Speed control, direction control, torque control, PWM modulation, and motor control algorithms for robotics

Motor Control Strategies

Controlling motors is fundamental to robotics. Whether you need variable speed, reverse direction, or precise torque, understanding control methods is essential.

Motor Control Parameters

The Three Core Controls

Speed Control

Method 1: Voltage Control

Reduce the voltage applied to the motor to reduce speed:

Full voltage (12V) → Full speed (5000 RPM)
Half voltage (6V) → Half speed (~2500 RPM)

Advantages: ✓ Simple ✓ No electrical noise

Disadvantages: ✗ Torque decreases significantly ✗ Inefficient (waste as heat) ✗ Hard to implement precisely with DC supplies

Method 2: PWM (Pulse-Width Modulation)

The industry standard for motor control:

Motor sees average voltage = (ON time / Total time) × Supply voltage

100% PWM (always on):    Motor at full speed
75% PWM (3/4 on, 1/4 off): Motor at 75% speed
50% PWM (1/2 on, 1/2 off): Motor at 50% speed

Example timing:

Total cycle: 20 ms (50 Hz)

100% duty: ON 20ms, OFF 0ms      → 12V average
75% duty:  ON 15ms, OFF 5ms      → 9V average
50% duty:  ON 10ms, OFF 10ms     → 6V average
25% duty:  ON 5ms, OFF 15ms      → 3V average

Advantages: ✓ Efficient (little wasted as heat) ✓ Maintains torque well ✓ Digital, precise control ✓ No noise

Disadvantages: ✗ Requires fast switching ✗ Slight acoustic noise (high frequency) ✗ More complex than voltage control

PWM Frequency Impact

Typical PWM frequencies:

Arduino: 490-980 Hz (too low for servos)
Motor controllers: 5-20 kHz (good)
Servo motors: 50 Hz (fixed)
High-speed ESCs: 32 kHz (racing)

Direction Control

Method 1: Mechanical Relay

Switch wires using electromagnetic relay:

Motor forward: Relay coil de-energized
               Wires connected: +12V to pin 1, GND to pin 2

Motor backward: Relay coil energized
                Wires swapped: GND to pin 1, +12V to pin 2

Disadvantages: ✗ Slow (~50ms to switch) ✗ Mechanical wear ✗ Audible click ✗ Power draw

Method 2: H-Bridge (Electronic)

Four transistors create bidirectional current control:

       +12V
        │
    ┌───┴───┐
    │Q1  Q2│
    │   │   │
Motor← ← → →Motor
    │   │   │
    │Q3  Q4│
    └───┬───┘
        │
       GND

Forward: Q1 and Q4 ON  → +12V to left, GND to right
Backward: Q2 and Q3 ON → GND to left, +12V to right
Stop: All OFF          → Motor coasts (unpowered)

H-Bridge Truth Table:

Q1	Q2	Q3	Q4	Result
OFF	OFF	OFF	OFF	Coast (unpowered)
ON	OFF	OFF	ON	Forward
OFF	ON	ON	OFF	Backward
ON	ON	OFF	OFF	Shoot-through (SHORT!) ❌

Never turn on Q1 and Q2 together → Short circuit!

Method 3: MOSFET H-Bridge with PWM

Combine H-bridge with PWM for variable speed control:

Forward at 50%: Q1 PWM 50%, Q4 ON
Backward at 75%: Q2 ON, Q3 PWM 75%

Torque Control

Understanding Motor Torque

Torque = proportional to current

High torque = High current
Low torque = Low current

Method 1: Current Limiting

Sense motor current and limit maximum:

Motor draws: 20A (jamming)
Limit set to: 10A
Response: Reduce PWM to stay at 10A
Result: Motor can't jam, protected

Implementation:

Shunt resistor measures current
ADC samples voltage drop
Feedback loop adjusts PWM

Method 2: Torque Feedback from Load

Use encoder or strain gauge to measure actual torque:

