Method of operating a traction system for a vehicle

Abstract: A method of controlling the driving wheels of a vehicle having an internal combustion engine wherein it is determined if a vehicle stuck or not and then, if stuck, the maximum permissible amount of wheel slip is increased with the intent of removing loose material, such as snow, underneath the tires. If the vehicle remains stuck and wheel slip continues, the amount of driving power and torque to the wheels is then reduced. Conversely, if the spin is not present at the subsequent gas pedal application (tip-in), and the vehicle is still stuck, the wheel torque limit is increased.

Description: This invention relates to electronic control of the driving wheels of a vehicle having an internal combustion engine. Traction control of various types is known. For example, it is known to reduce the power applied to the driving wheels through various combinations of controlling engine operating parameters such as spark advance, fuel injection, and throttle control. Braking may also be used to control driving wheel rotation. Even though such control of the driving wheels is known, there still is a need to improve the performance of traction control in various operating environments. Conditions such as low and medium speed operations on combined, split ice/snow surfaces are addressed, in part, by this invention. US-A-4 884 651 describes a vehicle slip control apparatus for preventing an excessive degree of slippage or spinning by a driven wheel on a road surface by controlling the torque transmitted to the driven wheel. This is desired so as to increase the safety of driving and to adapt to the skill of the driver. The apparatus includes an engine output torque adjusting means to adjust the engine torque and a brake force adjusting means to adjust the brake force of a brake for the driven wheel. A first slip control means controls the brake force adjusting means to a first target value of slippage and a second slip control means controls the engine output torque to a second target value of slippage. The control ratio between the first and second target values can be adjusted upon instruction to suit the driving requirements of the vehicle. If the vehicle becomes stuck, this is detected by observing a difference between the rotation of a left-hand driven wheel and a right-hand driven wheel and a stuck flag is set in the apparatus. An instruction is then sent to reduce the control ratio. Deliberate spinning of the wheels is avoided. According to the present invention, there is provided a method of operating a traction control system for a vehicle with an internal combustion engine including the steps of spinning the wheels to clear loose material; and reducing spinning of the wheels to drive the vehicle out of loose material. Further, according to the present invention there is provided a method of controlling traction of a vehicle with an internal combustion engine including the steps of establishing a predetermined amount of permissible wheel slip target; establishing a predetermined vehicle speed which indicates a stuck vehicle; determining the vehicle speed; comparing the vehicle speed to the predetermined vehicle speed; if the vehicle speed is less than the predetermined stuck speed, determining if wheel spin is greater than the predetermined amount of wheel spin needed to identify the surface friction coefficient; increasing the wheel slip target for permissible wheel slip for a predetermined amount of time; determining if the vehicle is still stuck; determining if a time duration since determining vehicle speed is greater than the predetermined amount of time; if yes, reducing the amount of engine torque applied to the wheels based upon the determined road surface friction coefficient; determining if the wheels are still spinning on the next gas pedal tip in; if yes, reducing the amount of engine torque applied to the wheels; and if not, increase the maximum allowable wheel torque limit. In accordance with an embodiment of this invention there is used an available preview of the surface friction conditions from increased slip targets to increase tire tractive forces up to a maximum possible at or near zero vehicle speeds, indicating a stuck vehicle. This invention is advantageous because there are increased chances for moving a vehicle from a snow bank, stuck in snow or similar stuck condition. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figs. 1A and 1B are graphic representations of wheel speed and torque during a spin on ice at or near zero vehicle speed indicating a stuck conditions; and Figs. 2A and 2B are a logic flow description of a traction control system in accordance with an embodiment of this invention. When trying to move a vehicle in deep snow the following traction control strategy in accordance with an embodiment of this invention is used. During the first attempts to remove the vehicle, such as the first couple of tip-ins, identify that the vehicle is stuck by monitoring of non-driven wheels and then start increasing the slip target. The slip target is the amount of wheel slip of the driving wheels allowed before reducing driving power to the driving wheels. This will cause more wheel slip and in some cases this could be beneficial. For example, this may help remove the loose material, such as snow, underneath the tire and bring the tire in contact with firmer, hopefully higher-mu ground layer. If the above approach does not cause the vehicle to move, then proceed with the approach based on "preview" of underlying surface which was provided by the above approach. Such an action has a potential to increase the tractive force up to 100% as shown in Figs. 1A and 1B indicating of initial wheel spin and torque on ice. Referring to Figs. 2A and 2B, logic flow starts at a block 20 and goes to a decision block 21 wherein it is asked if the vehicle speed is less than or equal to a stuck velocity for at least a predetermined number of seconds or if the stuck flag is on. If the answer is no, the logic flow goes to an exit block 22. If the answer is yes, logic flow goes to a block 23 wherein the vehicle stuck flag is set. Logic flow then goes to a decision block 24 wherein it is asked if wheel spin is greater than or equal to a limiting spin. The limiting spin is used to determine surface friction coefficients which are then stored for future use. The spin needs to be above such limiting value for the surface friction coefficient identification to be valid. If yes, logic flow goes to a block 25 wherein wheels are spinning, which facilitates identification of the road-mu. Logic flow then goes to a node 26 which then goes to a block 27 wherein there is gradually increased the slip target multiplier for t 2 . If the answer in block 24 is no, logic flow goes to node 26 and block 27. Logic flow then goes to a block 28 in Fig. 2B wherein it is asked if the velocity is less than or equal to the stuck velocity. If the answer is no, the stuck flag is reset at a block 29. If the answer is yes, logic flow goes to a block 30 wherein it is asked if the time is greater than the predetermined time. If no, logic flow goes to an exit block 31. Logic flow from block 29 also goes to exit block 31. If the answer in block 30 is yes, signifying that sufficient time has been spent in trying to move through an excessive wheel spin, the logic flow goes to a block 32 wherein the maximum engine torque based on estimated mu is clipped. Logic flow then goes to a decision block 33 wherein it is asked if the wheel is still spinning on the next gas pedal tip-in. If the answer is yes, logic flow goes to a block 34 wherein there is a reduced max clip engine torque. Logic flow then goes to a node 35 and then to a decision block 36. If at decision block 33 the answer was no, logic flow goes to a block 37 wherein there is an increased maximum clip engine torque. Logic flow from block 37 goes to node 35 and decision block 36. At decision block 36 it is asked if the vehicle speed is less than or equal to the stuck velocity, i.e. is the vehicle still in a stuck condition? If the answer is yes, logic flow goes to an exit block 37. If the answer is no, logic flow goes to a block 38 wherein there is reset the stuck flag and the target slip indicating that the vehicle is no more in a stuck condition. From block 38 logic flow goes to an exit block 39.

