Method and system of controlling braking pressure in a vehicle brake
Reference to related patents, the disclosure of which is hereby incorporated by reference: U.S. Pat. No. 3,888,550, U.S. Pat. No. 4,054,328. 
     The present invention relates to vehicle brake control systems, and to a method of controlling vehicle brakes, and more particularly to brakes using a compressed fluid, for example hydraulic fluid, which are installed in a system which prevents blocking of wheels upon application of braking pressure. 
     BACKGROUND 
     Various types of anti wheel blocking braking systems are known. Usually, such systems include transducers coupled to the wheels which provide output signals representative of speed of the wheel which are applied to suitable evaluation circuits and which process the signals, for example to obtain vehicle acceleration or deceleration signals, rate of change of acceleration/deceleration, and the like; the evaluation circuit then provides control signals applied to valves which, selectively, apply pressurized brake fluid to the wheel brake cylinders, drain pressurized brake fluid therefrom, or maintain a predetermined braking pressure by blocking both admission as well as drainage of brake fluid from the cylinders, once pressurized brake fluid has been applied thereto. Some systems of this type utilize control apparatus which provides change in braking pressure in steps, for example by pulses which, in pulsed intervals, increases the braking pressure. 
     Circuits and evaluation signals of this type are known, see, for example, the referenced U.S. Pat. Nos. 3,888,550 and 4,054,328. In accordance with the arrangement there disclosed, braking pressure is first applied, and, if the wheel tends to block, a relief of braking pressure is then commanded. After relief of braking pressure, that is, drop of braking pressure, a certain braking pressure may be maintained and then, again, increase gradually. The rate of increase of braking pressure, after a drop, and subsequent maintenance of braking pressure may be in accordance with a gradient or rate of change which is smaller than the initial braking pressure build-up. 
     Various circuits may be used to carry out the transfer functions desired between commanded braking pressure and desirable braking pressure in the light of vehicle characteristics and a predetermined program of build-up, relief, and/or maintenance of braking pressure. For example, in accordance with the disclosure of the referenced patents, a control pulse is generated which causes rapid increase of braking pressure, to be followed by a sequence of suitably arranged pulses which result in further increase of braking pressure at a lower rate, however, since there will be gaps between the pulses during which the braking pressure is maintained. In accordance with U.S. Pat. No. 3,888,550 a fixed timing interval is controlled, for example by a timing circuit which provides for a fixed time interval, which commands rapid increase of braking pressure. In U.S. Pat. No. 4,054,328, the rapid increase of braking pressure is made dependent on the degree of a prior build-up of braking pressure in a prior control cycle. The purpose, of course, is to optimize the increase in braking pressure and thus to optimize the overall braking effect to be obtained by the system. 
     THE INVENTION 
     It is an object to improve automatic brake anti-lock or anti-skid systems, and particularly to avoid possible vibrations or oscillations which might arise under some conditions if the braking cycles or sequences as previously proposed are carried out. 
     The invention is based on the discovery that, upon change in braking pressure after an initial rise and subsequent drop, and consequent acceleration of a previously strongly braked wheel, the wheel may undergo speed or acceleration/deceleration oscillations if stepped rise and drop in braking pressure is again commanded. Briefly, therefore, and in contrast to the prior solutions to prevent blocking of wheels, and hence skidding of a vehicle, a control pulse is generated which, after previous braking, controls the braking system to build up braking pressure only to a small extent, smaller than heretofore used, to be followed by a short pressure maintenance period, and only then to be followed by a further rise in braking pressure. The basis for this method is the discovery that a comparatively control pulse, resulting in a comparatively large increase in braking effort applied to any wheel--after the wheel has already excessively decelerated--may lead to an oscillatory sequence, particularly if the construction of the wheel, and the suspension thereof, is soft, and/or the wheel and the associated axle have compartively little inertia. 
    
    
     DRAWINGS 
     FIG. 1 is a pressure and speed diagram, with respect to time, illustrating anti-skid braking control in accordance with the prior art; 
     FIG. 2 is a diagram similar to FIG. 1, illustrating control in accordance with the present invention; 
     FIG. 3 is a schematic block circuit diagram illustrating an apparatus to carry out the braking sequence, or method, of FIG. 2; and 
     FIG. 4 is a schematic representation of a brake pressure fluid circuit. 
    
    
     DETAILED DESCRIPTION. 
     The operation of an anti-skid or anti-brake lock system in accordance with the prior art is shown in FIG. 1, which will be explained in detail since, then, the improvement effected by the present invention will be clear. The ordinate represents, respectively, speed or pressure, in accordance with the speed or pressure curves shown in the Figure, speed and pressure varying with respect to the time axis of the abscissa. The axes of FIG. 2 represent the same parameters. 
