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DE-112011101946-B4 - Capacitive measuring circuit with increased interference immunity to external alternating fields

DE112011101946B4DE 112011101946 B4DE112011101946 B4DE 112011101946B4DE-112011101946-B4

Abstract

Capacitive measuring circuit, comprising: a first AC signal generator (21) configured to generate a first AC voltage signal, a second AC signal generator (24) configured to generate a second AC voltage signal, wherein the second AC voltage signal has a lower frequency than the first AC signal, a first mixer (23) for multiplicative mixing of the first AC signal and the second AC signal and for generating a modulated AC signal, a control and evaluation unit effectively coupled to an antenna electrode or a separate receiver electrode, wherein the control and evaluation unit has a current measurement circuit configured to measure current signals, wherein the current signals have an amplitude and/or phase of a current flowing in the antenna electrode or in the separate receiver electrode, wherein the control and evaluation unit is configured to determine a capacitance to be measured based on the measured current signals and to output a signal indicating the determined capacitance; wherein the current measuring circuit comprises a capacitance-to-current or voltage transformer (27) coupled via the capacitance (28) to be determined, wherein the current or voltage transformer (27) has an input and an output (29), and a second mixer (30) with a first and a second input, which is effectively coupled via its first input to the output (29) of the capacitance-to-current or voltage transformer (27), and wherein one of the first AC signal generator (21) and the first mixer (23) is effectively coupled to the capacitance-to-current or voltage transformer (27) to provide the first AC voltage signal to the input of the capacitance-to-current or voltage transformer (27). or to supply the modulated alternating voltage signal, and the other is effectively coupled to the second input of the second mixer (30).

Inventors

  • Laurent Lamesch

Assignees

  • IEE INTERNATIONAL ELECTRONICS & ENGINEERING S.A.

Dates

Publication Date
20260513
Application Date
20110608
Priority Date
20100608

Claims (4)

  1. A capacitive measuring circuit comprising: a first AC signal generator (21) configured to generate a first AC voltage signal, a second AC signal generator (24) configured to generate a second AC voltage signal, wherein the second AC voltage signal has a lower frequency than the first AC signal, a first mixer (23) for multiplicatively mixing the first AC voltage signal and the second AC voltage signal to generate a modulated AC voltage signal, a control and evaluation unit effectively coupled to an antenna electrode or a separate receiver electrode, wherein the control and evaluation unit comprises a current measuring circuit configured to measure current signals, wherein the current signals have an amplitude and/or phase of a current flowing in the antenna electrode or in the separate receiver electrode, wherein the control and evaluation unit is configured to determine a capacitance to be measured based on the measured current signals and to output a signal indicating the determined capacitance; wherein the current measuring circuit comprises a capacitance-to-current or voltage converter (27) coupled via the capacitance (28) to be determined, wherein the current or voltage converter (27) has an input and an output (29), and a second mixer (30) with a first and a second input, which is effectively coupled via its first input to the output (29) of the capacitance-to-current or voltage converter (27), and wherein of the first AC signal generator (21) and the first mixer (23) one is effectively coupled to the capacitance-to-current or voltage converter (27) to supply the first AC voltage signal or the modulated AC voltage signal to the input of the capacitance-to-current or voltage converter (27), and the other is effectively coupled to the second input of the second mixer (30).
  2. Capacitive measuring circuit according to Claim 1 , wherein the second input of the second mixer (30) is a local oscillator input of the second mixer (30).
  3. Capacitive measuring circuit according to one of the Claims 1 until 2 , wherein a phase shifter (32) is coupled before the second input of the second mixer (30) in order to couple phase-shifted versions of the first AC signal or the modulated AC signal to the second input of the second mixer (30).
  4. Capacitive measuring circuit according to one of the Claims 1 until 3 , wherein an output signal at the output (31) of the second mixer (30) is amplified and filtered by a bandpass filter (33) and then mixed in a third mixer (35) with the second AC voltage signal of the second AC signal generator (24).

