Abstract:
An anti-skid control system for use with vehicle brakes which system comprises a control device which actuates a valve to interrupt the passage of brake pressure to a brake chamber in a cyclic manner should the wheel associated with the brake chamber tend to lock or skid. A pump generates a control signal according to the rotational speed of the wheel and the device compares this signal with a signal derived from the brake pressure. The control device operates in conjunction with a number of further valve devices to initiate the aformentioned cyclic interruption of brake pressure to the brake chamber should the control signal fall below a pre-set threshold level and this control sequence repeats itself so the brakes are successively applied and released until the wheel ceases to skid so that the control signal again rises above the threshold level.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system for automatically controlling a vehicle braking system to inhibit wheel locking, i.e., to a so-called anti-skid control system. The invention is particularly, but not solely, concerned with motor vehicles especially commercial vehicles with pneumatic braking systems. Various forms of anti-skid systems have been proposed which operate electrically. Generally such systems are expensive and a general object of this invention is to provide a non-electrical system which is relatively cheap and simple yet reliable and robust in operation. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a control system for automatic control of a vehicle braking system to inhibit wheel locking: said control system comprising means for generating a first pressure control signal from the rotation of a vehicle wheel, a valve incorporated in the brake pressure flow path and capable, when subjected to a second pressure control signal, to interrupt said flow path and means for automatically actuating said valve to periodically interrupt said flow path when the first pressure control signal indicates wheel locking said means including a control device which is fed with said first pressure control signal and with a further control signal dependent on the application of brake pressure, the control device producing said second pressure control signal to actuate the valve and interrupt said flow path when the further control signal is greater than the first control signal thereby signifying wheel locking. 
     Preferably the invention provides a control system for automatic control of a vehicle braking system to inhibit wheel locking: said control system comprising means for generating a pressure control signal in dependence upon the rotation of a vehicle wheel; first valve means with an input for receiving brake pressure and an output for application of said brake pressure to a brake chamber associated with said wheel, the first valve means being settable into a first state where communication between the input and output is established so that the brake pressure can be transferred through said valve means to said brake chamber or a second state where communication between the input and output is blocked and the passage for brake pressure interrupted; a control device having a first control input for receiving said control signal, a second control input for receiving pressure at least derived from the brake pressure and input and outputs, one of said outputs being connected to said valve means to cause the latter to change to the second state when pressure appears at said one output, said control device being settable into different states to establish selective communication between various groups of input and outputs in dependence upon the pressure differential between the first and second control inputs; and further valve means connected to at least some of the inputs and outputs of the control device and co-operating therewith to initiate a control sequence whereby the first valve means is changed from the first to the second state and back to the first state should said control signal signify wheel locking. 
     The generating means can be a high-frequency diaphragm pump and the control device can be a multi-port rapid-action diaphragm valve which can be set into one of two states with selective communication between the ports depending on the differential pressure between its control inputs. 
     Preferably the first valve means has a second output vented to atmosphere and this second output communicates with the first-mentioned output when the first valve means is in the second state. 
     The invention may be understood more readily and various other features of the invention may become more apparent from consideration of the following description: 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The present invention will be more fully understood by reference to the following detailed description thereof, when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements, and wherein: 
     FIG. 1 is a block schematic diagrammatic representation of a control system according to the present invention, and 
     FIG. 2 is a partial schematic partial diagrammatic representation of another embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     In the control system described hereinafter various pneumatic valves are provided and in the drawing and following description a somewhat pictorial reference is made to the valves as having connecting elements or paths to represent the operative state of the valves. It is to be understood that such references are not intended to relate to the actual operation or construction of the valves but are used for convenience in simplifying the understanding of the control system. It should also be appreciated that although the system is described primarily in connection with pneumatic braking systems it can equally well be adapted for use with hydraulic braking systems. 
