Capacitive type measurement transducer with improved electrode arrangement

Claim 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; first and second electrode arrays mounted on said first and second support members, respectively, in alignment with said measurement axis, for producing a varying capacitance pattern in dependence on the relative positions of said support members with respect to each other; said first electrode array comprising a series of discrete first electrodes uniformly spaced with a pitch P r defining a scale wavelength; said second electrode array comprising a series of discrete second electrodes disposed so as to be capacitively coupled to differing portions of said first electrode array in dependence on the relative posi­tions of said support members; and said second electrodes being arranged so as to define a plurality of measuring element groups separated from each other by a group pitch P g , and wherein each electrode group has N measuring elements, N being an integer greater than 2, and the second elec­trodes forming each of said electrode groups are posi­tioned within the respective groups so as to respectively occupy predetermined group positions which are spread over a distance greater than one scale wavelength, and each of which corresponds to the relative position of a different one of a group of relative scale wavelength segments obtained by dividing the group pitch into inter­vals corresponding to the scale wavelength and dividing each interval into N equal segments; signal generator means for applying at least one combination of N periodic excitation signals to the second electrodes in each of said electrode groups such that said excitation signals are respectively applied to corresponding ones of the second electrodes in each group in a relative order corresponding to the se­quence of relative scale wavelength segment positions occupied by the electrodes in each group; and detector means comprising: detector electrode means for sensing the output signals produced by the first electrodes capacitively coupled to said second electrode array in response to said at least one combination of excitation signals, and for producing an output signal which repre­sents a sum of the sensed first electrode output signals; and signal processing means for deriving measurement data from said detector electrode means out­put signal.