Multi-channel capacitive sensor device

Abstract

A multi-channel capacitive sensor device is disclosed. A capacitive sensor has multiple sensing inputs and a common reference input. The multiple sensing inputs are connectable to capacitive sensing electrodes. Each sensing input and the reference input can be placed in a measurement state, a ground state, or a shielded state. The sensor can be equipped with an external reference capacitor between each sensing input and the common reference terminal. The reference capacitor can be read individually by selectively pulling one of the input terminals to ground and driving the other to the same potential as the reference input.

Description

Technical Field

[0001] The present application relates to multi-input capacitance sensors. Embodiments relate to capacitance sensors with improved response stability in a small and economical package, and to portable electronic devices, such as smartphones, equipped with such sensors for detecting proximity to a part of a user's body. Background Art

[0002] In recent smartphones and connected personal devices, proximity sensing is a requirement, where it is used to limit radiation dose, prevent accidental input on touch displays, dim backlight panels, and in many other situations. Document EP 3796140 discloses a capacitive sensor that can be used, among other things, to detect the proximity of a user's body part in a portable phone, tablet, or similar portable device.

[0003] EP 3595176 and US Pat. No. 9582111 disclose multi-input capacitive sensors that can be used in smartphone applications. These devices have multiple sensing electrodes that can be used to improve sensing of specific situations, discern approaching direction, repel water or low-dielectric-constant objects, and other functions. These references disclose reference capacitors for calibrating the response of the sensing electrodes.

[0004] The difficulty with capacitive proximity sensing lies in the very small signal being sought. The approach of a hand or body part is detected by sensing changes in the self-capacitance of the electrodes, which are typically a fraction of a percent. This becomes even more challenging when you consider that the useful portion of the capacitive signal can be overwhelmed by strong noise and interference signals from the environment and from the smartphone itself. Thermal drift is significant and can easily obliterate the desired capacitance change.

[0005] Another problem with the devices used so far in these applications is that they must be integrated into highly miniaturized circuits where space is extremely limited.There is therefore a need for devices with a reduced number of terminals that can fit into a small package. Summary of the Invention

[0006] The purpose of the present device is to provide a device that overcomes the disadvantages and limitations of the prior art.

[0007] These objects are therefore achieved by the subject matter of claim 1 and in particular by a capacitance sensor device comprising a capacitance measurement circuit, a plurality of sensing inputs and a reference input, each sensing input terminal being connectable to a sensing electrode, the capacitance sensor device being configured for, in a first measurement mode, driving the reference input to be equipotential with one of the sensing inputs measured by the capacitance measurement circuit and determining a capacitance value seen by the sense input being measured unaffected by any capacitance between the sense input being measured and the reference input, and for, in a second measurement mode, measuring the capacitance seen at the reference input with the capacitance measurement circuit while holding one sensing input at ground and driving the other sensing inputs to be equipotential with the reference input, determining a value of a reference capacitor connected between the reference input and the sense input held at ground, unaffected by any capacitance between the reference input and the sensing input driven to be equipotential with the reference input.

[0008] In a variation, or in other words, the capacitance sensor device is configured to selectively set the sensing input and the reference input in a ground state, a shielded state, or a measurement state, whereby the input in the ground state is held at ground potential or a constant potential, the input in the measurement state is read by the capacitance measurement circuit, and the input in the shielded state is held at the same potential as the input in the measurement state, wherein the capacitance sensor device is configured to select a sensing input in a second measurement mode, set the selected sensing input in a low impedance state, set the other sensing inputs in a shielded state, set the reference input in the measurement state, and measure a reference capacitance seen at the reference input with the capacitance measurement circuit, and is configured to set the reference input in the shielded state and the selected sensing input in the measurement state in a first measurement mode, and measure an uncorrected capacitance seen at the selected sensing input with the capacitance measurement circuit, the capacitance sensor device further configured to generate a corrected capacitance for the selected sensing input based on the uncorrected capacitance and the reference capacitance.

