Motor types

Typical servo motor with encoder feedback

Position, velocity and torque/force control applications mainly rely on two main categories of motors: servo motors and stepping motors.

Servo motor

Electromechanical actuator with feedback allowing precision closed loop motion control and monitoring. Servo motor also enables precise torque limitation to gain safety.

Stepping motor

A low cost alternative to servo motors and can be operated without feedback devices.

Construction

Servo motor consists two main parts:

• Motor part - typically electromagnetic device that produces torque or force when driven with current
• Feedback device - typically electronic device that outputs measurement information such as shaft angle or velocity

Stepping motors can be used without any feedback device.

Shapes

Motors and feedback devices come mainly in two shapes:

• Rotary
• Linear

Shape of motor doesn't change their electromechanical principle so both types of motors can be driven with same drives.

Motor technologies

List of electromagnetic motor part types.

AC

Typical AC (alternating current) servo motor is a 3 phase permanent magnet syncronous machine. This type of motors are driven by 3 wires each driving one phase coil. In ideal case 3 phase AC servo is driven by sinusoidal current waveforms that are synchronized to the permanent magnet rotor.

BLDC

BLDC (brushless DC) is very similar to AC motor with only expection that it is desgined to be driven by trapezoidal current waveforms instead of sinusoidal. BLDC motors can always be driven with same drives regardless of current waveform matching.

Brush DC

Brush DC (direct current) motors typically have permanent magnet stator and rotor with mechanical commutator with brushes.

Stepping motor

Stepping motors are high pole count permanent magnet brushless motors with 2 or 3 phase windings. High pole count enables position control without feedback devices but with several drawbacks such as risk of lost position and possible resonance issues.

Comparison

Comparison of motor technologies

	Stepping motor	DC servo	BLDC servo	AC servo
Closed loop	No	Yes	Yes	Yes
Torque control	Yes with position feedback device	Yes	Yes	Yes
Velocity control	Yes	Yes	Yes	Yes
Position control	Yes	Yes	Yes	Yes
Available speed range	Low, Medium	Medium	High	Highest
Rated torque vs size	High	Low	Medium	Medium
Dynamic performance	Low, medium	Medium	High	High
Motion smoothness	Low to medium	Medium	Medium	High
Endurance	High	Medium	High	High
Energy efficiency	Low, medium	Medium	High	High
Safety concerns	No safe torque limit, loss of position, hot surface	Runaway if feedback lost

See also

• Feedback devices
• Resolution of motors

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