Patent Application
20130125651
This invention relates in general to inertial, or motion, sensors used in vehicle electronic safety control systems and in particular to a fail safe test bandwidth test for inertial sensor modules.
Electronic safety control systems for vehicles are becoming increasingly sophisticated. Such safety systems may include an Anti-Lock Brake System (ABS), a Traction Control (TC) System, a Vehicle Stability Control (VSC) System and airbag control units with rollover detection. The safety control system typically monitors vehicle motion parameters and is operable to selectively activate the vehicle wheel brakes and/or modify engine performance to avoid potential unwanted vehicle motions, such as, for example, a vehicle roll-over. The safety control system also may be operable to deploy airbags at an appropriate time.
Electronic safety control systems typically include a plurality of inertial sensors, such as accelerometers and angular rate sensors, that are utilized to sense vehicle motion. The signals generated by elements within the Electronic safety control systems sensors are typically modified by a signal conditioning circuit and then provided to a microprocessor in an Electronic Control Unit (ECU) of the electronic safety control system. The ECU microprocessor utilizes a stored algorithm to monitor the vehicle motion parameters, and, upon detecting a potential vehicle stability problem or crash/rollover condition, the microprocessor initiates corrective action by selectively activation the wheel brakes and/or deploying airbags.
The inertial sensors are typically packaged in a module with supporting signal conditioning circuitry, with the module containing one or more accelerometers and/or one or more angular rate sensors. Key to successful operation of the safety control system is proper functioning of the inertial sensors and signal conditioning circuitry. Accordingly, it is know to failsafe inertial sensor modules by applying a self test to the sensor module. Such self tests typically include applying an input signal to each one of the inertial sensors. The self test input signal is generated by the safety control system microprocessor and applied to a self test input that is provided on the motion sensor module. If the motion sensor is operating properly, a fixed offset will appear on the sensor output signal appearing at an output of the sensor module. If the microprocessor does not detect the offset after applying the self test activation signal, it is an indication of a sensor malfunction and the microprocessor will generate an error signal or code. However, during the self test activation, the self test signal may saturate the device, thus limiting the usefulness of the sensor during the self test. Additionally, the frequency of the self test technique may be limited by the bandwidth of the motion sensor module. Therefore, this type of self testing is typically done while the vehicle is standing still, such as upon initial start-up of the vehicle.
Some inertial sensor modules, such as the module 10 illustrated in
During a typical start-up self test, a supply voltage Vt is applied to the voltage supply port 14. In response, an output voltage builds up on the output port 16, reaching a steady state value. After the output port 16 reaches its steady state value, the test status port 12 goes high. A self test activation signal is then applied to the test activation port 18 by the safety control system microprocessor, which causes the test status port 12 to go low. In response to the test activation signal, the output voltage increases by an offset voltage. The offset voltage on the output voltage port 16 is compared to an acceptable offset voltage range by the microprocessor and, upon the offset voltage remaining below an acceptable threshold, the sensor is deemed as working satisfactorily. The test concludes with termination of the test activation signal, which causes the test activation port 18 to go low. If no problem has been detected with the sensor 10, the test status port 12 goes high while the output voltage decays to the original value.
Because the above described self test is typically only carried out at start up of the vehicle and not while the vehicle is in operation, the result does not consider the available output signal bandwidth of the inertial sensor. The inertial sensor bandwidth is important for the proper operation of vehicle electronic safety control systems, such as VSC. Changes in the sensor signal output bandwidth will change the behavior of VSC systems. While inertial sensor bandwidth is checked during manufacture, the bandwidth may drift with aging of the components or due to other component changes, such as failure of a component. Therefore, it is important to verify that the sensor signal output bandwidth is within specification in order to minimize unwanted phase and amplitude variation of the VSC system. Accordingly, it would be desirable to provide an output signal bandwidth self-test.
This invention relates to a fail safe test bandwidth test for inertial sensor modules.
