Tactile Sensors Overview
Tactile sensors, like vision systems, have been the object of intense research throughout the world. Indeed it is increasingly evident that the limitations suffered by vision systems could be overcome by the cooperative use of tactile sensors and that Robot Multisensory Feedback Systems, comprising at least one 2-D vision system, a range finder and a tactile sensor, are a good base for the development of future generations of robots (Ruocco and Seals, 1986).
Tactile sensors are based on direct measurement force transducers, that is devices which measure the mechanical deformation produced by a force acting directly on the transducer itself. This principle was illustrated in Figure 1. Tactile sensors are based on 2-D arrays of such measuring ‘cells’. Like their human analogue, in fact, robotic tactile sensors work by measuring the contact pressure between the object surface and the robot gripper; by dividing the contact area into an array of several measurement points (typically 16 x 16), a 2-D image of the contact pressure can be mapped, thereby producing a 3-D view, though partial, of the object being manipulated by the robot gripper.
It should be noted at this point that, following a generally accepted convention, ‘tactile sensing’ is herewith defined as the continuous measure ment of contact pressure within an array of so called ‘tactels’, or tactile elements, as distinct from ‘touch sensing’ which is defined as a single contact pressure measurement, as was the case with the force transducers.
Touch sensing can thus be accomplished by touch probes which, although can provide only single-point measurements, are often adequate for sensing the mere presence of an object and/or the force exerted on its surface. An example of such an application is ‘contour following’, where the touch probe is used to guide the robot gripper over the object surface in order to allow some other critical robot operation, such as deburring, welding or even an inspection task like the ultrasonic scan used to detect flaws beneath the object surface, as shown in Figure 2:
Tactile sensing allows continous measurement of the pressure distribution across the tactels array. As well as providing a pressure image of the object at any point in time, the tactile sensor can hence also allow the detection of slippage, another important parameter in the field of object manipulation. This is achieved by looking at the derivative of the pressure images, a task which in practice is achieved by simple consecutive image comparison. Slippage detection is particularly important when handling fragile objects since the forces exerted on their surface must be minimal, in other words the detection of slippage is required by the robot computer to allow the gripper to apply just enough pressure to prevent dropping the object in question.
There are three main types of tactile sensors technology-resistive, capacitive and optical.