A FAILSAFE SYSTEM AND A FAILSAFE METHOD FOR DETECTING VEHICLE SPEED SENSOR FAULT
FIELD OF THE INVENTION
The present invention relates to a failsafe system and method for detecting vehicle speed sensor fault/disconnection exclusively during vehicle running condition without neutral switch input. More specifically, the present invention relates to a system and method for detecting vehicle speed sensor fault in engine control unit using wheel speed information obtained from Anti‐ lock Braking System (ABS) control unit in vehicles.
BACKGROUND OF THE INVENTION
In the heavy‐duty vehicle application with electronically controlled engines, vehicle speed sensor input to engine control unit is crucial for functions like top speed limitation, gear detection algorithms, mass estimation and for diagnosis of other system malfunctions. Top speed limitation function is mandated by regulations such that even during the event of vehicle speed sensor input failure, the vehicle should not exceed the top speed limit. To meet the regulations, vehicle speed sensor fault detection by engine control unit becomes critical for ensuring vehicle speed within top speed limit. When vehicle speed sensor input is properly available to engine control unit, vehicle speed will be exactly limited close to top speed limit by PID control system without affecting the full engine power and engine speed operation in low gear ranges. When vehicle speed sensor fault is logged by engine control unit, vehicle speed will be limited well ahead of top speed limit by engine speed limitation, which will affect the full engine power and engine speed operating range in lower gears as well.
To detect the actual vehicle speed sensor fault in vehicle, engine control unit needs to distinguish between the situation of no pulse input in vehicle static

condition and the situation of no pulse input in vehicle running condition. In former situation, there is no failure in vehicle speed sensor input whereas in latter situation, there is failure in vehicle speed sensor input to engine control unit. To distinguish the static and running situations, in general, engine controller uses additional conditions like engine speed is above a threshold value and engine torque demand is above a maximum threshold value continuously for a predefined period of time. However, inaccurate vehicle speed fault logging can occur because of higher engine torque demand in stationary vehicle due to tipping load, air compressor load, higher engine friction in cold conditions, etc. Another possibility that may result in false vehicle speed fault code logging is leakage in charged air circuit, due to which higher torque demand during free acceleration can continue for more than the threshold period. The engine torque demand and continuous monitoring time thresholds are fixed such that vehicle speed fault code is logged before vehicle reaches the top speed limit while driving without load. So, there is a limitation in threshold values beyond which it cannot be increased to avoid false detection. A neutral switch input can be considered in addition to the above conditions to distinguish vehicle static condition and running condition. However, there is a failure mode when both neutral switch and vehicle speed sensor failure occur at the same time, then vehicle speed sensor fault logging will not happen, and vehicle may exceed top speed limit, violating the regulations.
The US patent No. 7286917 discloses a method of detecting vehicle speed sensor failures. The method is intended to prevent or discourage tampering with a vehicles speed limiting system. The method includes the steps of determining whether the transmission is in neutral or whether the transmission is in its converter mode to prevent false indications of speed sensor failure under certain circumstances.

However, the US patent detects the failure in vehicle speed sensor using transmission i.e., neutral switch input and torque converter mode. The use of neutral switch input is not a failsafe method. Furthermore, the torque converter mode is applicable only for automatic transmission.
Therefore, there is need in art to provide a system and a method to ensure failsafe fault detection in vehicle speed sensor during vehicle running condition. Furthermore, there is need in art to provide a system and a method to ensure failsafe fault detection in vehicle speed sensor which can be implemented in both the automatic and manual transmission.
All these problems of false detection in static condition and failure to detect in running condition are addressed by the invention as summarized below.
OBJECTIVES F THE INVENTION
The object of the present invention is to provide a failsafe system and method for detecting vehicle speed sensor fault.
Another object of the present invention is to provide a failsafe system and method for detecting vehicle speed sensor fault during vehicle running condition.
Further object of the present invention is to provide a failsafe system and method for detecting vehicle speed sensor fault which can be implemented in both the automatic and manual transmission.
SUMMARY OF THE INVENTION
According to the present invention, a failsafe method for detecting vehicle speed sensor failure by engine control unit exclusively during vehicle running condition without using neutral switch input is disclosed. The method includes several steps to conclude the actual vehicle speed sensor fault and to meet the regulations for top speed limit despite any failure mode that can occur in

vehicle. One step is to determine whether the vehicle speed value calculated from the pulse input of vehicle speed sensor is less than the minimum threshold value. Another step is to determine whether the engine speed and engine torque demand are higher than the respective threshold values. Further step is to ensure all the above conditions continue for a predefined time. The above‐mentioned steps are common practice followed in engine controllers. This common practice may include a step of determining whether the transmission is not in neutral using neutral switch input, but the failure of neutral switch has a possibility of not meeting the regulations during vehicle speed sensor failure. To overcome these failure modes, the innovative step taken in this invention is to use the wheel speed information obtained from Anti‐lock Braking System (ABS) control unit through CAN communication to determine whether the vehicle is in running condition. Also, in the event of failure in CAN communication while receiving wheel speed information, step has been taken to ensure vehicle speed fault logging and thereby meeting the regulations for top speed limitation in all possible failure modes. These aspects of the present invention will be better understood with reference to the attached drawing and following detailed description of the illustrated embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 depicts a flowchart of the failsafe system and method for detecting vehicle speed sensor fault during vehicle running condition in accordance with an embodiment of the invention.
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

