WO2015175882A1

WO2015175882A1 - Method for operating a parallel steering control system

Info

Publication number: WO2015175882A1
Application number: PCT/US2015/030977
Authority: WO
Inventors: Vishal Vijay MAHULKAR; David Samuel REMBA URIBE; Peter RECH; Michael Berne Rannow
Original assignee: Eaton Corporation
Priority date: 2014-05-15
Filing date: 2015-05-15
Publication date: 2015-11-19

Abstract

An aspect of the present disclosure relates to a steering system having first and second steering circuits in fluid communication with a fluid actuator wherein the steering circuits are disposed in parallel to each other. The system can operate with both the first and second steering circuits in fluid communication with the actuator wherein the first steering circuit includes a steering input and the second steering circuit supplements the hydraulic fluid flow to the actuator based on the steering input. The system can be configured with a position error compensation algorithm to minimize jerk feedback at the steering input when changing the variable steering ratio, and to ensure that the second steering circuit is actuated in the proper direction and is in cooperation with the first steering circuit.

Description

METHOD FOR OPERATING A PARALLEL STEERING CONTROL SYSTEM

RELATED APPLICATIONS

[0001] This application is being filed on May IS, 2015, as a PCT International Patent application and claims priority to U.S. Patent Application Serial No. 61/994,000 filed on May 15, 2014; U.S. Patent Application Serial No. 61/994,006 filed on May 15, 2014; and U.S. Patent Application Serial No. 61/994,398 filed on May 16, 2014, the disclosures of which are incorporated herein by reference in their entireties.

BACKGROUND

[0002] In many "off-highway" vehicles, such as tractors, loaders, line painting vehicles, sweepers, pavers, marine vehicles, etc., parallel steering circuits are used to control an actuator that steers the vehicle. In some applications, one of the parallel steering circuits is manually actuated using a steering wheel and another is actuated through an automation system. Typically, parallel steering systems are configured such that only one of the steering circuits is allowed to be actuated at any given time. For example, some systems give a first steering circuit priority over a second steering circuit such that the second steering circuit is deactivated upon an input being received by the first steering circuit. Additionally, each steering circuit is typically sized for the full required flow at the steering actuator(s) which results in a significant increase in cost in comparison to a steering circuit having a single steering circuit.

SUMMARY

[0003] One aspect of the present disclosure relates to a steering system having a first steering circuit in selective fluid communication with a fluid actuator. The first steering circuit defines a first flow path and includes a first proportional valve disposed in the first flow path. The steering system further includes a second steering circuit in selective fluid communication with the fluid actuator. The second steering circuit defines a second flow path that is in a parallel flow configuration with the first flow path of the first steering circuit.

[0004] In one aspect, the steering system is operable between a first, second, and third mode of operation. The first mode includes the first steering circuit being in fluid communication with the fluid actuator and the second steering circuit being isolated from the fluid actuator. The second mode includes the second steering circuit being in fluid communication with the fluid actuator and the first steering circuit being isolated from the fluid actuator. The third mode includes the first and second steering circuits being in simultaneous fluid communication with the fluid actuator with a steering input being provided via the first steering circuit. In one aspect, the third mode of operation includes the second steering circuit being configured to selectively provide a variable ratio steering effect to the fluid actuator which may be based on vehicle speed and/or the steering wheel W position. In one aspect, the third mode includes the second steering circuit being configured to selectively provide a load reaction feature to the fluid actuator such that a first side of the actuator is placed in fluid communication with a second side of the fluid actuator via the second steering circuit. In one aspect, the first steering circuit is provided with a first hydraulic fluid flow capacity that is less than a second hydraulic flow capacity of the second steering circuit.

[0005] Another aspect of the disclosure relates to a method for controlling a steering system having first and second steering circuits in fluid communication with a fluid actuator, wherein the steering circuits are disposed in parallel to each other. The system can operate with both the first and second steering circuits in fluid communication with the actuator wherein the first steering circuit includes a steering input and the second steering circuit supplements the hydraulic fluid flow to the actuator based on the steering input. The system can be configured with a position error compensation algorithm to minimize jerk feedback at the steering input when changing the variable steering ratio, and to ensure that the second steering circuit is actuated in the proper direction and is in cooperation with the first steering circuit.

[0006] A variety of additional aspects will be set forth in the description that follows. These aspects can relate to individual features and to combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad concepts upon which the embodiments disclosed herein are based.

