Drift suppression method, proximity sensor, and wireless device

Abstract

A portable device including a capacitive proximity sensor can suppress drift superimposed on the capacitive proximity signal and corresponding methods. A processor generates a baseline value by integrating a series of values derived from a slope of the proximity signal when the slope is within a prescribed limit, or integrating a fixed value when the slope is outside the prescribed limit.

Description

Technical Field

[0001] This invention relates to a filter arranged to suppress slow drift from a signal representing a quantity of interest. Embodiments of the invention relate to a digital implementation of the aforementioned drift suppression filter, and to a proximity detector that uses the filter to distinguish legitimate signals (e.g., the approach of a person) from drift generated by temperature changes or any other cause. However, these are not the only applications of the invention. Background Technology

[0002] In several measurement and sensing applications, relevant information is carried by electrical values ​​(i.e., voltage or current) representing the quantity of interest, superimposed with parasitic values ​​that may be constant or slowly drifting. Since the presence of spurious baselines limits the sensitivity and accuracy of measurements, several algorithms and filters have been designed to remove them from the signal of interest.

[0003] Capacitive proximity detectors are used in many modern portable devices, including mobile phones and tablets, to determine whether the device is near a user's body. This information is important in several ways: it is used to detect whether the phone is being actively operated by the user, whether the user is looking at a display, in which case the displayed information can be adjusted, and / or whether the device is switching from a low-power state to an active state. Importantly, this information is used to adjust the power level of the radio transmitter to comply with body dose limits. Capacitive proximity detection is also used in touch-sensitive displays and panels.

[0004] Capacitive proximity detection relies heavily on drift suppression. Typically, the capacitance of the user's body near the electrodes on the device is many times smaller than the background capacitance of the electrodes themselves. If the background value and its fluctuations are not eliminated, this background value and its fluctuations will completely mask the proximity signal.

[0005] If the signal to be measured changes much faster than the drift in the baseline, the drift in the baseline can be suppressed by a simple high-pass filter. In some cases, the same result is obtained by calculating the running average of the signal used to represent the drift and subtracting it from the original signal. Figure 1, discussed later, shows a digital implementation of this drift suppression process used in a capacitive proximity detector, provided that unit 60 implements a simple running average.

[0006] However, in many real-world situations, this simple approach is insufficient. Drift suppressors tend to reduce the desired signal as well as drift, especially when the signal of interest changes gradually, such as in portable devices where the user approaches the proximity sensor very slowly.

[0007] Documents US 10,136,399 B1, US 10,423,278 B2, US 10,298,280 B2 and US 2021 / 0083664 A1 disclose capacitive detectors that provide proximity sensing in portable devices such as mobile phones, as well as various circuits and algorithms for processing signals that take into account background and drift. Summary of the Invention

[0008] This invention proposes a method for removing drift from proximity signals, applicable to portable devices equipped with capacitive proximity sensors, but also effectively applicable to other uses. Capacitive sensors and portable devices thus equipped with them are also part of this invention.

[0009] Compared to other known solutions, the proximity sensor of this invention exhibits significantly better drift resistance and is therefore less sensitive to temperature changes. It maintains its proximity detection efficiency even when approaching slowly.

[0010] According to the present invention, these objectives are achieved by means of the objects of the appended claims. Attached Figure Description

[0011] Exemplary embodiments of the present invention are disclosed in the specification and illustrated in the accompanying drawings, wherein: Figure 1 schematically illustrates a capacitive proximity sensor with a digital drift suppression filter; Figure 2 A method for suppressing drift according to one aspect of the present invention is illustrated using a flowchart; Figure 3 These are curves of some signals used in this invention. Detailed Implementation

[0012] Figure 1 A capacitive proximity sensor with a drift suppression processor is schematically illustrated. For simplicity, this description will refer to capacitive proximity detectors in mobile phones or tablets, but the filters and methods of the present invention can be applied to different fields.

[0013] The detector is sensitive to the capacitance Cx of electrode 20, which increases slightly when the user's hand, face, or body approaches. As already discussed, the change due to body proximity is masked by the electrode's own capacitance, which is unstable. The capacitance signal is preferably amplified and processed by analog processor 23, which may also subtract a programmable offset and convert it to a raw digital value by A / D converter 25. The sample R(k) can be encoded as a 16-bit integer or in any other suitable format.

