Motor Types and Characteristics

Motors are the actuators of robotics - they convert electrical energy into mechanical motion. Different motor types have vastly different characteristics, control requirements, and applications. Choosing the right motor is critical for successful robot design.

Motor Comparison Overview

Figure: Common motor types in robotics applications

DC Brushed Motors

How It Works

Components:

• Stator: Permanent magnets
• Rotor (Armature): Rotating coil
• Commutator: Split ring conducts current
• Brushes: Carbon contacts for current switching
• Shaft: Output rotation

Operation:

Voltage applied → Current through coil → Magnetic field interaction 
→ Rotation → Commutator switches current direction → Continuous rotation

Characteristics

Advantages:

• Simple control (just apply voltage)
• Cheap
• High starting torque
• Wide voltage range
• Robust and forgiving

Disadvantages:

• Brush wear (limited life 1000-2000 hours)
• Electrical noise
• Less efficient (70-80%)
• Magnetic interference
• Heat generation

Specifications

Typical ratings for robotics:

Control Method

Simple: Apply voltage between motor terminals

Motor 0V → Stopped
Motor 6V → Medium speed
Motor 12V → Full speed
Motor -12V → Reverse direction

Using PWM:
0% PWM = 0V effective
50% PWM = 6V effective (assuming 12V input)
100% PWM = 12V effective

Example Application: Mobile Robot Wheel

Requirements: 12V robot wheel motor

Specifications found:
- Rated: 12V, 200 RPM, 5 N·m
- Free-running: 500mA
- Stalled: 8A

Wheel diameter: 0.1m (0.314m circumference)

Speed:
v = RPM × circumference / 60
  = 200 × 0.314 / 60
  = 1.05 m/s

Torque:
5 N·m at wheel → Force = 5 / 0.05 (radius) = 100 N
Max pull force: 100 N (≈ 10 kg climbing horizontally)

Figure: DC motor directly coupled to robot wheel for mobile base

Servo Motors

How It Works

Internal components:

• DC motor (usually)
• Gearbox (high reduction, 50:1 to 300:1)
• Position feedback (potentiometer)
• Control circuit (compares desired vs actual position)

Operation:

PWM signal (1-2 ms pulse) → Control circuit → Adjusts motor power
→ Gearbox drives output shaft → Feedback resistor measures position
→ If not at target: motor adjusts → Once at position: holds

Characteristics

Advantages:

• Position feedback (knows where it is)
• Holds position even when load applied
• Easy PWM control
• Compact
• Good precision

Disadvantages:

• Cannot do continuous rotation (standard servos)
• Limited speed (low RPM due to gearbox)
• Limited power (soft gears)
• More expensive than DC motors
• Torque drops as speed increases

PWM Control Signal

Standard servo control:

Pulse width determines position:
1 ms pulse = -90° (full left)
1.5 ms pulse = 0° (center)
2 ms pulse = +90° (full right)

Pulse must repeat at 50 Hz (every 20 ms)

Specifications

Common sizes:

Example Application: Robot Arm Joint

5-joint arm with servo control:

For each joint:
- Use standard 10 kg·cm servo
- Mount servo in joint housing
- Attach link via servo horn
- Control with PWM from microcontroller

Position command:
uint8_t angle = 90;  // 0-180 degrees
pulse_width = 1000 + (angle * 5.56);  // microseconds

Stepper Motors

How It Works

Structure:

• Stator coils (4 usually)
• Rotor with permanent magnet teeth
• Each coil step = small rotation (1.8° typical for NEMA 17)

Operation:

Pulse sequence on coil 1 → Rotor rotates 1.8°
Pulse sequence on coil 2 → Rotor rotates next 1.8°
(Continue for full rotation)

Step 0°, 1.8°, 3.6°, 5.4°, ...

Characteristics

Advantages:

• Exact positioning without feedback
• Can hold position indefinitely
• Precise angular control
• No drift
• Works at very low speeds

Disadvantages:

• Complex control circuit required
• Cannot go very fast
• Must have driver IC
• Can miss steps if overloaded
• Cogging (not smooth rotation)
• Heat generation at idle (holding current)

Specifications

Common stepper motors for robotics:

Control Signals

Two approaches:

1. Simple direction/speed:

DIR pin: 0 = clockwise, 1 = counter-clockwise
STEP pin: Pulse = one step
Frequency determines speed

Example: 1000 Hz = 1000 steps/sec
For NEMA 17: 1000/200 = 5 revolutions/sec = 300 RPM

2. Micro-stepping:

Divides each step into smaller steps (1/2, 1/4, 1/8, 1/16)
Results in: 
- Smoother motion
- Less cogging
- More precise positioning
- Takes more processing power

Example Application: 3D Printer Axis

X-axis stepper with 200 steps/rev and 5mm pitch leadscrew:

Each step moves: 5mm / 200 = 0.025 mm = 25 micrometers
Very precise!

With 1/16 micro-stepping:
25 micrometers / 16 = 1.56 micrometers per sub-step!

BLDC (Brushless DC) Motors

How It Works

Components:

• Stator coils (fixed)
• Rotor permanent magnets
• Hall effect sensors
• Electronic commutation

Operation:

Hall sensors detect magnet position → Drive electronics
→ Energize appropriate stator coil → Magnetic repulsion/attraction
→ Smooth continuous rotation (not stepped like stepper)

Characteristics

Advantages:

• High efficiency (85-92%)
• Long life (no brushes, 10K+ hours)
• Smooth operation
• High speed possible
• Less electrical noise
• Compact for power

Disadvantages:

• Complex drive circuit required (ESC)
• More expensive
• Harder to control precisely
• Requires Hall sensors or back-EMF detection

Motor Classes

Outrunner (hollow can):

• Can spins with rotor magnets
• Stator inside
• High torque, lower RPM
• Used in drones, direct drives

Inrunner:

• Rotor spins inside stator
• Higher RPM, lower torque
• Needs gearbox
• Smaller package

Specifications

Typical BLDC for robotics:

KV rating: RPM per volt (unloaded)

1000 KV at 12V = 12,000 RPM no-load
Actual RPM = (Voltage - Back_EMF) × KV

Motor Selection Flowchart

Summary

DC Brushed Motors:

• ✓ Simplest control, cheapest
• ✓ Good for wheels and constant rotation
• ✓ Limited life due to brush wear

Servo Motors:

• ✓ Position feedback, holds position
• ✓ Perfect for arm joints and steering
• ✓ Limited to ±90° range (usually)

Stepper Motors:

• ✓ Exact positioning without feedback
• ✓ Great for 3D printers, CNC
• ✓ Needs complex driver circuit

BLDC Motors:

• ✓ High efficiency and long life
• ✓ Smooth high-speed operation
• ✓ Needs ESC (more expensive)

Practical Recommendation:

• Starting robot: DC motors for wheels + servos for joints
• High-performance: BLDC motors with proper driver
• Precise positioning: Stepper motors with microstepping