The word servo has a vague definition and can encompass many types of subcomponents. If you want no-power position holding using an electromagnetic motor, then there are four options:
- High viscosity grease
- Dash pot
- Very high gear ratio
- magnetic brake shoe / brake band
EDIT: There are some additional possibilities, but it's sort of an effect of some motor designs:
- Permanent magnet pole holding torque, or detent torque
- Permanent magnet drag torque
The high viscosity grease and dash pot have similar functions; providing a high static friction to resist motion, which then breaks away into movement. (A dash pot is essentially a toothed gear inside a toothed wall container, and which is permanently filled with a high viscosity fluid.)
However both of these will add to the motor load, and when they get hot they tend to become thin and runny and lose holding ability.
A high gear ratio increases the torque required to back-spin the motor. Though note, if any gearbox could have a perfectly frictionless lubricant, no amount of gear reduction could stop back-spinning.
So really this is similar to the thick grease and dash pot, but gear reduction allows just the normal thin lubrication to have enough static breakaway friction, to stop back-spinning.
A magnetic brake gives you active control over the static breakaway friction, and can allow for very high holding ability vs intermittent load spikes on the output shaft. Typically with the power off, a spring engages the brake to hold position with no power.
Note, there are two different kinds of braking: static position holding, and deceleration of a moving mechanism.
The second type both dumps heat into the brake pads and friction plate, and wears down the pads and friction plate. If the brake overheats, the pads usually disintegrate, and the brake now won't hold position.
Unless we're talking about large machinery, usually servo brakes are simple designs without replaceable parts. When the brake fails, you pull the whole thing off and replace it ... including possibly the entire servomotor and gearbox too.
For long service life with magnetic braking, you need to actively decelerate the load using the servo, and then once it has stopped moving, then engage the brake to just do the job of holding position.
Using a magnetic brake for no-power static load holding only, will look like this:
- Power up motor, get current position, and begin actively holding position.
- Energize brake to release it
- Accelerate and move to new position
- Decelerate motor and hold end position
- Remove power from brake to engage it
- Turn off power to motor
The motor is powered up and goes into position holding first, so that the load doesn't suddenly slip in either direction, in the instant between the brake being released and the servomotor then becoming active.
EDIT #1: For some permanent magnet motors, even with the power off there is a resistance to rotation because in certain positions, the magnets and the pole pieces more closely attract to each other, known as detent torque.
This pole positioning is like a low-energy valley and it takes a bit of force to overcome the magnetic attraction and move the rotor in either direction using an external torque.
You can feel this in some brushless fans and stepper motors, where if you rotate the shaft unpowered, it resists continuous motion, and there seem to be high resistance points that the rotor does not want to cross. When spun, the rotor rapidly slows, and may fall into a back-forth oscillation as it settles into one of the magnetic attractive wells.
This can be used to resist external torques without a brake, but making use if it requires stopping the rotor in one of the magnetic wells rather than a specific position you may want. But if combined with a gearbox, the exact specific stopping position may not matter too critically, and there may be a number of wells next to each other where the rotor can stop that provide an acceptable end-effector alignment.
EDIT #2: Permanent magnet drag torque is an effect of shorting all the power wires together. When this is done it takes considerably more force to rotate the shaft than with the stepper coil wires disconnected from each other.
This is because with the coil wires all connected together, when the rotor turns it acts as a generator and current flows through the coils. This current flow creates what is known as counter electromotive force (CEMF), which counters the magnetic field of the permanent magnet.
This CEMF can provide additional resistance against rotation when the servo is unpowered. Although CEMF won't prevent motion, it can help resist continued motion and provide instant deceleration when intermittent load spikes are applied that try to move the unpowered servo.
All of the above is also generally true for linear motor servos. Those don't have gear reduction, but the motion carriage can be lubed with thick grease, be equipped with a dash pot, and have position holding magnetic brakes.