Abstract:
A controller for an electric brake system in a vehicle. The brake system utilizes switched reluctance electric motors to apply braking force to wheels. Such motors have non-ideal torque-speed characteristics, wherein excessive amounts of time and current are required to change torque delivered. The invention reduces the times, and currents, by adjusting the phase angle, and durations, of current pulses delivered to the coils of the motor. The adjustment is based on several parameters, including presently demanded torque, speed of the electric motor, deviation of vehicle system voltage from a norm, and deviation of motor temperature from a norm.

Description:
The invention concerns a control system for optimizing performance of switched reluctance electric motors, when utilized in a braking system in a vehicle. 
     BACKGROUND OF THE INVENTION 
     Switched reluctance electric motors have the torque-speed characteristics generally shown in FIG.  1 . If torque of the motor is to be increased from T 1  to T 2 , as when increased braking force is to be applied to a wheel of the vehicle, the operating point will generally be constrained to follow path P. However, following this path requires a relatively large amount of time, which is disadvantageous when emergency braking is required. Also, following this path consumes an excessive amount of current, which is not important during emergency braking, but does become significant during normal operation. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a control system for a switched reluctance motor, which reduces the time required to change torque, and also reduces current consumed during the change. 
     In one aspect, this invention comprises an anti-lock braking system comprising an electric motor which applies braking force to a wheel, and means for predicting a future torque which will be demanded of said motor. 
     In another aspect, this invention comprises an anti-lock braking system comprising an electric motor which applies braking force to a wheel, means for receiving a signal indicative of the amount of presently demanded braking force, means for predicting a future braking force FBF which will be demanded, and means for controlling phase angle and duration of current applied to the electric motor, based on the FBF. 
     In yet another aspect, this invention comprises a system for use within a vehicle, comprising a switched reluctance electric motor which has native torque-speed curves which require a time T to execute an acceleration from a slow speed S 1  to a higher speed S 2 , and means for reducing the time T. 
     In still another aspect, this invention comprises a braking system for a vehicle, comprising a switched reluctance electric motor which has native torque-speed curves which slope downward, and applies braking force to a wheel, means for receiving a demanded torque signal produced by a control within the vehicle; and means for causing slope, d(torque)/d(speed), of the native torque-speed curves to become closer to zero. 
     In yet another aspect, this invention comprises a braking system within a vehicle, comprising a brake controller which accepts input indicative of driver pedal position, lateral acceleration, yaw rate, and rotational speed of each of four wheels, and produces a demanded torque signal for each wheel, a switched reluctance motor for each wheel, which applies braking force to the wheel, a motor controller for each wheel which receives the demanded torque signal for its wheel, computes terms, and utilizes the two terms (tau, sigma) to control phase angle and duration of current within the coils of the switched reluctance motor for the wheel. 
     In one form of the invention, both the (1) phase angle and (2) duration of current pulses delivered to the coils of a switched reluctance motor are adjusted, based on selected operating conditions of the vehicle, in order to reduce the time required for a change in torque delivered by the motor. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a torque-speed diagram for a switched-reluctance motor; 
     FIGS. 2 and 3 illustrate one form of the invention; 
     FIG. 4 illustrates block  80  in FIG. 2, in greater detail; 
     FIG. 5 illustrates block  85  in FIG. 2, in greater detail; 
     FIGS. 6 and 7 illustrate sectors defined about a SHAFT of motor  40  in FIG.  2 . Parameters sigma and tau in FIG. 4 produce these sectors. When the rotational angle of the SHAFT enters one of the sectors, a current is triggered into a corresponding coil within the motor  40 ; 
     FIG. 8 illustrates an idealized plot of torque-versus-speed; 
     FIG. 9 illustrates two particular excursions along two particular paths, P and P 1 , taken on the plot of the type shown in FIG. 1; 
     FIG. 10 illustrates a family of plots P 5 -P 7 , of the type shown in FIG. 9; 
     FIG. 11 illustrates how parts of the plots of FIG. 10, taken together, form the idealized plots of FIG. 8; 
     FIG. 12 illustrates another form of the invention; 
     FIG. 13 illustrates a limit which a prior-art system encounters; 
     FIG. 14 illustrates a path P 1  which the invention is able to take, which avoids the limit of FIG. 13; and 
     FIG. 15 compares the time required by the prior art system of FIG. 13 to reach a given motor shaft position with the corresponding time required by one form of the invention, which follows path P 1  in FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a brake control  2 , commonly used in Anti-lock Braking systems (ABS). It receives eight inputs: 
     (1) an indication  3  of position of the driver&#39;s brake pedal; 
     (2) an input  6  which indicates the speed demanded by an Intelligent Cruise Control, ICC; 
     (3) an input  12  indicative of the rate of yaw of the vehicle, which is produced by a yaw rate sensor  15 ; 
     (4) an input  18  indicative of lateral acceleration, which is produced by lateral accelerometer  21  (lateral acceleration is straight-line acceleration perpendicular to the direction in which the vehicle is pointing; yaw is rotation of the vehicle in the horizontal plane); 
     (5)-(8) inputs  23 ,  24 ,  25 , and  26  which indicate rotational speed of each wheel. 
