METHOD FOR OFFSET COMPENSATION OF SENSOR SIGNALS, OFFSET COMPENSATION DEVICE AND SENSOR DEVICE

Description

[0001] The invention relates to a method for offset compensation of sensor signals according to the preamble of claim 1, an offset compensation device according to claim 7, a sensor device according to claim 8.

[0002] This invention concerns the analysis and correction of faulty signals from sensors, particularly wheel speed sensors (RDS) in electronic braking systems. RDS signals are modulated in the form of current level transitions or defined current pulses onto the positive and negative terminals of the RDS power supply.

[0003] RDS signals are essentially subject to two categories of error causes. Internal sensor errors (i.e., errors in the sensor element itself) are usually caused by electrical or mechanical influences on the sensor element. Defective sensor elements can directly cause a faulty RDS signal. Errors in the RDS signal transmission channel (especially caused by environmental influences, aging, mechanical interference, etc., and channel insulation) can also directly cause a faulty RDS signal.

[0004] The conditioning (capture and processing) of the RDS signals is typically performed by an analog signal interface, which represents the electrical link between the RDS and a microcontroller (MCU). This signal interface is responsible for ensuring the integrity of the signal transmission between the RDS and the MCU. For this purpose, special measures are implemented within the analog signal interface to measure the status of the RDS transmission channel.

[0005] Known technologies for testing the RDS transmission channel can be divided into two fundamentally different measurement methods. Invasive measurement methods transfer the transmission channel between the RDS and the analog signal interface to a defined, interference-free state during the measurement process. This requires decoupling the signal transmission between the RDS and the signal interface. The electrical parameters of the transmission channel are then measured under known external electrical conditions. Non-invasive measurement methods are performed by analyzing the RDS signal itself. This allows conclusions to be drawn about the state of the signal channel or the state of the RDS itself.

[0006] Error patterns manifest themselves as a shift (offset) and / or compression (attenuation) of the original RDS signal waveform. For early detection of the faulty condition, the signal interface must provide a measuring device that examines the signal-carrying nodes for the causes of signal errors. Depending on the measurement technology, the sensors may not be operational during the measurement period, which is why the measurement is performed outside of safety-critical periods. In vehicles, this usually means that the measurement process takes place immediately after the ignition is switched on and while the vehicle is stationary.

[0007] Document US 5,296,855 A describes a navigation system for a vehicle with a steering angle speed sensor for detecting the steering angle speed of the vehicle. The output of the steering angle speed sensor, which indicates the steering angle speed, is varied relative to a constant threshold, and the steering angle speed sensor has an offset value that indicates the output of the steering angle speed sensor while the vehicle is moving in a straight line. The sensor emits a pulsed signal. Furthermore, an offset calculator is provided for integrating the output data of the steering angle speed sensor during a stop period of the vehicle to determine an offset value of the steering angle speed sensor for drift correction. An offset correction is not provided.

[0008] In invasive measurement methods, the interruption of signal transmission between the RDS and the analog signal interface is particularly detrimental. Due to safety requirements for the RDS signal chain, this precludes error detection while driving.

[0009] It is therefore the object of the invention to provide a method and a device by means of which signal errors in the signal transmission chain between the sensor and the analog signal interface can be detected and compensated without interruption.

[0010] The object is achieved by the independent claims. According to the invention, a method for offset compensation of sensor signals by means of an offset compensation device is provided, wherein the offset compensation device comprises a shift register with N elements, an averager, and a low-pass filter, and wherein the following steps are carried out in the method: Storing sensor values ​​in the shift register, forming the difference between the first element of the shift register and the last element of the shift register, determining a plurality of mean values, wherein the following steps are carried out: a) starting the averaging by the mean value generator when the difference is greater than a threshold value S, b) stopping the averaging by the mean value generator when the difference is smaller than the threshold value -S, determining the lowest signal level by means of the low-pass filter based on the mean values, determining the offset by forming the difference between the lowest signal level and a reference current value, correcting the sensor values ​​using the determined offset.

