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STM Week 4: Motors & Drivers

C/C++

date: 2025-03-04

Adapted from 1

DRAFT

Concepts

1. Motor Types

Brushed DC Motors:

  • Use brushes for commutation
  • Simple, cheap, but require maintenance
  • Speed controlled by PWM voltage; direction by reversing polarity (H-bridge)

Brushless DC (BLDC):

  • Electronically commutated (ESC required)
  • Efficient, high-speed

Stepper Motors:

  • Move in discrete steps (e.g., 1.8° per step)
  • Used for precise positioning

Servo Motors:

  • Integrated control circuit + motor
  • Accepts PWM pulses (1–2ms pulse width) for angular control (0°–180°)

2. Stall Current Detection

  • When a motor is mechanically blocked, current spikes (“stall current”)
  • Detected via a shunt resistor or current sensor (e.g., ACS712). Critical for overload protection

3. Motor Transfer Functions

Mathematical model:

\[\frac{\omega(s)}{V(s)} = \frac{K}{(Ls + R)(Js + b)}\]

Where \(\omega\) is angular velocity, and \(V\) the input voltage

Simplified, this helps design PID controllers for speed/position

4. Driver ICs & H-Bridges

L298N (H-Bridge)

  • Controls DC motor direction/speed
  • IN1/IN2 pins set direction; EN pin uses PWM for speed

A4988/TMC2209:

  • Stepper motor drivers
  • Use STEP/DIR signals
  • TMC2209 offers quieter operation

PWM Duty Cycle:

  • Duty cycle (%) = (CCR / ARR) * 100
  • Controls average voltage to motor

5. Feedback Systems

Quadrature Encoders:

  • Two-phase output to track direction and position (e.g., 1000 PPR)

Potentiometers:

  • Used in servos for closed-loop angle feedback

6. API Design & Version Control

Abstraction Layer:

  • Create functions like motor_init(), motor_set_speed(), motor_set_angle() to hide hardware details

Version Control:

  • Use Git (plus different branches) to track code changes

Weekly Project Implementation

1. Brushed DC Motor with H-Bridge (L298N)

Hardware Setup:

  • Connect L298N IN1, IN2 to STM32 GPIOs (e.g., PA0, PA1)
  • Connect EN pin to PWM output (e.g., TIM2_CH1 on PA5)
  • Power motor from external supply (not STM32’s 5V)

STM32 Code

// PWM Setup (TIM2, 1kHz frequency)
LL_TIM_InitTypeDef tim_init = {0};
tim_init.Prescaler = 79; // 80 MHz / (79+1) = 1 MHz
tim_init.CounterMode = LL_TIM_COUNTERMODE_UP;
tim_init.Autoreload = 999; // 1 MHz / 1000 = 1 kHz
LL_TIM_Init(TIM2, &tim_init);
LL_TIM_EnableARRPreload(TIM2);
LL_TIM_OC_SetCompareCH1(TIM2, 500); // 50% duty (CCR=500)
LL_TIM_EnableCounter(TIM2);
LL_TIM_EnableAllOutputs(TIM2);

// GPIO Direction Control (IN1=HIGH, IN2=LOW)
LL_GPIO_SetOutputPin(GPIOA, LL_GPIO_PIN_0);
LL_GPIO_ResetOutputPin(GPIOA, LL_GPIO_PIN_1);

// Adjust speed via PWM duty cycle
void set_motor_speed(uint32_t duty) {
  LL_TIM_OC_SetCompareCH1(TIM2, duty * 10); // duty 0-100 maps to 0-1000 ARR
}

2. Servo Motor Calibration

Hardware Setup: Servo signal pin to STM32 PWM output (e.g., TIM3_CH1 on PA6)

STM32 Code:

// Servo PWM (50Hz, 20ms period)
LL_TIM_InitTypeDef tim_init = {0};
tim_init.Prescaler = 15999; // 80 MHz / (15999+1) = 5 KHz
tim_init.CounterMode = LL_TIM_COUNTERMODE_UP;
tim_init.Autoreload = 99; // 5 KHz / 100 = 50 Hz (20ms)
LL_TIM_Init(TIM3, &tim_init);
LL_TIM_OC_SetCompareCH1(TIM3, 5); // 1ms pulse (0°)
LL_TIM_EnableCounter(TIM3);

// Set servo angle (0°–180°)
void set_servo_angle(uint8_t angle) {
  uint32_t ccr = 5 + (angle * 10) / 180; // 5 (0°) to 25 (180°)
  LL_TIM_OC_SetCompareCH1(TIM3, ccr);
}

3. Unified Motor API

Example Header File (motor.h):

typedef enum { MOTOR_DC, MOTOR_SERVO } MotorType;

typedef struct {
  MotorType type;
  TIM_TypeDef *timer;
  uint32_t channel;
  GPIO_TypeDef *gpio1, *gpio2;
  uint32_t pin1, pin2;
} Motor;

void motor_init(Motor *m);
void motor_set_speed(Motor *m, int speed); // For DC
void motor_set_angle(Motor *m, int angle); // For Servo

Example Usage

Motor dc_motor = {MOTOR_DC, TIM2, LL_TIM_CHANNEL_CH1, GPIOA, GPIOA, LL_GPIO_PIN_0, LL_GPIO_PIN_1};
Motor servo = {MOTOR_SERVO, TIM3, LL_TIM_CHANNEL_CH1, NULL, NULL, 0, 0};

motor_init(&dc_motor);
motor_init(&servo);

motor_set_speed(&dc_motor, 70); // 70% speed
motor_set_angle(&servo, 90); // 90° position

Critical Steps

  1. Timer Configuration:
    • Use CubeMX or manual LL code to set PWM frequency (1kHz for DC, 50Hz for servo)
    • Enable GPIO clocks and timer clocks (LL_AHB1_GRP1_EnableClock / LL_APB1_GRP1_EnableClock)
  2. H-Bridge Wiring:
    • Double-check L298N connections (motor power ≠ STM32 power)
  3. Testing:
    • Test PWM with an LED before connecting motors
    • Use a logic analyzer or oscilloscope to verify pulse widths

DRAFT

https://www.engr.siu.edu/staff/spezia/Web438A/Lecture%20Notes/lesson14et438a.pdf

https://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling

https://www.reddit.com/r/ControlTheory/comments/1agwhvi/finding_transfer_function_for_dc_motor/

  • Driver ICs (A4988, TMC2209)
  • H-bridges, PWM, and ESC control
  • Reading and writing drivers for a motor
  • Abstraction layers, encoder feedback, and API design

week 9 https://books-library.net/files/books-library.net-07281709Ee1R6.pdf

FreeRTOS Beginner Guide

Credits

  1. Deepseek. ↩