Why Are Standard Servo Motors So Different?
Recently, I had to select a servo motor.Â We had already chosen to use a NEMA 23 mount with a 0.25″ shaft, and had other requirements such as maximum length, torque, speed, and voltage.
I was amazed at the different shaft diameters and lengths supposedly standard NEMA 23 motors have — I recall 0.25″, 8mm, and 0.375″ diameters, and can’t remember all the lengths.Â It was even worse when I had to select a NEMA 17 motor a while ago– at least one manufacturer’s supposedly NEMA 17 mount wasn’t the same as everyone else.
Then of course there are all the non-standard mount motors – but I only use those as a last resort, since I’ve already had motors go out of production twice; at least with a standard mount, there’s a possibility of finding a suitable replacement.
Maybe there are other standards: maybe the various European and Japanese 40mm, 60mm, and larger motors really do follow a standard, but I don’t have time to analyze them all.
Connectors are even worse.Â I can understand why manufacturers use different kinds of connectors, and I know there are no connector standards.Â A motor intended for harsh environments needs a tough but expensive connector, while a light duty motor is much better off with a cheap connector (such as Molex or Tyco/AMP crimp connectors) or flying leads.Â But, just like industrial Ethernet, it’s easy to think: do we really need all those choices?Â Couldn’t we have just three or four?
I personally like sub-D connectors a lot, since you can get high power (Combo-D), can use crimp pins or solder cups, and choose from a wide range of backshells (including straight, right angle, and 45 degree in metal, metalized plastic, and plastic) and manufacturers.
Of course, pin-outs are even more varied.Â For example, on the controller side, Galil (DMC-21×3/AMP205x0 combo), AMC (DX30, DX60, etc), and Copley (ADP series) all use HD15 connectors for feedback, but each one uses a different pin-out.
Oh, well, at least commutation and incremental encoder signals are pretty standard: three signals, either RS-422 differential or single ended (TTL or open collector) –Â except if you use some Japanese motors (e.g. Panasonic).Â And I’d better leave absolute encoders for another time…I’m not even sure how many “standards” there are for them.