Patent GB-1087132-A (GB)
Title: Improvements in or relating to scale reading means
Inventor: GODFREY CHARLES JOHN
IPC: G01D5/26
Filed: 1963-06-05
Abstract: 1,087,132. Photo-electric position determination. HILGER & WATTS Ltd. June 3, 1964 [June 5, 1963], No.22461/63. Heading G1A. A device for determining the position of a linear or circumferential scale bearing member comprises a spiral graticule for indicating fine subdivisions relative to the graduations on the main scale. Photo-electric means are provided for viewing a combined image of a scale graduation and a segment of the spiral and to avoid ambiguity when the value to be read is close to a main scale graduation the change in radial dimension between the end positions of the spiral as imaged on the detector is made less than the imaged spacing between the scale graduations. In one embodiment a linearly movable member 2 carries opaque scale graduations 4 and is illuminated by light from a lamp 6. Lens 8 produces an image of a small part of the scale, including a mark, in the plane of disc 10. The disc is opaque and has a transparent track 26 in the form of an Archimedian spiral. Superimposed images of the scale portion and the corresponding portion of track 26 are formed on photo-cell 14 by lens 14. The disc 10 may simply be rotated about axis 22 until the image of the trade coincides with that of the scale mark. This is indicated by the photo-cell output, as shown on meter 18, reaching a minimum. The position can then be read off from scale 4 and scale 28 marked round the edge of the disc. A small angular gap is left between the ends of the track 26 which does not cover the entire distance between two scale markings, this ensures that, although the field of view of the cell 18 at the disc is of finite size, ambiguities of one scale division close to the zero disc position do not arise. For control purposes it is preferred to obtain an alternating current output from the photo-cell by cyclically modifying the light path from the scale by oscillating the lens 8 as Indicated by the arrow A. This can conveniently be done by mounting it on leaf springs 42 (Fig. 7) and vibrating it by means of an A. C. energized electromagnet 46. The cell output then has a component of the same A. C. frequency which differs in phase according to the direction of deviation from the position of symmetry. By the use of a phase discriminator unit 16 fed with the A. C. energizing the vibrator a convenient signal can be derived for indication or control purposes. In Fig. 7 the control signal so obtained is used to operate a relay to switch off and arrest a motor driving disc 10 when the position of symmetry is reached. It may also actuate means 80, 84, for displaying or recording, preferably in digital form, the position as indicated by marks on the scale and edge of the disc. To ensure that a single main scale mark is always centred in the reading means (except for a brief period of changeover as the disc 10 passes through its zero position) the shaft carrying the disc 10 may also carry a cam cooperating with a cam-follower moving an optical plate in the viewing system for this purpose Figs. 5 and 6 (not shown). Alternatively where a motor system drives the disc 10, as in Fig. 7, it may be coupled to move the reading means to follow a scale division and have a quick return motion at zero disc position. To improve the accuracy of the device a plurality of spiral graticules and reading devices spaced apart an integral number of scale divisions may be used. The sum of the photo-cell outputs is then applied to the indicating and/or control system. This arrangement averages out any errors in the production of the spiral or main scale.