Abstract:
An estimated brake rotor temperature is provided including a cooling effect during active braking as well as when braking is inactive. The cooling effect is based on a difference between the rotor temperature and sensed ambient temperature and is also preferably based on wheel speed. In an active braking mode, a heating effect is provided based at least on sensed wheel speed. If no brake pressure signal is available, the heating effect is further based on vehicle deceleration. The heating effects for front and rear brake units are relatively compensated for differences in heat generation due to load shifts during braking, the compensation being preferably based on vehicle deceleration. The brake rotor temperature estimation is realized in a programmed digital computer but does not use computer resource hungry exponential functions.

Description:
TECHNICAL FIELD  
         [0001]    The technical field of this invention is vehicle systems incorporating vehicle wheel brakes, and particularly such systems for determining brake rotor temperature.  
         BACKGROUND OF THE INVENTION  
         [0002]    Braking performance can be significantly affected by the temperature rise in brake components such as rotors. For this reason, systems have been proposed for monitoring the temperature of brake components. For example, temperature sensors such as thermocouples have been used in brake system and/or component testing, although such sensors are not practical for mass-produced vehicle applications. In addition, brake component estimation algorithms have been proposed. Such monitoring allows the generation of a driver warning or even the automatic reduction of braking pressure of individual wheels in braking or traction control, as described in U.S. Pat. No. 5,136,508, issued Aug. 4, 1992.  
           [0003]    But the algorithm of the aforementioned patent has shortcomings. First of all, it is a method for estimating brake lining temperature; but brake lining temperature is much less well defined than brake rotor temperature. The linings, in a brake caliper pad, for example, are made of a substance having a much lower heat conductivity (higher temperature insulating effect) and a higher wear rate than the same characteristics of a brake rotor. Corroboration of any temperature estimation algorithm requires an accurately measured temperature test using a temperature sensor such as a thermocouple. Mounted in a brake lining, such a thermocouple must be placed a significant distance away from the rotor/lining interface so that, even with lining wear, the sensor will not be contacted and compromised by the metallic, fast spinning rotor. In the low heat conductivity environment of a brake lining, the distance of the thermocouple from the rotor/lining interface will provide a large temperature insulation between the interface and the sensor that leads to inaccuracy in sensing of the interface temperature. But a metallic brake rotor has a high heat conductivity and a low wear rate, in comparison with a lining. This allows a rotor mounted, corroborating thermocouple to be placed much closer, comparatively, to the rotor/lining interface if it is incorporated in a brake rotor rather than a brake lining. It will thus provide a more accurate reading of the temperature of the rotor/lining interface during braking.  
           [0004]    This effect is even more true in a dynamic temperature variation. As brake pressure varies and the temperature at the brake rotor/lining interface heats up and cools down, most (typically more than 95%) of the heat generated at the interface flows away through the highly conductive rotor. The temperature measured by the well insulated lining thermocouple will vary less and with much greater time delay. The temperature measured by the rotor mounted thermocouple will follow the dynamic variation of the temperature at the rotor/lining interface far better than the temperature measured by a lining mounted thermocouple. Thus, brake rotor temperature can be much more accurately defined with real time temperature measurement; and this provides a more accurate corroboration for a brake rotor temperature estimation algorithm than for a brake lining temperature algorithm. Since brake rotor temperature provides a more accurate indication of temperature at the rotor/lining interface, a temperature estimation algorithm can, and should, provide a more sophisticated, and thus more accurate, estimated value of the brake rotor temperature. For example, temperature effects due to fore and aft vehicle load shift due to braking and/or cooling effects during braking are significant and measurable.  
           [0005]    The temperature estimation algorithm of the aforementioned patent estimates a vehicle speed dependent cooling effect on the brake lining, but only when the brakes are not being applied. In reality, the cooling effect occurs at all times; and this is especially true for a brake rotor, which radiates heat more efficiently, both because of its metal structure and because of the great portion of its surface area that is not covered by the brake pad/lining and is thus exposed directly to the air.  
           [0006]    In addition, the algorithm of the aforementioned patent relies heavily on exponential functions, which greatly consume controller computer resources, especially memory. Thus, to be practical and accurate, a brake temperature estimation algorithm should preferably avoid the use of exponential and other memory intensive functions.  
         SUMMARY OF THE INVENTION  
         [0007]    This invention is a method and apparatus for providing an estimated brake rotor temperature. It provides for a cooling effect during braking as well as when braking is inactive, the cooling effect being based on a difference between the rotor temperature and sensed ambient temperature and also preferably based on wheel speed. In an active braking mode, it provides for a heating effect based at least on sensed wheel speed. If no brake pressure signal is available, it further bases the active braking heating effect on vehicle deceleration. Preferably, it provides compensation between front and rear brake units for differences in heat generation due to load shifts during braking, the compensation being preferably based on vehicle deceleration. It also does not use computer resource hungry exponential functions in determining the estimated brake rotor temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic diagram of a vehicle having a brake system with a brake rotor temperature estimation apparatus according to the invention.  