[0009] The dependent claims introduce useful but not necessary elements, such as correcting capacitance by subtracting a reference capacitance scaled by a determined factor from an uncorrected capacitance; repeatedly and sequentially sensing and correcting sense inputs; and a controllable input unit setting the corresponding sense input in a grounded state, a shielded state, or a measuring state according to a command received from a processor.

[0010] Each sense input can be coupled to a common reference cell with an external reference capacitor sized to provide a thermal coefficient close to that of the associated sense electrode but offering little or no sensitivity to the proximity of external conductive bodies. Sense electrodes are conventionally implemented as copper areas on a printed circuit board, and their thermal behavior is heavily influenced by the dielectric properties of the substrate. The traces to the sense electrodes contribute significantly to their self-capacitance and thermal drift. The device uses separate reference capacitors for each sense input, thus allowing for optimal drift compensation without constraints on the placement of the sense pads and reference capacitors, with a single additional input pin.

[0011] A capacitance measurement circuit can determine the capacitance seen at an input by connecting the input to a variable voltage source and measuring the corresponding change in charge.

[0012] The capacitive sensor can deliver a proximity signal based on the corrected capacitance to a host sensor, which can cause a predetermined action, such as changing the power of a radio transmitter, turning a display backlight on or off, or enabling or disabling a tactile input interface. One of the sensing inputs can be coupled to the host system's radio frequency antenna, which doubles as a proximity electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Exemplary embodiments are disclosed in the description and illustrated by the accompanying drawings, in which:

[0014] Figure 1 A capacitive proximity sensor of known type is illustrated schematically.

[0015] Figure 2 Show Figure 1 A multi-input variation of the single-channel circuit has multiple reference capacitors, each read by a separate input.

[0016] Figure 3 The embodiments are shown in schematically simplified form.

[0017] Figure 4 A possible configuration of a capacitance-to-voltage converter is shown.

[0018] Figure 5 The second embodiment is shown in schematically simplified form.

[0019] Figure 6 is a line graph of the signal in the device during the proximity of a conductive body.

[0020] Figure 7 Shows possible ways of using the sensor in a host system. DETAILED DESCRIPTION

[0021] Figure 1 A capacitive proximity sensor of a known type is shown in simplified form. Circuit 30 has a sense input M1 connected to capacitive pad 21. The circuit operates by monitoring the self-capacitance of pad 21 and detecting small changes attributable to the proximity of conductive body 18. The circuit includes a second input REF that can be connected to an external capacitor Cr and used for comparison and calibration.

[0022] Figure 1 The detector is typically part of a larger system such as a portable phone or tablet and communicates in any suitable manner with the host processor 50. A standard serial communication bus such as I2C is typically used to keep the pin count low, but dedicated lines for interrupts and data transfer are also possible. Figure 1 An interrupt line (INT) between the sensor circuit 30 and the host 50 is shown by way of example.

[0023] Figure 2 A multi-channel capacitance detector 30 is shown with six inputs. Three inputs M1, M2, M3 are used for capacitance electrodes 21, 22, 23, and three inputs R1, R2, R3 are used to read three independent reference capacitors 23, 24, 25. This solution allows each sensing input to be paired with an independent reference capacitor and sized in such a way as to compensate for the drift of the corresponding sensing electrode.

[0024] The capacitive electrodes and associated capacitors can be freely placed in the host system, and the connection traces on the circuit board can be paired for optical compensation. Importantly, different parts of the smartphone may not be at the same temperature, and this can be accounted for by placing the reference capacitors close to the corresponding sensing electrodes or where they are expected to experience the same temperature changes. However, this flexibility comes at the cost of a higher pin count: Figure 2 The sensor has the same six-channel sensor footprint.

[0025] Figure 3 There is shown an improved variant of a multi-channel capacitance detector 30. This detector has three sensing inputs M1, M2, M3 connected to as many capacitance sensing electrodes 21, 22, 23 and one common reference input R. Reference capacitors 24, 25, 26 are placed between each sensing line connecting the input to the corresponding electrode and the common reference input R.

