DISTURBANCE-RESISTANT CAPACITY DETERMINATION

Description

[0001] The present invention relates to a method for determining the capacitance of a capacitive element, wherein during each measurement cycle of a plurality of successive measurement cycles the capacitive element is charged, and subsequently a charge is transferred from the capacitive element to another capacitive element. After the plurality of measurement cycles, a measured value relating to the total charge transferred to the other capacitive element during the plurality of measurement cycles is determined, and the capacitance of the capacitive element is determined as a function of this measured value. The invention further relates to a corresponding measurement arrangement for determining a capacitance, a user input device with a capacitive element, and a computer program.

[0002] EP 3 474 452 A1 discloses a method for evaluating the capacitance value of a capacitive sensor electrode. The method comprises an evaluation process with the following steps: - Performing a charging process of the sensor electrode, - Evaluating a parameter dependent on the capacitance value of the sensor electrode. A first charging pulse duration, consisting of a first coupling duration and a first decoupling duration, of a first coupling and decoupling cycle, and a second charging pulse duration, consisting of a second coupling duration and a second decoupling duration, of a second coupling and decoupling cycle immediately following the first coupling and decoupling cycle, differ from each other in a first pulse duration deviation.

[0003] US Patent 6,466,036 B1 discloses a pulse circuit constructed from sets of three or more electrical switching elements arranged such that each switching element has one side electrically connected to either a supply voltage or an electrical ground. Furthermore, the circuit can be configured as a proximity switch requiring only a two-wire connection.

[0004] US Patent 2010 / 283485 A1 discloses a method for detecting changes in the capacitance of a sensor capacitor when a person approaches. The change is detected by charging the sensor capacitor through coupling the sensor to a power supply. During the charging process, the power supply is disconnected from a storage capacitor. The charge is transferred from the sensor capacitor to the storage capacitor. During the charge transfer, the power supply is disconnected from the storage capacitor. The charging process and charge transfer are repeated several times. The voltage of the storage capacitor is monitored. At least one value is determined that depends on the rate of change of the storage capacitor's voltage.

[0005] Document US 6,466,036 B1 describes a pulse circuit for measuring the capacitance of a sensor plate, which is capable of repeatedly transferring charge accumulated on the sensor plate to a capacitor and, after a large number of repetitions, reading out the amount of charge transferred or reading out a corresponding voltage in order to determine the capacitance of the sensor plate.

[0006] Such circuits can be used, for example, for capacitive user input detection, where the user's approach to or touch of the sensor plate changes the capacitance of the sensor plate, thereby enabling the touch or approach to be detected. In particular, such touch detection can be used for applications in the interior of motor vehicles, for example, for detecting the activation of touch-sensitive control panels or controls.

[0007] Since the individual charge transfers from the sensor plate to the subsequent capacitor occur repeatedly at a predetermined frequency, the corresponding circuits and measurement methods can be affected by external interference with similar frequencies. This can reduce the accuracy and reliability of the capacitance determination and, consequently, the reliability of touch detection.

[0008] Certain frequency ranges can be addressed using analog low-pass filters or digital filters. For example, with typical sampling frequencies on the order of 10 µs and a corresponding Shannon or Nyquist frequency on the order of 50 kHz, interference above the Shannon frequency and below a low-pass filtering threshold on the order of one or more MHz remains unaffected.

[0009] Against this background, an object of the present invention is to provide an improved concept for determining the capacitance of a capacitive element, which can reduce the influence of external disturbances, particularly in the form of electromagnetic waves.

[0010] This problem is solved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims.

[0011] The improved concept is based on the idea of ​​changing the sampling frequency or a corresponding duration of a large number of successive measurement cycles.

[0012] According to the improved concept, a method for determining the capacitance of a capacitive element is described. The capacitive element is charged, specifically partially charged, during each measurement cycle of a plurality of successive measurement cycles. Subsequently, within the same measurement cycle, a quantity of charge is transferred from the capacitive element to another capacitive element. After the plurality of measurement cycles, i.e., specifically after the completion of all measurement cycles of the plurality of measurement cycles, a measured value relating to the total quantity of charge transferred to the other capacitive element during the plurality of measurement cycles is determined, particularly by means of a processing unit. The capacitance of the capacitive element is then determined, particularly by means of the processing unit, as a function of the measured value. The plurality of measurement cycles comprises a first subset of measurement cycles and a second subset of measurement cycles.For the first subset, a first time period is determined using the computing unit, and for the second subset, a second time period is determined using the computing unit, which differs from the first time period.