         [0009]    [0009]FIG. 2 is a flow chart illustrating the operation of the brake rotor temperature estimation apparatus of the brake system shown in FIG. 1.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0010]    Referring to FIG. 1, a vehicle  10  is provided with wheels  12  and a brake system having brake units  14  associated with wheels  12 . The brake units  14  are preferably disk brake actuators in which brake rotors are clamped by calipers having brake pads or linings when the caliper is activated by a hydraulic piston. The brake units may be activated from a central master cylinder but may preferably include individual anti-lock (ABS) apparatus as is well known in the art for sensing wheel slip and modulating individual wheel brake pressure to reduce excessive wheel slip. Alternatively, the brake units may include their own hydraulic pressure source in a brake by wire system, or be activated by electric or air power. Brake units  14  are preferably provided with built in wheel speed sensors, commonly used for sensing wheel slip in ABS or traction control modes of operation and capable of providing sufficient information for derivation of vehicle speed and acceleration or deceleration.  
         [0011]    A brake rotor temperature estimator  16  is provided with the wheel speed signals from brake units  14  as well as an ambient temperature signal from ambient temperature sensor  18 . Sensor  18  may most economically be such a sensor already existing on the vehicle, such as an engine intake manifold air temperature sensor or a sensor for an outside temperature display; but a separate temperature sensor such as a thermocouple on the vehicle body close to the wheel is preferred. Estimator  16  is preferably part of a brake/traction control system controller, although it could alternatively be a stand alone device. Estimator  16  preferably comprises a digital computer having a microprocessor, memory and I/O apparatus and storing a program or routine controlling the process of estimating brake rotor temperature from the input signals and outputting the estimated brake rotor temperature and passing it on to apparatus requiring it, such as, for example, a traction control program in the same computer, wherein the estimated brake rotor temperature may permit the traction control routine to reduce or cancel traction control if the brakes become overheated. The signal may additionally be used in other ways as an over-temperature indicator or warning in vehicle  10 .  
         [0012]    The operation of estimator  16  is described with reference to the flow chart of FIG. 2. The routine operates individually on each brake unit of the vehicle and derives a brake rotor temperature for each brake unit independent of the other units. Routine BRAKE ROTOR TEMP starts at step  50  by reading the required inputs. These include wheel speed signals from brake units  14  and the ambient temperature signal from sensor  18 . From the wheel speed signals of undriven wheels, the routine can calculate vehicle speed (the average of the signals) and vehicle deceleration (derivative of the vehicle speed). The latter can be derived simply and reasonably accurately, if loop time and sample rate are sufficiently fast, by taking the difference between consecutive vehicle speed values.  
         [0013]    The embodiment described has two active braking modes: (1) ABS or anti-lock mode, wherein the ABS control of a wheel overrides the operator input as required to limit wheel slip, and (2) base brake mode, wherein a wheel is under operator control through the brake pedal input. The embodiment has, in addition, one inactive braking mode, in which the brakes are not activated. At any given time, different wheels of the vehicle may be in different active braking modes. For example, the operator may be controlling three of the four brake units in base brake mode while the ABS is in control of the fourth wheel, which has excessive slip due to a patch of ice.  
         [0014]    Proceeding to step  52 , the routine determines if ABS is active for the wheel. If it is not, an ABS Temp value is set equal to zero in step  54 . If ABS is active, an ABS Temp value is calculated for a front wheel in step  56  as follows: 
           ABS  Temp ( i )= ABS   1 *( ABS   2 +Decel)*Decel* WS  ( i ), 
         [0015]    wherein ABS 1  and ABS 2  are positive calibration constants, Decel is the determined vehicle deceleration (positive if decelerating) and WS is wheel speed for wheel i=1,2,3,4. Likewise, also in step  56 , an ABS Temp value is calculated for a rear wheel as follows: 
           ABS  Temp ( i )= ABS   3 *( ABS   4 −Decel)*Decel* WS  ( i ), 
         [0016]    wherein ABS 3  and ABS 4  are additional positive calibrated constants. In addition, ABS 4  is chosen so that the difference (ABS 4 −Decel) will be positive for any realizable value of Decel. These equations provide a contribution to brake rotor temperature increase that is compensated for load shift from rear to front wheels during vehicle braking. The load shift causes the front wheels to do more of the braking work and thus generate more heat. The amount increases with deceleration; and thus the value of deceleration is added to constant ABS 2  for the front wheel but is subtracted from constant ABS 4  for the rear wheel. For each wheel, the intermediate value obtained is multiplied times identical other factors: a scaling constant (ABS 1  or ABS 3 ), vehicle deceleration and wheel speed. The presentation as separate equations with all positive calibrated constants is for ease of reading and understanding. The same result may be accomplished more efficiently with a single equation as in the first equation above (i.e. using the sum: ABS 2 +Decel) with different sets of values for ABS 1  and ABS 2  for the front and rear wheels. The set of constant values for the front wheels will be positive; but the set of constant values for the rear wheels will be negative in order to produce the difference (ABS 2 −Decel).  