[0026] As will be explained below, the capacitance detector is configured to selectively pull one of the input terminals to ground and drive the other to the same potential as the reference input. In this way, reference capacitors 24, 25, and 26 can be read individually. In addition, capacitance detector 30 is configured to drive the reference input to the same potential as one of the measured sense inputs. In this way, the reference capacitor 21, 22, or 23 connected to the measured sense input does not change the measurement of the self-capacitance.

[0027] Internally, the capacitance detector 30 has an analog / digital converter 40 which converts the capacitance signal into a digital signal suitable for further processing in a digital processor 45. Preferably, a common converter is used to sequentially read the capacitance seen by the sensing electrodes through a multiplexer 35. However, this is not an absolute requirement.

[0028] Input units 31 and 32 are configured to set their corresponding input terminals to a desired state selected from a measurement state, a ground state, and a shielded state. In the measurement state, the potential of the input is variable, following the variable voltage source in the capacitive sensor device, and the resulting charge change is sent to the ADC to determine the capacitance value. In the grounded state, the input is held at a constant voltage—either ground or offset by a fixed value. In the shielded state, the voltage at the input follows the voltage of the other input in the measurement state, but ignores changes in charge.

[0029] Figure 4 A possible implementation of an input unit is shown in a very simplified form, which causes the voltage at the input to follow a variable voltage source and which can be used in the claimed device. This is for the sake of completeness of the present disclosure and is provided by way of example, but the claimed device also includes other structures of the input stage that can provide the desired functionality. The sensing electrode 21 is connected to the inverting input of an amplifier 75, while the other input is driven by a voltage source 72 that can provide a variable voltage or a constant voltage. Due to feedback, the inverting input of the amplifier 75 is a low impedance node and its voltage is the same as the voltage of the voltage source 72 (possibly with a constant offset that is not important in this application).

[0030] The ground state of input M1 can be achieved by causing source 72 to generate a constant voltage. Input M1 is then a virtual ground. The measurement state and the shielding state are achieved when source 72 generates a variable voltage. Input M1 then follows this voltage.

[0031] The capacitance of electrode 21 can be measured by causing source 72 to generate a series of steps. Capacitor 76 connected in the reaction loop provides an integrator, and the output of amplifier 75 will show steps proportional to the change in charge on electrode 21. The ratio between the charge, measurable as a step voltage at the output of amplifier 75, and the known voltage steps of source 72 is defined as the capacitance being sought. Switch 73 is used to periodically discharge reaction capacitor 76.

[0032] The output signal is digitized by an ADC converter 40, preferably synchronously with the pulses of the source 72. The multiplexer 35, preamplifier 38 and offset correction 39 are not shown in this figure but may be present.

[0033] Return to Figure 3 , the input stages 31, 32 generate voltage signals related to the capacitance seen at the respective input terminals M1-M3 and the reference terminal REF. The detector may include analog conditioning stages (e.g., for filtering and amplification) and / or an offset subtraction stage 39 prior to the ADC 40. The digital signal sampled by the ADC is then further processed in a digital processor 45 and made available to a host processor, for example, via an I2C bus.

[0034] As mentioned above, the input stage can have different structures. For example, the ground mode and the shield mode can foresee the activation of the switch connecting the input terminal to ground - respectively, to a suitable variable source.

[0035] Figure 5 Another embodiment is shown in which the sensing electrodes 21, 22, 23 are directly connected to the multiplexer 35, and the charge-to-voltage conversion is done after the multiplexer. In this variation, the inputs of the multiplexer 35 can be individually set to one of the measurement state, the shielded state, the ground state, or the high impedance state.