     The system as depicted in the drawing utilizes the rotation of a vehicle wheel to produce a pneumatic control pressure signal indicative of rotational speed. In the drawing the reference numeral 10 designates the vehicle wheel and the reference numeral 11 designates a rotation-sensing device adapted to drive a pump to generate the pressure signal. Usually each wheel would have such a device allocated to it. The device 11 may have a rotary element in frictional engagement with the brake drum of the vehicle wheel 10. This rotary element may be carried by a spindle which is coupled to an input shaft 70 of a high-frequency diaphragm pump 13. Other arrangements can be adopted to drive the pump 13 from the wheel 10. The pump 13 itself has an inlet 9 for admitting atmospheric air and an outlet 8 at which the aforesaid control pressure signal is generated. This outlet is connected via a non-return valve 14 to a conduit 63. The arrangement is such that the diaphragm of the pump 13 oscillates at an appreciable speed even when the wheel is rotating slowly as would be the case in heavy traffic. Gearing can be provided to increase the rotational speed of the input shaft 70 if desired. 
     As can be appreciated, in general, if the wheel 10 is stationary there will be no pressure signal on the outlet 8 of the pump 13 and hence in the conduit 63, whereas so long as the wheel rotates there is a pressure signal at the outlet 8 of the pump 13. There is also a gradual reduction in the pressure of the control signal at the pump outlet 8 as the rotational speed of the wheel 10 falls. 
     Turning now to the remainder of the system a notional main pressure supply or source 22, which is only active when the vehicle brakes are applied, is connected via a conduit 57 to an input port 35 of a valve device 20 and via a conduit 58 to an input port 85 of a valve device 19. A branch conduit 59 leads from the conduit 58 to an input port 41 of a valve device 18. The valve device 20 has an input/output port 32 which is connected through a conduit 62 to the brake chamber 15 associated with the wheel 10 and connected in a known manner to a brake-actuating mechanism denoted 12. The valve device 20 has a further output port 33 which is connected to atmosphere, i.e., to exhaust designated E. The valve device 20 has a connecting element or path 69 which can establish communication between the ports 32, 35 in a first position or between the ports 32, 33 in a second position. The two positions of the connecting element 69 represent the operative state of the device 20 and similar references are used hereinafter in connection with the other valve devices. The element 69 is biased by a spring 30 into the first position and the biasing action of the spring 30 can be overcome to set the element 69 to the second position when pressure is applied to a control input 34 to the valve device 20. With the element 69 in the first position the brake pressure can pass from the conduit 57 to the conduit 62 whereas with the element 69 in the second position the brake pressure is cut off and the conduit 62 is connected to atmosphere via the port 33. The outlet 8 of the pump 13 besides being connected to the conduit 63 is also connected to an adjustable bleed device 16 which vents to atmosphere and can be manually adjusted and pre-set to establish a correct range of operative pressure in the conduit 63. The conduit 63 is connected to a first control input 51 of a six port control valve device 17. A branch conduit 70 connects the conduit 59 to an input port 46 of the valve device 17 and the device 17 has a complementary output port 45 connected to atmosphere or exhaust. An input/output port 47 of the device 17 is connected via a branch conduit 71 to a conduit 60 which in turn is connected to the control input 34 of the valve device 20. The valve device 17 has an input port 49 which connects with a conduit 65 via a branch conduit 86 and an output port 50 which is connected to atmosphere. An input/output port 48 of the valve device 17 is connected to a conduit 64. The valve device 17 has two connecting elements or paths designated 72, 73. The element 72 can establish communication between the ports 48, 46 in a first position or between the ports 48, 45 in a second position. In an analogous manner the element 73 can establish communication between the ports 50, 47 in a first position or between the ports 49, 47 in a second position. The valve device 17 has a second control input 52 and incorporates a rapid-action diaphragm which is sensitive to the pressure differential between the inputs 51, 52 to set the elements 72, 73 into either their first or second positions. The control input 52 is connected via a conduit 66 to an input/output port 53 of a pre-set pressure regulating valve device 21. This device 21 has an input port 55 connected to the conduit 65 which also connects with an input/output port 36 of the valve device 19. The device 21 has an output port 56 connected to atmosphere and a connecting element or path 74 which can establish communication between the ports 53, 55 in a first position or between the ports 53, 56 in a second position. A pre-settable spring 88 serves to bias the element 74 of the device 21 into the second position and the action of the spring 88 is overcome when pressure prevails at the port 55 to cause the element 74 to adopt its first position. The spring 88 primarily serves however to adjust the pressure applied to the input 52 of the device 17 when the system is initially set up. 
