EP2877376B1 - Method for detecting and processing measurement values of a capacitive proximity sensor for initiating an operating function of a tailgate of a motor vehicle comprising an energy-saving evaluation mode - Google Patents

Description

The invention relates to a method for acquiring and processing measured values of at least one capacitive proximity sensor arranged in the ground-level rear area of a motor vehicle for triggering an actuating function of a tailgate by a predetermined foot movement of an operator, a measured value of the capacitive proximity sensor periodically being recorded after each sampling interval and a measured value being recorded corresponding digital measured value is stored in a FIFO memory and the stored digital measured values are fed to an algorithm for detecting a signal course corresponding to the predetermined foot movement of an operator,

It is known to arrange electrodes of capacitive sensors in the ground-level rear area of a motor vehicle, for example behind a plastic panel, in such a way that their capacitance changes relative to a reference potential (for example ground) when an operator standing behind the motor vehicle steps a foot in the direction of the rear area and momentarily swings under the rear area. An evaluation circuit is coupled to the electrode of the capacitive sensor, which outputs an analog output signal corresponding to the capacitance to an A / D converter of a microcontroller. The microcontroller is programmed to cause the A / D converter to periodically output digital values corresponding to the analog sensor output signal, which shall be referred to hereinafter as digital measurements. The digital measured values corresponding to the capacitance of the capacitive sensor are generated periodically, for example at intervals of 20 ms. The changes in the digital readings are evaluated, the microcontroller being programmed to recognize, based on the changes in the digital readings, whether a particular event, for example, the approach of a foot of the operator, has taken place. If the microcontroller determines that a specific event has taken place, it triggers an actuation function of a tailgate of the motor vehicle. Thus, the tailgate is opened when the operator standing behind the vehicle pivots his foot in a certain way under the rear of the vehicle. To improve the detection of the predetermined movement, two sensor electrodes are preferably arranged horizontally and vertically offset in a certain way. For example, it is elongated, formed by an outer conductor of a coaxial cable sensor electrodes, which are arranged offset transversely to the vehicle longitudinal direction and each other vertically and horizontally in the rear area. Such an arrangement is schematically shown in FIG FIG. 1 shown.

Usually, the digital measured values recorded at short time intervals are stored in a memory area of a RAM of the microcontroller. In this memory area, a predetermined number of digital readings are buffered before the microcontroller of the microcontroller accesses these digital readings for further processing. The further processing initially comprises a digital filtering of a predetermined number of stored digital measured values in order to obtain digital filter values for further processing. The filter values are calculated at the same time intervals that the digital readings are taken. Whenever a new digital reading is stored, the microprocessor calculates a new filter value from that and previously stored digital readings.

For reasons of energy saving, the further evaluation of the filter values for recognizing a signal which characterizes the predetermined movement of the body part of the operator should not be carried out with each new filter value, but only if a criterion is met, with little computational effort and thus with lower energy consumption can be determined. To reduce energy consumption, for example, a digital measured value is determined at longer intervals (lower sampling rate) in an energy-saving mode. From this digital reading and a few previously obtained with the lower sampling rate digital readings a filter value is determined in each case. To simplify the calculations, the filter value is merely compared with a threshold value, the exceeding of which thus represents the criterion which can be determined with little computational effort. As long as the threshold value of the filter value is not exceeded, this extraction of the digital measurements, the calculation of the filter values and the comparison with the threshold value are repeatedly performed, resulting in a relatively low energy consumption. However, once a filter value exceeds the threshold, it is considered a criterion that a predetermined movement of the operator body part might have occurred. The device then switches to a higher sampling rate, that is, the digital readings are taken at shorter intervals and subsequently filtered again. However, the filtering can only begin when a predetermined minimum number of digital measured values having the higher sampling rate has been recovered and stored in the buffer memory. The digital measurements previously obtained with the lower sampling rate can not be included in the further processing. This leads to a certain dead time after the threshold value has been detected, within which no first filter value can be calculated for further evaluation and analysis as to whether the predetermined foot movement of an operator has taken place.

From the DE 10 2010 011767 A1 A method for acquiring and processing measured values of at least one capacitive proximity sensor arranged in the ground-level rear region of a motor vehicle for triggering an actuating function of a tailgate by a predetermined foot movement of an operator is known, in which a measured value of the capacitive proximity sensor is periodically detected after each sampling interval and a measured value corresponding discrete sensor reading is stored in a FIFO memory. In a coarse evaluation, an increase in the sensor measured values is detected in each case after the storage of a predefined first number of sensor measured values, the increase is compared with a threshold value and then when the Threshold is exceeded, a fine evaluation activated. In the fine evaluation, the course of the sensor measured values is checked in each case after the storage of a predetermined number of sensor measured values, the sensor measured values being fed to a pattern recognition for detecting a signal course corresponding to the predetermined foot movement.