[0014] The device shown in Figure 1 also includes, although not necessary, a shielding electrode 21 located below the sensing electrode 20. The shielding electrode can be connected to a shielding control unit 24, which generates a voltage that is the same as or closely follows the potential of the sensing electrode. In this way, the shielding electrode 21 protects the sensing electrode 20 from the influence of the conductor below, which is a potential source of interference, without significantly contributing to the measured capacitance.

[0015] In a non-ideal world, the original sample R(k) also contains unwanted interference and noise that is attenuated by filter 30. Filter 30 can be a digital low-pass filter sized to account for the bandwidth of the desired signal or any other suitable filter. Filter 30 provides a series of samples U(k) for processing in successive stages.

[0016] Although filter 30 is shown as a simple block for the sake of simplicity in the accompanying drawings, it can actually include several processor units for noise reduction. It may include FIR low-pass filters, IIR low-pass filters, and nonlinear algorithms as needed. It has been found that the most important signal components for the functional purposes of controlling a portable telephone fall within, for example, the frequency spectrum of a few hertz.

[0017] Block 62 represents a drift suppression unit, which is used to estimate and suppress drift superimposed on capacitance changes, whether thermal or otherwise. Unit 60 is a baseline estimator that generates a digital signal A(k) of an instantaneous value close to the baseline, which is considered the effect of the drift. Subtracting the baseline digitally from the proximity signal U(k) yields the drift-suppressed signal D(k). The discriminator 50 then generates a flag signal 'PROX' indicating proximity to the user's hand, face, or body. However, the invention is not limited to binary output. In variations, the discriminator 50 can be a multi-level discriminator that generates several flags or any other useful variables depending on the degree of proximity. The detector's output does not need to be implemented in an output terminal: the flag can be encoded as a value stored in a register of the proximity sensor, which can be accessed via a communication bus (such as I...). 2 Access via C or SPI.

[0018] The various blocks that make up a proximity sensor are separated and represented differently in the diagram, but should actually be interpreted as functional units. In actual implementation, they can be implemented partially or entirely in software and can share code and data resources.

[0019] Capacitive proximity sensors can be part of a mobile phone, laptop, tablet, or other connected portable device. In this case, the capacitive electrode 20 can also be used as an RF antenna, part of the phone's metal casing, or simply a copper area on a PCB.

[0020] The generation of the baseline signal A(k) will now be explained with reference to Figure 2.

[0021] An important aspect of the method of the present invention is the estimation of the useful signal U. n The change. This change is caused by the quantity Δ. var It indicates that the quantity Δ var Preferably, this is calculated at each new useful sample U(k) (step 305). One possible way to estimate the change in U(k) is the difference between the sample and the previous sample, Δ. 01 =U(k)-U(k-1), or the running average Δ of the difference U(k)-U(k-1) within a suitable window. var For example, the last N received U(k) samples: Δ var =(U(k)-U(kN-1)) / N where N can take any suitable value, for example, N=8. In step 305, the method of the present invention receives a new value of the useful signal U(k) and calculates or receives the corresponding Δ var The new value.

[0022] Step 308 checks if the proximity indicator is raised, indicating the presence of a conductive object nearby. If the test result is positive, the method tests (step 130) a change Δ var Whether it is within the predetermined acceptance area. In this example, Δ var The upper threshold T1, which will be positive in most cases, is compared with the lower threshold T2, which may be negative, in step 322.

[0023] If the PROX flag is not declared, the method follows a right branch starting from test 308, which is completely symmetrical to the left branch. The change Δ var The comparison is made with an upper threshold T3, which will be positive in most cases (314), and a lower threshold T4, which may be negative (324). Numerically, T3 may be different from T1, and T4 may be different from T2, or they may be equal. Advantageously, this symmetrical treatment avoids the failure to detect strong negative changes (e.g., when the user suddenly moves away from the sensor).

