Method for offset compensation for a sensor signal, offset compensation device and sensor device - Patents.com

Abstract

The present invention relates to a method for offset compensation for a sensor signal by an offset compensation device (15), the offset compensation device comprising a shift register (17) containing N elements, an averaging unit (19) and a low pass filter (21), the method comprising the steps of: - storing the sensor values ​​in said 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 a plurality of average values, the average value comprising: a) initiating averaging by said averaging unit (19) if the difference is greater than a threshold S; b) stopping the averaging by the averaging unit (19) if the difference is less than a threshold value -S; and - determining a minimum signal level using said low pass filter (21) based on the average value; - determining an offset by calculating the difference between a minimum signal level and a reference current value; - correcting the sensor value using the determined offset; This includes executing: The invention further relates to an offset compensation device and a sensor device.

Description

[Technical Field]

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

[0002] The present invention relates to the analysis and correction of inaccurate signals from sensors, particularly wheel speed sensors (RDS) in electronic braking systems. The signals from the RDS are modulated on the positive and negative terminals of the RDS's power supply in the form of current level transitions or defined current pulses. [Background technology]

[0003] RDS signals are basically divided into two categories of error sources: Internal sensor errors (errors in the sensor element itself) are usually caused by electrical or mechanical influences on the sensor element. Any defects in the sensor element can quickly result in an inaccurate RDS signal. Errors in the transmission channel of the RDS signal (caused, among other things, by environmental influences, aging, mechanical disturbances, etc., as well as the channel's isolation) can also quickly result in an inaccurate RDS signal.

[0004] The RDS signal is usually conditioned (acquired and processed) by an analog signal interface, which is an 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 means are introduced into the analog signal interface to measure the state of the RDS transmission channel.

[0005] In this case, known techniques for checking the RDS transmission channel can be divided into two fundamentally different measurement methods. Invasive measurement methods convert the transmission channel between the RDS and analog signal interface into a defined state that is as interference-free as possible during the measurement process. This requires isolating the signal transmission between the RDS and signal interface. Therefore, the electrical parameters of the transmission channel are 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 RDS itself.

[0006] Error patterns manifest themselves as a shift (offset) and / or compression (attenuation) of the original RDS waveform. To detect inaccuracies early, the signal interface must be equipped with measurement equipment that probes the signal-carrying nodes for sources of signal error. Some measurement techniques do not allow sensors to be operational during the measurement period, so measurements are taken outside of safety-critical periods. In the case of vehicles, this usually means that the measurement process takes place immediately after the ignition is operated, when the vehicle is stationary.

[0007] For invasive measurement methods, interruptions to the signal transmission between the RDS and analog signal interfaces are particularly detrimental. The RDS signal chain must be secure, preventing error detection during operation. Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to provide a method and apparatus that allows for uninterrupted detection and compensation of signal errors in the signal transmission chain between a sensor and an analog signal interface. [Means for solving the problem]

[0009] According to the invention, a method for offset compensation for a sensor signal by an offset compensation device is provided, the offset compensation device comprising a shift register including N elements, an averaging unit and a low-pass filter, the method comprising: - storing the sensor values ​​in a shift register; - calculating the difference between the first element of the shift register and the last element of the shift register; - determining a plurality of average values, the average value comprising: a) if the difference is greater than a threshold S, initiating averaging by an averaging unit; b) stopping the averaging by the averaging unit if the difference is less than a threshold −S; and - determining a minimum signal level using a low pass filter based on the average value; - determining an offset by calculating the difference between a minimum signal level and a reference current value; - correcting the sensor values ​​using the determined offset; This includes performing the following.

[0010] Thus, a non-invasive measurement method is provided which has the following advantages: 1. As a result of the purely digital implementation, the method is very robust and exhibits a predictable response. 2. As technology improves, it becomes cheaper to implement (scaling effect). 3. Digital testing is automated, resulting in higher quality as a result of higher test coverage during manufacturing. 4. Analog circuit component tolerance compensation.

[0011] Even if the signal and sampling time are uncorrelated, a proper choice of sampling rate will ensure that the calculated amplitude corresponds to the signal amplitude after using the shift register. The calculated amplitude is determined by calculating the difference between the first and last elements of the shift register.

[0012] The low pass filter used is advantageously used to mitigate the effect of a single averaging: if multiple averagings are performed successfully, the output of the low pass filter will provide the value of the lowest signal level.

