Abstract: A system for measuring the angle of a beam of light is arranged so that a periodically varying pattern is formed on a photodetector, with a waveform property such as wavelength or phase of the pattern varying with the angle of the light. This may be done by creating interference between the light beam to be measured and a reference light beam having a fixed angle, or by forming a shadow on the detector array of one or more masks having a periodically varying transmission characteristic. One convenient way of processing the output of the photodetector array is to perform a fast Fourier transform, obtain therefrom one or more spatial frequency components, and extract therefrom a part which provides a measure of the angle of the light beam. The use of a periodically varying pattern on the photodetector array enables a substantial length of the array to be used in any particular measurement operation, and averages out individual performance differences between different elements of the photodetector array.

Abstract: A metrological apparatus has a movable arm 15 which moves up and down between two bearing members 13, both of which define datum positions for the arm 15. The arm 15 has air bearings on the bearing members 13, so that it adopts a position equidistant between the bearing members 13. In this way, the position of the arm 15 contains the average of the errors in the two datum positions defined by the bearing members 13. Where the bearing members 13 have symmetric errors, these will be cancelled. Since the path of maximum straightness of movement of the arm 15 is spaced from the bearing members 13, it is possible to put a workpiece 3 in this path and a turntable 5 for supporting the workpiece 3 is arranged to have its rotational axis in line with the path of maximum straightness.

Abstract: Apparatus for surface measurement has a grating interferometer with a curved diffraction grating carried on a pivotal support arm of a probe for contacting a surface. The probe is biased into contact with a surface by an electromagnetic coil acting on an armature, or a pair of such biasing arrangements. A laser diode illuminates the grating to produce a pair of first order diffracted beams of opposite sign which are reflected from internal faces of a prism, and combined by a diachronic central layer of the prism and a pair of beam splitters. Output signals from the beam splitters are supplied to a signal processing circuit having a fringe counter and an interpolator. The fringe counter detects zero crossings of the signals and the interpolator maintains a digital estimate of the phase of the signals and updates the estimate when the phase difference between the estimate and the input signals exceeds a predetermined threshold.

Abstract: Apparatus for measuring surface roughness utilizes diffraction patterns derived from the surface under test. The energies in the different orders are detected and processed utilizing various different algorithms dependent upon a number of factors such as the number of orders in the diffraction pattern and asymmetry in the diffraction pattern. Various filter functions both for the diffraction pattern itself and for values of surface roughness after measurement are disclosed. One algorithm for measuring surface roughness involves comparison of values representing the energy orders with reference values, preferably reference values obtained by an interpolation operation between one set of reference values representative of a diffraction pattern from a symmetrical surface and another set representative of a diffraction pattern from an asymmetrical surface.

Abstract: A metrological apparatus for measuring form and texture of a surface employs an inductive transducer producing an AC gauge signal, and a tracking AC analogue to digital converter. In order to avoid errors in the output signal at a relatively high speed of traverse of the surface due to a non-flat frequency response of the converter, the output signal is processed by a digital filter to provide a combined frequency response which is flat over a desired range. In order to avoid errors in the output signal due to non-linearity of the transducer, a square or cubic correction is applied to the signal. A method calibrating the apparatus to obtain coefficients for the square and cube terms in the correction process is also described.

Abstract: An interpolator, particularly for use with an optical grating for detecting position in a metrological apparatus, receives input signals in quadrature. One of the input signals is squared and the two signals are multiplied together to provide further signals in quadrature at twice the frequency of the input signals. One of the further signals is also squared and the further signals are multiplied together to provide output signals at four times the frequency of the input signals. Filtering is provided to remove DC components, such as due to drift. In the illustrated metrological apparatus, the zero crossing points of the output signals are detected to effect sampling, with high resolution, of a surface sensor output. Interpolation noise occurs at a high frequency and is removed by low pass filtering of the sample signal from the surface sensor.

Abstract: In order to measure the surface profile of a workpiece, a yoke having two fixed feet is held in contact with the surface as the workpiece is rotated. A dynamic probe is mounted on the yoke by means of a transducer which outputs a signal dependent upon the position of the dynamic probe relative to a transducer datum. The fixed feet and the foot of the dynamic probe have a similar shape and size so that they detect irregularities in the workpiece surface with equal sensitivity. Fourier analysis of the signal caused by movement of the dynamic probe directly by the workpiece and also by movement of the yoke enables the profile of the workpiece to be determined. In order to improve the frequency response of the transducer output over the range of harmonics considered, the angles between the dynamic probe and the fixed probes are unequal. The apparatus does not require a precision spindle type machine to rotate the workpiece and is therefore particularly suitable for in situ measurement of workpiece profiles.

