10 Servo Motors: Position, Motion, and Power

10.5.1 Internal Components

Servo motors integrate a complete feedback control system in a compact package:

1. DC Motor Core: Provides rotational force
2. Gear Reduction System: Amplifies torque, reduces speed
3. Potentiometer: Position feedback sensor
4. Control Circuit: Compares commanded vs actual position

The internal potentiometer continuously measures shaft position, and the control circuit compares this to the commanded position from the PWM signal. This closed-loop design maintains position accuracy even under varying loads.

10.5.2 PWM Position Control

Servo motors use pulse width, not motor-power duty cycle, to determine position. Inspect Figure 10.1 from the common 20 ms frame to the three different high-pulse widths before mapping any angle.

Read Figure 10.1 from the 1 ms pulse to 1.5 ms and then 2 ms: commanded angle advances while the repetition period stays 20 ms. The servo interprets high-time as a position target, so changing frame duty cycle without respecting the valid pulse window is not equivalent to controlling DC-motor power.

Key parameters:

• 50 Hz frequency (20 ms period) - Standard for hobby servos
• 1 ms pulse = 0 degrees (5% duty cycle)
• 1.5 ms pulse = 90 degrees (7.5% duty cycle, center position)
• 2 ms pulse = 180 degrees (10% duty cycle, maximum rotation)

Checkpoint: Pulse Width Means Position

You now know:

• Hobby servos repeat a 20 ms frame at 50 Hz; the high-pulse width, not average duty cycle, carries the angle.
• The standard map is 1 ms to 0 degrees, 1.5 ms to 90 degrees, and 2 ms to 180 degrees, with calibration required for real hardware.
• A 45 degree command is 1.25 ms, and a 1 second 0-180 degree sweep at 50 Hz sends 50 commands, or 3.6 degrees per frame.

Important: Servo libraries handle the timing. You just write servo.write(90) for 90 degrees!

10.7.1 Single Servo Control

#include <ESP32Servo.h>

Servo myServo;

#define SERVO_PIN 18

void setup() {
  Serial.begin(115200);

  // Attach servo (ESP32 uses different timers)
  myServo.attach(SERVO_PIN, 500, 2400);  // Min/max pulse width in microseconds

  Serial.println("Servo Controller Ready");
}

void loop() {
  // Sweep from 0 to 180 degrees
  for (int pos = 0; pos <= 180; pos++) {
    myServo.write(pos);
    Serial.print("Position: ");
    Serial.println(pos);
    delay(15);
  }

  delay(1000);

  // Sweep back from 180 to 0 degrees
  for (int pos = 180; pos >= 0; pos--) {
    myServo.write(pos);
    Serial.print("Position: ");
    Serial.println(pos);
    delay(15);
  }

  delay(1000);
}

10.7.2 Multi-Servo Robotic Arm

Read the arm example as a coordinated timing path. Start with the four named joint states, then follow moveToPosition() as it samples each current angle and advances every joint through the same number of 20 ms frames. Finally trace the pick-and-place sequence through approach, grip, transfer, release, and home. Coordination reduces sudden mechanism stress, but the shared supply must still tolerate simultaneous servo current.

#include <ESP32Servo.h>

Servo baseServo, shoulderServo, elbowServo, gripperServo;

struct RobotPosition { int base, shoulder, elbow, gripper; };

RobotPosition homePos  = {90, 90, 90, 90};
RobotPosition pickPos  = {45, 45, 45, 90};
RobotPosition placePos = {135, 45, 45, 45};

void setup() {
  Serial.begin(115200);
  baseServo.attach(18, 500, 2400);
  shoulderServo.attach(19, 500, 2400);
  elbowServo.attach(21, 500, 2400);
  gripperServo.attach(22, 500, 2400);
  moveToPosition(homePos, 1000);
  Serial.println("Commands: h=home, p=pick, l=place, s=sequence");
}

void loop() {
  if (Serial.available()) {
    switch(Serial.read()) {
      case 'h': moveToPosition(homePos, 1500); break;
      case 'p': moveToPosition(pickPos, 1500); break;
      case 'l': moveToPosition(placePos, 1500); break;
      case 's': pickAndPlaceSequence(); break;
    }
  }
}

// Smooth interpolated motion — all joints move simultaneously
void moveToPosition(RobotPosition target, int duration) {
  int cB = baseServo.read(), cS = shoulderServo.read();
  int cE = elbowServo.read(), cG = gripperServo.read();
  int steps = duration / 20;
  for (int i = 0; i <= steps; i++) {
    float p = (float)i / steps;
    baseServo.write(cB + (target.base - cB) * p);
    shoulderServo.write(cS + (target.shoulder - cS) * p);
    elbowServo.write(cE + (target.elbow - cE) * p);
    gripperServo.write(cG + (target.gripper - cG) * p);
    delay(20);
  }
}

void pickAndPlaceSequence() {
  moveToPosition(pickPos, 1500);   delay(500);  // Move to pick
  gripperServo.write(45);          delay(500);  // Close gripper
  moveToPosition(placePos, 2000);  delay(500);  // Move to place
  gripperServo.write(90);          delay(500);  // Open gripper
  moveToPosition(homePos, 1500);                // Return home
}

Checkpoint: Motion Code Has Limits

You now know:

• attach(pin, 500, 2400) records the pulse window the library should use before any write(angle) command.
• A sweep loop is easy, but coordinated motion needs shared timing; the robotic arm uses duration / 20 steps so joints arrive together.
• Home, pick, and place positions are explicit command records: {90, 90, 90, 90}, {45, 45, 45, 90}, and {135, 45, 45, 45}.