Basic theory on servo and/or step motor.

You should visit a guru www.galilmc.com

I'm going to say that a well-executed servomotor system will provide more power, smoother, quieter and higher performance motion control than a stepper of the same size/weight. Not to say there aren't appropriate places for a stepper system. Quantity would drive part of the decision.

Gearing is an interesting question; its a key, key, read:important piece of this system since it puts a lot of potentially painful variables between your actuator (the motor) and the load (the robot arm). I could well imagine different types of gearing in the system; some kind of planetary, relatively high ratio box on the motor driving a rack&pinion on the arm.... Feedback sensors on the motor and the load, 2 control loops, etc.

Harmonic gears might work, but they're very high ratio and you haven't given any information as to the (multiple) definition(s) of "high" in your original query. There is a pile of information that needs to be filled in, in order that a sensible system could be designed and executed.

Hugh

Try searching at google .. Servo Motors in pdf

The use of word pdf will provide you tons of info and can also help you in future research

Jhun

http://www.electro-craft.com/default.asp

Ah . . . Finally found a source for a book put out by Electro-Craft Corp. several years ago: http://www.directtextbook.com/publisher/electro-craft This is one of the best texts on motors and servo systems I have ever run across.

Galil, which Hendrik mentioned in his post, is also a good source of information, and it's on-line.