[0013] The duration of a measurement cycle corresponds to the time that elapses from the beginning to the end of the entire respective measurement cycle. The duration of a measurement cycle can therefore be understood as the inverse of the sampling frequency. The first and second durations are thus determined, in particular, by the processing unit before or when the respective measurement cycle begins.

[0014] In other words, each measurement cycle of the first subset has the first duration, and each measurement cycle of the second subset has the second duration.

[0015] The measurement cycles of the first time period do not all follow one another. Similarly, the measurement cycles of the second subset do not all follow one another. For example, after each measurement cycle of the first subset, a measurement cycle of the second subset may follow before another measurement cycle of the first subset, and so on. The multitude of measurement cycles can include not only the first and second subsets but also one or more further subsets, each with different time periods. Over the course of the multitude of measurement cycles, cycles with different time periods—that is, cycles from different subsets—can alternate, so that the respective time period of any two consecutive measurement cycles within the multitude of measurement cycles always differs from one another.

[0016] To determine the measured value, for example, the voltage applied to or across the other capacitive element can be measured. The measured value therefore corresponds to, or depends on, a corresponding voltage value of the other capacitive element. The voltage measurement for determining the measured value is performed only after the multitude of measurement cycles has been completed.

[0017] The amount of charge transferred from one capacitive element to another during a measurement cycle does not necessarily correspond exactly to the amount of charge that was applied to the capacitive element during the same measurement cycle when the capacitive element was charged. For example, charge equalization between the capacitive element and the other capacitive element may be prevented during charging, while charge equalization between the capacitive element and the other capacitive element may be allowed for the charge transfer.

[0018] While the charge accumulates on the capacitive element over the course of numerous successive measurement cycles to reach the total charge amount, the capacitive element can be discharged, i.e., reset, for example after each individual measurement cycle or within each individual measurement cycle.

[0019] A measurement cycle can therefore include several successive sections, corresponding to different states of the capacitive element and the next capacitive element, or of a circuit with the capacitive element and the next capacitive element. The capacitive element can, for example, be designed as a capacitor or comprise a single sensor area, where the capacitance of the capacitive element can then be determined, for example, by the intrinsic capacitance of the sensor area with respect to a ground potential.

[0020] The additional capacitive element can also be designed as a capacitor. The dimensions of this additional capacitive element are chosen such that its capacitance is many times greater than the maximum capacitance of the other capacitive element, for example, by a factor on the order of 10 to 1000, and in particular by a factor of 100 to 1000.

[0021] By determining the capacitance of the capacitive element indirectly via the total amount of charge transferred to the other capacitive element, the accuracy of the capacitance determination can be significantly increased, since it is not necessary to measure and evaluate the much lower voltages that are applied to the capacitive element itself.

[0022] Because the respective measurement cycles of the first and second subset have different durations, and the sampling frequency therefore changes over the course of the multitude of measurement cycles, the influence of external disturbances, corresponding to electromagnetic waves with an approximately monofrequency spectral characteristic, is reduced. In particular, while such an external disturbance can affect individual measurement cycles within the multitude of measurement cycles, it will generally not affect all measurement cycles, as these have correspondingly detuned durations or sampling frequencies. For example, if the external disturbance affects the first subset of measurement cycles because a dominant frequency of the external disturbance is approximately the inverse of the first duration, it will not affect the measurement cycles of the second subset, and vice versa.

[0023] According to an embodiment of the method according to the improved concept, each of the measurement cycles includes a charging section, wherein the charging of the capacitive element takes place during the charging section, in particular exclusively during the charging section. Each of the measurement cycles also has a transfer section, wherein the transfer of the charge quantity from the capacitive element to the other capacitive element takes place during the transfer section, in particular exclusively during the transfer section.

[0024] The transfer section follows the charging section of the same measurement cycle, but does not necessarily follow it directly. In particular, each measurement cycle, within the multitude of measurement cycles, comprises a multitude of successive sections, which include the charging section and the transfer section. The difference between the first and second time periods, for example, can be realized in a single section or distributed across several sections.

[0025] According to at least one embodiment, to determine the first duration, a first charging duration is defined for the respective charging section of the measurement cycles of the first subset, and to determine the second duration, a second duration is defined for the respective charging section of the measurement cycles of the second subset, wherein the second charging duration differs from the first charging duration.

[0026] In particular, the first charging time is the same for all charging sections of the measurement cycles of the subset, and analogously, the second charging time is the same for all charging sections of the measurement cycles of the second subset.

