Low-cost, high measurement speed capacitive sensing circuit for loading mode operation of capacitive sensors

Patent metadata

Field	Value
Publication number	US-12480983-B2
Application number	US-202118037011-A
Country	US
Kind code	B2
Filing date	Nov 15, 2021
Priority date	Nov 16, 2020
Publication date	Nov 25, 2025
Grant date	Nov 25, 2025

Abstract

An impedance measurement circuit for determining a complex impedance of a capacitive sensor having at least one sense electrode operable in loading mode and at least one guard electrode. The measurement circuit includes: a pulse generator unit for providing a periodic electric measurement signal and a periodic electric guard signal; a signal sensing circuit for sensing a sense current flowing through the at least one sense electrode or the sense electrodes in response to the pulse generator unit measurement signal; and a control and evaluation unit. The control and evaluation unit is configured for determining a complex impedance from the determined sense currents with reference to a complex reference potential.

First claim

The invention claimed is:

1 . An impedance measurement circuit for determining a complex impedance of a capacitive sensor having at least one sense electrode operable in loading mode and at least one guard electrode, comprising:
a pulse generator unit configured to provide a periodic electric measurement signal that comprises a plurality of different fundamental measurement frequencies, and for providing a periodic electric guard signal, a signal sensing circuit configured to sense a sense current flowing through the at least one sense electrode in response to the pulse generator unit measurement signal, a control and evaluation unit configured to determine a complex impedance from the determined sense currents with reference to a complex reference potential, and one or more remotely controllable electric switches for at least one out of selectively connecting one out of at least two provided reference potentials to a reference input port of the current measuring means, and selectively electrically operatively connecting at least one reference impedance of an a priori known impedance either in parallel to the at least one sense electrode or to the provided guard signal, wherein the control and evaluation unit is further configured for controlling the one or more remotely controllable electric switches according to a predefined sequence, and for determining a complex impedance from the determined sense currents with reference to a complex reference potential during predefined stages of the predefined sequence.

2 . The impedance measurement circuit as claimed in claim 1 , wherein the pulse generator unit is designed as a passive and amplitude-controlled generator unit that comprises a plurality of synchronized pulse width modulation units, wherein the pulse generator unit is configured to weight and to sum output signals of the plurality of synchronized pulse width modulation units having the plurality of different fundamental measurement frequencies, and a passive low-pass filter unit connected in series to the summed output signals.

3 . The impedance measurement circuit as claimed in claim 1 wherein the pulse generator unit is designed to have a digital-to-analog converter operatively connected to a digital data memory unit, and wherein the digital data memory unit is configured to provide data to the digital-to-analog converter that represent the periodic electric measurement signal.

4 . The impedance measurement circuit as claimed in claim 1 , wherein the pulse generator unit is further designed to provide a periodic reference signal to at least one reference impedance of an a priori known impedance, wherein a fundamental frequency of the periodic reference signal is different from any one of the plurality of fundamental measurement frequencies.

5 . The impedance measurement circuit as claimed in claim 1 , further comprising a remotely controllable variable attenuator circuit that is connected between the provided guard signal and at least another reference impedance having an a priori known impedance, wherein the at least another reference impedance is electrically connected to the at least one sense electrode.

6 . The impedance measurement circuit as claimed in claim 1 , wherein the control and evaluation unit comprises a microcontroller that includes a processor unit, a digital data memory unit, a microcontroller system clock, and an analog-to-digital converter unit having at least one analog-to-digital converter for at least converting an output signal of the signal sensing circuit.

7 . The impedance measurement circuit as claimed in claim 1 , wherein the control and evaluation unit is configured to execute an equivalent-time sampling method.

8 . The impedance measurement circuit as claimed in claim 1 , wherein the control and evaluation unit is configured for applying a software demodulation method to a voltage signal that is representative of a sense current and is digitally converted by the analog-to-digital converter unit, for determining a real part and an imaginary part of the complex impedance of the capacitive sensor.

9 . The impedance measurement circuit as claimed in claim 1 , wherein the control and evaluation unit is configured for applying a complex discrete Fourier transform DFT or a complex fast Fourier Transform FFT to a voltage signal that is representative of a sense current and is digitally converted by an analog-to-digital converter unit.

10 . A capacitive sensing device, comprising
an impedance measurement circuit as claimed in claim 1 , and a capacitive sensor having at least one sense electrode operable in loading mode and at least one guard electrode.

11 . A vehicle steering wheel comprising the capacitive sensing device of claim 10 .

12 . A vehicle seat comprising the capacitive sensing device of claim 10 .

Assignees

• Iee Sa

Inventors

• Lamesch Laurent
• Justinger Tobias
• Pütz Michael
• Faber Thomas
• Wendt Christoph
• Lorang Johnny

Classifications

• H03K17/62 Primary

  Switching arrangements with several input- output-terminals, e.g. multiplexers, distributors (logic circuits H03K19/00; code converters H03M5/00, H03M7/00) · CPC title

• H03K2217/960735

  characterised by circuit details · CPC title

• H03K17/955

  using a capacitive detector · CPC title

• B60N2210/12

  Capacitive; Electric field · CPC title

• B60N2/0021

  characterised by the type of sensor or measurement · CPC title