[0013] In a preferred development of the invention, the reference current value of the signal to be examined is set to 7 mA. This is particularly relevant for commercially used RDS variants, which have a current of 7 mA as the base level (for pulsed or three-level sensors) or low level (for standard two-level sensors). The difference between the measured and the reference current value results in the determined offset.

[0014] In a preferred development of the invention, the number N of shift registers is chosen so that it is possible to store at least all samples of an edge, N being greater than 2.

[0015] In another further development of the invention, the window of the averaging unit is selected to be longer than the phase with the lowest level in the protocol, so that averaging occurs after the protocol. The protocol involves each magnetic pole triggering a complete sequence of sensor pulses. These pulses have a defined length and contain further information about the sensor's status. Thus, the protocol sensor transmits a defined pulse sequence. The pause between protocols is determined by the wheel speed.

[0016] In a preferred development of the invention, the window of the averaging unit is selected to be shorter than the phase with the lowest level in the protocol, so that the averaging is completed multiple times during the protocol, which results in more robust and accurate results. As correction values ​​can appear earlier, as few transmission protocols as possible are lost.

[0017] In a preferred development of the invention, - supplying a voltage to at least one sensor by means of an evaluation circuit; - modulating a sensor current by the sensor; - measuring and evaluating the sensor current by means of an evaluation circuit; is executed.

[0018] In a preferred development of the invention, the evaluation circuit has a high-side path and a low-side path, - using high-side and low-side paths in parallel; - determining current offset values ​​for the high-side path and the low-side path; is executed.

[0019] By using the high-side and low-side paths in parallel and determining the current offset value, the type of error can be determined and / or its location can be pinpointed. Acquisition and conversion of the RDS signal by an analog-to-digital converter (ADC) is a hallmark of non-invasive measurement techniques. After the RDS signal has been appropriately conditioned (e.g., low-pass filtered), it is preferably converted into a digital waveform with sufficiently accurate resolution using an ADC (connected to the positive power supply = high side or negative power supply = low side, depending on the circuit architecture) or, optionally, using two ADCs (connected to the high side and low side). The signal characteristics are examined by processing the digitized RDS signal values.

[0020] In a preferred development of the invention, the offset values ​​determined for the high-side path and the low-side path are constructively overlaid, thereby forming an improved vector. Preferably, two bits with the same index from LVL_HS[x] and LVL_LS[x], respectively, are considered for this overlay. If LVL_HS[x] or LVL_LS[x] indicates a rising signal edge, the output bit LVL[x] is set to 1. If LVL_HS[x] or LVL_LS[x] indicates a falling signal edge, the output bit LVL[x] is set to 0.

[0021] This ensures that the evaluation path that first detects the signal edge is sufficient to detect the signal in the event of an error.

[0022] Particularly preferably, offset compensation is first performed separately for the high-side and low-side paths, and then the two partial results are overlaid.

[0023] In a preferred development of the invention, a hysteresis is additionally implemented for the individual detection thresholds, which advantageously further improves the robustness.

[0024] In a preferred further development of the invention, the thresholds are defined as ideally as possible between specific signal levels of the sensors.

[0025] This object is also achieved by an offset compensation device for compensating for an offset in a sensor signal, the offset compensation device comprising a shift register and an averaging unit. Preferably, the offset compensation device performs at least part of the method described above.

[0026] Furthermore, this object is achieved by a sensor device comprising an offset compensation device, an evaluation device and at least one sensor. In a preferred development, the sensor is a wheel speed sensor for an electric brake system. The sensor device is designed to perform the above-mentioned method.

[0027] Further preferred embodiments result from the dependent claims and the following description of exemplary embodiments on the basis of the drawings. [Brief explanation of the drawings]

[0028] [Figure 1] 1 shows the connection of a vehicle wheel speed sensor to an evaluation circuit (prior art). [Figure 2] 1 shows current curves for a wheel sensor according to a protocol with three different current levels (prior art); [Figure 3] 1 shows a schematic diagram of an evaluation circuit (prior art); [Figure 4] 1 shows a wheel sensor current curve similar to FIG. 2 but with offset error (prior art). [Figure 5] 3 shows an evaluation circuit similar to that of FIG. 3, but with an additional block for offset compensation according to the invention. [Figure 6] FIG. 1 shows an example study of the effect of ADC sampling rate and shift register length N on falling edges, in accordance with the present invention. [Figure 7] 1 shows an evaluation circuit when the HS path and the LS path are used in parallel according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention describes an algorithm for determining and calibrating the offset of an RDS signal within the context of a non-invasive measurement method.