[0027] According to at least one embodiment, to determine the first duration, a first transmission duration is defined for the respective transmission section of the measurement cycles of the first subset, and to determine the second duration, a second transmission duration is defined for the respective transmission section of the measurement cycles of the second subset, wherein the second transmission duration differs from the first transmission duration.

[0028] In particular, to determine the first and second durations, only the corresponding first and second transmission durations can be set differently from each other, whereas the charging duration is the same for all charging sections of the measurement cycles of the multitude of measurement cycles, i.e., in particular the first and second subset.

[0029] In principle, the duration of any section of the measurement cycle can be varied. Depending on the design of the corresponding circuit, it may be advantageous to change the transmission duration of the transmission sections to avoid leakage currents into the evaluation electronics.

[0030] According to an embodiment of the invention, each of the measurement cycles has an intermediate section between the charging section and the first transmission section, wherein the capacitive element is neither charged nor discharged during the intermediate section.

[0031] According to an alternative or additional embodiment of the invention, each of the measuring cycles has a further intermediate section before the charging section or after the transmission section, wherein the capacitive element is neither charged nor discharged during the further intermediate section.

[0032] For example, each measurement cycle can include or consist of the charging section, the intermediate section, the transfer section, and the next intermediate section in that order. Alternatively, each measurement cycle can include or consist of the next intermediate section, the charging section, the intermediate section, and the transfer section in that order.

[0033] As explained above, by appropriately connecting the capacitive element and the subsequent capacitive element, charge equalization between them can be prevented during the charging phase and permitted during the transfer phase. These intermediate sections prevent leakage currents from the subsequent capacitive element from distorting the measurement during the switch from the charging phase to the transfer phase or vice versa.

[0034] According to an embodiment of the invention, to determine the first time period, a first intermediate time period is determined for the respective intermediate section of the measurement cycles of the first subset, and to determine the second time period, a second intermediate time period is determined for the respective intermediate section of the measurement cycles of the second subset, wherein the second intermediate time period differs from the first intermediate time period.

[0035] According to an alternative or additional embodiment of the invention, to determine the first time period, a first further intermediate time period is determined for the respective further intermediate section of the measurement cycles of the first subset, and to determine the second time period, a second further intermediate time period is determined for the respective further intermediate section of the measurement cycles of the second subset, wherein the second further intermediate time period differs from the first further intermediate time period.

[0036] According to at least one embodiment, the computing unit sets a counter value for each measurement cycle of the plurality of measurement cycles and determines a time duration for the respective measurement cycle depending on the respective set counter value.

[0037] The time durations for the measurement cycles of the multitude of measurement cycles determined in this way include, in particular, the first time duration for the first subset of measurement cycles and the second time duration for the second subset of measurement cycles.

[0038] In particular, each set counter value is uniquely assigned to a corresponding time period. In other words, the counter can assume a predetermined finite set of counter values, where each counter value corresponds exactly to a time period. For example, the first time period corresponds to a first counter value, and the second time period corresponds to a second counter value that is different from the first.

[0039] The counter can be implemented as hardware or software. For example, it can be a binary counter capable of encoding a number of n bits, where n ≥ 1, preferably n ≥ 2, for example n = 3, n = 4, or n = 5. Accordingly, the number of counter values, and thus the number of encodeable time periods for the measurement cycles, is 2n< .

[0040] The number of measurement cycles within the plurality of measurement cycles can, in principle, be greater or less than 2n<, but is preferably greater than 2n<. For n = 3, n = 4, n = 5, or n = 6, the corresponding values ​​would be 2n< = 8, 2n< = 16, 2n< = 32, and 2n< = 64, respectively. The number of measurement cycles within the plurality of measurement cycles is preferably greater than or equal to 100, for example, greater than or equal to 200, greater than or equal to 300, greater than or equal to 400, or greater than or equal to 500. For example, the number of measurement cycles within the plurality of measurement cycles is in the range between 100 and 1000.

[0041] In embodiments where the number of measurement cycles is greater than 2n, the counter cycles through all its possible readings multiple times. Accordingly, in such embodiments, at least two of the multiple measurement cycles have the same duration. However, this is not necessarily the case. If, from an application-specific perspective, larger counters are considered, and in particular larger values ​​for n, 2n < n can also be greater than or equal to the number of measurement cycles, for example, for a number of measurement cycles equal to 300 and n ≥ 9.

