Metrological apparatus and calibration method therefor
First Claim
1. A metrological apparatus for measuring physical surface geometry, said apparatus comprising a transducer, said transducer producing an analogue electrical signal representative of a characteristic of a physical surface the geometry of which is to be measured, and an analogue to digital converter the gain of which varies with frequency over a particular frequency range, said analogus electrical signal being input to said analogue to digital converter to be analogue to digital converted thereby into a digital signal,said apparatus further comprising processing means for processing said digital signal to compensate for said variation of gain with frequency over said particular range.
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Accused Products
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.
79 Citations
21 Claims
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1. A metrological apparatus for measuring physical surface geometry, said apparatus comprising a transducer, said transducer producing an analogue electrical signal representative of a characteristic of a physical surface the geometry of which is to be measured, and an analogue to digital converter the gain of which varies with frequency over a particular frequency range, said analogus electrical signal being input to said analogue to digital converter to be analogue to digital converted thereby into a digital signal,
said apparatus further comprising processing means for processing said digital signal to compensate for said variation of gain with frequency over said particular range.
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11. A metrological apparatus comprising a transducer, said transducer producing an electrical signal representative of a surface characteristic and having a normal range over which said electrical signal is substantially linearly related to a position detected by said transducer and an extended range beyond said normal range, over which said electrical signal is non-linearly related to a position detected by said transducer,
said apparatus further comprising compensator means for processing said electrical signal to generate a compensated signal which is compensated for non-linearity over said extended range so that said compensated signal is substantially linearly related to the detected position over said normal and extended ranges of said transducer.
- 16. A metrological apparatus comprising a transducer, said transducer comprising a stylus and producing an electrical signal representative of a characteristic of a surface across which the stylus is traversed while in contact therewith, and the apparatus further comprising drive means for traversing said stylus across a surface in contact therewith and means for providing a traverse position signal in accordance with the position in the traverse direction of the point of contact of said stylus with said surface, the position of said point of contact in the traverse direction varying with the position of said point of contact in a direction towards and away from said surface, and the apparatus further comprising traverse signal compensator means to process the traverse position signal to compensate for said variations in the position of said point of contact.
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20. A method of calibration of an apparatus which apparatus comprises a transducer, means to traverse said transducer over a surface, and means to output a traverse position signal representative of the position of the transducer in the traverse direction, said transducer outputting a transducer output signal representative of a characteristic of said surface, and at least a first one of said transverse position signal and said transducer output signal undergoing a variation from a linear relationship with the property represented thereby with variation, at least over a part of the range of said transducer, in a position of said surface detected by said transducer, said apparatus further comprising compensator means for processing said first signal to compensate said first signal for said variation undergone by said first signal, said compensator means using at least one of said output signal, the square of the output signal and the cube of the output signal in said processing together with at least one calibration constant,
and said method of calibration comprising the steps of traversing said transducer over a surface of known form, storing a series of values of said output signal obtained during said traverse, setting an initial value for said at least one calibration constant, and determining a correction to said at least one calibration constant so that the stored signals as processed by said compensator means using said at least one calibration constant following correction tend to equal those signals which would be expected to be provided from a traverse of the surface of known form by an apparatus providing fully compensated signals.
Specification