Patent Application
20090201375
This invention relates in general to motion sensors and in particular to a fail safe test for motion sensors.
Electronic brake control systems for vehicles are becoming increasing sophisticated. Such braking systems usually include an Anti-Lock Brake System (ABS) and a Traction Control (TC) System. Additionally, a Vehicle Stability Control (VSC) System may be provided. A VSC 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. A plurality of motion sensors, such as accelerometers and angular rate sensors are utilized to sense vehicle motion. The signals generated by elements within the motion sensors are typically modified by a signal conditioning circuit and then provided to a microprocessor in an Electronic Control Unit (ECU) of the electronic brake control system. The ECU microprocessor utilizes a stored algorithm to monitor the vehicle motion parameters, and, upon detecting a potential vehicle stability problem, the microprocessor initiates corrective action.
The motion sensors are typically packaged with supporting circuitry, with the package containing one or more accelerometers and/or one or more angular rate sensors. The sensor packages may also include signal conditioning circuitry. Key to successful operation of the VSC system is proper functioning of the motion sensors and signal conditioning circuitry. Accordingly, it is know to failsafe motion sensors by applying a self test to the sensor package. Such self tests typically include applying an input signal to each one of the motion sensors. The self test input signal is generated by the brake system microprocessor and applied to a self test port that is provided on the motion sensor package. If the motion sensor is operating properly, a fixed offset will appear on the sensor output signal. If the microprocessor does not detect the offset after applying the self test signal, it is an indication of a sensor malfunction and the microprocessor will generate an error signal or code. However, during the self test, 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 package. Furthermore, lower cost motion sensor packages typically do not include a port, or connection pin, for application of the self test signal. Therefore, it would be desirable to provide an alternate approach to fail safe testing of motion sensors.
This invention relates to a fail safe test for motion sensors.
The present invention contemplates an electronic brake system that includes a motion sensor and an analog to digital converter having an analog signal input port, or pin, and a serial digital signal output port, the analog to digital converter being connected to the motion sensor and operable to generate a digital motion sensor output signal. The system also includes a microprocessor connected to the analog to digital converter and the analog to digital converter also includes an auxiliary signal input port.
The brake system microprocessor is operable to apply an analog test signal to the analog to digital converter auxiliary signal input port, or pin, with the analog to digital converter being operable to process the analog test signal and to serially combine the resulting digital test signal with the digital motion sensor signal. The analog to digital converter also is operable to send the combined signal back to the microprocessor. The microprocessor is operable to compare the digital test signal to a threshold that is a function of the analog test signal and to generate an error message upon the digital test signal being less than the threshold.
The present invention also contemplates a method for fail safe testing of an electronic brake system that includes providing the system components described above and then applying an analog test signal to the auxiliary input port, or pin, of the analog to digital converter. The analog test signal is processed through the motion sensor module and the processed analog test signal is then converted into a digital test signal. The digital test signal is transmitted to the microprocessor where the signal is compared to a threshold that is a function of the analog test signal. Finally, an error message is generated if the digital test signal is less than the threshold.
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.
Referring now to the drawings, there is illustrated in
The motion sensor module 10 includes two methods for output of the final acceleration signals. Analog signals are available at three analog signal output pins, or ports, that are labeled 28, 30 and 32 and are connected through the sensor signal conditioning circuitry to the X, Y and Z acceleration sensors, respectively. Alternately, the analog output signals pass through an Analog to Digital (A/D) converter 34 and the resulting digital signals are then supplied to a Serial Peripheral Interface (SPI) 36 for digital communication. The SPI 36 provides synchronous serial communication between the sensor module 10 and an electronic brake system microprocessor 38, which is shown in
The SPI 36 shown in
While the sensor module 10 is illustrated in block diagram form in
While the sensor module 10 does not include a self test input pin, the module does include an Auxiliary Input pin, or port, 50. The Auxiliary Input pin 50 is intended to utilize the Analog to Digital converter 34 in the sensor module 10 to reduce the Analog to Digital signal conversion load on the master microprocessor. For example, a digital accelerometer, such as the module 10, could receive an analog output signal from a rotational rate sensor (not shown), convert it to a digital signal and send the converted digital signal in a serial data format with the digital accelerometer data to the microprocessor over a digital bus (not shown).