The present invention contemplates a test method that includes taking at least one sample of an inertial sensor output. The sensor output sample is compared to at least one limit and an error flag is set if the sensor output sample is either greater than an upper limit or less than a lower limit. The invention also contemplates comparing the sensor output sample to both upper and lower limits with the error flag being set if the sensor output sample is outside of a range set by the upper and lower limits where the upper and lower limits are a function of the sensor bandwidth.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.
The output of an inertial sensor is set by an external bandwidth capacitor as well as internal sensor amplifiers. Additionally, the rate of rise of the sensor output signal is a function of the sensor bandwidth. However, as stated above, inertial sensor bandwidth may drift with aging of the components or due to other component changes, such as failure of a component. For example, an open bandwidth capacitor would have a signal that rises too fast and would be outside of the specified bandwidth for the sensor. Therefore, the present invention contemplates comparing amplitudes of samples of an inertial sensor output signal to acceptable limits based upon rates of change for the samples. Because the rates of change for the samples are a function of the sensor bandwidth, a sample that is outside of the limits would be indicative of sensor bandwidth being out of specification. Also, by knowing the sensor specification bandwidth and tolerances of supporting circuitry, it is possible to develop the limits as a function of the acceptable rise time of the output signal for maximum and minimum sensor bandwidth frequencies.
Referring again to the drawings, there is illustrated in
The present invention contemplates that the upper and lower limits 34 and 36 are derived from the circuit components with the following relationship:
V
c(t)=V(1−e−t/RC), where:
The limits may be calculated as being a predetermined percentage above or below the expected output value, as shown by the following formulas:
T
HN=(100+selected percentage)VC(tN), and
T
LN=(100−selected percentage)VC(tN), where
It is to be understood that the percentage utilized above is meant is to be illustrative and that other percentages may be used to calculate the upper and lower limits THN and TLN. Furthermore, different values for the percentage may be utilized, for example, the upper limit THN may be selected to be 10 percent greater than the expected sensor output voltage at the selected sampling time while the lower limit TLN may be selected to five percent less than the sensor output voltage at the same sampling time. It is also contemplated that a predetermined voltage may be added and subtracted from the expected voltage to establish the limits, as shown in the following formulas:
T
HN
=V
C(tN)+0.1, and
T
LN
=V
C(tN)−0.1.
In the above example, one tenth of a volt is added to the expected sensor output voltage at the selected sampling time to obtain the upper limit while one tenth of a volt is subtracted from the sensor output voltage at the same sampling time to obtain the lower limit.
It will be appreciated that the invention also may be practiced with different amounts being added and subtracted to the expected output voltage. For both the percentage and the addition and subtraction methods, either the lower or the upper limit may be equal to the expected voltage at the sampling time (not shown). Both the upper and lower limits may also be set equal to the expected voltage at the sampling time (not shown), if zero tolerance is to be allowed for the sensor output signal. It must be understood that the above methods of calculating the upper and lower limits are meant to exemplary and other methods than specific ones shown above may be used to calculate the limits THN and TLN.
While the present invention has been illustrated and described above as utilizing analog voltages as the senor output signal, it will be appreciated that the invention also may be practiced with any output format such as, for example, voltage expressed in a digital format, engineering units or digital counts. “Engineering units” refers to circumstances where a host microprocessor converts the sensor output to a digital signal that may be transmitted over a Controller Area Network (CAN) while “counts” refers to a digital sensor communicating over a bus, such as, for example, a Serial Peripheral Interface (SPI) (not shown). Furthermore, the application of the invention is not intended to be limited to sensors utilized with safety control systems. Indeed, it is contemplated that the present invention may be applied to any and all Inertial Measurement Units (IMUs) which also may be utilized as stand alone sensors that may feed output signals into a CAN bus or other data transfer system. Regarding the possible alternate sensor output signal formats, the invention further contemplates that the upper and lower limits would be adapted to the same format to allow comparison to the sampled output signal (not shown).
