Abstract:
A steering control system for directing fluid to steering actuators on a machine include a set of right and left manually-operated pilot pressure fluid control valves and a set of right and left electro-hydraulic (EH) pilot pressure fluid control valves, with both sets being coupled to a main steering control valve for controlling movement of the latter to effect operation of the steering actuators. An electronic control unit (ECU) includes a computer and a memory and receives signals representative of various operating parameters of the control system and of the vehicle ground speed and operates to control the EH pilot pressure control valves and a set of right and left solenoid-operated valves such that only flow from the set of manually-operated valves is connected for operating the main steering control valve to effect a least aggressive steering mode when the vehicle speed is indicative of road travel, and for also actuating the EH pilot pressure control valves for effecting a more aggressive steering mode when the vehicle speed is low indicating off-road travel. EH valve failure results in the control system defaulting to that for effecting the least aggressive steering mode, while failure of one or both of the manually-controlled valves results in the EH control valves being activated to provide that pilot pressure called for by the manually-operated valve.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a steering control system, and more particularly to an electro-hydraulic steering control system including manually-actuated pilot pressure control valves. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known to provide work vehicles, such as industrial articulated loaders, for example, with electro-hydraulic (EH) steering control systems. One advantage of electro-hydraulic systems over pilot controlled systems, e.g. systems that use a joystick to control the flow of pilot fluid to a pilot-controlled steering control valve, is that the metering curve of the EH systems can be easily modified and can even be changed during operation based on input from the operator or machine operating conditions. U.S. Pat. No. 7,283,900 granted Oct. 16, 2009 discloses an EH steering control system wherein a controller unit is used to store a plurality of flow-metering curves representing different steering ratios respectively corresponding to different vehicle speeds, with the controller unit acting in response to steering and vehicle speed signals for sending a representative steering control signal for effecting operation of a steering valve for effecting a desired steering response. Another EH steering control system is disclosed in U.S. Pat. No. 7,200,993 and includes redundant steering pilot valves which operate when primary pilot valves do not operate satisfactorily as determined by sensed pressures. 
         [0003]    A disadvantage of EH steering control systems is that they typically add additional failure modes to the machine. These failure modes make it difficult to use the EH steering control systems for “safety critical” functions like steering. 
         [0004]    The problem to be solved then is that of providing a steering control system which has the aforementioned advantage of EH steering control systems without requiring failure mode redundancies which add complexities and cost. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the present invention, there is provided an improved EH steering control system which includes a way for adjusting the metering of a pilot-controlled steering system while providing a simple control arrangement for providing steering in the event of a failure of a proportional EH steering control valve arrangement for routing pilot pressure fluid to a main steering control valve. 
         [0006]    The inventive result is obtained by providing a metering control that is adjusted with a proportional EH steering valve arrangement used in parallel with a manually-operated pilot pressure control valve arrangement. The steering control system includes a plurality of pressure sensors located strategically throughout the system to determine whether or not the proportional EH valve is working properly. When a failure of the proportional EH steering control valve is detected, then a solenoid-operated control valve arrangement is operated to block pilot pressure fluid from flowing from the proportional EH steering control valve to the main steering control valve. On the other hand, failure of the manually-operated pilot pressure control valve arrangement to supply pressure corresponding to that intended by the manual input results in the control unit acting to adjust the EH steering valve arrangement so that it supplies the deficiency of the manually operated pilot pressure control arrangement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic representation of an exemplary steering control system, with the system being shown with the main steering control valve positioned in a neutral centered position for straight ahead vehicle steering operation. 
           [0008]      FIG. 2  is a view like  FIG. 1 , but showing the steering control system in a condition corresponding to the joystick being operated for causing the vehicle to be steered to the right, with only the pilot pressure directed by the manually-operated pilot pressure control valve arrangement being available for shifting the main steering control valve from its neutral position. 