     The curve V F  represents vehicle speed, which is dropping smoothly as braking is controlled. The curve V R  represents the speed of an individual wheel which is controlled by the anti-skid or anti-block system. It is assumed that the vehicle in which the system is installed has a suspension construction such that the axles and wheels have low mass, and that the suspension is highly elastic, specifically longitudinally highly elastic. Upon increase of braking pressure subsequent to a deceleration/acceleration cycle of the wheel, oscillations may result as shown in the full-line position of wheel speed. The desired wheel speed curve is, however, in accordance with the broken-line portion of FIG. 1. As best seen in FIG. 1, the braking pressure is first increased up to a time t 1 , and then a drainage valve is controlled to permit a decrease in braking pressure, see curve P, time period t 1  to t 2 . Thereafter, a constant brake pressure maintenance period is commanded, time period t 2  to t 3 . When the wheel has accelerated to a predetermined level, or after a predetermined period of time, in accordance with the system which is selected, the braking pressure is again increased during a time interval T 1  which corresponds to the time period t 3  to t 4 . The time period T 1  is relatively long, and since the available pressure from a hydraulic or other fluid pressure source continues to be available at a level higher than that of the braking pressure P, the braking pressure P in the specific wheel will rise rapidly. This causes a deceleration of the wheel which, then, will again trigger a decrease in braking effort, see drop of braking pressure P to the time period t 5 . The actually required pressure increase will only occur subequently, that is, after time t 5 , see time period T 1&#39; , and subsequent stepped pressure increases, see T 2 . As can be clearly seen from FIG. 1, the pressure increase is in steps T 2  which are followed, each, by pressure maintenance phases so that the effective pressure increase, over a period of time, will be slow and gradual--see portion of curves beyond T 2 . 
     In accordance with the present invention--see FIG. 2--the pressure increase after period t 3  is much less than heretofore used, see FIG. 2, time period t 3  &#39;. The available pressure is shown in FIG. 2. Up to point A, that is, initial application of braking pressure to the wheel, the available and wheel pressure will be the same. The available pressure, for example from a hydraulic pressure reservoir, will then be disconnected, and the pressure of the fluid applied to the wheel will follow the curve P from point A. 
     In accordance with the present invention, thus, and starting at the time period t 3  &#39;, pressure is first raised only for a very short time interval T 2 , to be followed by a pressure maintenance period T 3 . Only after the initial small pressure rise during time T 2  and the subsequent maintenance phase T 3 , will the greater or more extensive pressure rise follow, as shown by the time T 1 . The pressure rise T 1  may correspond to that of T 1 , or T 1  &#39; of FIG. 1, or be slightly less. The pressure maintenance time T 0 , preferably, is longer than the time gap between time t 5  (FIG. 1) and the subsequent pressure rise, T 1&#39; . 
     In the system of the present invention, see FIG. 2, the time T 0  is then followed by small stepped pressure rises, see time T 2  and subsequent pressure maintenance phases or time periods T 4 . The behavior of the wheel will be different from that shown in FIG. 1, and, as is clearly seen in FIG. 2, the wheel will decelerate slightly, but the oscillatory or undulating behavior or the wheel will be eliminated. A very slight overshoot of braking effort may occur, but, since the wheel deceleration is more gradual, the overshoot is, effectively, eliminated, thus eliminating oscillations of wheel speed. 
     The system to carry out this sequence of steps is shown in FIG. 3, to which reference will now be made. A wheel speed sensor 1, of any suitable and customary construction--see, for example, the referenced patents--is coupled to a signal processing stage 2 in which the wheel speed signals are differentiated and otherwise processed to provide acceleration and deceleration signals +b and -b, to be applied to a control logic 3. The control logic 3 is controlled by a wheel slip or brake lock control stage or circuit 4, for example in accordance with the systems described in the referenced patents, to provide a signal to the logic circuit 3 when the wheel speed has a specific relative relationship with respect to calculated or derived reference levels. The logic circuit 3 controls respective amplifiers 5a, 5b which, in turn, control respectively opening and closing of a hydraulic fluid pressure inlet valve 6a and a hydraulic fluid pressure drain valve 6b. If valve 6b is closed while valve 6a is open, pressure will increase. Upon closing of valve 6a, the pressure will be maintained; upon opening of valve 6b, the pressure will drop due to drainage of pressurized fluid from a brake cylinder (not shown). 
     In accordance with a feature of the invention, two pulse generators 7, 8 are provided which furnish the pulses and pulse gaps T 2  and T 3 , as well as T 1  and T 2 , respectively. The term &#34;pulse generator&#34; is to be considered broadly. The physical structure need not necessarily be a specific pulse generator element, but it may be a suitably programmed microprocessor portion, for example a portion of an integrated microprocessor which includes the stage 2, control logic 3 and the wheel slip control stage 4, and provides, upon suitable recall of a program, pulses as defined above, namely with the pulses and pulse gaps T 2 , T 3  and T 1 , T 0 , respectively. Microprocessors which provide pulses of various widths are standard components, the programming of which is well known and likewise standard. Timing is provided by timing circuit 3&#39;. 