[0011] This provides a non-invasive measurement method which has the following advantages: 1. The purely digital implementation makes the method very robust and predictable. 2. Implementation becomes more cost-effective with technological advances (effects of scale). 3. Quality is improved due to high test coverage in production, as digital tests are automated. 4. Tolerances of analog circuit components are compensated.

[0012] Even if the signal and the sampling times are uncorrelated, a well-chosen sampling rate guarantees that the calculated amplitude after using the shift register corresponds to the signal amplitude. The calculated amplitude is determined by calculating the difference between the first and last elements of the shift register.

[0013] The low-pass filter is used to mitigate the influence of a single averaging operation. If the averaging is successfully performed multiple times, the value of the lowest signal level is available at the low-pass filter output.

[0014] In a preferred embodiment of the invention, the reference current value of the signal under investigation is set to 7 mA. This is particularly relevant for commercially used RDS variants, which have a current flow of 7 mA as the base level (for pulsed or three-level sensors) or as the low level (for standard two-level sensors). The deviation between the measured value and the reference current value results in the determined offset.

[0015] In a preferred development of the invention, the number N of the shift register is selected such that at least all sample values ​​of an edge can be stored and N is greater than two.

[0016] In In an alternative development of the invention, the window of the averager is selected such that it is longer than the lowest-level phases in the protocol, so that averaging occurs after the protocol. During a protocol, each magnetic pole triggers a complete sequence of sensor pulses. These pulses have a defined length and contain further information about the sensor status. A protocol sensor thus transmits a defined pulse sequence. The pauses between the protocols are determined by the wheel speeds.

[0017] In In a preferred embodiment of the invention, the window of the averaging unit is selected such that it is shorter than the lowest-level phases in the protocol, so that averaging is completed multiple times during a protocol. This achieves a more robust and accurate result. As few transmitted protocols as possible are lost, since the correction value can be adjusted significantly more quickly.

[0018] In a preferred embodiment of the invention, the following steps are carried out: Supplying at least one sensor with a voltage through an evaluation circuit, modulating a sensor current through the sensor, measuring and evaluating the sensor current through the evaluation circuit.

[0019] In a preferred embodiment of the invention, the evaluation circuit has a high-side path and a low-side path and the following steps are carried out Parallel use of the high-side path and the low-side path, determining the current offset values ​​for the high-side path and the low-side path.

[0020] By using the high-side and low-side paths in parallel and determining the current offset values, the type of error can be determined and / or the location of the error can be achieved. The acquisition and conversion of the RDS signal using an analog-to-digital converter (ADC) is characteristic of non-invasive measurement methods. After suitable conditioning of the RDS signal (e.g., low-pass filtering), it is converted into a digital signal with sufficiently precise resolution, preferably using an ADC (connected to the positive supply (high-side) or the negative supply (low-side), depending on the circuit architecture) or optionally using two ADCs (connected to the high-side and low-side). Investigations of the signal properties are carried out by processing the digitized RDS signal values.

[0021] In a preferred development of the invention, a constructive superposition of the determined offset values ​​for the high-side path and the low-side path is carried out, thereby forming an improved vector. For this superposition, two bits with the same index from LVL_HS[x] and LVL_LS[x] are preferably considered. The output bit LVL[x] is set to 1 if LVL_HS[x] or LVL_LS[x] show a rising signal edge. The output bit LVL[x] is set to 0 if LVL_HS[x] or LVL_LS[x] show a falling signal edge. This ensures that, in the event of an error, one evaluation path that detects the signal edge first is sufficient for signal detection. Particularly preferably, the offset compensation is first carried out separately for the high-side and low-side paths, and then the two partial results are superimposed.

[0022] In a preferred development of the invention, a hysteresis is additionally carried out at the individual detection thresholds, which advantageously further improves the robustness.

[0023] In a preferred development of the invention, the threshold values ​​are set as ideally as possible between the specific signal levels of the sensors.

[0024] The object is further achieved by an offset compensation device for compensating offsets in sensor signals, wherein the offset compensation device comprises a shift register and an averager. Preferably, the offset compensation device implements at least parts of the method described above.

