Motor types

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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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

List of electromagnetic motor part types.

AC[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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

Stepping motor[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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