[0030] Many sensors in a motor vehicle are connected to evaluation electronics via wiring harnesses. Figure 1 basically shows the connection of a vehicle wheel speed sensor 1 to an evaluation circuit 3 with evaluation logic 4. The sensor 1 is supplied with voltage by the evaluation circuit (evaluation IC) 3 via connection to a power supply KL 30. The sensor current flows through both the high-side driver (HS) 5 and the low-side driver (LS) 7 of the evaluation ASIC 3. During operation, the sensor 1 modulates the sensor current. This current can be measured and evaluated in the integrated circuit (evaluation circuit 3) using a high-side analog-to-digital converter (HS ADC) 9 and / or a low-side analog-to-digital converter (LS ADC) 11 (see Figures 3 and 5).

[0031] Various data protocols are used for wheel sensors in automobiles. These data are transmitted from the wheels to the electronic control unit via the vehicle's current interface. Figure 2 shows the current curves of wheel sensor 1 according to protocols with three different current levels. Furthermore, Figure 2 shows four thresholds S0, S1, S2, and S3.

[0032] Figure 3 shows a schematic diagram of the evaluation circuit 3. The wheel sensor signals are first converted to digital form using analog-to-digital converters (ADCs) 9, 11, and the results are compared with different thresholds S0 to S3. The thresholds S0 to S3 are selected to fit as closely as possible between the specified signal levels of the sensor 1. Each comparator 13 sets its digital output to 1 as soon as the value of the ADCs 9, 11 is greater than the tested threshold S0 to S3. When all output signals of these comparators are combined into a digital signal vector, the vector LVL[3:0] is obtained, which is used for further signal detection. In this regard, see also Figures 1 and 2, where the vectors are also shown.

[0033] However, a series of sensor errors can occur during vehicle operation: - Positive and negative offset errors - Amplitude error (sensor amplitude is too high or too low)

[0034] Figure 4 shows the signal curve for sensor 1 when all current levels are provided around a fixed absolute value. This error is called the offset error.

[0035] Since the sensor signal is no longer below threshold S1, signal detection to form vector LVL[3:0] fails. In addition, threshold S3 is exceeded, which is not intended for correct protocol detection of sensor 1. In this case, the sensor signal in the vehicle is lost and is no longer available for the control functions of the safety-critical control unit. However, in Figure 4, we can see that the sensor information is still fully preserved in the received signal; only the detection range has shifted.

[0036] The subsequent detection method, offset compensation, allows signal detection even in the presence of sensor errors, improving the availability of wheel sensor signals within the vehicle.First, only one of the two detection paths is considered, for example, the high-side path 5.

[0037] As shown in Figure 5, an additional block 15 for offset compensation is introduced into the evaluation circuit 3. A shift register 17 of length N stores the values ​​of the AD converter. The length N of the shift register 17 is selected so that at least all samples of the edge can be stored and N>2.

[0038] The difference ΔS is calculated from the first and last elements of the shift register 17. If the difference at the output of the shift register 17 is positive and greater than the edge steepness value (threshold) S, this indicates a rising edge, and if the difference is negative and less than the edge steepness value (threshold) −S, a falling edge is detected.

[0039] The definition of the length N of the shift register 17 is explained with reference to Figures 6a to 6c, where the influence of the sampling rate of the ADC and the length N of the shift register is examined for falling edges.

[0040] In Figure 6a, N=2, and the sampling rate increases with increasing edge steepness. The sampling time is indicated by an arrow in the signal curve. In this case, the value of ΔS decreases as the sampling rate of the ADCs 9 and 11 increases. The higher the sampling rate, the longer the shift register 17 should be selected.

[0041] In Figure 6b, a lower sampling rate is used. However, the time correlation between the sensor signal and the sampling time is unknown or random. In this case, the signal can be sampled exactly in the center of the edge. This results in ΔS being at most half the magnitude of the signal amplitude.

[0042] Figure 6c now shows the case where N=3. The sampling rate is chosen so that an entire edge occurs between two sample values. There is still no correlation between the signal time and the sampling time, but this ensures that the amplitude ΔS corresponds to the signal amplitude. This is the best possible result, and is the reason for introducing the shift register 17.

[0043] In addition to the shift register 17, an averaging unit 19 is implemented for the AD converter values ​​(see Figure 5). This averaging unit 19 does not operate continuously but is controlled by the shift register output. If the output difference is greater than a threshold S, averaging starts. If the output difference is less than a threshold -S, averaging stops. If the averaging is successfully completed after exactly M input elements, the result is forwarded to the next low-pass filter 21, whose output produces the lowest level in the signal curve.