[0042] By using the counter in the manner described, a particularly simple variation of the durations of the measurement cycles can be achieved.

[0043] For example, starting from an initial reading, the counter can be incremented by a predetermined amount, such as 1, for each of the multiple measurement cycles, as long as the counter reading is less than or equal to a predetermined maximum. The counter can be reset to the initial reading once it reaches the maximum.

[0044] The initial counter reading can be, for example, 0 and the maximum counter reading can be 2n-1<, where the increment is 1. The counter reading can be changed accordingly, for example, at the beginning, during, or shortly before each measurement cycle.

[0045] According to at least one embodiment, the relationship N ≤ M applies, where M denotes the number of measurement cycles of the plurality of measurement cycles, (N-1)*I is an integer representing a difference between the maximum counter reading and the initial counter reading, and I denotes the increment.

[0046] According to at least one embodiment, the computing unit determines the respective duration for each measurement cycle of the multitude of measurement cycles as the sum of a predetermined minimum duration, constant for all measurement cycles of the multitude of measurement cycles, and an additional duration dependent on the respective set counter value.

[0047] The additional time duration is directly proportional to the meter reading. The maximum meter reading therefore corresponds to a maximum additional time duration.

[0048] The maximum additional duration can be, in particular, smaller than the minimum duration. For example, the maximum additional duration is less than or equal to 10% of the minimum duration, less than or equal to 5% of the minimum duration, or less than or equal to 1% of the minimum duration. The minimum duration can, for example, be between 0.5% and 5% or between 0.5% and 2% of the minimum duration. Such variations in duration have proven to be well-suited to sufficiently reduce the impact of external disturbances.

[0049] According to at least one embodiment, the minimum duration is in a range of 0.5 µs and 30 µs, in particular in a range between 1 µs and 20 µs.

[0050] According to at least one embodiment, the respective additional duration is less than or equal to one tenth of the minimum duration, in particular less than or equal to one fiftieth of the minimum duration, for example less than or equal to one hundredth of the minimum duration.

[0051] According to the improved concept, a method for detecting user input is also specified, where the user input involves a user touching a capacitive element. The user input detection method according to the improved concept includes performing a procedure to determine the capacitance of the capacitive element. The capacitance thus determined is compared by the processing unit to a predefined threshold, and the user input or touch is detected by the processing unit depending on the result of the comparison.

[0052] According to the improved concept, a measuring arrangement for determining the capacitance of a capacitive element is also specified. The measuring arrangement has a connection for linking the capacitive element to the measuring arrangement and a circuit. The circuit is configured to connect the capacitive element to a voltage source during each measurement cycle of a plurality of successive measurement cycles in order to charge the capacitive element and subsequently transfer a quantity of charge from the capacitive element to another capacitive element of the measuring arrangement. The measuring arrangement has a processing unit configured to determine, after the plurality of measurement cycles, a total quantity of charge transferred to the other capacitive element during the plurality of measurement cycles and to determine the capacitance of the capacitive element as a function of the measured values.The computing unit is designed to define a first duration for a first subset of the multitude of measurement cycles and to define a second duration for a second subset of the multitude of measurement cycles, which differs from the first duration.

[0053] Generally, the capacitive element is not necessarily part of the measuring arrangement. However, in various embodiments, the measuring arrangement, particularly the circuit, can include the capacitive element. The additional capacitive element can, for example, also be part of the circuit.

[0054] Similarly, the voltage source is generally not necessarily part of the measuring setup. However, in various embodiments, the measuring setup can also include the voltage source.

[0055] To connect the capacitive element to the voltage source, or to transfer the charge from one capacitive element to another, the circuit can, for example, include appropriate switching elements and a controller for controlling these elements. The controller can also be integrated into the processing unit in various configurations.

[0056] The computing unit may, in particular, include a microcontroller.

[0057] According to at least one embodiment, the computing unit includes an analog-to-digital converter (ADC) which can be connected to the further capacitive element to determine the measured value.

[0058] Further embodiments of the measuring arrangement according to the improved concept follow directly from the various configurations of the method for determining a capacity according to the improved concept and the method for detecting a user input according to the improved concept, and vice versa. In particular, a measuring arrangement according to the improved concept can be configured to perform a method according to the improved concept or perform such a method.

[0059] According to the improved concept, a user input device, also referred to as a user input interface, is also specified, in particular a user input device for use in a motor vehicle interior. The user input device has a capacitive element arranged for touch by a user. The user input device includes a measuring arrangement for determining the capacitance of the capacitive element according to the improved concept.