     The wheels are indicated by the symbols RL (Right Left), RR (Right Rear), FR (Front Right) and FL (Front Left). 
     The control  2  controls each of these wheels in the same manner, so control of the FL (Front Left) wheel will be taken as exemplary of them all. 
     The control  2  produces a signal  30  which indicates the amount of torque demanded for the FL wheel. More specifically, a brake disc  34  is attached to the wheel FL. A caliper  37  applies braking force to the disc  34 . A switched-reluctance motor  40  moves the arms (not separately shown) of the caliper  37 , to thereby squeeze the disc  34 . The demanded torque in question is that demanded of the switched-reluctance motor  40  in this squeezing process. 
     The demanded torque signal is fed to a controller  60 , which controls the torque of the motor  40 . FIG. 3 illustrates this controller  60  in greater detail. The demanded torque signal  30  of FIG. 2 is fed to a summer S 1 . That summer S 1  also receives a signal  63  indicating the actual torque, presently being produced by the motor  40 . The actual-torque signal  63  is produced by block  64 . 
     Block  64  computes the actual-torque signal  63 , based on two parameters: (1) angular position, theta, of the shaft (not separately shown) of the switched-reluctance motor  40  and (2) current drawn by selected phases of the motor  40 . The angular position, theta, of the shaft is computed by block  70 , which represents an incremental shaft encoder (as opposed to an absolute shaft encoder), and is delivered on line  71 . Current is measured by an apparatus which is not shown, and is delivered on line  72 . 
     Summer S 1  computes a difference T(error) between (1) the demanded torque  30  and (2) the present actual torque. T(error) is fed to block  80 , which computes two parameters, sigma and tau, which are utilized by block  85 . FIG. 4 shows the detailed computation of delta and tau by block  80 . 
     FIG. 3 shows four computation blocks  90 A,  90 B,  90 C, and  90 D, which perform identical computations, but differ in the particular parameters and constants utilized. 
     Block  90 A (as well as the other three blocks  90 B,  90 C, and  90 D) implements a Proportional-Integral-Differential (PID) controller. Line  100  represents the proportional term, and carries a value of T(error) multiplied by gain K 1 . Line  105  represents the differential term, and carries a value of 
     
       
         {[present  T (error)−last  T (error)]×K 2 }/ DELTA —   T    
       
     
     wherein 
     present T(error) is the present torque-error signal; 
     last T(error) is the previous T(error) signal; 
     K 2  is a constant; and DELTA_T is the time-difference between present T(error) and last T(error), which corresponds to the time delay occurring one iteration of block  90 A and a subsequent iteration. 
     In overview, line represents the difference between present T(error) and the last T(error), divided by a time DELTA_T, which is a rate of change, or derivative. The derivative is scaled by K 2 . 
     Line  110  represents the integral term, and carries a value which represents a cumulative integral of the value of T(error). That is, for example, assume a sequence of T(error) signals, labeled T 1 , T 2 , T 3 , T 4 , T 5 , and T 6 . At the beginning of computation, line  110  will carry the signal of T 1 , because nothing will be stored by storage block  115 . Storage block  115  loads that value, T 1 . 
     At the next iteration, line  110  will carry T 2 −T 1 , and storage block  115  loads that value, T 2 −T 1 . 