[0036] When one of the terminals M1, M2, M2 or REF is in the measurement state, it is connected to the capacitance-to-voltage converter 37 through the multiplexer 35 and the offset correction unit 39. The capacitance-to-voltage converter may have Figure 4 The structure shown in FIG, however, is not a requirement, as other circuits for converting capacitance into voltage signals are known. As mentioned above, Figure 4 The potential of the input of the circuit is variable and follows the voltage (e.g. a series of pulses) generated by the variable voltage source 72 and integrates the charge received at the input. The output is a pulse signal whose amplitude is proportional to the capacitance seen at the input.

[0037] The inputs in the shielded state are connected to the outputs of buffer 70, which follow variable source 72. In the measurement state, they follow the potential of the inputs, but their capacitance is not measured. The phrase "follows the potential of the inputs in the measurement state" means that the potential difference between the inputs in the measurement state and the inputs in the shielded state is zero or at least essentially constant, even though the potential of the inputs in the measurement state is variable. In this way, the capacitance between these inputs is canceled and does not affect the measurement.

[0038] Inputs in the ground state are connected to ground through selectable switches in multiplexer 35, while inputs in the high impedance state are essentially unconnected (floating).

[0039] Figure 3 、 Figure 4 and Figure 5 The representation is a simplified version of a complete implementation of the claimed apparatus, wherein the blocks represent important functions, such as multiplexing, conversion to digital, conversion of capacitance to voltage, offset subtraction, etc. The reader understands that the claimed subject matter is not limited to these examples, but encompasses all variations within the scope of the appended claims, including those in which some of the functions presented herein do not correspond to physically identifiable subcircuits. For example, it may be the case that some functions are combined in one circuit with shared elements, or that they are partially or fully implemented by digital processing.

[0040] Importantly, the digital processor 45 is configured to specify the state of the input units 31, 32 in a manner to arbitrarily obtain the self-capacitance of the electrodes 21, 22, 23 and the capacitance of the reference capacitors 25, 26, as follows:

[0041] In order to measure the capacitance at one of the input electrodes (e.g. electrode 21), the corresponding input M1 is set in measurement mode and the reference input R is set in shielding mode to suppress the influence of the reference capacitor 24 (since this capacitor sees zero or constant voltage, it does not conduct current). The state of the inputs M2 and M3 is in principle irrelevant. They can be set in shielding mode, high impedance state or grounded. Tests have shown that setting unused inputs to ground generally provides the best performance, but other configurations may be advantageous in special cases. Preferably, the input unit is programmable and allows any desired state to be selected for the unconnected inputs.

[0042] The capacitance of the reference capacitor 24 may be measured by placing the reference input R in a measurement mode, placing the first input M1 in a ground mode, and placing the second input M2 and the third input M3 in a shielding mode.

[0043] The capacitance of the electrodes M2, M3 of the reference capacitors 25, 26, respectively, can be measured in the same way by using M2 or M3 instead of M1. The following table summarizes the possible combinations.

[0044]

[0045] Table 1

[0046] Each column of Table 1 shows the time taken to read a given sensing electrode (M1, M2, M3) or corresponding reference capacitor (R1, R2, R3, Figure 3 The configuration required for the capacitors (indicated as 24, 25, 26 in FIG. 1 ) is shown. The states of the inputs M1, M2, M3, and R are abbreviated, where "M" stands for measurement, "S" stands for shielding, "G" stands for grounding, and "H" stands for high impedance (floating state). As mentioned above, the state of the unconsidered inputs is not uniquely determined and can be selected from any of grounding, shielding, or high impedance that gives the best performance.

[0047] Figure 6 The line graph in shows the values ​​of capacitance measured using the method outlined above in a state where a conductive body, such as a part of a user's body, is close to one sensing electrode in the time interval between t1 and t2. The line graph has been drawn with an arbitrary offset to improve readability.

[0048] Graph 91 shows the capacitance seen at the first sensing electrode M1. It can be seen that the proximity of a body is marked by an increase in capacitance over a rise of approximately constant slope. This rise is actually the effect of temperature drift, which changes the capacitance and occurs on a significantly slower timescale than the proximity, appearing as a nearly linear rise.

