Method for detecting contact on a capacitive sensor element

Abstract

A method for detecting contact of a capacitive sensor includes transferring charge quantities in multiple successive cycles from the capacitive sensor to an integration capacitor having a known capacitance value. A voltage of the integration capacitor is measured. The measured voltage is processed to generate a sensor amplitude that is indicative of a capacitance value of the capacitive sensor. Contact of the capacitive sensor is detected based on a temporal behavior of the sensor amplitude. For instance, contact of the capacitive sensor is detected based on the rate of change of the sensor amplitude.

Claims

1. A method for detecting contact of a capacitive sensor, the method comprising: transferring charge quantities in multiple successive cycles from the capacitive sensor to an integration capacitor having a known capacitance value; measuring a voltage of the integration capacitor; processing the measured voltage to generate a sensor amplitude, the sensor amplitude being indicative of a capacitance value of the capacitive sensor; determining a rate of change of the sensor amplitude from a rise time for the sensor amplitude to increase from a predetermined change starting amplitude value to a predetermined change target amplitude value; and detecting contact of the capacitive sensor based on the rate of change of the sensor amplitude.

2. The method of claim 1 wherein: each of the predetermined change starting amplitude value and the predetermined change target amplitude value is a function of a physical implementation of the capacitive sensor.

3. The method of claim 1 further comprising: detecting contact of the capacitive sensor upon the rise time falling in a range between a predetermined minimum rise time threshold and a predetermined maximum rise time threshold.

4. The method of claim 3 further comprising: detecting contact of the capacitive sensor upon (i) the rise time falling in the range between the predetermined minimum rise time threshold and the predetermined maximum rise time threshold and (ii) at expiration of a predetermined wait time after the sensor amplitude exceeds the predetermined change target amplitude value, the sensor amplitude falling in a range between a predetermined amplitude minimum value and a predetermined amplitude maximum value.

5. The method of claim 4 wherein: the predetermined wait time is a function of a physical implementation of the capacitive sensor.

6. The method of claim 4 further comprising: detecting contact of the capacitive sensor upon (i) the rise time falling in the range between the predetermined minimum rise time threshold and the predetermined maximum rise time threshold, (ii) at expiration of a predetermined wait time after the sensor amplitude exceeds the predetermined change target amplitude value, the sensor amplitude falling in a range between a predetermined amplitude minimum value and a predetermined amplitude maximum value, and (iii) at expiration of a predetermined check time after expiration of the predetermined wait time, the sensor amplitude has not increased or decreased from its value at the expiration of the predetermined wait time more than an allowed positive tolerance or an allowed negative tolerance, respectively.

7. The method of claim 1 wherein: measuring the voltage of the integration capacitor includes measuring the voltage of the integration capacitor after each cycle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantageous embodiments result from the following description of an exemplary embodiment of a method according to the present invention. The method according to the present invention is described below with reference to the drawing in which:

(2) FIG. 1A illustrates a diagram of a known system for carrying, out an integration process to measure a capacitance value of the capacitive sensor; and

(3) FIG. 1 illustrates a time curve of the sensor amplitude A when a touch-sensitive capacitive sensor is approached and subsequently contacted during different first and second touch applications, the first touch application resulting in sensor amplitude A1 and the second touch application resulting in sensor amplitude A2.

(4) Referring initially to FIG. 1A, a diagram of a known system for carrying out an integration process to measure a capacitance value C.sub.M of a capacitive sensor 1 is shown. The system has an integration capacitor 2, a first switch S1, a second switch S2, an A/D converter 4, and a processor 5. A terminal of capacitive sensor 1 is electrically connected to a first terminal 2 of integration capacitor 2 at a shared circuit node 3. Shared circuit node 3 is also electrically connected to first switch S1. Shared circuit node 3, via first switch S1, is connectable to (i) a fixed supply voltage U.sub.V, (ii) potential-free, i.e., held open (NC), or (iii) the ground GND. A second terminal 2 of integration capacitor 2 is electrically connected to second switch S2. Second terminal 2 of integration capacitor 2, via second switch S2, is connectable to (i) the fixed supply voltage U.sub.V, (ii) an input of an A/D converter, or (iii) the ground GND. First and second switches S1 and S2 are switched in a known manner to implement the integration process in which small charge quantities are transferred from capacitive sensor 1 to integration capacitor 2 in multiple successive cycles. A/D converter 4 measures the voltage U.sub.CI(N) present at integration capacitor 2 after a number N of these charge transfers. Processor 5 processes the measured voltage to generate a sensor amplitude.

DETAILED DESCRIPTION

(5) Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

(6) FIG. 1 illustrates the time curve of the sensor amplitude A when a touch-sensitive capacitive sensor is approached and subsequently contacted, for two different applications. The first application results in sensor amplitude A1 plotted in FIG. 1 and the second application results in sensor amplitude A2 also plotted in FIG. 1.

(7) Thus, the top measurement curve shows the time curve of the detected sensor amplitude A1 when the capacitive sensor is approached and subsequently contacted in the center of the capacitive sensor by a large finger without a glove. In contrast, the bottom measurement curve shows the corresponding curve of the sensor amplitude A2 when the same capacitive sensor is approached and subsequently contacted at the edge of the capacitive sensor by a small finger with a glove.

