Absolute position transducers are necessary when the control system is subject to frequent power shut-down periods and when frequent resetting of the robot arm positions needs to be avoided (as would be required if incremental types were used), such as in high flexibility and high reliability position sensing systems. Absolute optical encoders are used, for example, in the UNIMA TE 2000 and 4000 series hydraulic robots.
Optical absolute encoder disks are based on a principle similar to that for incremental ones, producing an electrical signal proportional to the shaft angular position by light absorption using opaque lines on a transparent disk.
The difference lies in the opaque pattern used-where the incremental disk uses a channel of alternate opaque and transparent lines, the absolute encoder disk has several channels of either binary or Gray coded patterns with each channel requiring an LED-photodiode combination to detect the logic state of each bit, as shown in Figure 2 for a 4-bit Gray coded disk.
The output therefore is provided directly as a digital number representing the angular position measurement whose resolution depends on the number of bits, that is the number of channels used, as shown by eqn (2.5), where c is the number of channels required:
For instance, to design an absolute optical position transducer with a resolution of less than 5° of arc one would need an encoder disk with seven channels, since 360°/27 = 360°/128 = 2.81° which is physically bigger and more expensive than an incremental encoder of comparable resolution.
The encoded disk pattern usually employed is based on a Gray code because it mlmmlzes measurement errors since it allows only one bit to change state at any point in time. The alternative, namely a binary coded disk pattern, is not very popular because it may produce ‘race’ conditions in the digital counting hardware which would lead to measurement errors; this problem can occur in digital systems whenever more than one logic state changes at the same time, such as when counting from 01111111 to 10000000 when all 8 bits change. Figure 3 allows the comparison between a 4-bit absolute encoder disk based on Gray code and one based on binary code.