[0049] Graph 93 shows the capacitance of the first reference capacitor R1, which is dimensioned and connected in such a way that it is subject to the same temperature changes as the first sensing electrode and has a comparable temperature drift. It shows a rise like the first graph, but since it is insensitive (or much less sensitive) to proximity, there is no significant increase in the interval (t1, t2).

[0050] Graph 95 is the signal M obtained by combining the values ​​of M1 and R1 to compensate for thermal drift. * This can be obtained by a linear combination of M1 and R1, which can be expressed as M * =M1−k×R1, where k represents a coefficient that can be predetermined in a calibration step. This compensation can be performed in the processor 45.

[0051] Line graph 97 shows that by setting the value M * With threshold Cth The digital proximity signal obtained by the comparison can be sent to the host system 50.

[0052] Figure 7 The diagram illustrates how the innovative capacitive detector 30 can be used as a proximity detector in a personal connected device (such as a smartphone). The capacitive detector has three sensing inputs, M1, M2, and M3, which are connected to a number of capacitive sensing electrodes implemented as conductive areas on a circuit board (or implemented in any other suitable manner). The electrodes can take on many shapes and positions as needed. The number of input channels is also not limited and can be more or less than three as needed.

[0053] exist Figure 7 In the example illustrated above, the first sensing input M1 is connected to a flat antenna 21 which is coupled to the radio transceiver 54 and does double duty as a sensing electrode. In this configuration, the capacitance C1 is calibrated as disclosed above. * It can be used to determine whether a part of the user's body is close to the antenna 21.

[0054] The sensing inputs M2 and M3 are connected to two electrodes 22 and 23 that are close to or overlap each other. * and C3 * The comparative examination can give information about the approach of a part of the user's body and information about its approach direction.

[0055] The processor in the detector 30 converts the corrected capacitance into digital proximity markers and transmits these markers to the processor in the host system 50. The host system 50 is configured to take specific actions based on the activation or deactivation of the proximity markers. For example, proximity to the antenna can trigger a reduction in radio power to limit the absorbed dose. Proximity from the screen can cause the tactile interface to be disabled to avoid erroneous input, etc.

[0056] Reference symbols in the drawings

[0057] 18 conductive body; 21 sensing electrode; 22 sensing electrode; 23 sensing electrode; 24 reference capacitor; 25 reference capacitor; 26 reference capacitor; 30 capacitive sensor device; 31 input control stage; 32 input control stage; 35 multiplexer; 37 capacitance to voltage converter; 38 analog preprocessor; 39 offset subtraction; 40 ADC; 45 digital processor; 50 host; 54 transceiver; 57 display; 70 buffer; 72 voltage source; 73 reset switch; 75 amplifier; 76 feedback capacitor; 91 uncorrected sensing capacitance; 93 reference capacitor; 95 corrected sensing capacitance; 97 digital proximity signal; M1, M2, M3 sensing inputs; R reference input.

Claims

1. A capacitance sensor device comprising a capacitance measurement circuit, a plurality of sensing inputs, and a reference input, each sensing input being connected to a sensing electrode, the capacitance sensor device being configured to, in a first measurement mode, drive the reference input to equipotential with one of the sensing inputs being measured by the capacitance measurement circuit and determine a capacitance value seen by the sense input being measured, unaffected by any capacitance between the sense input being measured and the reference input, and being configured to, in a second measurement mode, measure the capacitance seen at the reference input using the capacitance measurement circuit while holding one sensing input at ground and driving the other sensing inputs to equipotential with the reference input, determine a value of a reference capacitor connected between the reference input and the sense input held at ground, unaffected by any capacitance between the reference input and the sense input driven to equipotential with the reference input, and generate a corrected capacitance for the selected sensing input based on the uncorrected capacitance and the reference capacitance, wherein the reference capacitor is sized and connected in such a way that it experiences the same temperature changes as the sensing electrode.