           [0009]      FIG. 3  is a view like  FIG. 2 , but showing the steering control system in a condition wherein, in addition to the pilot pressure fluid directed by the manually-operated pilot pressure control valve arrangement, further pilot pressure fluid is directed for controlling the main steering control valve by operation of the proportional EH pilot pressure control valve arrangement. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    Preliminarily, it is noted that the letters R and L are used in the following description to designate identical components or elements respectively used for controlling the flow of steering pilot pressure fluid or for transmitting steering control input or output signals for effecting right and left steering operation. 
         [0011]    Referring now to  FIG. 1 , there is shown a steering control system  10  including a source of steering fluid pressure, here depicted as a pump  12 , and a sump  14  connected to a vehicle steering actuator arrangement, here depicted as right and left steering cylinders  16  and  18 , respectively, by way of a pilot-operated, main steering control valve  20 . Specifically, the pump  12  is connected to a work port of the control valve  20  by a supply line  22 , while the sump  14  is connected to an exhaust port of the valve  20  be a return line  24 . The steering control valve  20  has a first feed/return port coupled by a feed/return line  26  to the head end of the steering cylinder  16  and to the rod of the steering cylinder  18 , and has a second feed/return port coupled by a feed/return line  28  to the rod end of the steering cylinder  16  and the head end of the steering cylinder  18 . 
         [0012]    As shown in  FIG. 1 , the pump  12  also serves to supply pilot fluid pressure to opposite ends of the steering control valve  20 . Specifically, the pump  12  is connected to a pair of identical manually-operated pilot pressure control valves  30 R and  30 L respectively by a pair of branch lines  32  and  34  extending from the supply line  22 . Similarly, the sump  14  is connected to the control valves  30 R and  30 L respectively by a pair of branch lines  36  and  38  extending from the return line  24 . The control valves  30 R and  30 L are both shown in a non-operated position blocking pressure fluid flow from the pump  12  while respectively coupling fluid supply lines  40  and  42  to the sump  14 . The control valves  30 R and  30 L are each manually controlled by a manually-operated steering control input device such as a joystick arrangement, for example, designated by the functional box  44 . The fluid supply/return lines  40  and  42  are respectively connected to upper and lower ends of the main steering control valve  20  by first and second branch lines  46  and  48  respectively containing check valves  50 R and  50 L oriented for preventing flow in the direction of the supply/return lines  40  and  42 . Thus, the manually-operated control arrangement  44 , pilot pressure fluid control valves  30 R and  30 L, supply/return lines  40  and  42 , branch lines  46  and  48 , and the check valves  50 R and  50 L comprise a first circuit for supplying pilot pressure fluid to the main steering control valve  20 . The control valves  30 R and  30 L operate to supply relatively low pilot pressure for establishing the least aggressive steering settings of the main steering control valve  20 , which are safe settings for operating the work vehicle for travel on the road. 
         [0013]    The pump  12  is coupled to the main steering control valve  20  by a second circuit, including a path parallel to that established by the pressure/return branch lines  46  and  48 , for establishing more aggressive steering settings of the main steering control valve  20  suitable for off-road steering operations of the vehicle. Specifically, the pump  12  is coupled to respective first ports of a pair of proportional, two-position electro-hydraulic (EH) pilot pressure control valves  52 R and  52 L by a pair of pressure supply branch lines  54  and  56  extending from the supply line  22 . Respective second ports of the EH pilot pressure control valves  52 R and  52 L are connected to the supply/return lines  40  and  42  by supply/return branch lines  58  and  60 . The control valves  52 R and  52 L may be shifted between normal, inactivated positions, as shown, wherein respective third ports are connected to the second ports, and activated positions wherein the third ports are respectively coupled to the first ports. A connecting line  62  extends between the third port of the EH pilot pressure control valve  52 R and a first port of a two-position solenoid valve  66 R, and a second connecting line  64  extends between the third port of the EH pilot pressure control valve  52 L and respective first ports of a two-position, solenoid-operated valve  66 L. Respective second ports of the valves  66 R and  66 L are respectively coupled to the supply/return lines  40  and  42 . Respective third ports of the valves  66 R and  66 L are respectively coupled to the supply/return branch lines  46  and  48  at respective locations between the steering control valve  20  and the check valves  50 R and  50 L by connecting lines  68  and  69 . The solenoid-operated valves  66 R and  66 L are shown in an inactivated condition wherein their respective third ports are connected to the second ports, thereby establishing connections between pilot pressure controllers at the top and bottom of the main steering control valve  20  and the pressure/return lines  40  and  42  so that a path is established for permitting pilot control pressure fluid to be relieved from the steering control valve  20 . 
