Efficency - stepper motors versus conventional motors

LoopyByteloose

I am just wondering, are stepper motors less efficent than regular DC motors?

They certainly make it easy to control slower speeds without gears, so I am wondering why I don't see many in robots.

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Dgswaner

No one has answered you yet so I'll tell you what I know. DC motors reach their greatest efficiency at high RMP and are meant to be run at the top end of their speed range, where Steppers are meant for accurate positioning. So I guess it depends on you application, also it depends on how you're determining efficiency, are you taking into account all of the circuitry to get the stepper to operate? I guess in gerneral I would say Steppers are Not as efficient as "regular" motors, because your trading accuracy for speed and efficiency.

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A complex design is the sign of an inferior designer. - Jamie Hyneman, Myth Buster

LoopyByteloose

Mostly, I am thinking efficency in terms of driving a small robot on 4 AA cells. I guess if you could get a small enough but adequate stepper for each wheel and compare that to R/C servos - you would answer my question. So far, all the steppers I have are too big.

For small robots, steppers have a beauty in their precision of motion that cannot be achieve without using encoders for feedback in the R/C servos.

I do understand that regular motors need to have the momentum of higher RPMs or the brushes really waste a lot of energy in the transfer of power. But, having to gear down to 60 to 300 RPM from 1000s of RPMs has some degree of power wasted. On the other hand, the gearing down provides some position holding drag. With a stepper, you have to provide power to hold position.

I guess I should pull a couple of 'tiny steppers' from some old 5 1/4 Floppy drives and build a bot with them to find out. No need for a fancy stepper controller board.

The BS2 can directly drive a UNL2008 that will drive two steppers and I don't think it wastes more program space that PWM. It does use more pins. But one can program an SX28 in SX/B to be a dedicated sequencer for the two stepper motors [noparse][[/noparse]you don't even need a crystal, just use the internal oscillator].

In that way you can just have a BasicStamp send directional commands and have it fully devoted to navigation rather than multi-tasking.

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"Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Phil Pilgrim (PhiPi)

Because DC motors in such applications (including R/C servos) are geared down, the available torque will be much higher than for a stepper of similar size. If torque per unit volume is any measure of efficiency, then the DC gearmotor wins.

-Phil

Whelzorn

steppers are less efficient. Not only do you have to power the circuitry to drive the stepper, but for maximum torque you need to run 2 phases at the same time. Plus, steppers resonate and do not build up momentum like a DC motor. So in order to maintain the torque, the motor needs to draw the "starting current" for each step. But if you want very precise movement of a robot for a good price, steppers are the way to go. If you've got some money to spend though, go with some good geared servo motors (the type with rotary optical encoders, not the square hobby-type servos).