The present invention contemplates utilizing the Auxiliary Input pin 50 for fail safe testing of the sensing module 10. As best seen in
The invention contemplates utilization of one of several analog test signals. For example, the test signal output pin may be simply switched from ground potential to a high potential, such as five volts, or any voltage between zero and five volts. The magnitude of the returned test signal would then be compared to the magnitude of the test signal sent out by the microprocessor 38. If the returned signal has the same magnitude as the sent signal, the sensor and associated circuitry would be judged to be operating satisfactorily. Conversely, if the returned signal does not have the same magnitude as the sent signal, the sensor and associated circuitry would be judged to have failed and an error message would be generated and/or an error flag set. Alternately, the returned test signal may be compared to a threshold value that is a function of the test signal, such as, for example, 80 percent of the test signal magnitude. With the alternative approach, if the returned test signal is greater than or equal to the threshold, the sensor and associated circuitry would be judged to be operating satisfactorily. Conversely, if the returned signal is less than the threshold, the sensor and associated circuitry would be judged to have failed and an error message would be generated and/or an error flag set.
Another test signal that may be utilized is a square wave 60, as shown in
As before, the returned test signal voltage values would be compared to the magnitude of the test signal sent out by the microprocessor 38 to determine whether the sensor module and associated circuitry are operating properly. Because two voltage levels are available in the square wave test signal, the comparison may be between only the maximum values, between only the minimum values or between both the maximum and minimum values. Alternately, the returned digital test signal would be compared to a threshold that is a function of the analog test signal. If the returned test signal is less than the threshold, an error message is generated. The threshold may be selected to be either a function of the maximum value of the analog test signal or a function of the minimum value of the analog test signal. The invention also contemplates utilizing two thresholds with one threshold being a function of the maximum value of the analog test signal and the other threshold being a function of the minimum value of the analog test signal. In this later case, if either of the maximum and minimum values of the returned test signal are less that the value of the corresponding threshold, an error message is generated. As another variation, the minimum value of the returned test signal may be compared to a range of values about the minimum value threshold with an error signal being generated if the minimum value of the returned test signal is outside of the acceptable range. Thus, if the minimum value of the returned test signal is too great, an error signal will be generated, in addition to an error signal being generated if the minimum value of the returned test signal is too low. In a similar manner, the maximum value of the returned test signal may be compared to a range of values about the maximum value threshold with an error signal being generated if the maximum value of the returned test signal is outside of the acceptable range.
The operation of the fail safe test is controlled by an algorithm that is stored in the microprocessor 38. The operation of the algorithm will now be described with reference to the flow chart shown in
In functional block 78, the returned digital test signal is compared to the original analog test signal. As described above, the comparison may be directly between the two signals or between the returned digital signal and a threshold that is a function of the analog test signal. Additionally, the returned digital signal may be compared to two threshold values that correspond to the maximum and minimum values of the analog test signal. Also, the comparison criteria may be an exact match or a within a predetermined range, such as, for example greater than or equal to 80 percent of the magnitude of the analog test signal. The predetermined test range may be included in the calculation of the threshold. Alternately, an acceptable range of values about one or both of the thresholds may be used for the comparison criteria. The algorithm then advances to decision block 80.
In decision block 80, it is determined whether or not the comparison criteria were met in functional block 78. If the comparison criteria were successfully met, the algorithm transfers to decision block 82. If, in decision block 80, the comparison criteria were not successfully met, the algorithm transfers to functional block 84 where an error signal is generated. The operation occurring within functional block 84 may include generation of an actual error message and/or setting an error flag (not shown). The algorithm then continues to decision block 82.
In decision block 82, the algorithm determines whether it should continue. Criteria that would be utilized in decision block 82 would depend upon operating conditions, such as for example, continuous testing, periodic testing or single testing. Additionally, the criteria may include whether the vehicle engine is running, the vehicle ignition is in an on position or whether the vehicle is in motion. If it is determined in decision block 82 that testing is to continue, the algorithm transfers back to functional block 72 and repeats the test. If it is determined in decision block 82 that testing is to not continue, the algorithm transfers to end block 86 and exits.
While the invention has been described and illustrated above with a motion sensor package that includes three accelerometers, it will be appreciated that the invention also may be practiced with a motion sensor package that includes one or more angular rate sensors. Additionally, the inventor contemplates that the invention also may be practiced with a motion sensor package that includes both accelerometers and angular rate sensors.
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.
