Motor types

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

Servo motor is a electromechanical mechanical actuator with feedback allowing precision closed loop motion control and monitoring.

Stepping motor is a low cost alternative to servo motors and can be operated without feedback device.

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

Stepper 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.

Feedback device types

Encoder

Quadrature encoders are nowadays the most common feedback device type in servo motors. Quadrature encoders are incremental sensors so they require position zeroing/homing to get absolute position feedback.

Another type of encoder is absolute encoder. Absolute encoders typically output serial data of absolute position thus they may not need to be zeroed. The drawback of absolute sensors is higher price and lower compatibility & interchangeability.

Resolver/synchro

Resolver is an analog technology based on rotary transformer that can provide absolute position (single turn absolute, for multiturn absolute position zeroing is still needed). Resolvers are good for harsh conditions but don't provide as high precision as encoders.

Tachometer

Tachometer is a small DC generator that outputs DC voltage proportional to rotation speed. It can be used as velocity feedback device but not as position sensor. Tachometers are often seen in dual-loop configurations with position sensor.

Hall sensors

Hall sensors are found in AC & BLDC motors only. Halls provide commutation information (drive current phase angle) for drive. Modern drives such as all GD drives don't require Hall sensors but can utilize them to make faster power-up possible. Hall sensors are too low resolution for position or high performance velocity control.

Comparison

Comparison of motor technologies

	Stepping motor	DC servo	BLDC servo	AC servo
Closed loop	No	Yes	Yes	Yes
Torque control	No, inpractical	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	Highest
Motion smoothness	Low to high	Medium	Medium	High
Endurance	High	Medium	High	High
Energy efficiency	Low, medium	Medium	High	High

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