Claims: A method of operating a traction control system for a vehicle with an internal combustion engine including the steps of: spinning the wheels (24,25,27) to clear loose material; and reducing spinning of the wheels (32,33,34) to drive the vehicle out of loose material. A method of operating a traction control system as claimed in claim 1 further comprising the steps of: determining the vehicle is stuck (21); and increasing the magnitude of a predetermined amount of slip required (27) before operating the traction control system (34) to reduce power applied to the driving wheels. A method of controlling traction of a vehicle with an internal combustion engine including the steps of: establishing a predetermined amount of permissible wheel slip target; establishing a predetermined vehicle speed which indicates a stuck vehicle; determining the vehicle speed; comparing the vehicle speed to the predetermined vehicle speed (21); if the vehicle speed is less than the predetermined stuck speed, determining if wheel spin is greater than the predetermined amount of wheel spin needed to identify the surface friction coefficient (24); increasing the wheel slip target for permissible wheel slip for a predetermined amount of time (27); determining if the vehicle is still stuck (28); determining if a time duration since determining vehicle speed is greater than the predetermined amount of time (30); if yes, reducing the amount of engine torque applied to the wheels based upon the determined road surface friction coefficient (32); determining if the wheels are still spinning on the next gas pedal tip in (33); if yes, reducing the amount of engine torque applied to the wheels (34); and if not, increase the maximum allowable wheel torque limit (37).

Inventors: HROVAT DAVORIN DAVID, TRAN MINH NGOC, YESTER JOHN LORING, HROVAT, DAVORIN DAVID, TRAN, MINH NGOC, YESTER, JOHN LORING
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1996
Main classification: B60K28
IPC codes: B60K28