     At the next iteration, line  110  will carry T 3 −(T 2 −T 1 ), and storage block  115  loads that value, T 3 −(T 2 −T 1 ), or, equivalently, T 3 −T 2 +T 1 . 
     At the next iteration, line  110  will carry T 4 −(T 3 −T 2 +T 1 , and storage block  115  loads that value, T 4 −T 3 +T 2 −T 1 ). 
     At the next iteration, line  110  will carry T 5 −(T 4 −T 3 +T 2 −T 1 ), and storage block  115  loads that value, T 5 −T 4 +T 3 −T 2 +T 1 . And so on. 
     Of course, the preceding was a simplification: the value carried by line  110  is scaled by the factor (K 3 ×DELTA_T). 
     Therefore, block  90 A receives, as input, the T(error) signal, and applies that input signal to a PID controller, to produce a parameter sigma. The other blocks  90 B,  90 C, and  90 D compute the parameter tau. 
     Bock  90 B receives, as input, w(t), which is the angular position of the shaft of motor  40 , which is equivalent to motor speed. 
     Block  90 C receives, as input, a voltage error signal. This represents the difference between the actual system voltage, produced by the combination of the vehicle&#39;s alternator and battery, and a reference voltage. 
     Block  90 D receives, as input, a temperature error signal, which indicates deviation of actual temperature from a reference temperature. The measured temperature  130  can be any temperature which provides a meaningful indication of the temperature of motor  40 . Temperature of the liquid coolant of the vehicle represents one candidate. 
     Blocks  90 B,  90 C, and  90 D apply these parameters to the respective PID controllers. The outputs of the PID controllers are added in summer  119 , which produces parameter tau. 
     The output of the computation of FIG. 4 are two parameters, delta and tau. These are fed to block  85  in FIG. 3, the details of which are shown in FIG.  5 . 
     The overall operation of the computation in FIG. 5 is this: the shaft of motor  40  in FIG. 3 continually travels through an angle theta, which runs in degrees from zero to 360, and then repeats. However, the units used in the computation are not degrees, but the units utilized by the shaft encoder  70  in FIG.  3 . Forty-eight encoder pulses correspond to 90 electrical degrees. The computation in FIG. 5 controls the timing of the current pulses fed to the phases of the motor  40 . For example, assume that delta equals 8 and tau equals 20. The output of summer  210  will be 16 (i.e., 20−0.5×8). The output of summer block  220  will be 24 (i.e., 20+0.5×8). 
     With these outputs of summer blocks  210  and  220 , the outputs of blocks  230 ,  235 ,  240  and  245  will be 16, 64, 112, and 160, respectively: the number 16 was added to zero in block  230 , to 48 in block  235 , to 96 in block  240 , and to 144 in block  245 . These values will be termed “sigma values.” 
     Similarly, the outputs of blocks  260 ,  265 ,  270 , and  275  will be 72, 120, 168 and 124, respectively. These values will be termed “rho values.” 
     Each sigma value is paired with a rho value, by the operation of the rank of comparator blocks  280 ,  285 ,  290 , and  295 . Each pair acts as a bracket, or upper-and-lower limit, in a process undertaken by the comparators  280 ,  285 ,  290 , and  295 . Each comparator  280 ,  285 ,  290 , and  295  inquires whether the present value of theta (i.e., angular shaft position of motor  40 ) lies within its bracket. 
     For example, assume shaft position to be  135 . The bracket-points for block  280  are  16  and  72 . The shaft position does not lie within that bracket. The phase corresponding to variable Sa is not fired. 
     The bracket-points for block  285  are  64  and  120 . The shaft position does not lie within that bracket. The phase corresponding to variable Sb is not fired. 
     The bracket-points for block  290  are  112  and  168 . The shaft position does lie within that bracket. The phase corresponding to variable Sc is now fired. 
     The bracket-points for block  295  are  160  and  24 . The shaft position does not lie within that bracket. The phase corresponding to variable Sd is not fired. Note use of digital or modulo base arithmetic, where  192  is the base (e.g., adding 4 and 190 equals 2). 
