Kenmore 158: Needle Position Sensing

Fancy new sewing machines can stop with the needle either up (so you can remove the fabric) or down (to nail it in place while you rotate it). This requires sensing the needle position, which prompted me to spend far too long contemplating all the mechanical gadgetry driven by the motor.

As nearly as I can tell, the crank counterweight behind the handwheel produces the most unambiguous position reports. Here’s what it looks like with the needle down:

Kenmore 158 - main shaft counterweight
Kenmore 158 – main shaft counterweight

As you’d expect, with the shaft rotated exactly 180° from that point, the needle is up.

The inviting space just above the shaft provides room for the bobbin winder that engages a knurled ring on the back of the handwheel, but the lower space seems to be available. The counterweight sits about halfway into the back of the handwheel, so the sensors must look at the frame side of the counterweight.

Two adjacent sensors could detect the edge of the counterweight, which would be enough to uniquely identify both positions. If they were spaced across the lower-left edge in that picture:

  • 01 = trailing edge = bottom dead center = needle down (as shown)
  • 00 = open air = needle rising
  • 10 = leading edge = top dead center = needle up
  • 11 = solid steel = needle falling

Either sensor gives you one pulse per handwheel revolution and the combination gives you a quadrature output of both position and direction. The top speed of 1000 RPM produces 17 Hz square waves.

An additional pulse/rev sensor on the motor shaft would give better control over the motor speed, as the handwheel runs at 1/10 the motor speed with belt slip built right in. Figure 10 kRPM → 170 Hz pulses.

From a cold start, you know the shaft angle to within a bit under 180°. If the motor can turn in both directions (as would a stepper or DC motor), you can always move the needle upward. If it turns only forward (as does the AC motor) and the needle is falling, then you probably don’t want to move the motor until you get a button push indicating that all fingers are clear.

A pair of Hall effect sensors might suffice to detect that big hunk of steel, perhaps with a pair of teeny magnets glued to the face or a magnetic circuit closed by the counterweight.

More pondering is in order.