Optical Tactile Sensors
Optical Tactile Sensors
Optical tactile sensors are gammg in popularity as the field of opto electronics expands from communications and displays to other engineering applications, bringing with it established and therefore easily available, low cost technology. The operation of a tactile sensor based on an optical principle is illustrated in Figure 1 and relies on changing the light absorption characteristics of the transparent medium under the effect of an applied force.
Several clear plastics and glass fibres exhibit this property, so one implementation of an optical tactile sensor is based on an x-y array of light sources and detectors communicating via glass fibres or arranged on the edge of a square sheet of the aforementioned clear plastic, as shown respectively in Figure 2 (a) and (b). Another optical technique which can be used to build a tactile array is shown in Figure 3 and is based on the slotted optoswitch, where the force is applied to a plunger within each individual tactel and partially interrupts the light path between an LED and a photodiode whose output thus provides a measure of the applied force. A tactile sensor based on this principle is available from the Lord Corporation and its agents worldwide.
The resolution and sensitivity of these optical tactile sensors varies considerably but, on the whole, is lower than those afforded by the resistive types. However, the ease of operation and the relatively inexpensive as sociated electronics are major advantages and may provide a good reason for use within a robot multisensory feedback system.
Another technique for optical tactile sensing has been described by researchers at the Massachusetts Institute of Technology (Schneiter and Sheridan, 1984) and provides a much higher tactel resolution at the expense of system complexity and cost. The proposed sensor, in fact, uses optical fibres and a standard television camera to provide a visual image of the mechanical deformations produced on the force sensitive ‘skin’ which is made up of a clear elastomer coated by a reflective silicon rubber. The optical fibres used are much smaller than the typical tactel based on the opto-switch or the capacitive principle and therefore afford a much higher tactel resolution (2100 touch sensitive ‘spots’ per square inch have been reported in the literature).