Abstract: A metrological instrument includes an inductive transducer having a stylus, all being disposed within a protective housing. Upon initiation of the measuring operation, the transducer is moved so that the stylus moves round an endless path such that the stylus is first projected at a high speed from the housing, is thereafter moved to engage the workpiece surface, is then moved at relatively slow speed across the workpiece surface for performing the measurements, and, following completion of the latter movement, is retracted at a high speed into the housing. These movements are effected by a series of cams driven by a single motor. Such an instrument may be located next to a production line without risk of damage and may used for the checking of each of a multiplicity of identical components produced in a mass production process.

Abstract: In a position encoder an interpolator derives from quadrature input signals a series of sine waves equispaced in phase. These are applied to respective comparators which output a binary zero or a binary one according to whether the respective sine wave is above or below the zero crossing point. Interpolation is achieved by identifying an adjacent pair of said sine waves of which one has a value above and the other a value below zero and a counter stores a number indicative of this pair of signals.

Abstract: Metrological apparatus, particularly for measuring form, has a turntable 4 and a transducer 14 and associated stylus 12 mounted so that the tip 12a of the stylus is displaceable radially and vertically relative to the turntable 4. The problem of obtaining both high resolution and large range of operation is solved by using a transducer/stylus 12,14 having a resolution of 12 nanometers and a range of only 0.4 mm and mounting the transducer 14 on an arm 10 which is driven radially and/or vertically by a computer 30 in response to the output from the transducer 14 to cause the stylus 12 to follow the workpiece surface. High resolution data defining the rotary position of the turntable 4 and the radial and vertical position of the transducer 14 is provided by a photodetector/optical grating arrangement 23, 21, 31; 27, 19, 43 and associated interpolators 42, 44, 46.

Abstract: Apparatus for measuring velocity utilizes quadrature input signals provided by a transducer and divides the derivative of one of the signals by the magnitude of the other to obtain a measure of velocity. The apparatus may be digital or analog. It is particularly suitable for the measurement of velocity where the quadrature signals are provided by an optical grating and transducer arrangement.

Abstract: In a metrological or other sensitive apparatus, the transducer or other operative member (22) is mounted on a support structure (14 to 18) which is carried on a chassis (2) by a first set of dampers or isolating means (10) and the chassis (2) is mounted on the floor by a second set of dampers or isolating means (6). The two sets (10, 6) have different natural frequencies and the first set (10) is arranged so that any vibration of the support structure tends to be rotational centered in the zone of operation (21) of the transducer or other member. This is achieved by arranging that the stiffness of the first set of dampers (10) is higher in the direction (24) passing through the zone of operation (21) than in the transverse direction.

Abstract: A bearing structure, preferably for supporting a rotary table (4) in metrological apparatus, includes both a journal bearing (34,36,38) and a thrust bearing (56,66). The journal bearing is made up of three dry pads (34,36,38), one (38) of which is radially adjustable, and the thrust bearing (56,66) comprises first and second air bearings. The axis of rotation of the bearing is vertical, and the lower (66) of the air bearings is less stiff than the upper one (56).

Abstract: To measure distances, light from a flash tube is plane polarized by a polarizing filter, then polarization modulated by an electro-optical crystal, then passed along a `wobbling` light path, then subjected to a relative retardation of about a quarter wavelength by a rhomb and then passed through a calibrated variable light path before being projected to a distant target reflector. The reflected light returns along the same path and is detected by a photomultiplier after passing through the polarizing filter. Adjustment of the variable light path until a null is obtained at the photomultiplier indicates that the length travelled by the light from the crystal to the reflector and back is an integral number of modulation wavelengths, possibly plus one half-wavelength. The quality of the null is adjustable by adjusting the orientation of the rhomb. Temperature correction may be provided by constructing a reference resonator defining the modulation wavelength of the same material as a structure being measured.

Abstract: Metrological apparatus is provided with a turntable (4) which is movable horizontally and pivotally for centering and levelling. The turntable (4) is supported at three points (A,B,P), one (P) of which is fixed and the other two (A,B) of which are vertically movable by motorized jacks. Centering and levelling is achieved automatically by means of a computer (72) which calculates the required horizontal and pivotal movements of the turntable (4) from data obtained by sensing the workpiece surface with a transducer (12).