This means the maximum speed of the machine is 13.333 Hz (not 10 as I had estimated), so it can do 800 stitches per minute.
  • We get vibration effects which get worse at high speeds: As the lever hits the board, it bounces off a few times. This, e.g., distorted the frequency estimated by the oscilloscope and will be a problem for any electronic equipment we will want to connect. Bernd suggested using a capacitor to alleviate this problem, but I will have to learn how...

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