     Therefore, as thus far described, at any given time, a single delta and a single tau are computed, and delivered, to the apparatus represented in FIG.  5 . That apparatus derives (1) four variables from delta and tau (the “sigma values”) and (2) four values from each delta and tau (the “rho values”). 
     Each sigma value is paired with a specific rho value, and forms a bracket-pair. Each bracket-pair controls the time-of-firing of one phase within the motor  40 . The control is accomplished by comparing angular position, theta, with the bracket-pairs. When angular position, theta, enters a bracket, a respective phase fires, and as the angular position leaves the bracket, the phase is de-activated. 
     From another point of view, four sectors, 90 degrees apart, are derived from each delta-tau pair. Tau determines the angular position of the sectors, and delta determines the angular spread of the sectors. FIG. 6 illustrates four such sectors. Sector A runs from zero to 90 degrees; sector B from 90 to 180 degrees; sector C from 180 to 270 degrees, and sector D from 270 to 0 degrees. The units used here are degrees, for ease of explanation. 
     Alteration of delta and sigma allow the sectors to be shifted, as in FIG.  7 . There, delta equals 10 degrees, and tau equals 5 degrees. Sectors A 1 , B 1 , C 1 , and D 1  illustrate the now-changed sectors. They have moved in position, and increased in angular length. 
     Each sector controls firing of a respective phase, or coil. When shaft angle enters one sector, a phase activates for the duration that shaft angle remains within the sector. Similar operation occurs for the other phases. 
     In FIG. 6, firing for sector A initiates at zero degrees, and terminates at 95 degrees. One coil, or phase, A within the motor  40  is fired accordingly. Sectors B, C, and D each control another coil, such as coil B, coil C, and coil D. 
     In FIG. 7, the firing of coil A has changed: sector A 1  now initiates firing at 10 degrees, and terminates it at 110 degrees. The other sectors have also shifted in position, and duration, by five and fifteen degrees in each case. 
     The rank of comparators  280 ,  285 ,  290 , and  295  produces a second influence. The comparators can be paired. Those pairs which represent brackets, or sectors, which are 180 degrees apart, cooperate together. That is, comparator  280  cooperates with comparator  290 , through OR-gate  310 . Comparator  285  cooperates with comparator  295 , through OR-gate  315 . 
     The OR-gates enable, or activate, operation of two sub-circuits  320  and  325  within block  330 . These blocks  320  and  325  comprise a pulse-width modulation comparator stage, to determine the state of variables Sac and Sbd. This comparator stage is contained within a Texas Instruments model TMS-320C-240 processor, which is commercially available, and described in the Texas Instruments Tutorial Manual for this processor. This processor is utilized to perform the computations of FIGS. 4, and  5 . 
     This comparator stage  330  is also fed the demanded currents, Iac and Ibd, which are produced by block  335  in FIG.  3  and delivered on line  338 . These currents represent the demanded torque, but in units of current, as opposed to foot-pounds. The comparator stage  330  is also fed the actual measured currents Iac and Ibd. 
     Based on these input signals, comparator stage  330  produces signals Sac and Sbd which, together with signals Sa, Sb, Sc, and Sd, are fed to the drive unit  350  in FIG.  3 . 
     The apparatus just described allows one to modify the firing angle, and firing duration, of each coil within motor  40  in FIG.  2 . This modification allows one to effectively alter the torque-speed characteristics of the switched-reluctance, brushless, motor. 
     Every individual block in FIG. 3, with the exception of blocks  80 , shown in detail in FIG. 4, and block  85 , shown in detail in FIG. 5, is known in the art. However, as stated blocks  80  and  85  are not known in the art, nor is the overall system of FIG.  3 . 
     FIG. 8 illustrates the torque-speed characteristics of an idealized motor. FIG. 1 illustrates the characteristics of an actual switched-reluctance motor, as explained in the Background of the Invention. The “V” symbols refer to field voltage, which indicate field current, which is indicative of torque. 
     FIG. 9 is an annotated version of the type of plot in FIG.  1 . In FIG. 9, if one wished to make an excursion from operating point X to operating point Y, one may increase field current, and follow path P. However, in practice, it is found that following path P is time-consuming. Also, a large field current is required. 