[0060] According to the improved concept, a computer program product with instructions is also specified. When executed by a computing unit of a measurement setup according to the improved concept, the instructions cause the measurement setup to perform a procedure according to the improved concept. The computer program product can, for example, be designed as a computer program containing the instructions. The computer program product can also be designed as a computer-readable storage medium containing a computer program that includes the instructions.

[0061] The invention is defined by the attached claims.

[0062] The figures show: Fig. 1a a schematic representation of an exemplary embodiment of a measuring arrangement according to the improved concept in an initialization state; Fig. 1b a schematic representation of the measuring arrangement made of Fig. 1a in an intermediate state; Fig. 1 is a schematic representation of the measuring arrangement. Fig. 1c und Fig. 1c in a charging state; Fig. 1 a schematic representation of the measuring arrangement made of Fig. 1a bis 1c in a transfer state: Fig. 1e a schematic representation of the measuring arrangement made of Fig. 1a bis Fig. 1d in a readout state; Fig. 2 a flowchart of an exemplary embodiment of a method according to the improved concept; and Fig. 3 a schematic representation of a measurement cycle in a method according to the improved concept.

[0063] In Fig. 1a bis Fig. 1e An exemplary embodiment of a measuring arrangement 2 according to the improved concept is shown, which is, for example, part of a user input device 1, for example for use in the interior of a motor vehicle (not shown). Fig. 1a bis Fig. 1e These represent different states of the measurement setup 2 corresponding to different sections of measurement cycles in a procedure according to the improved concept. A corresponding flowchart of a procedure for detecting user input according to the improved concept is shown in Fig. 2 depicted.

[0064] The user input device 1 has a capacitive element 3 that is positioned and configured for user touch. The capacitive element 3 can be designed, for example, as a touch-sensitive film or a touch-sensitive sensor plate. The user can, for example, touch a surface of the capacitive element 3 or bring a finger close to it to initiate user input. The user's approach to or touch of the capacitive element 3 changes its capacitance. The measuring arrangement 2 is capable of determining the capacitance of the capacitive element 3 and, based on this, inferring the presence of user input.

[0065] The measured capacitance of capacitive element 3 can therefore be understood as its own capacitance with respect to a mass potential. Therefore, in the figures Fig. 1a bis Fig. 1e the capacitive element 3 is also represented as a capacitor, the first terminal of which is connected to a reference potential terminal 9, in particular a ground terminal.

[0066] The measuring arrangement 2 has a connection 4 to connect the capacitive element 3, in particular the first connection of the capacitive element 3, to the measuring arrangement 2, as well as another capacitive element 5, which may, for example, be designed as a capacitor.

[0067] A second terminal of the capacitive element 3 is connected to a first terminal of the further capacitive element 5. The second terminal of the capacitive element 3 is, in particular, directly connected to or formed by the sensor plate, the touch-sensitive film, or the like. A second terminal of the further capacitive element 5 is connected to a first terminal 7 of a computing unit 6 of the measuring arrangement 2. A second terminal 8 of the computing unit 6 is connected to the second terminal of the capacitive element 3 as well as to the first terminal of the further capacitive element 5. The computing unit 6 may, for example, contain a microcontroller or another integrated circuit.

[0068] The measuring arrangement 2 also includes a circuit, which is shown in the figures. Fig. 1a bis Fig. 1e not shown separately. The circuit includes a controller and several switching elements that can be controlled by the controller. The controller can also be part of the computing unit 6. The switching elements are arranged and configured to connect or disconnect terminals 7 and 8 of the computing unit 6 from the reference potential terminal 9, depending on the current state of the measuring arrangement 2, when controlled by the controller. Furthermore, the controller can use the switching elements to connect or disconnect a voltage source 10, which may be part of the measuring arrangement 2 or the user input device 1, or may be external to these, from the second terminal 8 of the computing unit 6, depending on the state of the measuring arrangement 2.

[0069] The processing unit 6 also includes an analog-to-digital converter (ADC) 11, which can be connected to the second terminal 8. In particular, the circuit can connect or disconnect the ADC 11 from the second terminal 8 depending on the state of the measuring arrangement 2.

[0070] When carrying out a procedure according to the improved concept, various states S0, S1, S2, S3, S4 of the measuring arrangement 2, as described in Fig. 1a bis Fig. 1e are depicted, partly cyclically or repeatedly, as exemplified in Fig. 2 is shown. S0 denotes an initialization state of the measuring arrangement 2 during an initialization section of the procedure, and S4 denotes a readout state of the measuring arrangement 2 during a readout section of the procedure.

