Motor Control

You've got a motor. You want it to spin. But how you want it to spin depends on your task:

• Velocity control: "Spin at 100 RPM" (e.g., a conveyor belt, a fan)
• Position control: "Turn to 45 degrees" (e.g., a robot arm joint, a camera gimbal)
• Torque control: "Apply 2 Nm of force" (e.g., a robot gripper, a force-sensitive tool)

Each control mode uses different sensors, different control algorithms, and different ways of commanding the motor. Let's break them down.

The Three Control Modes

Each mode builds on the last — position control often uses velocity control internally, and velocity control uses torque (current) control at the lowest level.

Velocity Control: Constant Speed

Goal: Make the motor spin at a target speed, regardless of load.

You use a PID controller with an encoder as the feedback sensor.

The PID controller adjusts motor power to maintain constant speed even if:

• The load changes (robot climbs a hill)
• The battery voltage drops
• Friction varies

This is the most common control mode for driving wheels.

Position Control: Move to a Target Angle

Goal: Rotate the motor to a specific angle and hold it there.

You still use a PID controller, but now the error is the difference in position, not velocity. And the output is often a velocity command to a lower-level velocity controller.

This is a cascaded control design — position PID → velocity PID → motor PWM. Each layer of control handles one aspect of the motion.

Position control is essential for:

• Robot arms: Each joint must reach a precise angle
• Steering: Turn the wheels to a specific angle
• Servos: Camera gimbals, sensor platforms

Torque Control: Apply a Specific Force

Goal: Make the motor apply a precise force, regardless of speed or position.

Torque is proportional to current in a DC motor (Torque = Kt × Current, where Kt is the motor's torque constant). So torque control is really current control.

Torque control is critical for:

• Force-sensitive tasks: Polishing, assembly, medical robotics
• Compliant motion: The robot "gives" when it hits an obstacle
• Human-robot interaction: Collaborative robots (cobots) that are safe to touch

How Motors Actually Work: H-Bridge and PWM

Let's zoom down to the hardware level. How does your microcontroller actually control motor power?

The H-Bridge

An H-bridge is a circuit that lets you:

1. Spin the motor forward (positive voltage)
2. Spin the motor backward (negative voltage)
3. Brake the motor (short the terminals)
4. Let it coast (open circuit)

It's called an H-bridge because of the shape:

        +V
         │
    ┌────┼────┐
    │    │    │
   [S1] [S2] [S3] [S4]  ← Switches (transistors)
    │    │    │    │
    └────┼────┼────┘
         │    │
         M    M  ← Motor terminals
         │    │
        GND  GND

Forward: S1 + S4 closed → current flows left-to-right
Reverse: S2 + S3 closed → current flows right-to-left
Brake: S1 + S3 or S2 + S4 closed → motor terminals shorted
Coast: All switches open → motor spins freely

In practice, you use an H-bridge driver IC (like L298N, DRV8833, or TB6612FNG) that handles the switching for you. You just send direction and speed signals.

PWM: Pulse Width Modulation

How do you control the speed of the motor? You can't vary the voltage continuously — microcontrollers output digital signals (0V or 5V).

The solution is PWM (Pulse Width Modulation). You rapidly switch the voltage on and off. The duty cycle (percentage of time the signal is high) controls the average power.

100% duty cycle (full speed):
████████████████████████  (always on)

50% duty cycle (half speed):
████    ████    ████    ████

25% duty cycle (quarter speed):
██  ██  ██  ██  ██  ██  ██  ██

0% duty cycle (stopped):
                        (always off)

The motor's inductance smooths out the pulses, so the motor sees an average voltage proportional to the duty cycle.

A typical PWM frequency for motors is 20-50 kHz — fast enough that the motor doesn't "feel" the individual pulses, but slow enough to minimize switching losses.

What's Next?

So far, we've been controlling individual motors or joints. But what if you want to control the robot's overall motion — like "drive forward at 0.5 m/s" or "turn left at 30 degrees/second"?

That's where velocity commands come in — and they require translating high-level motion goals into individual wheel speeds. We'll cover that in the next lesson.