[0024] If Δ var If the value is neither higher than the upper threshold nor lower than the lower threshold, then regardless of whether a proximity flag is declared, the baseline value is increased (step 330) by the value derived from the proximity signal. Preferably, the increment will be a value related to the slope or derivative of the proximity signal, such that continuous increments return to the original shape through integration. In possible variations, the baseline signal A(k) is increased by the average slope signal Δ var Alternatively, the baseline signal is increased by an instantaneous slope Δ 01=U(k)-U(k-1). However, other expressions are also possible.

[0025] If Δ var If the value is above the upper threshold or below the lower threshold, the baseline increases by a fixed amount, while U(k) and Δ... var The value is irrelevant. This occurs in one of units 316, 326, 318, and 328, depending on the sign and flag state of the threshold being exceeded. The operation performed is always the same, but the values ​​of the fixed quantities D1, D2, ..., D4 may differ.

[0026] It is important to periodically reset the value of the baseline A(k) to the value of the proximity signal U(k). It has been found that doing this is advantageous whenever the value of the baseline exceeds the value of the proximity signal (units 335 and 338).

[0027] Figure 3 A conceptual simulation of key signals in the processor and method of this invention is shown. Each curve is scaled, but the units of the vertical axis are not the same for all curves. These curves may have been resized and shifted vertically by any amount to fit in the graph. However, the horizontal axis (showing time or sample index) is shared, and the temporal relationships between the curves are respected. In reality, the time axis could cover spans of several seconds.

[0028] Curve 130 is the proximity signal U(k) from filter 30. It exhibits a steeply rising step, indicating that a conductor is approaching the sensor, followed by a descending step after some time, indicating that the conductor has been removed. This proximity pattern is superimposed on the slowly changing baseline, as it is likely due to thermal drift. Curve 140 is the corresponding average slope Δ obtained over a window with 8 samples. var In this curve, the rising and falling steps are marked by positive and negative peaks.

[0029] Thresholds T2 and T1 are marked by dashed line 144. Within intervals 148 and 149, the baseline signal A(k) increases by a fixed value and has a linear slope, unless it is limited by an interventional reset test (335 and 338 in Figure 2). Elsewhere, the baseline increases by an average slope Δ var (As seen in curve 150) or increase the instantaneous slope Δ 01 (As seen in curve 155), and closely follows the changes in the proximity signal U(k).

[0030] Subtracting the baseline from the proximity signal provides the drift-suppressed signal. Curve 160 is the average slope Δ as the baseline increases. var This suppression is a result of the baseline increasing by an instantaneous slope Δ. 01At the same time, curve 165 is the same. Due to the limited resolution of the numerical algorithm, the former retains some noise, while the latter is very close to the square signal and has minimal noise.

[0031] Please note that this tracking method can be improved by using a dejitter and / or limiter on the signal to avoid Δ var A very large Δ occurs when the value is momentarily crossed to approximately 0 (e.g., during a rapid tap). 01 value.

[0032] In portable wireless devices, the sensors and methods disclosed herein can be advantageously used to detect when the device is near, for example, a user's head. In this case, the device can be configured to take specific actions, such as turning off the display, disabling tactile input, or adjusting radio transmission power.

[0033] Reference symbol 20 Sensing Electrodes 21. Active blocking 23 Analog front-end / back-end amplifiers 24 shielding controller 25 A / D converter 30 filter 40 adders 50-Differentiator 60 baseline extractor 62 drift suppression units 139 Proximity Signal 140 changes 144 threshold line 148 Non-tracking interval 149 Non-tracking interval 150 baseline signal 155 baseline signal 160 Δ var drift correction signal 165 Δ 01 drift correction signal 220 proximity signal 230 baseline signal 240 Drift-corrected signal 305 New Value 308 Approach Marker Test 312 Upper Threshold 314 Upper Threshold 316 Fixed Increment 318 Fixed Increment 322 lower threshold 324 lower threshold 326 Fixed Increment 328 Fixed Increment 330 Tracking 335 test 338 Reset.