The object of the invention is to provide a method for acquiring and processing measured values, which allows a faster detection of the predetermined foot movement of an operator and at the same time is less prone to interference.

This object is achieved by a method having the features of claim 1.

In this method for acquiring and processing measured values of at least one capacitive proximity sensor arranged in the ground-level rear region of a motor vehicle for triggering an actuating function of a tailgate by a predetermined foot movement of an operator, a measured value of the capacitive proximity sensor is periodically acquired after each sampling interval and a digital measured value corresponding to the measured value is acquired stored in a FIFO memory. The FIFO memory may, for example, be a separate component or part of a memory of a microcontroller. In a first, energy-saving evaluation mode, a filter value is calculated in each case after the storage of a predefined first number of digital measured values, that is to say after each time interval corresponding to the product of sampling period and first number, from the currently stored digital measured value and a predetermined second number of previously stored digital measured values each of these stored digital readings is multiplied by an associated first factor and the results are added together. Then the filter value is compared to a threshold. If the filter value exceeds the threshold value, a second evaluation mode is activated. In the second, normal evaluation mode, in each case after the storage of a predetermined third number of digital measured values which is less than the first number, that is to say at shorter time intervals, from the currently stored digital measured value and a predetermined fourth number previously stored digital readings calculates a filter value by multiplying each of these stored digital readings by an associated second factor and adding the results, and applying the filter value to an algorithm for detecting a waveform corresponding to the operator's foot movement.

This procedure has the advantage that the sampling frequency (sampling rate) or sampling period duration remain constant during the first and the second evaluation mode, so that it is not necessary to refill the FIFO memory with samples of a changed sampling rate after the second evaluation mode has been started when the threshold value is exceeded , Although the higher sampling rate initially entails that somewhat more energy is consumed for the A / D conversion and the intermediate storage in the FIFO memory; However, this is compensated by the improved possibility of filtering. In addition, the filter values in the first, energy-saving evaluation mode are obtained at greater intervals than in the second, normal evaluation mode. The filter calculations therefore need slightly less energy in the energy-saving mode. The possibility of being able to use the already buffered (buffered) digital measured values immediately in the second mode allows the immediate beginning of the algorithm for detecting a signal course corresponding to the predetermined foot movement of an operator and thus a faster detection of this event.

The predefined first number of digital measured values preferably corresponds to at least twice the predetermined third number of digital measured values. The larger the first number, the greater the energy saving because the filter calculation and comparison with the threshold occur less frequently. However, the distance must not be so great that there is a risk of "overlooking" a triggered by the foot movement of an operator predetermined signal change. In one embodiment, the predetermined first number of digital measurements is between 4 and 16; preferably it is equal to 8.

In one embodiment of the method for acquiring and processing measured values, the second number is equal to the fourth number and corresponds to the memory depth of the FIFO memory. For example, this is equal to 32. In this embodiment, the full memory depth is always utilized in both the first, energy-saving mode of operation and in the second, normal mode of operation in order to obtain the best possible filtering.

In a preferred embodiment, the sampling interval is in the range between 1 ms and 10 ms, preferably in the range between 2 ms and 6 ms, for example about 4 ms. This sampling interval allows-with a corresponding number of the measured values included-a good filtering of the signal curve (smoothing and noise suppression) for a subsequent recognition of a signal sequence corresponding to the predetermined foot movement of an operator.

In a development of the invention, the second factors determining the type of filtering in the second evaluation mode are different from the first factors determining the type of filtering in the first evaluation mode, so that the measured values are differently filtered in the first and second evaluation modes. Thus, the first filtering may be adapted to the lower rate of generating a filter value and to the task of detecting a threshold overshoot. The second filtering may be adapted to the higher rate of generation of a filter value and to the task of detecting a waveform corresponding to the predetermined foot movement of an operator.

The fourth number (number of previously stored measured values included in the filtering) and the second factors (filter coefficients) are preferably selected so that signals are filtered out in a frequency range that corresponds to the frequency range of the capacitance change occurring during the foot movement of the operator. This is preferably a frequency range up to 5 Hz.