[0071] States S1 to S3 constitute a measurement cycle, whereby a multitude of successive measurement cycles are performed to carry out the procedure. How this develops Fig. 2 As can be seen, to determine the capacity of the capacitive element 3, or to detect the user input, the measuring arrangement 2 is first brought into the initialization state S0, then the multitude of measurement cycles is carried out, and afterwards the measuring arrangement 2 is brought into the readout state S4. The in Fig. 2 The outlined process can also be referred to as a measurement process, whereby the measurement process can also be repeated during the operation of the measuring arrangement 2 or the user input device 1 in order to achieve continuous monitoring of the capacitive element 3 with regard to contact by the user.

[0072] In one embodiment of the method as described in Fig. 2 As shown, each measurement cycle comprises an intermediate stage, a subsequent charging stage, another intermediate stage, and a subsequent transfer stage. Accordingly, during a measurement cycle, the measurement setup 2 transitions from an intermediate state designated S1 to a charging state designated S2, followed by another intermediate state S1, and then a transfer state designated S3. After completion of the measurement cycle, the processing unit 6 checks whether the specified number of measurement cycles per measurement process has been reached. If so, the measurement setup 2 is brought to the readout state S4; otherwise, the next measurement cycle begins.

[0073] In state S0, the controller connects the first terminal 7 and the second terminal 8 to the reference potential terminal 9 via the circuit. This connects the first and second terminals of capacitive element 3 and the other capacitive element 5 to the reference potential terminal 9. Accordingly, during the initialization state S0, both capacitive element 3 and the other capacitive element 5 are discharged. In the subsequent first intermediate phase, the measuring arrangement 2 is brought into the intermediate state S1, which can also be referred to as the switching state or changeover state. Here, terminals 7 and 8 of the processing unit 6 are disconnected from the reference potential terminal 9. The measuring arrangement 2 remains in the intermediate state S1 for a duration that is, for example, less than 1 µs, such as between 0.1 µs and 1 µs.

[0074] The control circuit then transfers the measuring arrangement 2 to the charging state S2. In contrast to the intermediate state S1, the second terminal 8 of the processing unit 6 is connected to the voltage source 10 to charge the capacitive element 3. By internally setting the first terminal 7 to a high impedance, for example, the amount of charge on the other capacitive element 5 remains unchanged. The charging state S2 is maintained for a predetermined charging duration, which can be, for example, in the range of 1 µs to 10 µs. The measuring arrangement 2 is then returned to the intermediate state S1 during a further intermediate step. The duration of this further intermediate step can, for example, be equal to the duration of the intermediate step.

[0075] During a subsequent transfer phase, the measuring arrangement 2 is brought into the transfer state S3, also known as the transfer state, by the control circuit. Here, the second terminal 8 of the processing unit 6 is set to a high impedance, and the first terminal 7 is connected to the reference potential terminal 9. This allows charge equalization to occur between the capacitive element 3 and the other capacitive element 5. The capacitance of the other capacitive element 5 is chosen to be many times higher than the maximum capacitance of the other capacitive element 3, so that effectively a charge quantity determined by the two capacitances is transferred from the capacitive element 3 to the other capacitive element 5.

[0076] The transmission duration of the transmission segment can, for example, be in the range of 0.5 µs to 5 µs or in the range of 0.5 µs to 2 µs, or similarly. The readout duration of the readout period can, for example, be in the range of 0.5 to 5 µs or in the range of 0.5 µs to 2 µs.

[0077] The states S1, S2, S1, S3 are in this order, as in Fig. 2 The measurement process is initiated and repeated cyclically until a predetermined total number of measurement cycles has been completed, for example, 300 to 500 measurement cycles. After this total number of measurement cycles, a total charge has been transferred from capacitive element 3 to the other capacitive element 5 in the respective transfer sections. This total charge is then read out in the subsequent readout section using the processing unit 6. For this purpose, the measurement arrangement 2 is brought into readout state S4 by means of the controller and the circuit. The first terminal 7 of the processing unit 6 remains connected to the reference potential terminal 9, and the second terminal 8 of the processing unit 6 is connected to the ADC 11 to measure the voltage between terminal 4 of the measurement arrangement 2 and the reference potential terminal 9, thus determining a measured value that represents the total charge.

