2. Robot hardware
Robot components
Recall from last time: Robotics problems exist at the intersection computation, action, and sensing.
Types of actuators
- A DC motor rotates at a high velocity with low torque and limited control over the amount of rotation.
- A gearhead motor has gears attached to a DC motor, decreasing velocity and increasing torque.
- A stepper motor rotates in small fixed increments, providing precise control over the amount of rotation.
- A servo is a motor equipped with electronics to sense and control the position of the rotor, but without the ability to complete a full rotation.
- Linear actuators generate translational motion in various ways.
Modes of locomotion
Locomotion refers to process or means by which robots move through their environments. There are many modes of locomotion, depending on the environment.- Terrestrial: wheels, legs, hovercraft, ...
- Underwater: thrusters, flippers, ...
- Airborne: propellers, fixed-wing, ...
- Space: rockets, solar sails, ...
Types of sensors
- Encoders measure the amount of rotation in a joint or wheel, often using an encoder wheel and an emitter-detector pair.
- Infrared sensors measure distance by emitting IR light and measuring the intensity of the signal reflected back into the sensor.
- Ultrasonic (sonar) sensors measure distance by emitting a pulse of sound and measuring its time-of-flight.
- Lidar measures distance using phase shifts in highly coherent light.
- Cameras record the color and intensity of visible light, possibly aided by mirrors of various shapes.
- RGBD (red-green-blue-depth) sensors provide images with an additional channel representing the distance to the nearest object in that direction, usually by projecting a pattern of infrared dots onto the scene and measuring the distortion in how the dots appear.
- Compasses measure orientation with respect to Earth's magnetic field.
- GPS (Global Positioning System) receivers use satellite signals to determine a device's position on the Earth.
- Inclinometers measure the relative direction of gravity.
- Inertial measurement units (IMUs) combine 3 accelerometers and 3 gyroscopes to measure linear and angular acceleration.
Active vs. passive sensing
Sensors can be classified as either active or passive.- Active sensors emit energy into or directly modify the environment.
- Passive sensors receive energy from the environment.
Proprioception vs. exteroception
Sensors can measure quantities that are either internal or external to the robot.- Proprioceptive sensors provide information about the robot's internal state.
- Exteroceptive sensors provide information about the robot's environment.
Evaluating sensors
An individual sensor can be judged according to several criteria:- Speed of operation
- Cost
- Error rate
- Robustness
- Computational requirements
- Power consumption
- Size and weight
Which of these criteria is the “most important”? It depends on the
task we intend the robot to complete. We should choose sensors
that provide information helpful for completing that task.
Tradeoffs are everywhere!
- space
- weight
- power
- cost
- durability
- maintainability
- accuracy
- reliability
- ...
Two strategies
When choosing the hardware to include on a robot, we can take at least two different approaches. Option 1: We can (try to) to eliminate issues by adding or improving devices.
Talos, MIT; Kuka manipulator
Roomba, iRobot; Sawyer, Rethink Robotics