A preferred embodiment of the method is characterized in that measured values of two in the vehicle longitudinal direction offset arranged capacitive proximity sensors detected and the measured values corresponding digital readings are stored in two sensors associated FIFO memory areas.

Preferably, in the first, energy-saving evaluation mode, only the filter values for one of the two sensors are calculated and compared with the threshold value and in the second evaluation mode the filter values for both sensors are calculated and fed to the algorithm for detecting the signal course corresponding to the predetermined foot movement of an operator.

Advantageous and / or preferred developments of the invention are characterized in the subclaims.

• Figur 1 eine schematische Darstellung der das erfindungsgemäße Verfahren ausführenden Einrichtungen; und
• Figur 2 eine schematische Darstellung der zeitlichen Abläufe der Digitalmesswertgewinnung und Filterberechnungen.

The invention will be described in more detail below with reference to a preferred embodiment illustrated in the drawings. In the drawings show:

• FIG. 1 a schematic representation of the method according to the invention exporting devices; and
• FIG. 2 a schematic representation of the timing of digital data acquisition and filter calculations.

The method according to the invention detects and processes measured values of capacitive proximity sensors arranged in the ground-level rear region of a motor vehicle. In FIG. 1 the rear region 1 of a motor vehicle is shown, which has the electrodes 2A and 2B of two capacitive proximity sensors near the ground (behind a diaphragm). Preferably, it is elongated sensor cable, which is transverse to the vehicle longitudinal direction, that is transverse to the plane of the plane FIG. 1 extend. The others in FIG. 1 shown devices are drawn only for the purpose of illustration outside the motor vehicle, but are actually arranged in the motor vehicle. In each case, a supply cable 3 extends from the sensor electrodes 2A, 2B to an evaluation circuit 4A and 4B of the capacitive sensor which generates at its output an analog output signal corresponding to the capacitance of the sensor, in particular a voltage corresponding to the capacitance. The voltages corresponding to the capacitances of the two sensor electrodes are each an input port of a microcontroller 5, wherein an analog-to-digital converter 7A and 7B is connected to each of the input ports. The microcontroller 5 includes a microprocessor 6, a ROM 10 for storing operating programs, a RAM 8 as a working memory and other interface circuits, one of which is exemplified as a port 11. The analog-to-digital converters 7A, 7B and the other circuits mentioned are connected to the microprocessor 6 via a bus 9. Programmatically, the microprocessor 6 causes the reading of the digital measured values from the analog-to-digital converters 7A, 7B and their storage in the RAM 8. For storing the digital measured values, a specific memory area 12 is provided in the RAM 8, in which the digital measured values after the FIFO Principle are filed. The time control of the measured value acquisition and the further processing is determined by the processed in the microprocessor 6 programs.

FIG. 2 illustrates the timing of the acquisition of the digital measurements and the calculation of filter values according to a preferred embodiment of the method according to the invention. First, for example, the microcontroller causes a digital reading to be taken every 4 ms for each sensor by causing the A / D converter to output a corresponding digital value, which is then stored in the FIFO area 12 of the RAM 8. The FIFO area 12 may receive 32 digital readings per sensor in the preferred embodiment shown here. The 32 values stored in the FIFO area are in FIG. 2 represented as 32 small squares and correspond to one line of this representation. A column corresponds to a measured value. Physically different embodiments of such a FIFO memory are conceivable. A measured value preferably remains stored in its memory area until it is overwritten by a new measured value after 32 measured value acquisitions. A pointer circulating in the FIFO memory area determines in which memory location a new measured value can be written in each case.

FIG. 2 illustrates the further procedure in the first, energy-saving evaluation mode. After each 8 new digital readings are written in the FIFO memory area 12 have been made, the microcontroller causes the calculation of a filter value. The calculation of the filter value (filter calculation) is thus performed every 8 * 4 ms = 32 ms. However, not only the new last 8 measured values, but the last 32 ms are included in the filter calculation which takes place every 32 ms. Each stored measurement thus participates in a total of four filter calculations. On the one hand, the fact that the filter calculation is performed only every 8 ms reduces the power consumption. On the other hand, the inclusion of the complete 32 digital measurements stored in the FIFO area allows more accurate filtering, especially noise suppression.

In the energy-saving mode, the filter value obtained in this way is then compared with a threshold value. If an exceeding of the threshold value is detected, a second evaluation mode is activated.

In the second evaluation mode, the distances of the filter value calculation are reduced. For example, a filter value is calculated every 2 or 4 measured values. The filter values now obtained at shorter intervals are fed to an algorithm for detecting a signal course corresponding to the predetermined foot movement of an operator.