[0025] Furthermore, the object is achieved by a sensor device comprising the offset compensation device, the evaluation device, and at least one sensor. In a preferred embodiment, the sensor is a wheel speed sensor for an electric braking system. The sensor device is designed to carry out the method described above.

[0026] Further preferred embodiments emerge from the subclaims and the following description of embodiments with reference to figures.

[0027] They show in schematic representation Fig. 1 the connection of a wheel speed sensor in the vehicle and the evaluation circuit (state of the art); Fig. 2 the current flow of a wheel sensor with protocol with three different current levels (state of the art); Fig. 3 a schematic diagram of the evaluation circuit (state of the art) Fig. 4 the current flow of a wheel sensor as in Fig. 2 but with offset error (state of the art) Fig. 5the evaluation circuit as Fig. 3 but with an additional block for offset compensation according to the invention Fig. 6 an example investigation of the influence of the sampling rate of the ADC and the shift register length N for a falling edge according to the invention Fig. 7 the evaluation circuit with parallel use of HS and LS path according to the invention.

[0028] The invention describes an algorithm for determining and calibrating offsets in RDS signals using a non-invasive measurement method.

[0029] Many sensors in the vehicle are connected to an evaluation electronics via the wiring harness. Fig. 1 shows the principle of connecting a wheel speed sensor 1 in the vehicle to an evaluation circuit 3, which has an evaluation logic 4. The sensor 1 is supplied with voltage via the evaluation circuit (the evaluation IC) 3 via the connection to a power supply KL30. The sensor current flows through both a high-side driver (HS) 5 and a low-side driver (LS) 7 in the evaluation ASIC 3. During operation, the sensor 1 modulates the sensor current. This current can be converted in the integrated circuit (evaluation circuit 3) to the high-side analog-digital converter (HS ADC) 9 and / or the low-side analog-digital converter (LS ADC) 11 (see Fig. 3 and 5 ) are measured and evaluated.

[0030] Different data protocols are used for wheel sensors in motor vehicles. This data is transmitted from the wheels to the electronic control unit via a power interface in the vehicle. Fig. 2 shows the current curve of wheel sensor 1 with protocol with three different current levels. Furthermore, Fig. 2 four threshold values ​​S0, S1, S2, S3 are shown.

[0031] Fig. 3 shows a schematic diagram of the evaluation circuit 3. The wheel sensor signal is first digitally converted using an analog-to-digital converter (ADC) 9, 11, and the result is compared with various threshold values ​​S0 to S3. The threshold values ​​S0 to S3 are selected so that they lie as ideally as possible between the specified signal levels of the sensors 1. Each comparator 13 sets the digital output to 1 as soon as the value of the ADC 9, 11 is greater than the examined threshold S0-S3. Combining all output signals from these comparators into a digital signal vector yields the vector LVL[3:0], which is used for further signal detection. See also Fig. 1 and 2 , where the vector is also shown.

[0032] However, a number of sensor errors can occur during vehicle operation: Offset errors in positive and negative directions Amplitude errors (sensor amplitude too high or too low)

[0033] In Fig. 4 The signal curve for sensor 1 is shown, where all current levels are offset by a fixed amount. This error is called the offset error.

[0034] The signal detection for forming the vector LVL[3:0] fails because the sensor signal no longer falls below threshold S1. Additionally, threshold S3 is exceeded, which is not intended for correct protocol detection of sensor 1. In this case, the sensor signal in the vehicle fails and is no longer available for the control functions in safety-critical control units. However, one can see in Fig. 4 that the sensor information is still completely contained in the received signal, only the detection area is shifted.

[0035] With the following detection method, offset compensation, signal detection is possible even in the presence of sensor errors, thus achieving increased availability of the wheel sensor signals in the vehicle. Initially, only one of the two detection paths, e.g., high-side path 5, is considered.

[0036] As in Fig. 5 As shown, an additional block 15 for offset compensation is introduced into the evaluation circuit 3. A shift register 17 of length N stores the AD converter values. The length N of the shift register 17 is selected such that at least all sample values ​​of an edge can be stored and N > 2.