DRAWINGS

[0007] Figure 1 is a schematic of a steering system having a hydrostatic steering circuit and a parallel electrohydraulic steering circuit with features that are examples of aspects in accordance with the principles of the present disclosure. [0008] Figure 2 is a schematic of the steering system of Figure 1 showing an exemplary hydrostatic steering circuit configuration and an exemplary parallel electrohydraulic steering circuit configuration, wherein the steering system is in a variable ratio steering mode of operation.

[0009] Figure 3 is a schematic of the steering system of Figure 2 in a load reaction mode of operation.

[0010] Figure 4 is a schematic showing an enlarged view of the hydrostatic steering circuit shown in Figures 2 and 3.

[0011] Figure 5 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1-3. [0012] Figure 6 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1-3.

[0013] Figure 7 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1-3.

[0014] Figure 8 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1-3.

[0015] Figure 9 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1 -3.

[0016] Figure 10 is a schematic showing an exemplary electrohydraulic steering circuit usable with the steering system shown in Figures 1-3. [0017] Figure 11 is a graph showing a system change in the variable steering ratio.

[0018] Figure 12 is a graph showing position and response time errors.

[0019] Figure 13 is a process flow diagram showing which compensation mode in which the system should be placed. [0020] Figure 14 is a process flow diagram showing implementation of the

compensation modes.

DETAILED DESCRIPTION

[0021] Reference will now be made in detail to the exemplary aspects of the present disclosure that are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like structure.

Steering System and Circuit

[0022] Referring now to FIG. 1 , a hydraulic schematic of a steering system, generally designated 10, for controlling the steered wheels 25 an off-highway vehicle (e.g., combine, loader, tractor, paver, line painter, sweeper, marine vehicle, etc.) is shown. The steering system 10 includes a steering circuit, generally designated 12, having a reservoir 14, and a pump 16 having an inlet in fluid communication with the reservoir 14. As shown, pump 16 is a constant volume pump. A priority valve (not shown) may also be provided to operate to ensure that the pressure delivered to the steering circuit 12 is both prioritized and at the desired pressure for efficient operation through a load-sense circuit Instead of a constant volume pump and priority valve combination, a pressure compensated pump could also be utilized.

[0023] The steering circuit 12 further includes an electrohydraulic steering circuit, generally designated 18, and a hydrostatic steering circuit, generally designated 20. Each of the electrohydraulic steering circuit 18 and the hydrostatic steering circuit 20 provide selective fluid communication between the pump 16 and a fluid actuator 22. While the fluid actuator 22 is shown in FIG. 1 as being a single fluid actuator, it will be understood that the term "fluid actuator" in the specification and the claims includes at least one fluid actuator.

[0024] The electrohydraulic steering circuit 18 defines a first flow path 24 while the hydrostatic steering circuit 20 defines a second flow path 26. The first flow path 24 of the electrohydraulic steering circuit 18 is disposed in a parallel flow configuration with the second flow path 26 of the hydrostatic steering circuit 20. In operation, the

electrohydraulic steering circuit 18 selectively communicates fluid from the pump 16 to the actuator 22 through the first flow path 24 while the hydrostatic steering circuit selectively communicates fluid from the pump 16 to the actuator 22 through the second flow path 26.

[0025] As will be described in greater detail later, the steering system 10 and circuit 12 is configured to be placed in three general operational modes: (1) a hydrostatic steering only mode in which the actuator 22 is actuated only by fluid passing through the hydrostatic steering circuit 20 via the second flow path 26 based on a steering input to the hydrostatic steering circuit 20; (2) an electrohydraulic steering only mode in which the actuator 22 is actuated only by fluid passing through the electrohydraulic steering circuit 18 via the first flow path 24 based on a steering input to the electrohydraulic circuit 18; and (3) a hybrid steering mode in which the actuator 22 is actuated by fluid passing through both the hydrostatic and electrohydraulic steering circuits 18, 20 via flow paths 24, 26 based on a steering input to the hydrostatic circuit 20. In one aspect, the hydrostatic steering circuit 20 is sized for less than the full flow ordinarily required by the actuator 22, but with enough capacity to allow an operator to still effectuate limited steering in the case of a failure of the steering system 12. As such, the hydrostatic steering only mode is not a mode in which the system would be placed under ordinary operating conditions. In the hybrid steering mode, the electrohydraulic circuit 18 is configured to supply the remaining amount of hydraulic fluid required by the actuator 22 that is not provided by the hydrostatic circuit 20. Accordingly, the disclosed system represents a potential cost savings in that a smaller hydrostatic steering circuit 20 can be provided instead of a circuit that is sized for the full actuator flow, as is the case with typical parallel steering systems.