The present invention also contemplates that the fail-safe sensor bandwidth test will be included in the control algorithm stored in the safety control system Electronic Control Unit (ECU) (not shown). The safety control system microprocessor controls the operation of the safety control system microprocessor that is responsive to sensor signals to initiate corrective action by selectively activating the vehicle wheel brakes and/or deploying airbags. A flow chart for the bandwidth test is shown in
If the output at the test status port 12 is high in decision block 46, the sensor reset has been released and the sensor 10 is ready for testing; but if the status port output is low, the sensor reset has not been released and the sensor is not ready for testing. If the test status port 12 is low, the algorithm transfers back to functional block 44 where the sensor reset condition is again released and the sensor output test status port 12 status is again checked. If the test status port 12 is high, the algorithm transfers to functional block 48.
In functional block 48, the bandwidth self test is commanded. The algorithm then continues to decision block 50 where the sensor test status port 12 is again checked. If the test status port is still high, the self-test command has not been acknowledged and the algorithm transfers to functional block 52 where a delay is introduced to allow the test command to be acknowledged. The algorithm then transfers back to decision block 50 where the sensor test status port 12 is rechecked. If, in decision block 50, it is determined that the test status port is now low, the self-test command has been acknowledged and the algorithm transfers to functional block 54.
In functional block 54, an index N that represents the number of sensor output samples to be checked during the test is set to unity. The algorithm then advances to function block 56 where the sensor output voltage amplitude OUTN is measured. The algorithm then continues to decision block 58 where the sensor output voltage amplitude is compared to predetermined upper and lower limits THN and TLN, respectively, that are a function of the bandwidth and the sensor output index N. Thus, the upper and lower limits THN and TLN, respectively, are also a function of time and are derived as a function of the sensor output formula presented above. Solution of the formula provides an expected value and then the upper and lower limits may be taken THN and TLN calculated for a specific time t from the expected sensor output value at the time t.
If the sensor output voltage amplitude is outside of the range defined by the predetermined upper and lower limits TLN and THN, the sensor bandwidth is out of specification and the algorithm transfers to functional block 60 where an error flag is set. Setting the error flag will cause an error signal, visual and/or audio, to be activated to warn the vehicle driver. Additionally, setting the error flag also may result in the system that utilizes the sensor output being taken out of service. Once the error flag is set, the test is completed and the algorithm exits through block 62.
If, in decision block 58, the sensor output voltage amplitude is within the range defined by the predetermined upper and lower limits TLN and THN, the sensor bandwidth is within specification and the algorithm transfers to functional block 64 where the sensor output index N is increased by one. The algorithm then advances to decision block 66 where the current sensor output index N is compared to a maximum sample threshold NMAX. The maximum sample threshold NMAX is equal to the desired number of senor output samples to be utilized in the test. In the preferred embodiment, four samples are used; however, more or less samples than four also may be used. If the current sensor output index N is greater than the maximum sample threshold NMAX, the desired number of senor output samples have been utilized and the algorithm exits through block 62 without setting an error flag. If, on the other hand, the current sensor output index N is less than or equal the maximum sample threshold NMAX, the desired number of senor output samples have not been examined and the algorithm transfers back to functional block 56 where the next sensor output voltage amplitude OUTN+1 is measured. The algorithm than continues as described above.
The algorithm shown in
As shown in
If, in decision block 74, If the number of errors NER is less than the maximum allowable number of errors, MAXNER, the algorithm transfers to function block 64 and continues as described above. Similarly, returning to decision block 58, if the sensor output voltage amplitude is within the range defined by the predetermined upper and lower limits TEN and THN, the sensor bandwidth is within specification and the algorithm transfers to functional block 64 and continues as described above.
While the algorithm illustrated in
Because the present invention utilizes the sensor output signal, it is contemplated that the test may be run while the sensor is in service. Thus, the test is not limited to only times when the vehicle is being started. Additionally, the test may be run continuously, being restarted once the number of the number of sensor output samples to be checked N is exceeded, or on a periodic basis (not shown). With regard to periodic running to the test, the test may be run either with a predetermined time period between test and/or at random time intervals (not shown).
It will be appreciated that the flow charts shown in
In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.