         [0014]    An electronic control unit (ECU)  70 , which includes a microprocessor and memory, is provided for effecting electro-hydraulic (EH) control of the main steering control valve  20 . Various sensors are provided for sensing operating conditions and generating signals representing the measured quantity and inputting this signal to the ECU  70  for processing, Specifically, a position sensor  72  is provided for measuring movement of the steering input device  44 , with the generated signal being connected to the ECU  70  by an input lead  74 , with the ECU  70  determining from information located in the memory relating the sensed position of the device  44  to a pilot pressure conveyed by the valve  30 R. Alternatively, a first pair of pressure sensors  76 R and  76 L are respectively provided in the pressure/return lines  40  and  42 , with the respective generated signals being connected to the ECU  70  by signal input leads  78  and  80 . A second pair of pressure sensors  82 R and  82 L are respectively coupled for sensing the pressure in the connecting lines  62  and  64 , with the generated signals being connected to the ECU  70  by signal input leads  84  and  86 . A vehicle ground speed sensor  88  senses either the actual ground speed (radar device for example) or the speed of a rotating element corresponding to ground speed (transmission out put shaft or axle rotation, for example), with the generated speed signal being coupled to the ECU  70  by a signal input lead  90 . Finally, an operator input device  92  is coupled to the ECU  70  by an input lead  94  for permitting the operator to “key in” input data or to select a desired steering aggressiveness that is stored in memory, which aggressiveness may be stored as a value related to vehicle speed. Using the input signals and data in memory, the ECU  70  generates corresponding control signals which are respectively coupled to the proportional EH pilot pressure valves  52 R and  52 L by valve operating signal leads  96  and  98  for effecting shifting of the valves amounts corresponding to the magnitude of the control signal. Additionally, the ECU  70  is connected, as by leads  100  and  102 , for respectively selectively activating the solenoid operated valves  66 R and  66 L. 
         [0015]    The operation of the steering control system  10  is as follows. Assuming that control system  10  is associated with a vehicle that can be steered to the right by extending the cylinder  16  while retracting the cylinder  18 , or can be steered to the left by extending the cylinder  18  while retracting the cylinder  18 , a right turn can be effected by the operator using the manually operated device  44  to shift the pilot pressure control valve  30 R to an operated position connecting the pump  12  to the pressure/return line  40 , as shown in  FIG. 2 . The amount of pilot fluid pressure corresponds directly to the amount of movement imparted to the valve  30 R by the steering input control device  44  with the movement of the device  44  being sensed by the sensor  72  which generates a representative signal that is sent to the ECU  70  which uses this signal in conjunction with position/pressure data in memory to determine the corresponding control pressure that is supposed to be sent to the line  40  by the valve  30 R. In addition, or alternatively, the pressure in the line  40  is sensed by the pressure sensor  76  which generates a representative signal that is sent to the ECU  70 . In addition, the vehicle ground speed sensor  88  generates a speed signal representative of the sensed ground speed and sends it to the ECU  70  which uses this signal to determine whether or not the vehicle speed is too high to permit an aggressive metering of work fluid to the steering cylinders  16  and  18  from the main steering control valve  20 . If the ECU  70  determines more aggressive metering of work fluid is not warranted, no signal is sent to activate the proportional EH pilot pressure control valve and steering is performed at the rate established by the position of the valve  30 R. Thus, the pilot fluid flows to the top (as viewed in  FIG. 2 ) of the steering control valve  20  shifting it downwardly so as to connect the right steering section of the valve  20  to the steering cylinders, causing extension of the cylinder  16  and retraction of the cylinder  18 . 
