Motors and Movement
Driving motors and calibrating wheel steer angles
Definition
Motors convert electrical energy from the battery into rotational force, while gears reduce speed and increase torque for control. Key concepts include Voltage Signal.
Think of Motors and Movement as:
Just as your brain receives sensory feedback from your skin and signals muscles to react, Motors and Movement manages feedback loops.
Real-Life Example
Just as humans rely on physical organs and reflexes, Motors and Movement operates through specific electrical and mechanical rules:
- 1 Identify the physical parameter (like light, touch, or distance).
- 2 Convert this into a voltage change on the controller pin.
- 3 Execute motor actions to adjust the robot's physical position.
Key Highlights:
- Physical detection
- Electrical mapping
- Mechanical feedback
Interactive Diagram
Launch the interactive diagram to see this in action.
Open Interactive DiagramThe interactive diagram for this chapter demonstrates Motors and Movement. It shows different motor types (DC, servo, stepper) with a robot chassis showing movement directions.
What to explore:
- click each motor type to see how it works; control the robot direction; watch wheel and joint movement
- motors convert electrical energy into movement — different motor types are suited for wheels, arms, and precise positioning
Introduction
A robot that can sense and think but cannot move is just a statue. Motors and Movement systems are what transform a robot from a thinking machine into an acting one. Whether it is rolling on wheels, walking on legs, or flying through the air, every robot needs some form of actuation to interact with the physical world.
Electric motors are the most common Actuator in robotics. They convert electrical energy into rotational motion, which can then drive wheels, arms, propellers, or tracks. Different types of motors — DC motors, servo motors, stepper motors — offer different trade-offs between speed, torque, precision, and control complexity.
In this chapter, you will explore how motors work, how they are controlled, and how different movement mechanisms (wheels, tracks, legs, rotors) enable robots to navigate diverse environments. You will also learn about motor drivers, gearboxes, and the physics of robot locomotion.
How It Works
A DC (direct current) motor is the simplest type. It has two wires: apply voltage one way and the shaft spins clockwise; reverse the voltage and it spins counterclockwise. The speed depends on the voltage applied — higher voltage means faster spinning. Inside, a DC motor uses magnets and wire coils to convert electrical current into rotational force (torque).
A servo motor is a DC motor with built-in feedback control. It has three wires: power, ground, and signal. The signal wire receives a PWM pulse that tells the servo what position to go to. The servo continuously monitors its actual position using a built-in potentiometer and adjusts the motor until it matches the commanded position. Servos can hold a position even against moderate external force.
Household Object Analogy
Think of a DC motor like a water wheel. Water flowing over the wheel (electric current) makes it turn. More water (higher voltage) makes it spin faster. If you try to stop the wheel (apply a load), you feel the force of the water pushing — that is torque. Put a gear on the wheel, and you can trade speed for even more pushing force, like a car's transmission.
Deeper Dive
Stepper motors are a third type that move in precise increments called steps. A typical stepper motor has 200 steps per revolution, meaning it can turn exactly 1.8 degrees per step. By counting the number of steps sent, the controller knows the exact shaft position without needing a feedback sensor. This makes stepper motors ideal for 3D printers, CNC machines, and precision robotics.
Motor drivers are electronic circuits that control how much power flows to the motor. A Microcontroller's output pins cannot supply enough current to run a motor directly — they would be damaged. A motor driver uses the Microcontroller's low-power signal to control high-power transistors that deliver battery voltage to the motor. Popular motor driver chips include the L298N, L293D, and TB6612.
A gearbox is a set of gears attached to a motor that changes its speed and torque characteristics. A high-ratio gearbox (like 50:1) makes the output shaft spin 50 times slower but with 50 times more torque. This allows a small, lightweight motor to move a heavy robot. The trade-off is speed — a robot with high gear reduction moves slowly but can push through obstacles.
Key Insight
The most efficient electric motors convert over 95% of electrical energy into mechanical motion. In contrast, internal combustion engines (gasoline car engines) are only about 20-30% efficient. This is why electric vehicles and robots are inherently more energy-efficient than gas-powered machines.
Advanced
Brushless DC motors (BLDC) eliminate the brushes that wear out in traditional DC motors. Instead, they use electronic commutation — the controller electronically switches the magnetic field to keep the rotor spinning. BLDC motors are more efficient, quieter, and last longer than brushed motors, but require more complex controllers. They are used in drones, electric cars, and high-performance robots.
Wheel slip is a common problem in mobile robotics. When a robot tries to accelerate or turn too quickly, its wheels may lose traction and spin without moving the robot. This causes the robot's odometry position estimate to drift. Robots use IMU sensors (accelerometers and gyroscopes) to detect slip and adjust motor power accordingly.
Differential drive is the most common wheel configuration for mobile robots. Two independently powered wheels on opposite sides, with a third caster wheel for balance, allow the robot to move forward, backward, and turn by varying the speed and direction of each wheel. To turn left, the right wheel spins faster than the left wheel. To spin in place, the wheels turn in opposite directions.
Vocabulary Table
| Term | Definition |
|---|---|
| Motors and Movement | The primary technological concept explaining how components interact within the context of How Robots Work. |
| Voltage Signal | An electrical signal representing data values based on pressure or intensity. |
| Microcontroller | A tiny computer chip designed to process inputs and steer physical circuits. |
| Actuator | A physical mechanical device (like a motor) that creates movement. |
Fun Facts
The fastest electric motors used in robotics can spin at over 100,000 RPM. These are used in high-speed machining and drone racing.
Some robot motors have built-in encoders that measure position with a resolution of 1,000 counts per revolution, allowing precision within 0.36 degrees.
The most powerful robot motors can produce over 100 kilowatts (134 horsepower) — enough to move a full-sized car. These are used in heavy industrial robots.
Piezoelectric motors use crystals that vibrate when electricity is applied. They can achieve nanometer-level positioning precision and are used in microscope stages and camera lens mechanisms.
Shape-memory alloy Actuator change shape when heated and return when cooled. They are used in soft robots because they are lightweight, silent, and can mimic muscle movement.
Common Misconceptions
Misconception: Bigger motors always make a stronger robot.
Truth: A motor's torque depends on its design, not just its size. A small motor with a high-ratio gearbox can lift more than a large motor without gears. Gearing is often more important than motor size.
Misconception: A stalled motor is not drawing current.
Truth: A stalled motor (prevented from turning while power is applied) can draw many times its normal operating current, potentially burning out the motor or damaging the driver. Software should detect stall conditions and cut power.
Misconception: All motors use electricity.
Truth: Some robots use pneumatic (compressed air) or hydraulic (pressurized fluid) Actuator for high-force applications. These are common in industrial robots that need to lift heavy loads.
Misconception: A motor turns at the same speed regardless of load.
Truth: Motor speed decreases as load increases. A motor rated at 100 RPM with no load might drop to 80 RPM under its maximum rated load. This is why robots need feedback control to maintain consistent speed.
Knowledge Check
1. What is the primary role of Motors and Movement?
Answer: To capture or process physical feedback
2. What does PWM stand for in motor speed control?
Answer: Pulse Width Modulation
3. Which unit converts physical attributes into electrical values?
Answer: A sensor