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Capacitance-type measuring device for absolute measurement of positions

  • US 5,053,715 A
  • Filed: 12/28/1990
  • Issued: 10/01/1991
  • Est. Priority Date: 04/04/1986
  • Status: Expired due to Term
First Claim
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1. Capacitive-type measuring apparatus comprising:

  • first and second support members, said support members being relatively displaceable with respect to each other, and at least one of said support members being displaceable relative to a measurement axis;

    electrode array means disposed on said first and second support members in relative alignment with said measurement axis for providing a plurality of discrete signal transmission paths each having a capacitive transfer function which varies in dependence on the relative positions of said first and second support members with respect to each other;

    said capacitive transfer function having a first component which varies over a first predetermined wavelength Wc according to the position of the associated transmission path relative to a reference position on one of the support members, and a second component which varies according to a predetermined function of the displacement between said first and second support members over a second predetermined wavelength Wm shorter than said first predetermined wavelength;

    said electrode array means comprising;

    a transmitter electrode array having at least one group of N adjacent transmitter electrodes spaced from each other relative to the measurement axis, wherein N has a value which is an integer multiple of four;

    an array of first receiver electrodes disposed on said second support member in alignment with said measurement axis and such that differing portions of said first receiver electrode array are capacitively coupled with said transmitter electrode array in dependence on the relative positions of said supporting members;

    said first receiver electrodes being spaced from each other relative to the measurement axis by a pitch Pr1 defining a scale wavelength Wf,said at least one transmitter electrode group defining a transmitting wavelength Wt, andthe transmitter electrodes in each group being positioned within the group so as to respectively occupy predetermined group positions which span a distance greater than one wavelength Wf, and such that each group position corresponds to the relative position of a different one of a group of relative fine wavelength segment positions obtained by dividing the transmitting wavelength Wt into intervals corresponding to the fine wavelength, and dividing each interval into N equal segments; and

    detector electrode means for producing first and second outputs in response to excitation signals applied to said at transmitter electrode array, said detector electrode outputs individually varying in accordance with said first transfer function component and producing when combined a signal which varies in accordance with said second transfer function component;

    excitation signal generating means for selectively generating groups of N excitation signals for application to the respective N electrodes of said at least one group of transmitter electrodes in a selected one of first and second spatial orders;

    said first spatial order being defined by a first order of connection according to the sequence of positions of the transmitter electrodes relative to each other in said at least one transmitter electrode group, and said second spatial order being defined by a second order of connection according to the sequence of relative fine wavelength segment positions in which the respective transmitter electrode group positions are arranged;

    each of said groups of excitation signals comprising two sets of excitation signals which are phase inverted with respect to each other and in spatial phase quadrature when applied to said transmitter electrodes, and wherein the relative spatial phase positions occupied by the respective sets of excitation signals in each group of excitation signals successively incrementally change from one group to the next;

    first control means for controlling said excitation signal generating means to selectively apply a pair of first and second ones of said groups of excitation signals to said at least one group of transmitter electrodes;

    output signal selecting means for selecting a signal corresponding to said first and second detector electrode outputs or to said combined detector electrode output signal as an electrode array means output signal, and thereby producing successive first and second electrode array means output signals in response to said pair of excitation signal groups;

    means for demodulating said first and second output signals to produce first and second demodulation signals, and for performing a dual ramp integration of said first and second demodulation signals wherein said first demodulation signal is integrated for a predetermined time interval by integrator means, and said second demodulation signal is integrated by said integration means until the integrator output returns to a reference level, said first and second ones of said groups of excitation signals being selected such that said integration of said second demodulation signal causes integration of the integrator output in the reverse direction from said integration of said first demodulation signal output;

    means for measuring the integration time of said second demodulated signal integration and for producing an output when the integration time exceeds a predetermined limit value;

    second control means responsive to said output of said integration time measuring means for resetting said demodulation and dual ramp integration means, for repetitively selectively applying a different further pair of first and second ones of said groups of excitation signals to said at least one group of transmitter electrodes, and for causing said demodulation and dual ramp integration means to integrate the resultant first and second demodulation signals until there is no output from said integration time measuring means;

    means for deriving a scale position value from the integration time of the second demodulation signal integration which does not produce an output from said integration time measuring means;

    master control means for controlling said excitation signal generating means and said output signal selecting means so as to perform a measurement cycle for sequentially producing;

    a first scale position value from a second demodulation signal produced from a second electrode array means output signal corresponding to said first and second detector electrode outputs produced in response to a group of excitation signals generated in said first spatial order;

    a second scale position value from a second demodulation signal produced from a second electrode array means output signal corresponding to said combined detector electrode output signal produced in response to a group of excitation signals generated in said first spatial order; and

    a third scale position value from a second demodulation signal produced from a second electrode array means output signal corresponding to said first and second detector electrode outputs produced in response to a group of excitation signals generated in said second spatial order; and

    means for combining said first, second and third scale position values to produce an absolute position measurement.

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