Your right that you will/could learn a lot designing and building your own servo. One can of course just need a functional model servo for a project and get them really cheap these days, $10 and up, maybe less on E-bay. As far as the motor, a simple DC motor will work, rated to the voltage you have available to drive it. You will need bi-directional control so some kind of H-drive circuit is called for. There is a need for position feedback to the controlling electronics. This is usually pot mechanically attached to the gear train to measure actual position of the servo output. The controlling electronics takes the feedback measurement and the desired position command and decides if there is an error between those two measurements and decides what direction and how hard to drive the H-drive motor driver circuit. A common control method is to utilize A PID algorithm.
This can all be quite complex for a first time project or for a beginner. And of course you will be spending tons more for the parts and will never make it as small and compact as a store bought servo. However the reward of learning how a servo actual works and getting your own design to function can be worth a lot.
Technically, I think “servo” refers to the presence of a position feedback mechanism, rather than a particular type of motor. MOST servos of the sort used in hobbyist radio control things use ordinary DC motors (thus the “simplicity” of conversions to continuous rotation), but some proudly proclaim their use of “coreless” motors. Large industrial servo motors are likely to have custom-manufactured motors carefully tuned to the needs at hand (an awful lot of industrial motors seem to be near-custom, whether used in servos or otherwise.) It would be possible (and an interesting exercise) to make a servo using a stepper motor. Microservos use motors that look very much like pager motors.
Usually the hard part of a servo is the gear-train. the electronics is conceptually simple, and the motor choice is flexible. (and there have been some interesting results in micro-realm by getting creative with the gears and mechanical construction.)
An electric motor generally uses electromagnetism to create motion. If current goes through a wire, it creates a magnetic force. If a wire moves through a magnetic field, an electrical current is produced in the wire. Both of these physics concepts are leveraged (pun intended) to get the axle moving. What you’ll find inside a motor are a number of windings of fine wire around a metal slug, and some magnets arrayed around the case. Some motors are “inside out” with the windings on the case and the magnets in the middle. The rest of the motor is just the use of ball bearings or pin bearings to allow the axle to turn, and some form of contacts to allow current to be applied to each winding or to detect when current is generated from the motion.
Well the newest thing I’ve come across in recent years is the wide availability and reliability of brush-less DC motors used in the radio control aircraft industry. By utilizing now inexpensive driving electronics using efficient MOSFET switching transistors they can get very impressive performance these days.
So the combination of brush-less speed controllers (work like servo control using PPM command) coupled with newer rare earth magnets in the motors and Li-poly batteries has really advanced the state of the art in electric motor flight that can rival gas powered IC engines.
I’m sure it could be utilized in robot design, however there is a price premium paid for the higher power/total flight weight performance not necessarily needed for robots.
Retro whats the torque & load compared to those muscle memory actuators? or Pneumatic hydraulic servos?
Are the brushless dc motors compatible with an arduino? or do they need a special interface etc ?
Muscle memory actuators are not real powerful, but I guess you could parallel a lot of them and increase pulling power. I’ve only read of them mostly, although they were used as meter pointer movers in process controllers I have seen in a refinery.
Now Pneumatic actuators are the bad boys for actuator force, they can handle any load you can imagine. You just multiply the maximum air pressure you have available times the square inch area of the piston it’s pushing against to get the force available. A 3" piston with 15 psi available will generate over 100 pounds of force.
And of course electric motors can come in any size required, limited only by your cubic dollars available. The speed controllers used with brushless motor are controlled just as if they were a servo, so an Arduino can drive them using the servo library.