     FIG. 4, schematically, shows a brake fluid circuit in which a pressure source P provides pressurized brake fluid through the respective valves 6a, 6b (FIG. 3) to a wheel brake B. The delivered pressure is measured by a first pressure sensor S1; the actual braking pressure is measured by a second pressure sensor S2. The differential measured pressure, determined in a differentiator D, is then applied to control the respective timing intervals. 
     Operation, with reference to FIGS. 2 and 3: Let it be assumed that the operator applies braking pressure, causing the wheel to decelerate from an initial speed value to point I (FIG. 2). If this deceleration is sensed by the wheel slip control stage 4 to be in excess of a predetermined level, wheel braking pressure is then dropped, for example by opening of valve 6b and closing of valve 6a. The wheel speed will still decrease, however, due to inertia effects, until a point II is reached, at which time the wheel speed will again increase. Closing of valve 6b, with valve 6a likewise closed, will then maintain the pressure, causing an increase in wheel speed, to point III. 
     In accordance with the present invention, and as soon as the acceleration signal which is due to the increase of the wheel speed between points II and III terminates, for example at a given increase rate, or after a predetermined period of time, the pulse generator 7 is activated over control 9 from the signal processing stage 2, to provide, over line 11, a pulse T 2 , followed by a pulse gap T 3 , in order to first open the valve 6a for a very short period of time, thus resulting in only small increase of pressure, and then keeping both valves 6a, 6b closed to maintain the pressure for a somewhat longer time interval T 3 . The period of time after the wheel has again accelerated subsequent to deceleration, that is, a predetermined time interval subsequent to point II (see FIG. 2) can be controlled by timing circuit 3&#39;. 
     This predetermined time, occurring after termination of the deceleration signal -b, derived for example from the control logic, is available to energize the pulse generator 7 over control line 10. Thus, either termination of the acceleration signal +b, or a predetermined time after termination of the deceleration signal -b, will cause the pulse generator 7 to provide the pulse T 2  and the subsequent pulse gap T 3 . 
     Upon termination of the time interval T 3 , the pulse generator 8 is started, which now generates a pulse T 1  with subsequent pulse gap T 0 , which is transferred via line 12 to the control logic 3 to suitably control the amplifiers 5a, 5b and hence the valves 6a, 6b, to first open the pressure inlet valve 6a during time T 1 , and then close it, with valve 6b closed, to maintain pressure. Termination of the pulse T 0  reactivates the pulse generator 7 over line 14. The pulse generator 7, upon being activated over line 14, provides, however, for a longer pulse gap period, so that, rather than the pulse gap T 3 , a pulse gap T 4  will be commanded after the next pressure rise T 2 . The pulse generator 7, thus, controls, via line 13, the pulse generator 8 and, in turn, is controlled from pulse generator 8 by line 14. Alternatively, of course, a third pulse generator (not shown) may be provided, which has the pulse sequence T 0  -T 4  as the output, rather than a switchable pulse gap time, T 3  or T 4 , as shown. Since the pulse periods and pulse gap or pressure maintenance periods all can be controlled by program command, only the simplest form, with a switchable pulse generator system or arrangement, is shown in the drawings. 
     The pulse - pulse gap or pulse pause relationships with the time constants T 2  /T 4  then provides for gradual, slow increase of braking pressure, as shown in the period between points III and IV. If the wheel speed then should decrease too much, suddenly, the pressure can again be dropped after point IV, as shown by the pressure curve P. 
     Suitable time periods for a hydraulic braking system of a typical mid-size sedan passenger motor vehicle are: 
     T 0  : 60 milliseconds 
     T 1  : 6 ms 
     T 2  : 3 ms 
     T 3  : 15 ms 
     T 4  : 60 ms. 
     Various changes and modifications may be made within the scope of the inventive concept. 
     For example, the difference between then pertaining actual pressure at a brake of a wheel, which is represented by a signal in the control logic 3 or in the wheel slip control stage 4, can be compared with the available pressure from the standard vehicle pressure source. This pressure is connected, for example, to the wheel slip control stage 4 at a terminal AP&#39;. The available pressure curve AP (FIG. 2) deviates from the wheel pressure curve (P) starting at point A. This difference can be used to control the time intervals T 2 , T 1 , respectively, and/or also the time intervals T 3 , T 4 . Preferably, however, it is used primarily to control the length of the time interval T 2  to reliably insure that the wheel will not decelerate too much so that the actual wheel speed curve will be as shown in FIG. 2, and thus closely approach the curve shown in broken lines in FIG. 1. 
     The respective timing intervals of the brake signals can be sensed in elapsed-time circuits, such as a circuit 3&#39;, forming part of the control logic 3, or of any other suitable circuit component of the system.