         [0017]    From step  54  or step  56  the routine proceeds to step  58 , wherein it is determined if the Base Brake mode is active for the wheel. This mode is active for a wheel when the operator is applying the brakes and the operator chosen braking force is not overruled by another braking mode such as ABS at the wheel. If the Base Brake mode is not active, a value Base Temp is set equal to zero at step  60 . But if the Base Brake mode is active, a value of Base Temp is calculated at step  62 .  
         [0018]    The calculation of Base Temp is similar to that of ABS Temp. For the front wheel: 
         Base Temp ( i )=Base 1 *(Base 2 +Decel)*Decel* WS  ( i ), 
         [0019]    wherein Base 1  and Base 2  are positive calibration constants, Decel is the determined vehicle deceleration (positive if decelerating) and WS is wheel speed for wheel i=1,2,3,4. Likewise, also in step  56 , Base Temp is calculated for a rear wheel as follows: 
         Base Temp ( i )=Base 3 *(Base 4 −Decel)*Decel* WS  ( i ), 
         [0020]    wherein Base 3  and Base 4  are additional positive calibrated constants. In addition, Base 4  is chosen so that the difference (Base 4 −Decel) will be positive for any realizable value of Decel. As with the similar equations for ABS Temp, these equations provide a contribution to brake rotor temperature increase that is compensated for load shift from rear to front wheels during vehicle braking; and a similar result may be accomplished more efficiently with a single equation in the same manner.  
         [0021]    From step  60  or step  62 , the routine proceeds to step  64 , wherein a value for the cooling effect of air on the rotor is calculated. A value of Cool Temp (i) is derived for each wheel by the following equation: 
         Cool Temp ( i )=(Cool 1  +Cool 2 * WS  ( i ))*(Rotor Temp ( i )−Amb Temp), 
         [0022]    wherein Cool 1  and Cool 2  are calibrated constants, Rotor Temp (i) is the most recent value of the estimated rotor temperature and Amb is the ambient temperature as reported by sensor  18 . The cooling effect is essentially proportional to the wheel speed and to the difference between the rotor temperature and ambient temperature.  
         [0023]    From step  64 , the routine proceeds to step  66 , wherein the value of estimated rotor temperature is updated according to the following equation: 
         Rotor Temp ( i )=Rotor Temp ( i )+ ABS  Temp ( i )+Base Temp (i)−Cool Temp ( i ). 
         [0024]    This is an updating equation in which the new value of Rotor Temp, for each wheel, is the sum of the previous value of Rotor Temp with updating factors for ABS Temp, Base Temp and Cool Temp. For a given wheel at any time, if the braking mode is inactive (no braking), the first two of these factors will be zero; otherwise, there will be a non-zero value for the braking mode that is active and a zero value for the one that is not active. It is not anticipated that ABS Temp and Base Temp will both be non-zero, since by definition the ABS mode and the Base Brake mode are mutually exclusive. But the cooling effect is always present when the brake temperature is greater than ambient and wheel speed is greater than zero.  
         [0025]    In addition, the equation given above for Rotor Temp (i) may be expanded to include other braking modes, including traction control, vehicle stability control, etc. This is done most simply by the addition of a new term in the equation for each additional, mutually exclusive braking mode and additional code for determining the value of the term based on the main factors producing brake rotor heat in that braking mode and some calibrated constants.  
         [0026]    The routine is repeated recursively on a specified time basis so that the estimated brake rotor temperature Rotor Temp will closely approximate the changing actual brake rotor temperature for each wheel. The recursive process may be begun when vehicle operation is initiated by setting the estimated rotor temperature Rotor Temp equal to the ambient temperature Amb Temp.  
         [0027]    Some vehicles may be provided with brake pressure sensing capability: either a single pressure sensor of master cylinder pressure or separate pressure sensors for the individual brake units  14 . In the case of individual brake units, a brake pressure signal from such a sensor would preferably replace the use of vehicle deceleration in the Base Brake mode and the ABS mode. If a single, master cylinder based pressure sensor is used, it would preferably replace the use of vehicle deceleration in the Base Brake mode but could not replace it in the ABS mode. Moreover, for those braking modes wherein brake pressure signals indicate the actual brake pressure in front and rear brake units, there is no need to compensate for load shift in braking. But the cooling effect would still be required at all times, whether a braking mode is active or not.