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

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Andermo, Nils I.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Mueller, Robert W.
• Application Number:
  US07/635,960
• Time in Patent Office: 277 Days
• Field of Search: 324/662, 324/661, 324/660, 324/658, 324/681, 324/686, 324/688, 324/690, 324/725, 340/870.37, 341/15, 33/125 C
• US Class Current: 324/662
• CPC Class Codes: G01B 7/003 for measuring position, not involving coordinate determination coordinate measuring G01B7/004
• CPC Class Codes: G01D 5/2415 adapted for encoders
• CPC Class Codes: H03M 1/645 for position encoding, e.g. using resolvers or synchros H03M1/485 takes precedence

Abstract

Capacitive-type measuring apparatus for making absolute measurements comprises first and second support members which are relatively displaceable with respect to a measurement axis; an array of first electrodes disposed on the first support member; an array of second electrodes disposed on the second support member such that differing portions of the second electrode array are capacitively coupled with the first electrode array in dependence on the relative positions of the support members; an array of third electrodes disposed on the second support member so as to be electrically connected to corresponding second electrodes and spatially offset therfrom by an amount which varies according to the position of the third electrodes from a reference position; and a fourth electrode arrangement disposed on the first support member for providing in combination with the other arrays a plurality of discrete signal transmission paths each having a capacitive transfer function with two components, one of which varies according to a predetermined function over a first wavelength, and the other of which varies according to a predetermined function over a second wavelength shorter than the first. The fourth electrode arrangement permits selective spatial filtering of the two components, thereby permitting different resolution measurements to be made with the same electrode arrays, which measurements can be combined to obtain high accuracy absolute position measurements over an extended measuring range.

Claims

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 W_c 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 W_m 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 P_r1 defining a scale wavelength W_f,

said at least one transmitter electrode group defining a transmitting wavelength W_t, and

the 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 W_f, 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 W_t 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.

2. 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 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 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 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 P_r1 defining a scale wavelength W_f,

said at least one transmitter electrode group defining a transmitting wavelength W_t, and

the 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 W_f, 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 W_t 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;

measuring 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 producing a scale position value when said measuring means does not produce an output;

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

a first scale position value of M_c 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 M_m 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 M_f 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;

said master control means being configured such that;

each group of N excitation signals is assigned a value K, where K ranges between 1 and N, and at the beginning of each portion of the measurement cycle for generating successive pairs of excitation signal groups to produce a scale position value, the first one of said excitation signal groups in the initial pair of excitation signal groups is selected according to a predetermined initial value of K, and thereafter the value of K is incremented or decremented in accordance with the polarity of the first demodulation signal integration result to produce a new value of K for selecting the next first one of said excitation signal groups in the next further pair of excitation signal groups;

said M_c scale position value is computed according to the formula M_c =A*K_c +P_c *T_c, unless the value so computed is outside a predetermined range, in which case a further wrap-around calculation is performed to convert the computed value to a value within the range, where A is a predetermined constant selected according to the relationship between the wavelengths W_c and W_m, K_c is the value of K which resulted in said measuring means not producing an output during the portion of the measurement cycle for producing said M_c value, P_c is the polarity of the first demodulation signal integration result for the next further pair of excitation signal groups causing said measuring means not to produce an output, and T_c is the time required for integration of the second demodulation signal for the pair of excitation groups causing said measuring means not to produce an output;

said M_m scale position value is computed according to the formula M_m =B*K_m +P_m *T_m, unless the value so computed is outside a predetermined range, in which case a further wrap-around calculation is performed to convert the computed value to a value within the range, where B is a predetermined constant selected according to the relationship between the wavelengths W_m and W_f, K_m is the value of K which resulted in said measuring means not producing an output during the portion of the measurement cycle for producing said M_m value, P_m is the polarity of the first demodulation signal integration result for the pair of excitation signal groups causing said measuring means not to produce an output, and T_m is the time required for integration of the second demodulation signal for the pair of excitation groups causing said measuring means not to produce an output; and

said M_f scale position value is computed according to the formula M_f =C*K_f +P_f *T_f, unless the value so computed is outside a predetermined range, in which case a further wrap-around calculation is performed to convert the computed value to a value within the range, where C is a predetermined constant selected according to the desired measurement resolution, K_f is the value of K which resulted in said measuring means not producing an output, P_f is the polarity of the first demodulation signal integration result for the pair of excitation signal groups causing said measuring means not to produce an output, and T_f is the time required for integration of the second demodulation signal for the pair of excitation groups causing said measuring means not to produce an output.

3. The capacitive-type measuring apparatus of claim 2 wherein the initial value of K for the portion of the measuring cycle for producing M_m is selected according to the formula K=M_c -N*Int(M_c /N);

and the initial value of K for the portion of the measuring cycle for producing M_f is selected according to the formula K=M_m -N*Int(M_m /N).

4. The capacitive-type measuring apparatus of claim 3 wherein the value of M_c is incremented or decremented during the portion of the measuring cycle for producing M_m according to the value of the associated first demodulation signal integration result each time said measuring means produces an output.

5. The capacitive-type measuring apparatus of claim 4 wherein the value of M_m is incremented or decremented during the portion of the measuring cycle for producing M_f according to the value of the associated first demodulation signal integration result each time said measuring means produces an output;

and if the value of M_m so incremented or decremented is outside the predetermined range for M_m, the value of M_c is adjusted correspondingly to a new value.

6. The capacitive-type measuring apparatus of claim 5 wherein the M_f value, and the M_m and M_c values obtained as a result of producing the M_f value, are weighted with respect to each other in accordance with the relative measurement dimensional value represented by each increment of the scale position values, and the weighted scale position values are summed to derive said absolute position measurement.