Robot Primer 4: Motion Technologies
Before digging into robotics, I am going to take a short detour to look at typical ways to move (motion technologies) and common ways to control (automation controllers).Â This sidetrack will help place robots into the wider automation world.
I am going to be pretty brief; for more information, start with wikipedia, internet searches, or books such as Industrial Automation: Hands On.
Pneumatic motion systems use compressed air to extend and retract cylinders. Pneumatic motion systems are easy to control (just turn the valve off or on), and are inexpensive to build. Providing a desired force is easy too: just use a regulator to control the pressure and use F=PA (force = pressure x area). On the downside, pneumatic cylinders are normally limited to two fixed positions (extend or retract), and compressed air can be expensive.
Stepper motors are brushless DC motors with rotors and stators designed so that a full rotation is divided into a number of equal steps. Since the stepper motor can move a step at a time or hold a position (assuming the motor has enough torque for the application), they can be run open loop (without feedback).
Since no feedback such as an encoder or resolver is required, stepper motors can be significantly cheaper than servo motors, they do not have servo dither (when the servo motor keep moving back and forth and never settles into position) and they are easy to control. You can buy a single IC stepper driver (from Allegro Microsystems, ST, TI, etc), connect it to power and the step motor, and get moving by giving the driver step and direction commands.
On the other hand, with standard drive technology, stepper motors quickly loose torque at higher speeds, waste energy (because the same amount of power is always used; it does not depend on the load), cannot dynamically respond to the load, and can lose position without warning.
Servo motors are brushed or brushless DC motors with feedback, typically encoder or resolver. They can be rotary or linear motors. They require a complex closed loop control algorithm (such as the classic PID method). Normally the control loop has to be tuned, and servo dither can be a problem.. Due to the added control and feedback, typically servo systems are more expensive than stepper systems.
Servo motors typically have a peak torque of 3-10x the continuous torque, their torque curve is much flatter than the stepper curve, and the maximum speeds are much higher. Peak torque is a great thing; often, a system just needs extra power for a short time to accelerate, overcome friction, or such.
Of course, there are other options such as hydraulics, proportional pneumatics, piezo motors, solenoids, and voice coils.