[0044] When combined with a shift register, the following reaction is obtained in the system: if the input signal from the ADC is constant, the shift register 17 will be filled with approximately identical values, the difference calculation at the output will result in neither exceeding nor falling below either of the two thresholds S or -S, and the state of the averaging unit 19 will not change.

[0045] When the input signal has a falling edge, the shift register 17 is first filled with larger values, then with smaller values. As soon as the entire edge is stored in the shift register 17, the difference at the output is maximum. During a falling edge, the threshold S may be exceeded multiple times. In this case, the averaging unit 19 is restarted each time, discarding all previous partial results. Only at the end of the falling edge is the threshold S no longer exceeded and averaging continues. This ensures that averaging only begins at the end of a falling edge. The advantage of this method is that it is not necessary to know when an edge starts or ends. Since the averaging unit 19 is no longer interrupted, the end of the edge is automatically detected.

[0046] If no further edges (falling or rising) are detected during averaging, the averaging of M values ​​ends and the result is transferred to the next low-pass filter 21. However, if a rising edge occurs during operation, the shift register 17 is first filled with a lower value and then with a higher value. In this case, the threshold value -S is below, so the averaging ends immediately. All intermediate results are discarded and nothing is transferred to the low-pass filter 21.

[0047] This method ensures that averaging begins and occurs exactly at the end of the falling edge, unless the input signal changes again after that. The next low-pass filter 21 is implemented to mitigate the effect of a single averaging. If averaging is performed successfully multiple times, the output of the low-pass filter 21 provides the value of the lowest signal level.

[0048] Calculating the difference using the lowest threshold expected level of sensor 1 gives the deviation of the sensor signal from this expected value. This difference is used to correct the value of the AD converter for sensor evaluation. Therefore, sensor evaluation works even in the presence of offset errors.

[0049] The discovered offset error is further used as a software monitoring parameter: if the value becomes too large, the system software may react and notify the driver or workshop of the error.

[0050] When designing an averaging unit, the following general conditions must be observed:

[0051] The longer the value M of the averaging unit 19 is selected, the more robust and accurate the determined results will be. However, the available waveforms must also be taken into account. In the case of the sensor protocol of Figure 2, it is possible to determine whether an averaging unit 19 is selected that is short enough that averaging is completed multiple times during the protocol. In this case, correction values ​​can appear earlier, so that as few transmission protocols as possible are lost.

[0052] If the averaging is chosen to be longer than the phase with the lowest level in the protocol, the averaging can only be performed after the protocol, which would take a long time before the correct correction value is calculated. Either implementation is possible.

[0053] As shown in Figure 7, when high-side and low-side paths 5, 7 are used in parallel within the evaluation IC 3, the currently determined offset values ​​of the high-side or low-side paths 5, 7 are continuously acquired during operation. These values ​​can be used to identify the type and location of errors. In addition, the determined data LVL_HS and LVL_LS can be constructively overlaid, resulting in an improved vector LVL. The overlay can be performed by an overlay unit 23.

[0054] Two bits with the same index from LVL_HS[x] and LVL_LS[x], respectively, are considered for this overlay. If LVL_HS[x] or LVL_LS[x] indicates a rising signal edge, the output bit LVL[x] is set to 1. If LVL_HS[x] or LVL_LS[x] indicates a falling signal edge, the output bit LVL[x] is set to 0.

