Position Transducers
Position Transducers
The control structure of a robot needs to know the position of each joint in order to calculate the position of the end effector (e.g. gripper) thus enabling the successful completion of the programmed task. The movements of the joints can be angular and/or linear depending on the type of robot; they are illustrated in Figure 1 for each of the four main robot coordinate systems. A suitable algorithm for calculating the end effector position in any of the coordinate systems can then easily be deduced, as shown in Figure 2 for the polar coordinates type robot, thus permitting the development of the necessary robot control software.
It should be noted that future developments in the robotics field may one day allow the use of external sensory feedback, or exteroceptors, as
Figure 1 Robot coordinates systems (courtesy of PERA)
Figure 2 Schematic representation of polar coordinates robot and relative end effector position formulae
shown in Figure 3, to actually ‘measure’ the end effector posItIon in relation to its surroundings, thereby dispensing with the need to calculate it from the joint positions. Indeed some commercial systems have successfully proved the worth of this technique in industrial applications. This approach, however, based on an adaptive technique similar to the way the human body
Figure 3 Block diagram of second generation robot system, showing the use of exteroceptive feedback
operates, is still largely at the research stages and may not be implemented generally on commercial machines for some time.
Internal position transducers (proprioceptors) therefore remain, at least for the present, the most accurate and reliable way of determining the end effector position within a robot control structure. 
There are two main types of position transducers: absolute and incremental (or ‘relative’). The absolute position transducers are themselves further subdivided into resistive and optical types, as illustrated in Table 2.1. Other types of position transducer are also employed in industry, such as differential transformers and synchros, but their larger size and/or higher cost have precluded their use within robot structures and will not therefore be discussed here.