Hydrostatic Steering Circuit

[0026] Referring now to Figures 2-4, the hydrostatic steering circuit 20 is shown in further detail. The hydrostatic steering circuit 20 includes a valve assembly 30 and defines a fluid inlet port 106 in fluid communication with the pump 16, a fluid outlet port 108 in fluid communication with the reservoir 14, a first control port 100, and a second control port 102. The first and second control ports 100, 102 are in fluid communication with the first and second ends 46, 48, respectively, of the fluid actuator 22 (shown in Figures 1 and 2). The valve assembly 30 is disposed between the fluid inlet and outlet ports 106, 108 and the first and second control ports 100, 102. In one embodiment, the valve assembly 30 is an ORBITROL® steering valve manufactured by Eaton Corporation of Cleveland, Ohio. [0027] In the subject embodiment, the valve assembly 30 includes a valving assembly 70 having a rotary valve 70a (e.g., spool) and a follow-up valve member 70b (e.g., sleeve). In the subject embodiment, the spool rotates within a bore of the sleeve as a result of manual actuation of a steering actuation member S (e.g., a steering wheel W, a joystick, etc.).

[0028] The valve assembly 30 is movable from a neutral position N_H to a right turn position R_H or a left turn position L_H through the manual actuation of the steering actuation member S. With the valve assembly 30 in the right turn position R_H or the left turn position L_H, fluid is communicated from the pump 16 to one of the first and second ends 46, 48 of the fluid actuator 22 through a fluid meter 71 (e.g., a gerotor gear set), as explained in further detail in the following paragraphs.

[0029] In the subject embodiment, valve 30 is a three-position, six-way valve having ports 30a, 30b, 30c, 30d, 30e, and 30f. In the embodiment shown, port 30a is in fluid communication with control port 100, ports 30b and 30c are in fluid communication with the fluid meter 71, port 30d is in fluid communication with control port 102, port 30e is in fluid communication with reservoir 14 via outlet 108, and port 30f is in fluid

communication with pump 16 via outlet 106.

[0030] In the subject embodiment, the neutral position N_H is a closed position. It will be understood that the term "closed position" refers to a position in which fluid communication between the inlet 106 and one of the first and second control ports 100, 102 is blocked by the valve 30. In other words, with the valve assembly 30 in the neutral position N_H, fluid from the pump 16 cannot be communicated through the second flow path 26 to the fluid actuator 22 via the control ports 100, 102. In the exemplary embodiment shown in FIG. 3, ports 30e and 30f are blocked while ports 30a and 30b are placed in fluid communication with each other, as are ports 30c and 30d.

[0031] In the subject embodiment, the left turn position L_H enables pumped fluid to be provided to the first end 46 of fluid actuator 22 via control port 102 and fluid meter 71 by placing port 30F in fluid communication with 30b and by placing ports 30c and 30d in fluid communication with each other. Fluid is returned to the reservoir 14 by placing ports 30a and 30e in fluid communication with each other. Likewise, the right turn position R_H enables pumped fluid to be provided to the second end 48 of fluid actuator 22 via control port 100 and fluid meter 71 by placing port 30f in fluid communication with port 30c and by placing ports 30a and 30b in fluid communication with each other. Fluid is returned to the reservoir 14 by placing ports 30d and 30e in fluid communication with each other. [0032] In the subject embodiment, the fluid meter 71 is dual functional. The fluid meter 71 functions as a metering device that measures the proper amount of fluid to be fed to the appropriate control port 100, 102 of the steering circuit 20 in response to rotation of the steering actuation member S. The fluid meter 71 also functions as a follow-up device that provides follow-up movement to the valving assembly 70 such that the valving assembly 70 is returned to the neutral position N_H after the desired amount of fluid has been directed through the fluid meter 71 to the fluid actuator 22. In the subject embodiment, this follow- up movement is achieved by a mechanical link 72 (e.g., a drive, etc.) that connects the fluid meter 71 to the valving assembly 70. Centering springs 31 may also be provided at both ends of the spool 70a to maintain the spool 70a in the neutral position NH when no input to the actuation member S is provided.