     The invention allows one to follow path P 1 , by adjusting tau and sigma appropriately, to thereby adjust the positions of the sectors, shown in FIGS. 6 and 7, wherein the respective coils fire. That is, the invention allows one to alter the native curves, shown in FIGS. 1 and 9, which are inherent in the switched reluctance motor, to the idealized curves shown in FIG.  8 . 
     To explain this, the Inventor points out that Pi in FIG. 9 is a generalized path. A family of such paths P 5 -P 7  is shown in FIG.  10 . If one considers only parts of those paths shown in Figure, then it is clear that the invention causes an ordinary switched-reluctance motor, having the torque-speed curves shown in FIG. 1, to exhibit the more idealized torque-speed curves of FIG. 8, as FIG. 11 indicates. Appropriate selection of sigma and tau can stretch out the plots shown in FIG. 11, so that they resemble those in FIG.  1 . 
     These idealized torque plots, which are substantially horizontal when torque is plotted as a function of speed, are attainable by selection of appropriate sequences of tau and sigma, which are the parameters shown in FIG.  4 . 
     Prediction of Demanded Torque 
     The control system of FIG. 3 utilized presently demanded torque as an input parameter. FIG. 12 illustrates a system in which presently demanded torque is, in effect, replaced by a predicted future torque. 
     The components below double arrows  400  represent a standardized control system for motor-driven brakes. FIGS. 2 and 3 represent one system which can be used. 
     However, in FIGS. 2 and 3, presently demanded torque is the input. In contrast, in FIG. 12, a future, predicted torque demand is utilized as an input. That is, the predicted torque signal on line  405  in FIG. 12 replaces the torque input  30  in FIG.  3 . The predicted torque signal  405  in FIG. 12 is derived from the present torque demand signal  410 , which is produced by the brake controller  2  in FIG.  2 . 
     Restated, the brake controller  2  produces a presently demanded torque signal on line  30 . That signal  30  is fed to line  410  in FIG.  12 . Predicted torque  405  is produced, and is fed as the positive input to summer S 1  in FIG.  3 . 
     The computation represented by the components above double arrow  400  in FIG. 12 produces the predicted torque demand. In a simple case, the predicted torque is a linear extrapolation of previous torque demanded. An example of this extrapolation is obtained by setting parameters a 2  and a 3  in FIG. 12 to zero. That done, if a 0  is set to 2, and a 1  set to negative 1, then the computation becomes: 
     
       
         predicted  TD= 2(present  TD )−previous  TD    
       
     
     wherein 
     TD is torque demanded, 
     present TD is the torque presently demanded by the control system  2  in FIG. 2, and 
     previous TD is the torque demanded by the control system  2  during the last iteration. 
     This predictive approach introduces a type of time-shifting. That is, presently computed tau and sigma, which control firing angle and duration, become computed based on anticipated torque demand. 
     Additional Considerations 
     1. The plots of FIG. 1 refer to a “native” switched reluctance motor. The term “native” refers to the motor operated in its switched-reluctance mode, wherein speed is controlled by the timing of the pulses applied to the motor, and torque is controlled by the current applied in those pulses. 
     The invention allows the motor to attain a given transient in torque, such as that indicated by plot P 1  in FIG.  9 . This approach provides two primary benefits. One is that the time of the transient is reduced, compared with path P. A second is that current consumed by the motor is reduced, also compared with path P. 
     2. FIG. 1 is an idealized plot. FIG. 13 illustrates a real-world limit which motor speed encounters, in the absence of the tau-sigma control strategy of the invention, that is, with a fixed field voltage. Path P shows a torque excursion from T 1  to T 2 , using a prior-art system, and the limit which is encountered. 
     FIG. 14 illustrates a path P 1  which torque may take by use of the invention. Path P 1  is superimposed over a plot of the type shown in FIG. 8, and shows a torque excursion from T 1  to T 2 , using one form of the invention. 
     FIG. 15 compares the times taken by the excursions P and P 1  of FIGS. 13 and 14, in order to reach a given motor shaft position PM, which corresponds to a given amount of caliper compression, which corresponds to a given amount of braking torque. Using one form of the invention, path P 1  requires a shorter time to reach position PM, compared with path P. 
     While the methods herein described, and the forms of apparatus for carrying these methods into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claims.