[0037] A difference ΔS is calculated from the first and last elements of shift register 17. If the difference at the output of shift register 17 is positive and greater than a slope value (threshold) S, this indicates a rising edge; if it is negative and smaller than a slope value (threshold) -S, a falling edge is detected.

[0038] The definition of the length N of the shift register 17 is based on Fig. 6a-c explained. In the Figuren 6a bis 6c For a falling edge, the influence of the sampling rate of the ADC and the shift register length N is investigated.

[0039] In Fig. 6a N = 2 and the sampling rate is high relative to the slope. The sampling times are indicated by arrows in the signal waveform. In this case, the value ΔS decreases the higher the sampling rate of the ADCs 9, 11 increases. The higher the sampling rate, the longer the shift register 17 should be.

[0040] In Fig. 6b A lower sampling rate was used. However, the temporal correlation between the sensor signal and the sampling times is unknown or random. In this case, it can happen that the signal is sampled exactly in the middle of the edge. This results in ΔS being at most half the signal amplitude.

[0041] In Fig. 6c The case with N = 3 is now shown. The sampling rate is chosen so that the entire edge can occur between two samples. Although the signal and the sampling times are still uncorrelated, this guarantees that the amplitude ΔS corresponds to the signal amplitude. This is the best possible result and the reason for the introduction of shift register 17.

[0042] In addition to the shift register 17, an averager 19 is implemented for the AD converter values ​​(see Fig. 5 ). This averager 19 does not run continuously, but is controlled by the shift register output. If the output difference is greater than a threshold S, averaging is started. If the output difference is less than a threshold -S, averaging is stopped. If averaging is successfully completed after exactly M input elements, the result is passed to the following low-pass filter 21, at whose output the lowest level in the signal curve is established.

[0043] Together with the shift register, this leads to the following behavior in the system: If the input signal from the ADC is constant, then the shift register 17 fills with almost equal values, when the difference is formed at the output neither of the two thresholds S or -S is exceeded or undershot and the status of the averager 19 does not change.

[0044] If the input signal has a falling edge, the shift register 17 is first filled with larger and then with smaller values. The difference at the output is maximum as soon as the entire edge is stored in the shift register 17. During a falling edge, the threshold S may be exceeded multiple times. In this case, the averager 19 is restarted each time and all previous partial results are discarded. Only at the end of the falling edge is the threshold S no longer exceeded and the averaging continues. This ensures that the averaging only begins at the end of a falling edge. This method has the advantage that you do not need to know when an edge begins or ends. The end of the edge is found automatically because the averager 19 is no longer interrupted.

[0045] If no further edge (falling or rising) is detected during averaging, the averaging of M values ​​is terminated, and the result is passed to the following low-pass filter 21. However, if a rising edge occurs during the operation, shift register 17 is first filled with lower values ​​and then with higher values. In this case, the threshold -S is exceeded, and averaging is terminated immediately. All intermediate results are discarded, and nothing is passed to the low-pass filter 21.

[0046] This method ensures that averaging starts exactly at the end of a falling edge and continues as long as the input signal remains unchanged. The following low-pass filter 21 is implemented to mitigate the influence of a single averaging. If averaging is successfully performed multiple times, the value of the lowest signal level is available at the output of low-pass filter 21.

[0047] By calculating the difference between the expected level and the lowest threshold of sensor 1, we obtain the deviation of the sensor signal from this expected value. This deviation is used to correct the A / D converter values ​​for sensor evaluation. This allows the sensor evaluation to function even with an existing offset error.

[0048] The detected offset error is also used as a monitoring parameter for the software. If the value becomes too large, the system software can react and inform the driver or workshop about the error.

[0049] When designing the averager, the following constraints must be considered: The longer the value M of the averager 19 is chosen, the more robust and accurate the determined result will be. On the other hand, however, the existing signal shape must also be considered. In the case of the sensor protocol in Fig. 2 You can decide whether to set the averager 19 to a value that is short enough so that the averaging is completed multiple times during a protocol. In this case, as few transmitted protocols as possible are lost, since the correction value can be adjusted much more quickly.