     The conduit 60, which connects to the port 47 of the valve device 17 as well as the control input 34 of the valve device 20 is also connected to a control input 44 of the valve device 18. In addition to the input port 41 the device 18 has an output port 42 connected to atmosphere and an input/output port 43 connected via a conduit 61 to a control input 39 of the valve device 19. In a similar manner to the device 20, the device 18 has a connecting element or path 75 which can establish communication between the ports 43, 42 in a first position or between the ports 43, 41 in a second position. A spring 40 biases the element 75 into the first position and the device 18 has a control input 44 which is connected to the conduit 60, which it will be recalled connects via the conduit 71 with the port 47 of the valve device 17. Pressure at the input 44 serves to overcome the biasing action of the spring 40 to set the element 75 to the second position. The valve device 18 however is designed to have a time delay so when pressure is applied to the input 44 to change the element 75 from the first to the second position this change over takes place after a pre-set time. 
     Finally, the valve device 19 has, in addition to the ports 85, 36 and the control input 39, a control input 38 which is connected to the conduit 64 leading to the port 48 of the valve device 17 and an output port 37 which is connected to atmosphere. The valve device 19 has a connecting element or path 76 which can establish communication between the ports 37, 36 in a first position and between the ports 85, 36 in a second position. The presence of pressure at the input 39 causes the element 76 to adopt the first position and the presence of pressure at the input 38 causes the element 76 to adopt the second position. The control system as described functions to prevent the wheel 10 from locking when the brakes are applied to thereby inhibit skidding. To achieve this the control system continually monitors the rotational speed of the wheel 10 and if the speed decreases faster than a predetermined amount, signifying tendency to lock, the system momentarily cuts-off the braking pressure and exhausts the brake chamber 15 before allowing the pressure to be re-applied. This cut-off and re-application of the brake mechanism may occur rapidly a number of times in succession until the locking of the wheel ceases. 
     The operation of the system is as follows: 
     Assume initially that the wheel 10 is rotating so that pressure prevails in the conduit 63 due to the pump 13 and the brakes are not applied. Under these conditions the pressure at the input 51 of the valve device 17 causes the connecting elements 72, 73 to adopt the first position shown in full outline. Since the brakes are not applied no pressure prevails in the conduits 57, 58 or 59. The connecting element 69 of the valve device 20 is maintained by its spring 30 in the first position shown in full outline. Similarly the connecting element 75 of the valve device 18 is also maintained by its spring 40 in the first position shown in full outline. No pressure is applied to either input 38, 39 of the valve device 19 but the connecting element 76 of this device 19 is biased under these conditions to adopt the first position shown in full outline. No pressure is applied to the input 55 of the regulator device 21 so the connecting element 74 of this device assumes the second position shown in dotted outline. Assume now that the brakes are applied so that pressure is now present in the conduits 57, 58 and 59 and no wheel locking occurs under braking. Under these conditions the brake pressure is conveyed through the conduits 57, 62 and the valve device 20 to the brake chamber 15. The pressure in conduit 59 is applied to the input port 46 of the valve device 17 and is conveyed from the port 48 thereof to the conduit 64 and thence to the control input 38 of the valve device 19. This causes the connecting element 76 of the device 19 to adopt the second position shown in dotted outline so that the pressure applied to the input port 85 of the valve device 19 by the conduit 58 is conveyed to the port 36 and thence through the regulator device 21, the connecting element 74 of which assumes its first position, in full outline, to the control input 52 of the valve device 17. Since no locking has occurred the pressure at the input 51 exceeds that at the input 52 and this condition can be ensured by appropriate adjustment of the devices 21, 16. It follows therefore that the connecting elements 72, 73 remain in their first positions. The connecting element 75 of the valve device 18 also remains in its first position. If during braking the wheel 10 should suddenly lock so its rotational speed decreases rapidly the pressure at the input 51 of the device 17 will fall until the pressure at the input 52 of the device 17 exceeds it and the connecting elements 72, 73 will now assume their second positions shown in dotted outline. In this case the pressure prevailing in the conduit 65 will be conveyed from the input port 49 of the device 17 to the port 47 and thence via the conduit 71 to the control input 34 of the device 20. 