The filter calculation takes place, for example, with an FIR algorithm. In this case, all 32 digital measured values are multiplied by a factor and from these products a sum is formed, which corresponds to the filter value. In the preferred embodiment, the type of filtering differs in the first, energy-saving mode from the type of filtering in the second, normal mode for detecting foot movement. This is also expressed in different filter coefficients. The filter coefficients are selected, for example, in the second, normal evaluation mode so that, in particular, signal characteristics in the frequency range up to about 10 Hz, preferably up to 5 Hz, are filtered out. It has been found that, in particular, signals in these low-frequency areas reproduce the movement of the operator's body part (foot) to be detected. In addition, the filter coefficients in the first and second modes adapted to the respective rate of extraction of the filter values. For example, in the energy-saving, first evaluation mode, the filter values are obtained every 32 ms (every 8 samples), while in the normal evaluation mode, for example, they are obtained every 8 ms (after every two samples).

Within the scope of the invention defined in the claims numerous alternative embodiments are conceivable. For example, the sampling period may differ from 4 ms and may be in the range of 1 to 10 ms, for example. The spacing of the filter calculations in the first and in the second evaluation mode can vary, of course with the proviso that it is lower in the second evaluation mode. Similarly, the depth of the FIFO memory may differ from 32.

Claims (11)

1. A method for detecting and processing measurement values of at least one capacitive proximity sensor arranged in the rear area of a motor vehicle near the floor for triggering an actuating function of a tailgate by a preset foot movement of an operator,
   wherein periodically after a scanning interval in each case a measurement value of the capacitive proximity sensor is captured and a digital measurement value corresponding to the measurement value is stored in a FIFO memory,
   wherein in a first energy-saving evaluation mode after every storing of a preset first number of digital measurement values

   a filter value is calculated from the currently stored digital measurement value and a preset second number of previously stored digital measurement values in that each of these stored digital measurement values is multiplied in each case with an associated first factor and the results are added,

   the filter value is compared with a threshold value and in particular when the filter value exceeds the threshold value, a second evaluation mode is activated,

   wherein in the second, normal evaluation mode in each case following the storing of a preset third number of digital measurement values, which are lower than the first number,
   a filter value is calculated from the currently stored digital measurement value and a preset fourth number of previously stored digital measurement values in that each of these stored digital measurement values is multiplied in each case with an associated second factor and the results are added, and
   the filter value is supplied to an algorithm for detecting a signal path corresponding to the preset foot movement of an operator.
2. The method for detecting and processing measurement values according to Claim 1, characterized in that the preset first number of digital measurement values corresponds at least to double the preset third number of digital measurement values.
3. The method for detecting and processing measurement values according to Claim 2, characterized in that the preset first number of digital measurement values is between 4 and 16.
4. The method for detecting and processing measurement values according to Claim 3, characterized in that the preset first number of digital measurement values is equal to 8.
5. The method for detecting and processing measurement values according to any one of the Claims 1 to 4, characterized in that the second number is equal to the fourth number and corresponds to the memory depth of the FIFO memory.
6. The method for detecting and processing measurement values according to any one of the Claims 1 to 5, characterized in that the scanning interval is in the range between 1 ms and 10 ms.
7. The method for detecting and processing measurement values according to any one of the Claim 6, characterized in that the scanning interval is in the range between 2 ms and 6 ms.
8. The method for detecting and processing measurement values according to any one of the Claims 1 - 7, characterized in that the second factors are distinct from the first factors so that the measurement values in the first and the second evaluation mode are filtered differently.
9. The method for detecting and processing measurement values according to any one of the Claims 1 - 8, characterized in that the fourth number and the second factors are selected so that signals are increasingly filtered out in a frequency range which corresponds to the frequency range of the capacity change that occurs during the foot movement of the operator.
10. The method for detecting and processing measurement values according to any one of the Claims 1 - 9, characterized in that measurement values of two capacitive proximity sensors which are arranged offset in vehicle longitudinal direction are detected and digital measurement values corresponding to the measurement values are stored in two FIFO memory regions allocated to the sensors.
11. The method for detecting and processing measurement values according to Claim 10, characterized in that in the first energy-saving evaluation mode only the filter value for one of the two sensors is calculated and compared with the threshold value and in that in the second evaluation mode the filter values for both sensors are calculated and supplied to the algorithm for detecting the signal path corresponding to the preset foot movement of an operator.

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