         [0016]    However, the above-described operation may be automatically modified in the event of a failure of the manually operated pilot pressure control valve  30 R. Such a failure is indicated if the pressure sensor  76 R senses a pressure which is lower than that which should correspond to the sensed position of the manually operated control device  44 . This deficiency in fluid pressure is added by the action of the ECU  70  sending a signal of an appropriate strength to the proportional EH pilot pressure control valve  52 R to cause it to shift an amount necessary for connecting the pilot pressure supply branch line  54  to the connecting line  62  to produce the deficient pressure there for being added to that established by the valve  30 R. Concurrently, the ECU  70  connects a signal for activating the solenoid-operated valve  66 R so that it shifts to connect the connecting line  62  to the top end of the steering control valve  20  by way of the connecting line  68 . The condition of the control system  10  is then that shown in  FIG. 3 . When the desired steering operation is over, the operator releases the control device  44  which permits the valve  30 R to return to the position shown in  FIG. 1 , the ECU  70  then receiving a position signal from the sensor  72  indicating that the steering operation is over whereupon the actuating signals to the EH pilot pressure control valve  52 R and the solenoid-operated control valve  66 R are turned off. Thus, the pilot pressure fluid at the top of the steering control valve  20  is connected to sump by way of the connecting line  68 , the deactivated valve  66 R and the pressure/return line  40 . 
         [0017]    Assume that, during the normal steering operation described above, the sensed vehicle speed is sufficiently low that a more aggressive metering of steering fluid from the main steering control valve  20  to the steering cylinders  16  and  18  is appropriate. The ECU  70  will determine from the stored information relating metering aggressiveness to vehicle speed, the aggressiveness and related pilot fluid pressure corresponding to the sensed speed. Taking in account the pressure reading of the pressure sensor  76 R, the ECU  70  will send an appropriately valued signal to the proportional EH pilot pressure control valve  52 R for causing the latter to establish a pilot fluid pressure at the connecting line  62  which, when added to that supplied by the pilot pressure control valve  30 R equals the desired pressure for obtaining the desired metering aggressiveness. Concurrently, the ECU  70  will automatically send a signal energizing the solenoid operated valve  66 R. The condition of the control system  10  is like that shown in  FIG. 3 . When the steering operation is over, the operator will release the manually-operated device  44  so that it and the operated pilot pressure control valve  30 R return to their unactuated positions shown in  FIG. 1 . 
         [0018]    A failure of the EH control valve  52 R is determined by comparing the pressure sensed by the pressure sensor  82 R (P 1 ) to the sum of the pressure sensed by the pressure sensor  76  (P 2 ) and the pressure corresponding to the electrical signal sent to activate the proportional EH control valve  52 R (P 3 ). If P 1 &gt;P 2 +P 3 , then the ECU  70  operates to deactivate the solenoid operated valve  66 R leaving only the pilot pressure control valve  30 R to provide pilot pressure for controlling the steering control valve  20  for providing the least aggressive metering control. In other words, the steering control system defaults to the least aggressive steering control upon failure of the proportional EH control valve. 
         [0019]    Operation for left steering operation is similar to that described above for right steering and for the sake of brevity is not included here. 
         [0020]    Thus, it will be appreciated that metering aggressiveness can be easily modified by the EH pilot pressure control valves  52 R and  52 L, while maintaining a reliable safety feature afforded by the low metering aggressiveness made available when the EH pilot pressure control valves fail. In addition, the control circuitry makes it possible to use the EH pilot pressure control valves to provide pilot control pressure to the steering control valve for effecting low metering aggressiveness in the event of a failure of one or the other of the manually-operated pilot pressure control valves 
         [0021]    Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.