Patent GB-1079495-A (GB)
Title: Improvements in or relating to instrument  reading
IPC: G01D5/30
Filed: 1964-11-02
Abstract: 1,079,495. Photo-electric indicator reading; recording apparatus. A. ARNOUX. Nov. 2, 1964 [Nov.13, 1963; Oct. 9, 1964], No. 44546/64. Heading G1A and G1J. A reading device for pointer-type indicators comprises a reading head 1 in which are mounted two photo-electric cells 7 and 8 associated with means, such as tunnels 9, 10, directing the axes of the light beams falling on them along paths have axes intersecting at a point outside the head. The cells are connected in a Wheatstone bridge including resistors 11 and 12 and balancing potentiometer 13. The output across one diagonal of the bridge is fed to detector unit 15. Preferably a lamp 5 is mounted in the head. The angle between the photocell viewing axes is preferably greater than 90‹ and is bisected by the perpendicular from the head face 20 through their point of intersection. If a source 5 is included it is positioned along this perpendicular. The reading head is positioned by means of a support which may include clamps, rubber-bands, suckers, or brackets against the front of an indicator so that the cell viewing axes intersect at a point approximately on the dial immediately behind the position of the pointer 4 it is required to sense. As the pointer moves through the field of view in the direction 5 the bridge output signal applied to the detector 15 is as shown in Fig. 2. The detector is so arranged that it produces no output change at point U o , due to bias applied to its first stage, but gives an output signal from a point shortly beyond U 10 . The signal may be applied to a relay or relays controlling indicator lamps and control circuits. The reading head containing the photo-cells and light source may be made quite small and connected by a multicore cable with a unit containing the rest of the apparatus. As described so far the unit may contain means for energizing a relay when the pointer is immediately to the left of the point A, it may also contain a similar circuit, oppositely biased, to indicate when the pointer is to the right; the two relays concerned releasing each others holding circuits in accordance with the direction the pointer moves through the point A. To avoid the system being affected by external radiation when the reading head is made small for convenience in positioning and to minimize obstruction of direct viewing, the head may be surrounded by an annulus of polarizing material. The part under the head and particularly under the lamp passage is left clear whilst the cell passages are screened by polarizers crossed with respect to the annulus. To check the balance of the bridge the head support may be arranged so that the head can be withdrawn so that the intersection of the photo-cell viewing axes lies in the plane of pointer movement. The pointer then has no effect on the balance of the bridge wherever it is. Mechanism may be provided to automatically lift the head to checking position at regular intervals, the bridge output at such times being applied to a motor to adjust potentiometer 13 to rebalance the bridge. The reading head may be mounted so that it repeatedly scans across the dial of the indicator; a sawtooth voltage waveform being generated in synchronism with the scan. When the reader passes through the pointer position it energizes a relay, as described above, to sample the voltage of the sawtooth and thus obtain an electrical output proportional to pointer position. For recording, the head 1 may be scanned repeatedly over the dial of meter 4 by means of motor 147, eccentric 149 and arm 143. The arm is provided with a springy downward extension 154 bearing a stylus 155 co-operating with disc 156 driven at a slow speed and carrying the recording medium. At each sweep of the head the stylus marks a fine arcuate line but when the head detects the pointer an actuating device is energized to press bar 159 against the arm 154 and cause the stylus to make a bold mark such as 168.