Target torque: 10 N·m
Measured: 8 N·m (load light)
Action: Increase current to reach 10 N·m

Measured: 10 N·m (load heavy)
Action: Reduce current, motor will slip controlled

Method 3: Stall Protection

Prevent motor from drawing excessive current at startup:

Soft-start: Gradually increase from 0 to 100% over 0.5 seconds
Result: Smooth acceleration instead of massive spike

Motor Control Algorithms

Open-Loop Control (Simple)

Just set PWM and hope for best:

Command: "Set speed to 50%"
Action: Set PWM to 50%
Reality: Might be 48% if load changes (no feedback)

Used in: Toys, simple robots Problem: No correction for external disturbances

Closed-Loop Control (PID)

Sense actual speed and correct:

Target speed: 1000 RPM
Measured speed: 950 RPM
Error: 50 RPM difference

PID controller:
P (Proportional): Increase PWM by 5%
I (Integral): Remember past errors, add correction
D (Derivative): Predict overshoot, add damping

New PWM: Smoothly reaches 1000 RPM

Used in: Robots, quadcopters, industrial motors Advantage: Maintains speed despite load changes

Acceleration Ramping

Prevent sudden current spikes:

Command: Go to full speed
Linear ramp: 0 → 100% over 1 second (100%/sec)

Quadratic ramp: S-curve for smoother

Result: Smooth startup, less mechanical stress

Motor Control IC Comparison

Feature	L298N	L293D	BTS7960	DRV8833
Max Current	2A	600mA	43A	2A
Voltage	5-35V	4.5-36V	5-27V	2.7-10.8V
PWM support	Yes	Yes	Yes	Yes
Cost	$	$	$$	$
H-bridge	Yes	Yes	Yes	Yes
Best for	Medium motors	Small motors	Large motors	Micro motors

Practical Examples

Example 1: Robot with Two Motors

Left motor control:
  Pin EN1 (PWM) → Speed 0-255 (PWM duty)
  Pin IN1/IN2 (Digital) → Direction (fwd/back)

Right motor control:
  Pin EN2 (PWM) → Speed 0-255
  Pin IN3/IN4 (Digital) → Direction

Code:
  Forward: EN1=150, IN1=HIGH, EN2=150, IN2=LOW
  Turn left: EN1=100, IN2=150
  Backward: EN1=150, IN1=LOW, EN2=150, IN2=HIGH

Example 2: Servo Motor

Servos are pre-controlled with internal electronics:

Just send PWM signal: 50 Hz frequency
Pulse width determines angle:
  1.0 ms → 0°
  1.5 ms → 90°
  2.0 ms → 180°

No need for full H-bridge, just single PWM line!

Common Problems and Solutions

Problem	Cause	Solution
Motor doesn't start	Dead zone in PWM	Increase minimum PWM (15-20%)
Jerky motion	PWM too low frequency	Increase to 5+ kHz
Excessive current	No current limiting	Add shunt resistor, limit in software
Motor gets hot	Continuous high torque	Reduce duty cycle or load
Uncontrolled spin	No feedback	Add encoder, implement PID
Oscillation	PID gains too high	Tune P/I/D gains

Summary

Speed Control: ✓ PWM is the standard method ✓ Efficient, precise, smooth ✓ Use 5+ kHz frequency for audible noise control

Direction Control: ✓ H-bridge circuit required ✓ Electronic (MOSFET) > mechanical (relay) ✓ Never shoot through (short circuit)

Torque Control: ✓ Measure current, limit if needed ✓ Soft-start prevents spikes ✓ Stall protection saves motor

Control Algorithm: ✓ Open-loop simple but imprecise ✓ Closed-loop PID for stability ✓ Ramping for smooth acceleration

Implementation: ✓ Choose IC based on current/voltage needs ✓ Add feedback for critical applications ✓ Test thoroughly before deployment

Motor Control Strategies

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