     The effect of this pressure at the input 34 of the device 20 causes the connecting element 69 thereof to adopt its second position, shown in dotted outline, to connect the brake chamber 15 to the port 33 and to atmosphere to momentarily cut off the brake pressure. The pressure in the conduit 60 is also connected to the control input 44 of the valve device 18 to cause the connecting element 75 of this device to assume its second position shown in dotted outline after a certain time delay. The effect of this is to convey the pressure in the conduit 59 through the ports 41, 43 of the device 18 and to the conduit 61 and thence to the control input of the valve 39. The pressure formerly applied to the input 38 of the device 19 is now no longer present since the ports 46, 48 of the device 17 are not connected so that the connecting element 76 of this device 19 now assumes the first position shown in full outline to thereby remove the pressure from the input 52 of the device 17. The pressure at the input 51, albeit low now prevails to trip the connecting elements 72, 73 back to their former first positions to remove the pressure in the conduits 60 and to allow the connecting elements 75, 69 of the devices 18, 20 to revert to their first positions. The brake pressure is then re-applied via the conduits 57, 62. The pressure at the input 39 of the device 19 is removed while the pressure at the input 38 is re-established to cause the connecting element 76 to change back to its second position and thereby pressure is re-applied to the input 52. The pressure at the inputs 51, 52 of the device 17 are again effectively compared and if the pressure at the input 52 is still greater the above sequence is repeated and indeed many cycles of operation, with a periodic time determined by the inertia of the mechanisms and the time delay of the valve device 18, may take place with the effect that the brake pressure is rapidly applied and exhausted to inhibit the tendency to lock whereupon the pressure at the input 51 should eventually prevail until the static condition described above re-establishes itself. 
     In the case where the wheel 10 is stationary and no braking pressure is established then the connecting elements 69, 75 of the valve devices 20, 18 assume their first positions while the connecting element 74 of the device 21 assumes its second position. Although no pressure is present at the inputs 51, 52 of the device 17 or at the inputs 39, 38 of the device 19 the device 17 is biased under these conditions so that the connecting elements 72, 73 assume their second position and, as mentioned previously, the device 19 is biased so that the connecting element 76 assumes its first position. In this way the supply of pressure during braking takes place normally through the valve device 20 since the input/output port 47 leading to the control input 34 of the valve device 20 is connected to the output 37 of the valve device 19. Only when the wheel rotates does the system become enabled for control and the above-mentioned operative conditions establish themselves. 
     In order to improve the speed of response of the system to wheel locking the brake chamber 15 can incorporate a diaphragm and two working chambers one of which is pressurized to cause the application of the brake and the other of which is pressurized to cause the positive release of the brake. A further valve device, similar to the device 20 but operating in a contrary sense thereto would feed the release working chamber so that the application of pressure to the input 34 of the device 20 which causes the exhaust of the main working chamber occurs simultaneously with the application of pressure to the corresponding control input of this other valve device to cause the latter to open communication between the conduit 57 and the release working chamber. This modification is represented in the scrap view at the bottom of the drawing where the reference number 90 denotes the release chamber, 91 the main working chamber, 92, the diaphragm, and 93 the associated brake mechanism. The additional valve device is designated 99 and has a control input 94 leading to the conduit 60, an input/output port 95 connected to the release chamber 90, an input port 96 connected via a conduit 97 to the conduit 57 and an output port 98 connected to atmosphere. The valve device 99, similarly to the device 20, has a connecting element 100 which is biased by a spring 101 into a first position, shown in full line, to connect the ports 95, 98. The element 100 can be switched into a second position, shown in dotted outline, by the application of pressure to the control input 94. 
     The operation of the modified system can be readily appreciated from the description above.