Patent GB-1328386-A (GB)
Title: Instrument with indicating and relay modes of operation
IPC: G05B11/01
Filed: 1970-08-12
Abstract: 1328386 Indicating-apparatus; photo-electric value monitor WESTON INSTRUMENTS Inc 12 Aug 1970 [13 Aug 1969] 38824/70 Headings G1A G1J Apparatus providing a light display representing the value of a variable by displacing a beam of light along an elongated, light transmitting surface 16 is also provided with at least one flexible film 44 having an elongated light transmitting portion 44A at least partially covering a corresponding portion 16A of the surface 16 and having mounted thereon adjacent the portion 44A means 45 for producing an electrical signal when activated by the light beam at a predetermined position along the surface 16, the signal being conducted to utilizing means e.g. relay 57. White light from an incandescent lamp 22 behind a wall 21 is concentrated by a lens 25 in an aperture in the wall, reflected by an inclined mirror 26 and focussed by a concave mirror 27 on to the light-diffusing screen 16. In accordance with the variable a meter 30 (e. g. of D'Arsonoval type) moves a vane 28 in the light path between the mirrors 26, 27. The vane 28 is perpendicular to the focal axis and formed by a film acting as a colour filter e. g. passing red light so that a red light bar 29 with leading edge 29A is sharply defined and moves along the surface 16 over its full width. Viewed from the front an opaque scale plate 19 obscures the lower portion of the screen 16 and has graduation marks against which the edge 29A indicates the value of the variable. The film 44 supports near its free end the photo-detector 45 and has a corresponding index mark 60 indicating against the scale to facilitate adjustment of the detector to a desired set point. The detector is a solid state light switch e.g. a photo-sensitive silicon-controlled rectifier which changes to a conductive state when the level of radiation incident thereon exceeds a threshold level and vice versa. The three terminals of the detector are connected to three fixed terminals 51, 52, 53 on a plate 61 (to which the fixed end of this film 44 is attached by adhesive) by three flexible copper foil strips bonded to the film 44. The relay 57 is arranged in a circuit with the terminals 51, 52, 53 and an A. C. source 58 and follows the conductive and non-conductive state of the detector 45 to change over the contacts 59A, 59B of a switch 59. With the vane 28 acting as a red filter the threshold level of detector 45 is chosen so that the state of the detector depends on the position of the red light bar 29. To provide for upper and lower set points the arrangement is duplicated, using films 44, 44<SP>1</SP>. The films may be made of a polyester thermoplastic a few mils thick and have holes to engage sprocket wheels that can be manually driven by knobs 71, 71<SP>1</SP> to adjust the set points. The plates 61, 61<SP>1</SP> are spaced from side walls 12, 11 against which loops of the films tend to spring, thus ensuring smooth paying in and out. The housing is sealed against the entry of ambient light and dust and its interior is coated with light absorbing material. A modification of the vane is described, Fig. 6 (not shown).