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When hardware hackers mess with industrial sewing machine, Ladyada writes -

A few years ago, after my plastic portable machine broke, I bought a Singer 20u73, light industrial sewing machine. I was pretty tired of slow, weak machines with low torque. Going through 4 layers of fabric was a struggle so of course I went all out and got this nice machine. Unlike little portables which turn on the DC motor when the foot-lever is pressed, industrials use a clutch motor. The motor is running the moment you turn it on, and when the foot-lever is pressed, it moves the clutch towards the motor, engaging it. This means higher torque when you turn it on, thus getting rid of the frustrating "have to help the machine along" stuff necessary with small sewing machines. Great. only problem is that the motor runs at a perky 1750 RPM and the clutch is very sticky. This means it requires some practice to get used to the foot pedal: instead of being linearly related to stitch speed, its much more 'exponential'. Seems like either nothing is happening or its going at a ridiculous 2000-2500 stitches a minute -- much too fast for someone out of practice.

The solution? Basically everyone says "you'll get used to it with lots of practice." Which is another way of saying "this design really sucks". (There's DC servo motors that have no clutch because they can give high instantaneous torque but I'm not 100% sure they solve the speed-control issues)