Sensors and Transducers Grant Agreement No LLP UK-LEONARDO-LMP Project acronym: CLEM Project title: Cloud services for E-Learning in Mechatronics Technology

Sensors and Transducers Sensors and actuators play an important role in robotic manipulation and its applications. They must operate precisely and function reliably as they directly influence the performance of the robot operation. A transducer, a sensor or actuator, like most devices, is described by a number of characteristics and distinctive features. In this section, we describe in detail the different sensing and actuation methods for robotic applications, the operating principle describing the energy conversion, and various significant designs that incorporate these methods. This section is divided into four subsections, namely, tactile and proximity sensors, force sensors, vision, and actuators.

Sensors and Transducers By definition, tactile sensing is the continuously variable sensing of forces and force gradients over an area. This task is usually performed by an m ´ n array of industrial sensors called forcels. By considering the outputs from all of the individual forcels, it is possible to construct a tactile image of the targeted object. This ability is a form of sensory feedback which is important in development of robots. These robots will incorporate tactile sensing pads in their end effectors. By using the tactile image of the grasped object, it will be possible to determine such factors as the presence, size, shape, texture, and thermal conductivity of the grasped object. The location and orientation of the object as well as reaction forces and moments could also be detected. Finally, the tactile image could be used to detect the onset of part slipping. Much of the tactile sensor data processing is parallel with that of the vision sensing. Recognition of contacting objects by extracting and classifying features in the tactile image has been a primary goal. Thus, the description of tactile sensor in the following subsection will be focused on transduction methods and their relative advantages and disadvantages.

Sensors and Transducers Proximity sensing, on the other hand, is the detection of approach to a workplace or obstacle prior to touching. Proximity sensing is required for really competent general-purpose robots. Even in a highly structured environment where object location is presumably known, accidental collision may occur, and foreign object could intrude. Avoidance of damaging collision is imperative. However, even if the environment is structured as planned, it is often necessary to slow a working manipulator from a high slew rate to a slow approach just prior to touch. Since workpiece position accuracy always has some tolerance, proximity sensing is still useful. Many robotic processes require sensors to transduce contact force information for use in loop closure and data gathering functions. Contact sensors, wrist force/torque sensors, and force probes are used in many applications such as grasping, assembly, and part inspection. Unlike tactile sensing which measures pressure over a relatively large area, force sensing measures action applied to a spot. Tactile sensing concerns extracting features of the object being touched, whereas quantitative measurement is of particular interest in force sensing. However, many transduction methods for tactile sensing are appropriate for force sensing.

Sensors and Transducers In the last three decades, computer vision has been extensively studied in many application areas which include character recognition, medical diagnosis, target detection, and remote sensing. The capabilities of commercial vision systems for robotic applications, however, are still limited. One reason for this slow progress is that robotic tasks often require sophisticated vision interpretation, yet demand low cost and high speed, accuracy, reliability, and flexibility. Factors limiting the commercially available computer vision techniques and methods to facilitate vision applications in robotics are highlights of the subsection on vision.

Sensors and Transducers Resistive tactile element.

Sensors and Transducers Mechanical/capacitive tactile element.

Sensors and Transducers Magnetoresistive tactile element.

Sensors and Transducers Optical proximity sensing.

Sensors and Transducers Schematic of piezoelectric sensor for a soft fingertip

Sensors and Transducers Piezoelectric force rate sensor

Sensors and Transducers Schematic of a vacuum diode force sensor.

Sensors and Transducers Quartz force sensor.

Sensors and Transducers Schematic of a flexible integrated vision system.

Sensors and Transducers Vision system for robotic applications.

Sensors and Transducers Basic elements of an incremental optical rotary encoder.

Sensors and Transducers Exploded view of an incremental optical rotary encoder showing the stationary mask between the code wheel and the photodetector assembly.

Sensors and Transducers Channels A and B provide bidirectional position sensing. If channel A leads channel B, the direction is clockwise; if channel B leads channel A, the direction is counterclockwise. Channel Z provides a zero reference for determining the number of disk rotations.

Sensors and Transducers Binary-code disk for an absolute optical rotary encoder. Opaque sectors represent a binary value of 1, and the transparent sectors represent binary 0. This four-bit binary- code disk can count from 1 to 15.