Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1984-05-25
1987-08-25
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
250226, 250227, 250237G, G01D 534
Patent
active
046894853
DESCRIPTION:
BRIEF SUMMARY
This invention relates to displacement measuring apparatus and has uses in the measuring of displacement of a carriage relative to a supporting track in machine tools.
BACKGROUND OF THE INVENTION
It is known in displacement measuring apparatus, e.g. for machine tools, to provide a mark-space grid on one of two relatively movable members and a reading head on the other of said members. The reading head is connected to a control unit situated on the one member, the latter usually being stationary, by a flexible cable for two-way transmission of electrical signals. Further, the reading head includes an optoelectronic system for converting an electrical signal received from the control unit into an optical signal for illumination of the grid, and for converting optical signals generated by the marks and spaces of the grid into electrical signals for transmission to the control unit, there to be converted into actual dimensions of measurement. Also, the head usually embodies means for determining the sense of direction of the movement. It has been found that the reading head can be relatively bulky or sensitive to vibration, or that the apparatus can be needful of relatively expensive connector and cable means for transmission of the electrical signal between the head and the control unit. It is generally an object of this invention to overcome or reduce those difficulties.
SUMMARY OF THE INVENTION
According to this invention there is provided apparatus for measuring displacement between a fixed and a movable member, comprising an elongate mark-space grid secured to the fixed member, a reading head secured to the movable member and adapted to traverse the grid along the length thereof during movement of the movable member, the reading head having two input stations, each adapted to receive radiation emanating from said grid in the form of a wave signal corresponding to said marks and spaces thereof, the two stations being spaced apart in the direction of the length of the grid by a distance p(n +a) where p is the pitch of the marks, n is a whole number, and a is a value of approximately 1/4 p so that during movement of the reading head the two wave signals are out of phase by approximately 90.degree., means for counting the periods or fractions thereof of at least one of the wave signals thereby to determine the relative displacement of the reading head and the grid during said movement, and means for detecting which of the two wave signals is leading the other thereby to determine the sense of direction of said displacement, characterised in that the detecting means is situated on fixed structure in a position remote from the said reading head and is connected thereto by an elongate flexible radiation guide adapted to transmit said wave signals from said reading head to said detecting means.
Preferably said radiation guide is a single such guide, said radiation insofar as it is derived from said stations has different frequencies, said reading head has means for combining said frequencies for transmission by said single guide, and said detecting means include means for discriminating between said two frequencies.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagrammatic side elevation of parts of a machine tool embodying the apparatus,
FIG. 2 is an enlarged detail of FIG. 1 showing an optical part of the apparatus,
FIG. 3 is a section on the line III--III in FIG. 2,
FIG. 4 is a block circuit diagram of an electronic part of the apparatus,
FIG. 5 is a signal diagram.
FIG. 6 shows a modification of FIG. 2.
DESCRIPTION OF PREFERRED EMBODIMENTS
The machine (FIG. 1) comprises a carriage 10 movable along a track 11 and supporting a reading head 12 movable together with the carriage relative to an elongate grid 13 secured to the track. The reading head 12 is connected by a single light guide 14 to a control unit or input/output unit 15 mounted on the track or other fixed structure of the machine.
The unit 15 (FIG. 2) has a random light source 20 illuminating the wide end of a first convergent transparent mem
REFERENCES:
patent: 4096383 (1978-06-01), Mancini et al.
patent: 4162399 (1979-07-01), Hudson
patent: 4204115 (1980-05-01), Boldridge, Jr.
patent: 4242574 (1980-12-01), Grant
patent: 4356396 (1982-10-01), Ruell
patent: 4546246 (1985-10-01), Bechtel
Gatto James G.
Nelms David C.
Renishaw plc
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