Abstract:
An anti-skid brake control system and a failsafe circuit therefor wherein an open in either one of two wheel velocity pick-up coils is detected by comparing the sum of the coil currents with a constant current, and suppressing, in response thereto, a controller override on the manual. The controller is, thus, disabled when it should be, i.e. when its input from one or both of the coils is erroneous because of one or more opens therein.

Description:
This application is a division of copending patent application Ser. No. 495,227 filed Aug. 6, 1974, now U.S. Pat. No. 3,917,359. The benefit of the filing date of said copending application is therefore, hereby claimed for this application. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to brake control systems, and more particularly, to a system for preventing skids, and a failsafe circuit therefor. 
     In the past, anit-skid brake control systems have suffered from malfunctions when opens developed in the coils of the wheel speed sensors. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the above-described and other disadvantages of the prior art are overcome by the use of an open circuit detector. 
     According to an outstanding feature of the present invention, the current through two velocity pick-ups is added together and compared with a constant current. When the sum falls below a minimum indicating an open in either one of the coils or in both of them, system controller output is cut off and the system returned to manual. This is a safety measure because it is not at all safe for the system to operate on erroneous data provided by a coil which has an open. Further, manual control is much preferred over defective automatic control. 
     The above-described and other advantages of the present invention will be better understood from the following detailed description when considered in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which are to be regarded as merely illustrative, the single FIGURE is a diagrammatic view of one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawing, a motor vehicle is illustrated at 10 having wheels 11 and 12. Permanent magnets 13 and 14 are fixed relative to the chassis, not shown. Permanent magnet 13 has a pick-up coil 15 wound therearound. Permanent magnet 14 has a pick-up coil 16 wound therearound. A toothed wheel 17 is fixed to wheel 12. A toothed wheel 18 is fixed to wheel 11. 
     The arrangement of permanent magnets 13 and 14, coils 15 and 16 and toothed wheels 17 and 18 is entirely conventional and is employed to produce output pulses of a pulse repetition frequency directly proportional to wheel speed. Pulses are produced across coil 15 at a frequency directly proportional to the speed of wheel 12. Pulses are produced across coil 16 of a frequency directly proportional to the speed of wheel 11. 
     The vehicle air brakes are illustrated at 19 connected from an air brake valve 20 via a conduit 21. An air supply 22 is connected via conduits 23 and 24 to valve 20 and a brake pedal assembly 25, respectively. 
     The air brakes 19 are operated, as is conventional, by depressing a brake pedal in assembly 25. This supplies air to valve 20. Valve 20, in turn, supplies air to the brakes 19. 
     It is also conventional to override the manual control which is achieved through the use of the brake pedal. This override comes in the form of an antiskid brake system controller 26. Valve 20 has an electrical input lead 27 which is connected from an electrical output lead 28 of controller 26 through a switch 29. Switch 29 may be a relay or an electronic switch such as a transistor switch. The output of controller 26 is disabled under a coil open condition. In other words, switch 29 is opened by an output from a circuit 30 taken from a junction 31, the connection to switch 29 from junction 31 being over an input lead 32 to switch 29. 
     Various junctions are provided throughout circuit 30 including junctions 33, 34, 35, 36, 37, 38, 39, 40, 41, and 42. 
     Controller 26 is connected from junctions 35, 36 and 37. A regulated source of potential 43 is provided having one output lead grounded at 44, and another output lead 45 connected to junction 33. A voltage divider 46 is provided. A resistor 47 is connected between junctions 33 and 35, and a resistor 48 connected from junction 35 to ground. 
     Junctions 34 and 35 are connected together. Coil 15 is connected between junctions 34 and 37. Coil 16 is connected between junctions 34 and 36. A resistor 49 is connected between junctions 36 and 38. A resistor 50 is connected between junctions 37 and 38. Junctions 38 and 39 are connected together. Transistors of the NPN conductivity type are provided at 51 and 52. Transistor 51 has a collector 53 connected from junction 38, an emitter 54 connected to ground and a base 55 connected from junction 42. 
     Junctions 41 and 42 are connected together. A diode 56 is connected from junction 42 to ground and poled to be conductive in a direction toward ground. 
     Transistor 52 includes a collector 57 connected to junction 40, an emitter 58 which is connected to ground, and a base 59 which is connected from junction 39. 
     A non-inverting amplifier 60 is provided having an input lead 61 connected from junction 40 and an output lead 62 connected to junction 31. A capacitor 63 is connected between junctions 31 and 39. A junction 64 is connected from junction 33. A resistor 65 is connected between junctions 40 and 64. A resistor 66 is connected between junctions 41 and 64. A resistor 67 is connected from juntion 31 to junction 41. 
     A number of component parts of the circuit 30 may be considered to be what is known as a Norton amplifier. The current supplied from junction 64 through resistor 66 determines the amount of collector current transistor 51 has. Diode 56, with the remainder of the circuit, makes the collector current of transistor 51 substantially the same as that which flows through resistor 66. This means that when the sum of the currents in resistors 49 and 50 fall below a predetermined minimum, the circuit 30 latches and opens switch 29, disabling controller 26. 
     Resistor 67 is a positive feedback resistor which enhances latching. 
     Capacitor 63 prevents latching due to the receipt of asynchronous pulses from coils 15 and 16. This can happen because, in conventional anti-skid brake control systems, each of the wheels 11 and 12 may rotate independently because of the use of a conventional differential or for any other reason. 
     source 43, controller 26, valve 20, air supply 22, assembly 25, and brakes 19 all may be entirely conventional. The same is true of switch 29. As an example of the foregoing, air brake valve 20 may be that disclosed in copending application Serial No. 359,653 filed May 14, 1973, by Roger Greenwood for COMBINATION VALVE, assigned to the assignee of this application. The entire contents of said copending application is, by this reference hereto, incorporated herein as though fully set forth herein hereat. 
     Switch 2a is an entirely conventional, self-locking relay or latching electronic or transistor switch which latches in the open position when actuated to that position by amplifier 60 and, upon removal of power and the subsequent reapplication thereof, moves the switch back to its unlatched position. For example, switch 29 is shown in its unlatched position. 
     The phrase &#34;latching switch&#34; is hereby defined for use in the claims to include either a self-locking relay or a latching electronic or transistor switch or other latching switch.