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.
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.
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 par- ticular interest in force sensing. However, many transduction methods for tactile sensing are appropriate for force sensing.
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.
Resistive and Conductive Transduction
This technique involves measuring the resistance either through or across the thickness of a conductive elastomer. As illustrated in Figure 14.5.1, the measured resistance changes with the amount of force applied to the materials, resulting from the deformation of the elastomer altering the particle density within it. Most commonly used elastomers are made from carbon or silicon-doped rubber, and the construction is such that the sensor is made up of a grid of discrete sites at which the resistance is measured.
A number of the conductive and resistive designs have been quite successful. A design using carbonloaded rubber originated by Purbrick at MIT formed the basis for several later designs. It was constructed
from a simple grid of silicon rubber conductors. Resistance at the electrodes was measured, which corresponds to loads. A novel variation of this design developed by Raibeit is to place the conductive sheet rubber over a printed circuit board (PCB) which incorporates VLSI circuitry, each forcel not only transduces its data but processes it as well. Each site performs transduction and processing operations at the same time as all the others. The computer is thus a parallel processor.
