Patent GB-811616-A (GB)
Title: Direction sensing and displacement detection apparatus
IPC: G01D5/20
Filed: 1957-08-16
Abstract: 811,616. Electrical displacement detectors. HUGHES AIRCRAFT CO. Aug. 16, 1957 [Aug. 16, 1956], No. 25989/57. Class 40 (1). Apparatus adapted to detect the direction and extent of the movement of a slider 12, Fig. 1, comprises a number, say four reading heads 17-20 which produce binary signals according to their disposition relative to a linear scale member 10, the heads being spaced apart by one-eighth of the distance of one cycle along the scale, a first register 30 for storing samples of a last pattern of signals from the reading heads, means including a second register for sampling and storing samples of a present pattern of signals, the sampling being made before the movement has exceeded the distance covered by the heads and means for detecting and counting the number of disagreements between the bits of the last pattern of signals and the bits of the present one, the number of disagreements being equal to the number of positions between the positions corresponding to the last and present signal patterns. This number is added to or subtracted from a number in an accumulator to obtain the value of the present position, the direction of movement being determined by a subtracting circuit controlled by a selected bit to subtract individual bits of the past and present patterns according to certain rules whereby a negative movement is indicated by the production of carry signals. The scale 10 is magnetic and formed by grooves and ridges which may be each a thousandth of an inch in width. The reading heads have pointed cores and are each connected to circuits 26-29 which produce a binary " 1 " or " 0 " according to the inductance of the coil of the head. When a head is opposite a groove, a binary " 0 " is produced and a " 1 " if opposite a ridge. The pattern of signals produced from the four heads represents their disposition relative to a cycle of the scale, one of the signals changing for a movement of one position. The binary value corresponding to the present position of the slider 12 relative to the scale is held in a circulating register 32 which is a magnetic drum also producing a train of clock pulses 100, Fig. 2. These are applied over line 43 to the stages Q 1 -Q 4  of the shifting register 30 and to bit counter 38. This counter produces control pulses 102, 104, Fig. 2, at the fourth and twentieth bit times (t 4 , t 20 ). These pulses pass to the set and reset inputs of control flip-flop Q P  which divides the operation of the apparatus into a " serial shift " phase (intervals t 1 -t 4  during which the four signals from the reading heads are passed into the shift register 30) and an "accumulator" phase (intervals t 5 -t 20 , during which the number of changes is added or subtracted from the value in the accumulator). Sampling gates 62-65 are opened at time t 20  and the bits enter the stages Q 1 - Q 4  previously reset by the set signal Qp of flip-flop Qp. In the next four bit times t 1 -t 4  the contents of register 30 are shifted out through the end stage Q 1  which gives an " A " signal for a " 1 " and an " A for a " O." These signals are passed through Or gates 70, 72 to the " write " flip-flop Qw and recorded on the drum. At the same time the four binary signals representing the previous pattern from the reading heads are read out from drum 32 on lines B or B and compared digit by digit with the present pattern represented by signals A, A. This is done in a gating matrix 40 designed to pass a signal to a two-stage counter 34 whenever the bits of the two patterns disagree. The value in the counter 34 is applied with the value read out from the accumulator track of the drum 32 to an adder-subtractor 36 and the resulting sum or difference is re-recorded through the write flip-flop Qw. Whether the values are to be added or subtracted is determined by a direction flip-flop C d  which is controlled by a subtracting circuit 42 in which the individual bits of the present and last patterns are subtracted according to rules determined by a control bit. In this case the control bit is selected from reading head 17 and taken from register stage Q 4 . The rules are such that when the subtraction produces one or more carries, a negative movement is indicated and the production of zero carries indicates none or a positive movement. This is effective by applying the set and unset outputs of the Q 4  stage (signals Q 4  and Q 4 ) to And gates with A and B and A and B signals respectively. Coincidence then indicates a carry and applies a setting signal to direction flip-flop C d .