[0055] This ensures that the evaluation path that first detects the signal edge is sufficient to detect the signal in the event of an error. Additional hysteresis at the individual detection thresholds further increases robustness. The present application relates to the invention described in the claims, but also includes the following as other aspects. 1. A method for offset compensation for a sensor signal by an offset compensation device (15), said offset compensation device comprising a shift register (17) containing N elements, an averaging unit (19) and a low pass filter (21), said method comprising the steps of: - storing the sensor values ​​in said 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 a plurality of average values, the average value comprising: a) initiating averaging by the averaging unit (19) if the difference is greater than a threshold S; b) stopping the averaging by the averaging unit (19) if the difference is less than a threshold value −S; and - determining a minimum signal level using said low pass filter (21) based on said average value; - determining an offset by calculating the difference between a minimum signal level and a reference current value; - correcting the sensor value using the determined offset; The method includes: 2. 2. The method according to claim 1, wherein the number N of the shift registers (17) is selected so as to be able to store at least all samples of an edge, and N is greater than 2. 3. 3. The method according to claim 1 or 2, wherein the window of the averaging unit (19) is selected to be longer than the phase in the protocol with the lowest level, so that the averaging is performed only after the protocol. 4. 4. The method according to any one of 1 to 3 above, wherein the window of the averaging unit (19) is selected to be shorter than the phase with the lowest level in the protocol, so that the averaging is completed multiple times during the protocol. 5. below, - supplying a voltage to at least one sensor (1) by means of an evaluation circuit (3); - modulating a sensor current by said sensor (1); - measuring and evaluating the sensor current by means of the evaluation circuit (3); 5. The method according to any one of 1 to 4 above, wherein the following is carried out. 6. The evaluation circuit (3) has a high-side path (5) and a low-side path (7), and - using the high-side path (5) and the low-side path (7) in parallel; - determining the current offset values ​​of the high-side path (5) and the low-side path (7); 6. The method according to claim 5, wherein the following is performed: 7. below, - constructively overlaying the determined offset values ​​of the high-side path (5) and the low-side path (7) to form a vector. 7. The method according to any one of 1 to 6 above, wherein the following is carried out. 8. An offset compensation device (15) for compensating for an offset in a sensor signal, comprising: The offset compensation device (15) comprises a shift register (17) and an averaging unit (19), and is designed to perform the method according to any one of 1 to 4 above. 9. A sensor device comprising an offset compensation device (15) as described in item 8 above, an evaluation device (13) and at least one sensor (1), the sensor device being designed to carry out the method according to any one of items 1 to 7 above. 10. 10. The sensor device according to claim 9, wherein the sensor (1) is a wheel speed sensor for an electric brake system. [Explanation of symbols]

[0056] 1 Wheel speed sensor 3 Evaluation circuit 4. Evaluation logic 5 High-side drivers 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 Averaging Unit 21 Low-pass filter 23 Overlay Unit S0, S1, S2, S3 thresholds Connecting to the KL30 power supply

Claims

1. A method for offset compensation for a sensor signal by an offset compensation device (15), said offset compensation device comprising a shift register (17) containing N elements, an averaging unit (19) and a low pass filter (21), said method comprising: - feeding a plurality of sensor values ​​digitally converted by a high-side analog-to-digital converter (9) and a low-side analog-to-digital converter (11) to said shift register (17) and said averaging unit (19); - calculating the difference between the first element of said shift register (17) and the last element of said shift register (17); - determining an average value of a plurality of sensor values ​​by: a) starting averaging by the averaging unit (19) if the difference is greater than a threshold S; and b) stopping averaging by the averaging unit (19) if the difference is less than a threshold −S; - feeding the average value of the sensor values ​​to a low-pass filter (21); the lowest signal level value among the average values ​​of the supplied sensor values ​​is output from the low-pass filter (21); determining an offset by calculating the difference between the value of the minimum signal level and the reference current value; - correcting the sensor value using the determined offset; The method includes:

2. 2. The method of claim 1, wherein the number N of the shift registers (17) is selected so as to be able to store at least all samples of an edge, and N is greater than 2.

3. 3. The method according to claim 1 or 2, wherein the window of the averaging unit (19) is selected to be longer than the phase in the protocol with the lowest level, so that the averaging is performed only after the protocol.

4. 4. The method of claim 3, wherein the window of the averaging unit (19) is selected to be shorter than the phase with the lowest level in the protocol, so that the averaging is completed multiple times during the protocol.

5. below, - supplying a voltage to at least one sensor (1) by means of an evaluation circuit (3); modulating the sensor current by said sensor (1); - measuring and evaluating said sensor current by means of said evaluation circuit (3); The method according to claim 1 or 2, wherein the following is performed:

6. The evaluation circuit (3) has a high-side path (5) and a low-side path (7), and - using said high-side path (5) and said low-side path (7) in parallel; - determining the current offset values ​​of the high-side path (5) and the low-side path (7); The method of claim 5 , wherein:

7. below, constructively overlaying the determined offset values ​​of the high-side path (5) and the low-side path (7) to form a vector; The method of claim 6, wherein:

8. An offset compensation device (15) for compensating for an offset in a sensor signal, comprising:

3. An offset compensation device (15), characterized in that it comprises a shift register (17) and an averaging unit (19) and is designed to carry out the method according to claim 1 or 2.

9. 9. A sensor device comprising an offset compensation device (15) according to claim 8, an evaluation device (13) and at least one sensor (1), the sensor device being designed to carry out the method according to claim 1.

10. 10. The sensor arrangement according to claim 9, wherein the sensor (1) is a wheel speed sensor for an electric brake system.

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