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AC Motors

High power, single- or multi-phase, constant torque and speed, large loads

DC Motors

Normal DC motors almost linear characteristics:

• rotation speed determined by applied DC voltage
• output torque determined by current

Speed: few rpm (revolutions per minute) to many thousands rpm

Brushed Motor

Cheap, small, easy to control

Brushless Motors

Use Hall effect switches for stator field rotation sequence, smaller, more efficient, better torque/speed characteristics, more expensive than brushed motors

Servo Motors

Brushed motor with positional feedback control, can be controlled by PWM.

A DC (RC) Servo Motor consists of a DC motor, gearbox, position feedback device and error correction (closed loop control).

Stepper Motor

Accurate positioning, fast response to starting, stoping and reversing.

Low-Level Control

A motor usually doesn’t produce the exact force or torque requested. So different approaches are used to control the motors at low-level.

Current Control of Direct-Drive Motor

• Motor with no gearhead
• Controlled by an amplifier
• Torque: $\tau = k_t I$
  • Torque is (approximately) proportional to the current through motor
• Current sensor used to measure actual current
• Local feedback runs at higher rate than high-level control loop
  • Typical example:
    • $10 kHz$ for low-level control
    • $1 kHz$ for high-level control
• Cons:
  • Ungeared motors are large for sufficient torque

Current Control of Geared Motor

• Similar to Current Control of Direct-Drive Motor
• Motor with gearhead increase available torque (gear ratio $G \gt 1$)
• Pros:
  • Smaller motors for necessary torque
  • Motor operates at higher speed: more efficient
• Cons:
  • Backlash and friction due to gearhead
  • Backlash can be nearly eliminated by harmonic drive gears

Current Control of a Geared Motor with Local Strain Gauge Feedback

• Similar to Current Control of Geared Motor
• Harmonic drive gearing instrumented with strain gauges to sense torque
• Pros:
  • Sensor at output allows compensation of friction
• Cosn:
  • Additional complexity

Series Elastic Actuator (SEA)

• Electric motor with
  • Gearhead (often harmonic drive gearhead)
  • Torsional spring attaching output of gearhead to output of actuator
• Pros:
  • Torsional spring makes joint naturally “soft”
  • Protects gearing and motor from shocks at output
• Cons:
  • Additional complexity
  • More challenging to control high-speed or high-frequency at output

Literature

Modern Robotics: Mechanics, Planning, and Control by Kevin M. Lynch and Frank C. Park, Cambridge University Press, 2017