[0078] After the readout section, the processing unit 6 can compare the measured value thus determined with a predefined threshold value to detect whether the capacitive element 3 has been touched by the user or not. If the measured value is greater than the predefined limit or threshold value, then, for example, touching can be assumed.

[0079] According to the improved concept, the total durations of the individual measurement cycles across the multitude of measurement cycles are not constant. This is shown schematically for an exemplary embodiment in Fig. 3 sketched. In the embodiment according to Fig. 3The intermediate durations of the intermediate segment and the subsequent intermediate segment, as well as the charging duration of the charging segment, are each constant. The transmission duration of the transmission segment consists of a predetermined, constant minimum transmission duration (i.e., constant for all measurement cycles) and an additional duration dT that varies for different measurement cycles. The sum of the minimum transmission duration, the charging duration, and the respective intermediate durations of the intermediate segments corresponds to a minimum duration Ts for the measurement cycles. In other words, the duration of a measurement cycle is given by the sum of the minimum duration Ts and the additional duration dT. The additional duration dT can be systematically varied, particularly between 0 and a maximum additional duration.

[0080] It should be noted that varying the duration of the measurement cycle does not necessarily require changing the transmission duration of the transmission section, nor does it necessarily require changing only the transmission duration. In other embodiments, the respective durations of one or more of the further sections can also be varied.

[0081] For example, the processing unit 6 can set a counter value for each measurement cycle of the multitude of measurement cycles, so that the counter value increases stepwise from 0 to a maximum counter value. The additional time duration dT can then be determined by the processing unit 6 depending on the respective counter value for the corresponding measurement cycle. This allows, for example, the duration (Ts + dT) for successive measurement cycles to be increased by increasing the additional time duration dT according to the respective counter value.

[0082] A software-based 4-bit or 5-bit counter can be used as the counter. Other counter sizes are, of course, also possible. In the purely illustrative case of a 4-bit counter, the counter can therefore assume 16 counter values ​​from 0 to 15. For a given counter value k, the additional time dT can thus be defined, for example, as dT = (k / 16*ΔTs), where ΔTs corresponds to the maximum additional time. Alternatively, dT = ([(n+1) / 16]*ΔTs) can also be used as the additional time, so that the additional time dT ranges from ΔTs / 16 to ΔTs.

[0083] Assuming, for example, 300 to 500 measurement cycles per measurement process, a 4-bit counter will traverse its entire value range many times during a single measurement. The counter does not necessarily need to be reset for the next measurement.

[0084] By gradually varying the duration (Ts + dT) of the measurement cycles of a measurement process, or possibly also of several successive measurement processes, it is possible to prevent a significant influence on the accuracy of the capacitance measurement in the event of an external disturbance with a more or less constant frequency on the order of 1 / Ts.

[0085] As described, particularly with regard to the figures, the improved concept allows the capacitance of a capacitive element to be determined with higher reliability and, in particular, with reduced influence from external disturbances. This is achieved by artificially extending the corresponding measurement cycles. In principle, this extension can be performed in any state of the measurement setup or at any point during the measurement cycle. The transfer state is particularly suitable for this purpose, as it helps to avoid current leakage. However, the extension could also be performed during the charging state or intermediate states. In various embodiments, the duration of the measurement cycle can be continuously increased by a few percent, for example, 1 to 3%. External disturbances therefore have only a fraction of the influence they would without the improved concept.This can prevent faulty touch detection.

Claims

1. Method for determining a capacitance of a capacitive element (3), wherein, - during each measurement cycle of a plurality of successive measurement cycles, the capacitive element (3) is charged and an amount of charge is subsequently transferred from the capacitive element (3) to a further capacitive element (5); - following the plurality of measurement cycles, a measured value relating to a total amount of charge transferred as a whole to the further capacitive element (5) during the plurality of measurement cycles is determined; and - the capacitance of the capacitive element (3) is determined depending on the measured value; wherein, by means of a computing unit (6), - a first period is defined for a first subset of the plurality of measurement cycles; and - a second period is defined for a second subset of the plurality of measurement cycles, said second period differing from the first period; - each of the measurement cycles contains a charging section, wherein the capacitive element (3) is charged during the charging section; - each of the measurement cycles contains a transfer section, wherein the amount of charge is transferred from the capacitive element (3) to the further capacitive element (5) during the transfer section; - each of the measurement cycles contains an intermediate section between the charging section and the transfer section, wherein the capacitive element (3) is neither charged nor discharged during the intermediate section; and / or - each of the measurement cycles contains a further intermediate section upstream of the charging section or downstream of the transfer section, wherein the capacitive element (3) is neither charged nor discharged during the further intermediate section; characterized in that - a first intermediate duration is defined for the respective intermediate section of the measurement cycles of the first subset in order to define the first period and a second intermediate duration is defined for the respective intermediate section of the measurement cycles of the second subset in order to define the second period, wherein the second intermediate duration differs from the first intermediate duration; and / or - a first further intermediate duration is defined for the respective further intermediate section of the measurement cycles of the first subset in order to define the first period and a second further intermediate duration is defined for the respective further intermediate section of the measurement cycles of the second subset in order to define the second period, wherein the second further intermediate duration differs from the first further intermediate duration.