[0050] If the averaging period is chosen to be longer than the lowest-level phases in the protocol, the averaging can only be performed after the protocol. In this case, it takes longer to calculate the correct correction value. Both implementations are possible.

[0051] If you use, as in Fig. 7 As shown, if the high-side and low-side paths 5, 7 are parallel in the evaluation IC 3, the currently determined offset value for the high-side and low-side paths 5, 7, respectively, is continuously obtained during operation. These values ​​can be used to determine the type and location of the fault. Additionally, it is possible to constructively superimpose the determined data LVL_HS and LVL_LS to form an improved vector LVL. The superimposition can take place in a superimposition unit 23.

[0052] For this overlay, two bits with the same index from LVL_HS[x] and LVL_LS[x] are considered. The output bit LVL[x] is set to 1 when LVL_HS[x] or LVL_LS[x] show a rising signal edge. The output bit LVL[x] is set to 0 when LVL_HS[x] or LVL_LS[x] show a falling signal edge.

[0053] This ensures that in the event of a fault, a single evaluation path that detects the signal edge first is sufficient for signal detection. Additional hysteresis at the individual detection thresholds further improves robustness. List of reference symbols:

[0054] 1 Wheel speed sensor 3 Evaluation circuit 4 Evaluation logic 5 High-side driver 7 Low-side driver 9 High-side analog-to-digital converter (HS ADC) 11 Low-side analog-to-digital converter (LS ADC) 13 Comparator 15 Offset compensation block (offset compensation device) 17 Shift register 19 Averager 21 Low-pass filter 23 Overlay unit S0, S1, S2, S3 Threshold values ​​KL30 Connection to power supply

Claims

1. Method for offset compensation for sensor signals by means of an offset compensation device (15), wherein the offset compensation device comprises a shift register (17) containing elements in the number N, an averaging unit (19) and a low pass filter (21), and wherein the method involves the following steps being carried out: - storing sensor values in the shift register (17), - calculating the difference between the first element of the shift register (17) and the last element of the shift register (17), - determining multiple mean values, the following steps being carried out: a) starting the averaging by the averaging unit (19) if the difference is greater than a threshold value S, b) stopping the averaging by the averaging unit (19) if the difference is less than the threshold value -S, - determining the lowest signal level by means of the low pass filter (21) based on the mean values, - determining the offset by calculating the difference between the lowest signal level and a reference current value, - correcting the sensor values by means of the determined offset.

2. Method according to Claim 1, wherein the number N for the shift register (17) is chosen such that at least all samples of an edge can be stored and N is greater than two.

3. Method according to either of the preceding claims, wherein the window of the averaging unit (19) is chosen such that it is longer than the phases having the lowest level in the protocol, with the result that the averaging takes place only after the protocol.

4. Method according to one of the preceding claims, wherein the window of the averaging unit (19) is chosen such that it is shorter than the phases having the lowest level in the protocol, with the result that the averaging is completed multiple times during a protocol.

5. Method according to one of the preceding claims, wherein the following steps are carried out: - supplying at least one sensor (1) with a voltage by way of an evaluation circuit (3), - modulating a sensor current by way of the sensor (1), - measuring and evaluating the sensor current by way of the evaluation circuit (3).

6. Method according to Claim 5, wherein the evaluation circuit (3) has a high-side path (5) and a low-side path (7) and the following steps are carried out - using the high-side path (5) and the low-side path (7) in parallel - determining the present offset values for the high-side path (5) and the low-side path (7).

7. Method according to any one of the preceding claims, wherein the following step is carried out - constructively overlaying the determined offset values for the high-side path (5) and the low-side path (7) and forming a vector.

8. Offset compensation device (15) for compensating for offsets in sensor signals, characterized in that the offset compensation device (15) has a shift register (17) and an averaging unit (19) and is designed to perform the method according to one of Claims 1 to 4.

9. Sensor device comprising the offset compensation device (15) according to Claim 8, an evaluation device (13) and at least one sensor (1), wherein the sensor device is designed to perform the method according to one of Claims 1 to 7.

10. Sensor device according to Claim 9, wherein the sensor (1) is a wheel speed sensor for an electric braking system.