DETAILED DESCRIPTION
The present invention as embodied by a “a failsafe system and a failsafe method for detecting vehicle speed sensor fault" succinctly fulfils the above‐ mentioned need(s) in the art. The present invention has objective(s) arising as a result of the abovementioned need(s), said objective(s) being enumerated below. In as much as the objective(s) of the present invention are enumerated, it will be obvious to a person skilled in the art that, the enumerated objective(s) are not exhaustive of the present invention in its entirety, and are enclosed solely for the purpose of illustration. Further, the present invention encloses within its scope and purview, any structural alternative(s) and/or any functional equivalent(s) even though, such structural alternative(s) and/or any functional equivalent(s) are not mentioned explicitly herein or elsewhere, in the present disclosure. The present invention therefore encompasses also, any improvisation(s)/modification(s) applied to the structural alternative(s)/functional alternative(s) within its scope and purview. The present invention may be embodied in other specific form(s) without departing from the spirit or essential attributes thereof.
Throughout this specification, the use of the word "comprise" and variations such as "comprises" and "comprising" may imply the inclusion of an element or elements not specifically recited.
A failsafe system for detecting vehicle speed sensor fault comprises a vehicle speed sensor. The vehicle speed sensor transmits the measured pulse signal of gearbox output shaft rotations to an engine control unit from which vehicle speed is calculated. The engine control unit receives the information about the engine speed which is measured by an engine speed sensor. The engine control unit calculates engine torque based on the inputs of driver demand through accelerator pedal, engine speed, intake air pressure and fuel pressure. Furthermore, the engine control unit obtains the wheel speed information

continuously during its operation.
In an exemplified embodiment, the failsafe system for detecting the vehicle speed sensor fault, the engine control unit determines whether the vehicle speed is below the minimum threshold vehicle speed. If the vehicle speed sensor is below the minimum threshold vehicle speed, the engine control unit then checks for the engine speed criteria. If the engine speed is below the threshold engine speed, the failsafe system aborts and restart the above process. If the engine speed is above the threshold engine speed, the engine control unit checks for the engine torque criteria.
If the engine torque is below the threshold engine torque, the failsafe system aborts and restart the above process. If the engine torque is above the threshold engine torque, the engine control unit thereon proceeds to check for the wheel speed criteria. The engine control unit firstly determines that there is no fault in communication from the ABS control unit. The communication from the ABS control unit to the engine control unit is by means of the CAN communication.
If there is any fault in communication from the ABS control unit to the engine control unit, the timer for fault logging starts incrementing and repeat the above process from the start till timer reaches the predefined time period. During repetition of the above process, if any one of the criteria is not meeting the threshold, the failsafe system aborts, resetting the timer to zero and restart the above process. Once the timer reaches the predefined time, the vehicle speed sensor fault logging and subsequent limitation for maximum engine speed occurs. Optionally the engine control unit may initiate the vehicle speed sensor fault alarm.
If thereisnofault incommunication from the ABScontrolunit totheengine
control unit, the engine control unit compares the speed of all the wheels

obtained from the ABS control with the predefined wheel speed threshold value. If the wheel speed of all or at least one wheel is more than the threshold wheel speed, the timer for fault logging starts incrementing and repeat the above process from the start till timer reaches the predefined time period. During repetition of the above process, if any one of the criteria is not meeting the threshold, the failsafe system aborts, resetting the timer to zero and restart the above process. Once the timer reaches the predefined time, the vehicle speed sensor fault logging and subsequent limitation for maximum engine speed occurs. Optionally the engine control unit may initiate the vehicle speed
If the wheel speed of all or at least one wheel is not more than the threshold wheel speed for the predefined time period, the failsafe system aborts and the engine control unit performs the above process again to determine the vehicle speed sensor fault.
According to FIG. 1, a flowchart depicting the steps involved in the method for detecting vehicle speed sensor fault using input from ABS control unit is illustrated. In the illustrated embodiment, the engine controller logic begins at 10 by calculating vehicle speed from pulse input of vehicle speed sensor using the inputs of number of pulses per shaft rotation and driveline parameters like rear axle ratio and tire rolling radius. This calculated vehicle speed is then monitored at 12 for exceeding the minimum threshold limit and the logic proceeds for further diagnosis at 14 only if the vehicle speed is less than the threshold. At 14, the logic monitors for engine speed exceeding the threshold value and if it exceeds, then the logic proceeds for further diagnosis at 16, where the engine torque demand is monitored for exceeding the threshold limit. If the engine torque demand exceeds the limit, then the logic proceeds for further diagnosis at 18, where the diagnostic fault checks done for CAN communication from ABS control unit. One of the fault checks is done by monitoring timeout error for Wheel Speed Information – EBC2 message, which

