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Actuators and Sensors: The Building Blocks of Movement

Overview​

A humanoid robot's ability to move, interact with its environment, and perceive the world is fundamentally determined by its hardware. This section explores the two most critical hardware components: actuators, which produce motion, and sensors, which gather information. We will examine the most common types of actuators and sensors used in modern humanoid robotics, and discuss their respective advantages and disadvantages.

Detailed Explanation​

Actuators: The Muscles of the Robot​

Actuators are the components responsible for moving and controlling a robot's mechanisms. They convert energy (usually electrical) into mechanical motion. The choice of actuator is a critical design decision, as it affects the robot's strength, speed, precision, and overall cost.

1. Electric Motors​

Electric motors are the most common type of actuator used in robotics.

  • DC Motors: Simple and inexpensive, DC motors provide continuous rotation. They are often used in wheeled robots or for simple joint movements.
  • Stepper Motors: These motors move in discrete steps, which allows for precise positioning without the need for a feedback sensor. They are often used in 3D printers and other applications that require precise control.
  • Servo Motors: A servo motor is a combination of a DC motor, a gearbox, a potentiometer (for position feedback), and a control circuit. They are widely used in robotics for controlling joints in robotic arms and legs, as they allow for precise control of the angular position.

2. Hydraulic Actuators​

Hydraulic actuators use a fluid (usually oil) to generate motion. A pump pressurizes the fluid, which is then used to move a piston in a cylinder. Hydraulic systems can generate very large forces, making them suitable for heavy-duty applications. Boston Dynamics' Atlas robot is a famous example of a robot that uses hydraulic actuation to achieve its impressive dynamic movements. However, hydraulic systems are also complex, messy, and less energy-efficient than electric motors.

3. Pneumatic Actuators​

Pneumatic actuators are similar to hydraulic actuators, but they use a gas (usually compressed air) instead of a liquid. They are fast, clean, and relatively inexpensive. However, they are less precise than electric or hydraulic actuators and are not as capable of generating large forces.

Sensors: The Senses of the Robot​

Sensors are the robot's connection to the world. They provide the robot with information about its own state and the state of its environment.

1. Proprioceptive Sensors​

Proprioceptive sensors measure the internal state of the robot.

  • Encoders: These sensors are used to measure the angular position of a motor shaft. They are essential for closing the loop on joint control.
  • Inertial Measurement Units (IMUs): An IMU typically combines an accelerometer (to measure linear acceleration) and a gyroscope (to measure angular velocity). IMUs are crucial for balance and navigation, as they allow the robot to sense its own orientation and movement relative to the ground.

2. Exteroceptive Sensors​

Exteroceptive sensors measure the state of the environment.

  • Cameras: Cameras provide rich visual information about the world. They are used for object recognition, navigation, and human-robot interaction.
  • LiDAR (Light Detection and Ranging): LiDAR sensors use lasers to create a 3D map of the environment. They are highly accurate and are a key sensor for autonomous navigation and obstacle avoidance.
  • Tactile Sensors: Tactile sensors, or touch sensors, allow a robot to sense physical contact with an object. They are essential for grasping and manipulation tasks, as they provide information about the force being applied and the shape of the object.

Hands-on Exercise: Choosing a Servo for a Robotic Arm​

This exercise is a research task to help you understand the trade-offs involved in selecting an actuator for a specific application.

The Scenario​

You are tasked with designing a small, 3-DOF (Degrees of Freedom) robotic arm for a hobbyist project. The arm needs to be able to pick up and move small objects weighing up to 50 grams. Your task is to choose a suitable servo motor for the "shoulder" joint of the arm, which will experience the highest load.

Your Task​

  1. Research three different servo motors that are commonly used in hobbyist robotics. Some popular brands to look for are Futaba, Hitec, and Tower Pro.
  2. For each servo, find the following specifications:
    • Torque: Usually measured in kg-cm or oz-in. This is the most important specification, as it determines the lifting capacity of the arm.
    • Speed: Usually measured in seconds per 60 degrees. This determines how fast the arm can move.
    • Voltage Range: The operating voltage of the servo.
    • Gears: Metal or plastic? Metal gears are more durable but also more expensive.
    • Price: The approximate cost of the servo.
  3. Create a comparison table of the three servos.
  4. Based on your research, write a short paragraph recommending one of the servos for the robotic arm project and justify your choice. Consider the trade-offs between torque, speed, durability, and cost.

Case Study: The Design of ASIMO​

Introduction: Honda's ASIMO (Advanced Step in Innovative Mobility) is one of the most iconic humanoid robots ever created. First introduced in 2000, ASIMO was the culmination of decades of research and development at Honda. This case study examines some of the key hardware design choices that made ASIMO's remarkable mobility possible.

Actuation: ASIMO's fluid and human-like movements were achieved through a combination of powerful and precise servo motors. Each of ASIMO's legs has 6 degrees of freedom, and its arms have 7, allowing for a wide range of motion. The servo motors used in ASIMO were custom-designed by Honda and featured a number of advanced features, including:

  • High power-to-weight ratio: This was essential for keeping the robot's weight down while still providing enough power for dynamic movements like walking, running, and climbing stairs.
  • Harmonic drives: These are a type of compact, high-ratio gearbox that provides zero-backlash performance, which is critical for precise and repeatable movements.
  • Brushless DC motors: These motors are more efficient and have a longer lifespan than traditional brushed DC motors.

Sensing: ASIMO was equipped with a sophisticated suite of sensors that allowed it to perceive its environment and its own body.

  • Vision: ASIMO's head contained two cameras that provided stereoscopic vision, allowing it to perceive depth and recognize objects and faces.
  • Balance: A key to ASIMO's ability to walk and run was its advanced balance control system. This system used an IMU (Inertial Measurement Unit) to sense the robot's orientation and acceleration, and force sensors in its feet to measure the ground reaction forces. This information was used to make real-time adjustments to the robot's posture to maintain balance.
  • Tactile Sensing: ASIMO's hands were equipped with tactile sensors that allowed it to grasp and manipulate objects with a delicate touch.

Legacy: Although the ASIMO project was officially retired in 2018, its legacy lives on. ASIMO was a powerful demonstration of what was possible in humanoid robotics and inspired a generation of researchers and engineers. Many of the technologies developed for ASIMO, particularly in the areas of bipedal locomotion and human-robot interaction, have found their way into other robotic systems. ASIMO remains a landmark achievement in the history of robotics and a testament to the importance of a holistic approach to robot design that considers the tight integration of actuators, sensors, and control systems.