2. The capacitive sensor device of claim 1 , wherein the capacitive sensor device is configured to selectively set a sensing input and a reference input in a grounded state, a shielded state, or a measurement state, whereby an input in the grounded state is connected to a low impedance node connected to ground, an input in the measurement state is read by the capacitance measurement circuit, and an input in the shielded state is held at the same potential as the input in the measurement state, wherein the capacitive sensor device is configured to select a sensing input in a second measurement mode, set the selected sensing input in a grounded state, set the other sensing inputs in a shielded state, set the reference input in a measurement state, and measure a reference capacitance seen at the reference input using the capacitance measurement circuit, and to set the reference input in a shielded state, set the selected sensing input in a measurement state, and measure an uncorrected capacitance seen at the selected sensing input using the capacitance measurement circuit in a first measurement mode.

3. The capacitive sensor device according to claim 1, comprising: a multiplexer unit having a plurality of input ports, each of the plurality of input ports being connected to one of the sense input and the reference input; and an output port connected to a capacitance measurement circuit, wherein the capacitance measurement circuit determines the capacitance seen at the input by connecting the input to a variable voltage source and measuring a corresponding charge change, wherein the input port of the multiplexer is in a ground state, a high impedance state, a measurement state in which the input port is connected to the output port, or a shielded state in which the input port is not connected to the output port and follows the variable voltage source, the capacitance sensor including a controller configured to measure the capacitance seen by the selected sensing input by setting a port of the multiplexer connected to the selected sensing input in a measurement mode and an input port of the multiplexer connected to the reference input in a shielded mode, and configured to measure a reference capacitor between the selected sensing input and the reference input by setting an input port of the multiplexer connected to the reference input in a measurement mode, an input port of the multiplexer connected to the selected sensing input in a ground mode, and an input port of the multiplexer connected to the unselected sensing inputs in a shielded mode. 4 . The capacitive sensor device of claim 1 , wherein the generating of the corrected capacitance comprises multiplying a reference capacitance by a predetermined factor and subtracting the resulting value from the uncorrected capacitance. 5 . The capacitance sensor device of claim 1 , configured to repeatedly and sequentially select sensing inputs and generate a corrected capacitance for each sensing input.

6. The capacitive sensor device according to claim 1 , comprising a processing unit, the sensing input having a controllable input unit, the controllable input unit being configured to accept a command from the processing unit and to set the corresponding sensing input in a ground state, a shielding state or a measuring state based on the command. 7 . The capacitive sensor device according to claim 1 , the reference input having a controllable input unit configured to accept a command from the processor and to set the reference input in the shielding state or the measuring state based on the command.

8. The capacitive sensor device of claim 1, wherein each sensing input is coupled to a reference input through an external reference capacitor.

9. The capacitive sensor device of claim 8, wherein the sensing input is coupled to a capacitive sensing electrode, the self-capacitance of the capacitive sensing electrode changes when the conductive body is in proximity, and the external reference capacitor has a temperature coefficient that is the same as or proportional to the temperature coefficient of the capacitive sensing electrode.

10. The capacitive sensor device of claim 9 , wherein the capacitive sensing electrode is a conductor area on a printed circuit board, and the external reference capacitor comprises a trace and / or conductor area on the same board or on a board having the same characteristics, and the capacitance of the external sensing electrode is less affected by the proximity of the conductive body.

11. A portable electronic device equipped with the capacitive sensor device according to claim 1, wherein the capacitive sensor is configured to generate a proximity signal based on the corrected capacitance, and the portable electronic device is configured to activate a predetermined action when the proximity signal is generated and / or when the proximity signal is invalid.

12. The portable electronic device of claim 11, wherein the action is one of: changing the power of a radio transmitter, turning a display backlight on or off, enabling or disabling a tactile input interface.

13. The portable electronic device of claim 11, wherein the at least one sensing input is coupled to a radio frequency antenna, the self-capacitance of the radio frequency antenna changing when the conductive body is in proximity.

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