SENSORS IN ROBOTICS
The general topic of sensors as components in control systems is discussed in previews pages . Here we discuss sensors as they are applied in robotics. Sensors used in industrial robotics can be classified into two categories: (1) internal and (2) external. Internal sensors arc those used for controlling position and velocity of the various joints of the robot. These sensors form a feedback control loop with the robot controller. Typical sensors used to control the position of the robot arm include potentiometers and optical encoders. To control the speed of the robot arm, tachometers of various types are used.
External sensors are used to coordinate the operation of the robot with other equipment in the cell. In many cases, these external sensors are relatively simple devices. such as limit switches that determine whether a part has been positioned properly in a fixture or that indicate that a part is ready to be picked up at a conveyor. Other situations require more advanced sensor technologies, including the following:
Tactile sensors. Used to determine whether contact is made between the sensor and another object. Tactile sensors can be divided into two types in robot applications:
touch sensors and (2) force sensors, Touch sensors are those that indicate simply
that contact has been made with the object. Force sensors are used to indicate the magnitude of the force with the object. This might be useful in a gripper to measure and control the force being applied to grasp an object.
Proximity sensors. Indicate when an object is close to the sensor, When this type of sensor is used to indicate the actual distance of the object, it is called a range sensor.
Optical sensor. Photocells and other photometric devices can be utilized to detect the presence or absence of objects and are often used for proximity detection.
Machine vision. Used in robotics for inspection, parts identification, guidance, and other uses. In Section 23.6, we provide a more-complete discussion of machine vision in automated inspection.
Other sensors. This miscellaneous category includes other types of sensors that might be used in robotics, including devices for measuring temperature, fluid pressure, fluid flow, electrical voltage, current, and various other physical properties.
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