STM32 Mbed CodeIDE (7/8): servo position control

Some outputs should move, not only light up. Servos are used when a controller needs to point, open, steer, latch, or position something in a repeatable way. In this lesson, you will connect the servo position command to a potentiometer reading.

PWM for an LED changes average brightness. A servo also uses repeated pulses, but the important value is the pulse width:

- about 700 microseconds: one broad side of travel
- about 1500 microseconds: middle
- about 2300 microseconds: the other broad side of travel

In Mbed, the servo signal is produced with PwmOut. servo.period_ms(20) sets the usual 20 ms repetition period, and servo.pulsewidth_us(...) changes the high-pulse width that the servo interprets as position.

The exact mechanical angle can vary. This lesson deliberately stays inside the conservative 700-2300 us exercise range, which is inside the wider range already exercised on this remote setup. Do not command values outside 700-2300 us. If the arm chatters continuously, appears to press against a hard stop, or does not settle after a move, end the run and tell your instructor instead of widening the range.

Your job is to show a controlled relationship: low potentiometer value gives one broad position, high value gives a different position, and the serial output matches your mapping.

The camera may show the servo only as a small arm on the training board. Treat angle as an estimate calculated by your code. The strongest evidence is the combination of serial pulse_us, serial angle estimate, and a visible change in broad servo position.

Pin and evidence checklist for this lesson:

- Servo signal output: PA_15
- Potentiometer used later: Pot. pot2 -> PC_1
- Required serial evidence: pulse_us
- Camera evidence: broad servo position change, not a precise measured angle

#include "mbed.h"

PwmOut servo(PA_15);

void set_servo_us(int pulse_us) {
    servo.pulsewidth_us(pulse_us);
}

int main() {
    servo.period_ms(20);
    printf("Servo three-position test start\n");

    while (true) {
        set_servo_us(700);
        printf("pulse_us=700 position=low\n");
        ThisThread::sleep_for(2000ms);

        set_servo_us(1500);
        printf("pulse_us=1500 position=middle\n");
        ThisThread::sleep_for(2000ms);

        set_servo_us(2300);
        printf("pulse_us=2300 position=high\n");
        ThisThread::sleep_for(2000ms);
    }
}

Fill three rows, one for each fixed pulse width: 700 us, 1500 us, and 2300 us. Pulse width is a number in microseconds (us); Printed position is the exact console label; and Observed motion or position is only a broad camera description. If motion is unclear, choose unclear and describe what you could see instead of inventing a precise angle. Leave extra unused rows completely blank.

Now map AnalogIn PC_1 to an approximate angle and pulse width. Use the Pot. pot2 slider for PC_1.

#include "mbed.h"

AnalogIn pot(PC_1);
PwmOut servo(PA_15);

int main() {
    servo.period_ms(20);
    printf("Potentiometer servo control start\n");

    while (true) {
        float reading = pot.read();
        int percent = (int)(reading * 100.0f);
        int angle = (int)(reading * 180.0f);
        // TODO 1: replace 700 with the full 700-2300 us mapping.
        int pulse_us = 700;

        // TODO 2: replace 700 with the mapped pulse_us variable.
        servo.pulsewidth_us(700);

        printf("percent=%d angle=%d pulse_us=%d\n",
               percent,
               angle,
               pulse_us);
        ThisThread::sleep_for(500ms);
    }
}

Fill the three required cases: low, middle, and high potentiometer positions. Serial percent has unit %; Serial angle estimate is a calculated estimate in degrees, not a camera measurement; Serial pulse width is in microseconds (us); and the camera field records only broad position. Leave any extra unused rows completely blank.