(8) These two measurement curves or sensor amplitudes A1 and A2 thus represent borderline cases with extremely different conditions. These different conditions are, however, to be handled using the same recognition method for detecting contact on the capacitive sensor.

(9) Before a finger approaches the capacitive sensor, the sensor amplitude A for both measurement curves remains at a base amplitude value A.sub.B that is essentially constant or changes only slowly due to external influences such as temperature fluctuations. As soon as the finger approaches the capacitive sensor, the sensor amplitude A begins to slowly increase. The rate of change A/t of the sensor amplitude A is used to obtain a necessary criterion for recognizing actual contact of the capacitive sensor.

(10) In this exemplary embodiment, the rate of change A/t of the sensor amplitude A is specifically ascertained by determining a rise time t.sub.a. The rise time t.sub.a is the time needed for the sensor amplitude A to increase from a change starting amplitude value A.sub.S at point in time T.sub.S to a change target amplitude value A.sub.Z at point in time T.sub.Z. This rise time t.sub.a=T.sub.ZT.sub.S is inversely proportional to the rate of change A/t of the sensor amplitude A and may thus be used as a measure of the sought rate of change.

(11) In this exemplary embodiment, the change starting amplitude value A.sub.S and the change target amplitude value A.sub.Z are predetermined parameters. Particularly, the change starting amplitude value A.sub.S and the change target amplitude value A.sub.Z for the present physical implementation of the capacitive sensor are experimentally determined in such a way that they are used to detect the rate of change A/t of the sensor amplitude A in a range in which this is as similar as possible for most applications. In this regard, in the example shown in FIG. 1 of the two measurement curves with the sensor amplitudes A1 and A2, the rate of change A/t of sensor amplitudes A1 and A2 for the two diverse applications is identical in the selected range.

(12) As a necessary criterion for the recognition of actual contact of the capacitive sensor, the rate of change A/t of the sensor amplitude A must be between a rate of change minimum value (A/t).sub.min and a rate of change maximum value (A/t).sub.max. That is, the determined rise time t.sub.a must be in a predefined range between a minimum time value t.sub.a min and a maximum time value t.sub.a max.

(13) Such a criterion that responds to the dynamics of the signal curve thus naturally has a very sensitive response to possible interferences of the signal. For this reason, a further criterion is necessary for reliably recognizing actual contact of the capacitive sensor.

(14) The sensor amplitude A itself is considered for defining such a criterion. In contrast to an interference, caused by electromagnetic pulses, for example, during actual contact of the capacitive sensor, the sensor amplitude A must have a value that is greatly above the base amplitude value A.sub.B. On the other hand, a sensor amplitude A that is significantly higher than a maximum value to be expected based on the physical characteristics would once again indicate interference. As such, a maximum amplitude value A.sub.max is defined, below which the sensor amplitude A must lie to assume a signal without interference.

(15) The further criterion therefore requires that the sensor amplitude A is between an amplitude minimum value A.sub.min and the amplitude maximum value A.sub.max at a waiting time T.sub.W. The waiting time T.sub.W corresponds to the expiration of a predefined time period t.sub.w after the point in time T.sub.Z that the change target amplitude value A.sub.Z is exceeded. The time period t.sub.w, which corresponds to a waiting time until an at least essentially constant sensor amplitude A is reached, is set by multiplying the previously determined rise time t.sub.a by a constant factor w. The constant factor w is a function of the physical implementation of the capacitive sensor and must be experimentally determined, so that the following must apply: t.sub.w=w*t.sub.a, and thus, T.sub.W=T.sub.Z+w*t.sub.a.

(16) Although the sensor amplitudes A1 and A2 for the two measurement curves are greatly different at the waiting time T.sub.W, which is attributed to the described different physical conditions for the two applications, both sensor amplitudes A1 and A2 are within the range defined by the amplitude minimum value A.sub.min and the amplitude maximum value A.sub.max, so that valid contact of the capacitive sensor is established in both cases.

(17) To further ensure the reliability of the sensor evaluation, and particularly more reliably avoid false positive recognitions, a third criterion may be defined. The aim of the third criterion is to confirm that the sensor amplitude A at the waiting time T.sub.W is not only between the amplitude minimum value A.sub.min and the amplitude maximum value A.sub.max but is also essentially constant. For this purpose, a check is made at a check time T.sub.P as to whether the sensor amplitude A has not increased or decreased from its value at waiting time T.sub.W by more than an allowable positive tolerance A+ or a negative tolerance A, respectively.

(18) The check time T.sub.P should be as close as possible to the waiting time T.sub.W but should allow at least enough time for the sensor amplitude A to assume a debounced state after reaching an essentially constant value. Therefore, a test time period t.sub.p is specified that meets this requirement for the capacitive sensor arrangement, and the check time is set to T.sub.P=T.sub.W+t.sub.p.

(19) At this check time T.sub.P, the following must apply for the sensor amplitude A:
(AA)<A<(A+A+).

(20) Contact of the capacitive sensor that is recognized as valid according to the above-described criteria is therefore rejected as invalid in this last step if, at check time T.sub.P, the sensor amplitude A differs from its value at waiting time T.sub.W by more than the allowed positive tolerance A+ upwards or by more than the allowed negative tolerance A downwards.

(21) While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.