Electrohydraulic steering system

An electrohydraulic steering system has a pair of proportional valves for controlling displacement of a steering valve for normal steering functions. The proportional valves are selectively energized by a position sensor in response to rotation of a steering wheel. However, should an electrical malfunction occur, rotation of the steering wheel beyond the normal operating range causes a steering control member to mechanically move an armature of the proportional valves through a spring to provide mechanical/hydraulic steering. Making the armatures of the proportional valves responsive to a mechanical input through the springs provides a simple, cost effective mechanical/hydraulic backup for the primary electrohydraulic steering control.

TECHNICAL FIELD 
This invention relates generally to a steering system for an industrial 
machine and, more particularly, to an electrohydraulic steering system 
having an auxiliary mechanical/hydraulic steering arrangement interfaced 
therewith. 
BACKGROUND ART 
Electrohydraulic steering controls are being developed for heavy-duty 
industrial machines because of their potential versatility over mechanical 
and hydraulic steering systems. Such electrohydraulic steering systems 
commonly use solenoid operated pilot valves for directing pilot fluid to a 
primary steering valve for steering the machine in response to manual 
manipulation of a steering control member, i.e. a steering wheel. The 
solenoid pilot valves are energized in response to electrical signals 
outputted from a sensor when the steering control member is moved in 
opposite directions from a neutral position. One of the concerns from a 
manufacturer's viewpoint is that customers are somewhat reluctant to 
purchase large machines in which the steering is dependent solely upon 
electrohydraulic circuitry. 
In view of the above, it would be desirable to have an electrohydraulic 
steering system for the normal steering functions while having a simple 
mechanical/hydraulic steering arrangement interfaced therewith for 
steering the machine should a malfunction occur in the electrical 
circuitry. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention, an electrohydraulic steering system 
has a manually operated steering control member movable in opposite 
directions from a neutral position and a pilot operated steering valve 
having opposite ends. The system includes a pair of solenoid valves 
individually hydraulically connected to the opposite ends of the steering 
valve with each valve including a valving portion, an electromagnetic 
coil, and an armature disposed within the coil and connected to the 
valving portion. A position sensor is disposed to energize one of the 
coils upon movement of the steering control member in a first direction 
and to energize the other coil upon movement of the steering control 
member in a second direction. A means is provided for individually 
mechanically moving the armatures upon movement of the steering control 
member in either direction beyond a predetermined operating range thereof 
so that the solenoid valves can be selectively mechanically actuated to an 
operating position should an electrical malfunction occur.

BEST MODE FOR CARRYING OUT THE INVENTION 
An electrohydraulic steering system 10 includes a steering wheel 11 
disposed to impart pivotal movement to a steering control member 12, and a 
pair of hydraulic steering actuators 13,14 suitably connected to the 
steering mechanism of a machine. Each of the actuators has head end and 
rod end chambers 15,16. A pilot operated steering valve 17 has opposite 
ends 18,19 and is connected to the head end and rod end chambers in the 
usual manner. Alternatively, the steering wheel 11 can be replaced with a 
lever or a lever and linkage arrangement for imparting pivotal movement to 
the steering control member 12. 
Each of a pair of solenoid actuated proportional valves 21,22 have a 
valving portion 23 hydraulically connected to a source of pilot pressure 
such as a pump 24 and individually connected to the opposite ends 18,19 of 
the steering valve. Each of the proportional valves also includes an 
electromagnetic coil 25 connected to a position sensor 26 disposed to 
energize one of the coils upon movement of the steering control member 12 
in a first direction and to energize the other coil upon movement of the 
steering control member in an opposite or second direction. An armature 27 
is disposed within each of the coils for moving the valving portion 23 to 
an operating position when the associated coil is energized. 
A means 28 is provided for individually mechanically moving the armatures 
27 upon movement of the steering control member 12 in either direction 
beyond a predetermined operating range thereof so that the proportional 
valves can be selectively mechanically actuated to an operating position 
should an electrical malfunction occur. In this embodiment, the means 
includes a pair of coil springs 29,30 disposed between the opposite ends 
of the steering control member and the armatures 27. 
INDUSTRIAL APPLICABILITY 
In use, steering in a first direction is initiated by rotating the steering 
wheel 11 clockwise. The sensor 26 detects the angular position of the 
steering wheel and directs an electrical signal to energize the coil 25 of 
the proportional valve 22 with the strength of the electrical signal being 
representative of the detected angular position. The electrical signal 
energizes the coil 25 of the proportional valve 22 causing the armature 27 
to move the valving portion 23 downwardly to direct pressurized pilot 
fluid to the end 19 of the steering valve 17 with the fluid pressure being 
proportional to the strength of the electrical signal directed to the coil 
25. The pressurized pilot fluid moves the steering valve to the left an 
appropriate amount to direct main system fluid into the rod end chamber 16 
of the actuator 13 and the head end chamber 17 of the steering actuator 
15. 
Rotating the steering wheel counterclockwise results in the proportional 
valve 21 being energized to direct pilot fluid to the end 18 of the 
steering valve 17 resulting in main system pressure being directed to the 
head end chamber 15 of the actuator 13 and the rod end chamber 16 of the 
actuator 14. 
Rotating the steering wheel 11 clockwise, for example, also pivots the 
steering control member 12 clockwise. In this embodiment, the spring 30 is 
selected so that it has substantially no effect on the armature 27 of the 
proportional valve 22 during rotation of the steering wheel within a 
predetermined normal operating range. Alternatively, the steering wheel 
can be connected to the steering control member through a lost motion 
connection so that the steering control member is pivoted only after the 
steering wheel is rotated beyond a predetermined operating range. 
Should an electrical malfunction occur, steering can be achieved by 
rotating the steering wheel beyond the predetermined operating range. This 
causes the spring 30 to exert a downward force on the armature 27 of the 
proportional valve 22 for moving the valving portion 23 downwardly causing 
leftward movement of the steering valve 17 as described above. Rotating 
the steering wheel counterclockwise will similarly result in rightward 
movement of the steering valve 17. 
In view of the foregoing description, it is readily apparent that the 
present invention provides an improved electrohydraulic steering system 
which utilizes proportional valves as the normal or primary steering 
control. However, should an electrical malfunction occur, 
mechanical/hydraulic steering becomes operative simply by rotating the 
steering wheel beyond its normal operating range. This imparts a 
mechanical force to the armature of the proportional valve through a 
spring thereby causing the armature to move the valving portion of the 
proportional valve to an operating position directing pilot fluid to the 
steering valve. Making the armatures of the proportional valves responsive 
to a mechanical input through the springs provides a simple, cost 
effective mechanical/hydraulic backup for the primary electrohydraulic 
steering control. 
Other aspects, objects and advantages of this invention can be obtained 
from a study of the drawings, the disclosure and the appended claims.