Claims

1. A method for suppressing drift superimposed on a proximity signal, the proximity signal being generated by a capacitive sensor, the method comprising: • Receive a series of samples of the proximity signal. • Repeatedly calculate the average slope of the proximity signal from the received samples. • Compare the average slope with a threshold. • The baseline value is updated by adding a value derived from the proximity signal when the average slope is below the threshold, or by adding a fixed value when the average slope is above the threshold. • A drift-corrected proximity signal is obtained by subtracting the baseline value from the proximity signal, wherein: When comparing the average slope with a threshold, the average slope is compared with an upper threshold and a lower threshold, and when updating the baseline value, the value added when the average slope is between the upper threshold and the lower threshold is derived from the proximity signal, the value added when the average slope is below the lower threshold is a first fixed value, and the value added when the average slope is above the upper threshold is a second fixed value, wherein the first fixed value and the second fixed value are different.

2. The method according to claim 1, wherein, The value derived from the proximity signal is either the instantaneous slope of the proximity signal or the average slope of the proximity signal.

3. The method according to claim 1, comprising the following steps: The baseline value is compared with the proximity signal, and if the baseline value exceeds the proximity signal, the baseline value is set to be equal to the proximity signal.

4. The method according to claim 1, comprising the following steps: A drift-suppressed proximity signal is compared with a proximity threshold to generate a logical proximity flag, wherein the values ​​of the upper threshold and / or the lower threshold and / or the first fixed value and / or the second fixed value are changed when the logical proximity flag is raised, and are set back to the previous values ​​when the logical proximity flag is lowered.

5. The method according to claim 1, wherein, The first fixed value and / or the second fixed value are positive, zero, or negative.

6. A capacitive proximity sensor for a portable device, comprising a sensing electrode and a readout circuit configured to read the self-capacitance of the sensing electrode and provide a proximity signal consisting of a series of capacitance values, the capacitive proximity sensor including a drift suppression unit configured to: • The average slope of the proximity signal is repeatedly calculated from the capacitance value. • Compare the average slope with a threshold. • The baseline value is updated by adding a value derived from the proximity signal when the average slope is below the threshold, or by adding a fixed value when the average slope is above the threshold. • A drift-suppressed proximity signal is obtained by subtracting the baseline value from the proximity signal, wherein: When comparing the average slope with a threshold, the average slope is compared with an upper threshold and a lower threshold, and when updating the baseline value, the value added when the average slope is between the upper threshold and the lower threshold is derived from the proximity signal, the value added when the average slope is below the lower threshold is a first fixed value, and the value added when the average slope is above the upper threshold is a second fixed value, wherein the first fixed value and the second fixed value are different.

7. The proximity sensor according to claim 6, wherein, The value derived from the proximity signal is either the instantaneous slope of the proximity signal or the average slope of the proximity signal.

8. The proximity sensor of claim 7, wherein the drift suppression unit is configured to compare the baseline value with the proximity signal and set the baseline value to be equal to the proximity signal if the baseline value exceeds the proximity signal.

9. The proximity sensor of claim 7, wherein the drift suppression unit is configured to compare a drift-suppressed proximity signal with a proximity threshold to generate a logical proximity flag, wherein the values ​​of the upper threshold and / or the lower threshold and / or the first fixed value and / or the second fixed value are changed when the logical proximity flag is raised, and are set back to their previous values ​​when the logical proximity flag is lowered.

10. The proximity sensor according to claim 8, wherein, The first fixed value and / or the second fixed value are positive, zero, or negative.

11. A portable wireless device comprising the proximity sensor according to claim 6.

12. The portable wireless device of claim 11, wherein the proximity sensor is configured to compare the drift-suppressed proximity signal with a proximity threshold to generate a logical proximity flag, wherein the portable wireless device is configured to take a specific action when declaring the logical proximity flag.

13. The portable wireless device of claim 12, wherein the action is one of the following: reducing the transmission power of the wireless data interface, turning off or reducing the brightness of the display, or disabling tactile input.

14. The portable wireless device according to claim 13, wherein, The values ​​of the upper threshold and / or the lower threshold and / or the first fixed value and / or the second fixed value are changed when the logical proximity flag is raised, and are set back to their previous values ​​when the logical proximity flag is lowered.

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