is being transmitted from ABS control unit in accordance to SAE J1939 CAN standard for Parameter Group Number (PGN) 65215. The timeout error is logged in engine control unit if the Wheel Speed Information - EBC2 message is not received continuously for a predefined time. This can happen if the wiring connection between engine control unit and ABS control unit is disconnected. Another fault check is done by monitoring the values received in CAN message for within range. If all the wheel speed sensors are disconnected from ABS control unit, then the wheel speed information values received will be out of range. If either the timeout condition or values out of range condition is active at 18, then the logic proceeds directly to 22, otherwise the logic proceeds for further diagnosis at 20. This is to ensure that even if there is a failure in receiving the wheel speed information, the vehicle speed sensor fault logging will continue to meet the top speed limit regulations. The wheel speed obtained from ABS control unit is monitored for exceeding the threshold limit at 20 to ensure the vehicle is actually in running condition. If it exceeds, then the logic proceeds to 22, where the timer for diagnosis fault logging is monitored for exceeding the time limit. If the timer is within the limit indicating that the above conditions have not existed for enough period to conclude sensor fault, timer is incremented further at 24 and the steps are repeated from 10. If the timer exceeds the limit at 22, then the logic concludes vehicle speed sensor fault at 26, where fault logging and subsequent limitation for maximum engine speed occur. This limitation in engine speed is applied to ensure the vehicle does not exceed top speed limit even during the event of vehicle speed sensor failure as per regulations.
It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations, and improvements without deviating from the spirit and the scope of the invention may be made by a person skilled in the art.

We Claim:
1. A failsafe system for detecting vehicle speed sensor fault, comprises,
a. a vehicle speed sensor;
b. an engine control unit, said engine control unit in which vehicle
speed measured by the vehicle speed sensor;
c. an engine speed sensor, said engine speed sensor measures the
engine speed and sends the measured engine speed to the engine
control unit;
d. an engine torque model in the engine control unit, said engine torque
model derives the engine torque from the inputs of driver demand
through accelerator pedal, engine speed, intake air pressure and fuel
pressure; and
characterized in that
e. an ABS control unit, said ABS control unit communicates the wheel
speed information of one or more than one wheel to the engine
control unit to determine the vehicle speed sensor fault.
2. The failsafe system as claimed in claim 1, wherein the communication
from the ABS control unit to the engine control unit is by means of a CAN communication.
3. The failsafe system as claimed in claim 1 or 2, wherein the
communication from the ABS control unit to the engine control unit do
not have any fault, the engine control unit, for logging fault of vehicle
speed sensor, determines whether the wheel speed of one or more than
one wheel of the vehicle is more than the threshold speed, continuously
for a predefined time period.

have any fault, the engine control unit, for logging fault of vehicle speed sensor, determines whether fault in communication is continuously for the predefined time period.
5. The failsafe system as claimed in claim 1 or 2, wherein the communication from the ABS control unit to the engine control unit do not have any fault, the wheel speed of one or more than one wheel of the vehicle is more than the threshold speed, not continuously for a predefined time period, the failsafe system aborts, the failsafe system performs the detection of vehicle speed sensor fault from the beginning, and the predefined time period is adjusted.
6. The failsafe system as claimed in claim 1 or 2, wherein the communication from the ABS control unit to the engine control unit do have any fault, not continuously for a predefined time period, the failsafe system aborts, the failsafe system performs the detection of vehicle speed sensor fault from the beginning, and the predefined time period is adjusted.
time period is either incremented or decremented. 8. A failsafe method for detecting vehicle speed sensor fault, comprising of,
a. determining vehicle speed is less than the threshold vehicle speed;
b. determining engine speed is more than the threshold engine speed;
c. determining engine torque is more than the threshold engine torque;
d. determining fault in communication from an ABS control unit to an
engine control unit;

wherein, vehicle speed is less than threshold vehicle speed, engine speed is more than the threshold engine speed, engine torque is more than the threshold engine torque and fault in communication from the ABS control unit to the engine control unit, continuously for a predefined time period,
e. logging of vehicle speed sensor fault; and
f. limiting the maximum engine speed.
9. A failsafe method for detecting vehicle speed sensor fault, comprising of,
a. determining vehicle speed is less than the threshold vehicle speed;
b. determining engine speed is more than the threshold engine speed;
c. determining engine torque is more than the threshold engine torque;
d. determining no fault in communication from an ABS control unit to
e. determining the wheel speed information obtained from the ABS
control unit is more than the threshold wheel speed;
wherein, vehicle speed is less than threshold vehicle speed, engine speed is more than the threshold engine speed, engine torque is more than the threshold engine torque, no fault in communication from the ABS control unit to the engine control unit and obtained information about wheel speed is more than the threshold wheel speed, continuously for a predefined time period,
f. logging of vehicle speed sensor fault; and
g. limiting the maximum engine speed.
10. The failsafe method as claimed in claim 8 or 9, wherein fault in communication from the ABS control unit to the engine control is not continuously for the predefined period, or obtained wheel speed is not more than the threshold wheel speed, for the predefined period, the

failsafe system performs the method from the beginning and the predefined time period is adjusted.
period is either incremented or decremented.