2. Method according to Claim 1, characterized in that - a first charging duration is defined for the respective charging section of the measurement cycles of the first subset in order to define the first period and a second charging duration is defined for the respective charging section of the measurement cycles of the second subset in order to define the second period, wherein the second charging duration differs from the first charging duration; and / or - a first transfer duration is defined for the respective transfer section of the measurement cycles of the first subset in order to define the first period and a second transfer duration is defined for the respective transfer section of the measurement cycles of the second subset in order to define the second period, wherein the second transfer duration differs from the first transfer duration.

3. Method according to either of the preceding claims, characterized in that, by means of the computing unit (6), a counter reading of a counter is set for each measurement cycle of the plurality of measurement cycles, and a period is defined for the respective measurement cycle depending on the respective set counter reading.

4. Method according to Claim 3, characterized in that, - starting from an initial counter reading for each of the measurement cycles, the counter reading is increased by a predefined increment for as long as the counter reading is lower than or equal to a predefined maximum counter reading; and - the counter reading is reset to the initial counter reading if the counter reading has reached the maximum counter reading.

5. Method according to Claim 4, characterized in that the relationship N ≤ M applies, wherein M designates a number of measurement cycles of the plurality of measurement cycles, (N-1)*I is an integer that designates a difference between the maximum counter reading and the initial counter reading, and I designates the increment.

6. Method according to one of Claims 3 to 5, characterized in that, by means of the computing unit (6), for each measurement cycle of the plurality of measurement cycles, the respective period is defined as the sum of a predefined minimum period that is constant for all the measurement cycles and an additional period that is dependent on the respective set counter reading.

7. Method according to Claim 6, characterized in that the minimum period is in a range between 0.5 µs and 30 µs, in particular in a range between 1 µs and 20 µs.

8. Method according to either of Claims 6 and 7, characterized in that the respective additional period is shorter than or equal to a tenth of the minimum period, in particular shorter than or equal to a fiftieth of the minimum period, for example shorter than or equal to a hundredth of the minimum period.

9. Method for detecting a user input that includes a user touching a capacitive element (3), characterized in that - a method for determining a capacitance of the capacitive element (3) according to one of the preceding claims is carried out; - the determined capacitance of the capacitive element (3) is compared with a predefined threshold value by means of the computing unit (6); and - the user input or the touching is detected by means of the computing unit (6) depending on a result of the comparison.

10. Measuring arrangement for determining a capacitance of a capacitive element (3) that is configured to carry out the method according to one of Claims 1 to 9, the measuring arrangement (2) having - a connection (4) to connect the capacitive element (3) to the measuring arrangement (2); - a circuit that is configured, during each measurement cycle of a plurality of successive measurement cycles, - to connect the capacitive element (3) to a voltage source (10) in order to charge the capacitive element (3); and - to subsequently transfer an amount of charge from the capacitive element (3) to a further capacitive element (5) of the measuring arrangement (2); and - a computing unit (6) that is configured, following the plurality of measurement cycles, to determine a measured value relating to a total amount of charge transferred as a whole to the further capacitive element (4) during the plurality of measurement cycles and to determine the capacitance of the capacitive element (3) depending on the measured value; characterized in that the computing unit (6) is configured - to define a first period for a first subset of the plurality of measurement cycles; and - to define a second period for a second subset of the plurality of measurement cycles.

11. User input device having a capacitive element (3) that is arranged so as to be touched by a user, characterized in that the user input device (1) contains a measuring arrangement (2) for determining a capacitance of the capacitive element (3) according to Claim 10.

12. Computer program product having instructions, which, when executed by a computing unit (6) of a measuring arrangement (2) according to Claim 10, cause the measuring arrangement (2) to execute a method according to one of Claims 1 to 9.