Description

Field of invention The present invention relates generally to the technical field of capacitive measuring circuits and in particular to a capacitive measuring system with one or more electrodes, wherein the features of a conductive body, such as the shape and the placement, are determined by capacitive coupling via the electrically conductive body. Background of the invention Capacitive measurement and/or detection systems have a wide range of applications and are frequently used, among other things, to detect the presence and/or position of a conductive body near an electrode of the system. A capacitive sensor, sometimes called an electric field sensor or proximity sensor, is a sensor that generates a signal that responds to the influence of something being sensed (a person, a part of a person's body, a pet, an object, etc.) on an electric field. A capacitive sensor generally includes at least one antenna electrode to which an electrical oscillation signal is applied, causing it to radiate an electric field into an area in space near the antenna electrode while the sensor is operating. The sensor has at least one measuring electrode, which itself may have one or more antenna electrodes at which the influence of an object or living being on the electric field is detected. An example of such a capacitive measurement system is described in the publication [reference to publication]. US 6 025 726 A known. The technical document entitled “ “Electric Field Sensing for Graphical Interfaces” by JR Smith, published in Computer Graphics I/O Devices, May/June 1998 issue, pages 54-60 This paper describes the concept of electric field measurement as it is used to perform non-contact three-dimensional position measurements, and in particular to measure the position of a human hand for the purpose of inputting three-dimensional positions into a computer. Within the general concept of capacitive measurement, the author distinguishes between distinct mechanisms, which he refers to as "loading mode,""shuntmode," and "transmit mode," corresponding to different possible paths for the electric current. In loading mode, a voltage oscillation signal is applied to a transmitting electrode, which establishes an electric oscillation field at ground. The object being measured modifies the capacitance between the transmitting electrode and ground. In "parallel mode," a voltage oscillation signal is applied to the transmitting electrode, establishing an electric field at a receiving electrode, and the displacement current induced at the receiving electrode is measured, thereby modifying the displacement current through the measured object. In "transmit mode", the transmitting electrode is brought into contact with the user's body, who then becomes a transmitter relative to a receiver, either through a direct electrical connection or via capacitive coupling. Capacitive coupling is generally achieved by applying an AC voltage signal to a capacitive antenna electrode and measuring the current flowing from the antenna electrode in coupling mode, either to ground (in charging mode) or to the second electrode (receiving electrode). This current is typically measured by a transimpedance amplifier connected to the measuring electrode, which converts the current flowing into the measuring electrode into a voltage proportional to the current flowing into the electrode. 1 shows a typical state-of-the-art circuit configured to measure an unknown capacitance in so-called "charging mode", which means that the capacitance between an electrode of a capacitive sensor and ground or earth is measured. An AC voltage source 1 generates an AC voltage signal of known frequency and amplitude, for example, a periodic sine wave of 100 kHz and a peak amplitude of 1 V. The output node 2 of the AC voltage source 1 is connected to the non-inverting input of an operational amplifier 3. The operational amplifier 3 is configured as a transimpedance amplifier. Due to the feedback effect of the associated feedback impedance 4 (preferably a capacitor connected in parallel with a resistor, where the impedance of the capacitor at operating frequency is at least 10 times lower than the resistor), the operational amplifier 3 maintains essentially the same potential at its inverting input as at its non-inverting input, thus keeping the read node 5 at the same potential as the output 2 of the AC voltage source. Therefore, the voltage of the AC voltage source is applied to the unknown capacitor 6 to be measured across its plates. The current flowing through the unknown capacitance 6 is then given by its capacitance and the known voltage of the AC voltage source. given, where the current also flows through the feedback impedance 4 when the input current into the non-inverting input of the amplifier 3 is essentially zero. The voltage at output 7 of amplifier 3 responds to the voltage of the AC voltage source and the unknown capacitance. This amplifier output voltage is then mixe