GB1286490A - Incremental measuring system including an error detecting circuit - Google Patents

Abstract

1286490 Incremental measuring systems DR JOHANNES HEIDENHAIN 12 Sept 1969 [12 Sept 1968] 45199/69 Heading G4H In an incremental measuring system in which signals T4, T5 phase displaced by 90 degrees are counted positively or negatively according to the direction of movement in a counter 9, there being four counting pulses for each cycle of one of the signals T4, T5, to detect errors in the count the signals are additionally fed to logic circuitry for comparision with the count. As described signals T4, T5 are derived from photo-cells 14, 15 sensing a grating 20, 11. A bi-stable 20 delivers a "1" or a "0" input to an exclusive OR gate 16 which receives at its other input the signal T4 so that the output signal on conductor 21 fed to an exclusive OR gate 17 is either in phase or out of phase with the signal T4 in dependence on the bistable output. The gate 17 also receives the signal T5 so that it delivers an output signal on conductor 22 at twice the frequency of the signals T4, T5 to an exclusive OR gate 18, the phase of the signal on conductor 22 is inverted by a change in state of the bistable. The gate 18 also receives a signal on conductor 23 representing the state of the lowest stage of the counter (that is representing the instantaneous count mod. 2) this signal being of twice the frequency of the signal T4, T5. When the signals on conductors 22, 23 are in phase no output is obtained from gate 18 but when they are out of phase a lamp 28 is lit and bi-stable 20 is switched to bring the inputs to gate 18 in to phase. The lamp is extinguished by operation of switch 30. A pulse forming unit 34 delivers pulses of logical value zero on conductor 41 at the edges of pulses from gate 17 to prevent errors of the switching instants of the counting pulses. Capacitors or conductive or magnetic scanning may alternatively be used.