Electrohvdraulic Steering Circuit

[0033] Referring back to Figure 1, the electrohydraulic steering circuit 18 may also be configured to control the actuator 22. For example, the electrohydraulic steering circuit 18 may be provided with one or more valve assemblies that initiate a left hand steering mode by placing ports 116 and 112 in fluid communication with each other such that the pump 16 is placed in fluid communication with the first end 46 of the actuator 22. Likewise, the electrohydraulic steering circuit 18 may be provided with one or more valve assemblies that initiate a right hand steering mode by placing ports 116 and 114 in fluid

communication with each other such that the pump 16 is placed in fluid communication with the second end 48 of the actuator 22. A neutral position can also be obtained by blocking fluid communication between ports 1 16, 118 and ports 112, 114.

[0034] Referring to Figures 2-3, the electrohydraulic steering circuit 18 may be provided with a valve assembly 28 that accomplishes the above stated normal steering functions, but that can be further placed into a variable ratio steering mode and a load reaction mode when the hydrostatic steering circuit is operational in the hybrid steering mode. [0035] Figure 2 shows the valve assembly 28 in the variable ratio steering mode. In this mode, the valve assembly 28 selectively adds and removes and additional amount of hydraulic fluid to the actuator 22 via variable orifices 28a, 28b. Accordingly, this flow is an augmentation to the flow already being provided to the actuator 22 by the hydrostatic steering circuit 20. In operation, the additional flow amount selectively effectuates a change in the relationship between the amount of change in the steering input position and the amount of hydraulic fluid provided to the actuator 22. The amount of additional flow provided by the valve assembly 28 at any given time can be based on a number of different factors and inputs, and combinations thereof. For example, the variable ratio steering can be controlled based on vehicle speed, vehicle load, and terrain. It is noted that where it is desired for the valve assembly 28 to not affect variable ratio steering (or at least to not affect the steering input provided by the valve assembly 30), the valve assembly 28 can be controlled to ensure that hydraulic fluid is added/subtracted in the same ratio as that being provided/removed to the actuator 22 by the hydrostatic steering circuit 20.

Regardless of the degree to which variable ratio steering is provided by the valve assembly 28, the valve assembly 28 can be controlled to ensure that the same amount of fluid that is added to the actuator 22 during a steering operation is subsequently removed from the actuator by the valve assembly 28 when the steering input S is returned to that same position. This ensures that that operator is provided with consistent position feedback of the wheels 25 or actuator 22 through the steering input S.

[0036] With continued reference to Figure 2, it can be seen that the additional flow provided by the electrohydraulic steering circuit 18 is provided by placing the pump port 116 with either of ports 112, 114 and by placing the reservoir port 118 with the other of ports 112, 114. In order for the valve assembly 28 to work cooperatively with the valve assembly 30, the valve assembly 28 is controlled to place the pump port 1 16 in fluid communication with port 112 when the valve assembly 30 has placed the pump port 106 in fluid communication with port 100. Similarly, when the valve assembly 30 has placed the pump port 106 in fluid communication with port 102, the valve assembly 28 is controlled to place the pump port 1 16 in fluid communication with port 114. When the valve assembly 30 is in the neutral position N_H, the valve assembly 28 is commanded to prevent flow between the pump 16 and the actuator 22. [0037] In one embodiment, the actuator 22 is configured to receive about 200 cubic centimeters (cc) of hydraulic fluid for a discrete movement of the steering input S (e.g. a complete rotation of a steering wheel W) with the hydrostatic steering circuit 20 being configured to deliver about 64 cc of hydraulic fluid for the movement and the

electrohydraulic steering circuit 18 being configured to supply the remaining amount. In one embodiment, the electrohydraulic steering circuit 18 is also configured to supply the total amount of hydraulic fluid to the actuator 22. Accordingly, at least in some embodiments, the hydrostatic steering circuit 20 is smaller and has a lower flow capacity than the electrohydraulic steering circuit 18.

[0038] With reference to Figure 3, the valve assembly 28 is shown in a load reaction mode. In the load reaction mode, all of the fluid entering the port 1 12 or 114 is returned to the other of port 112 or 114 via a controlled orifice 28c for delivery back to the actuator 22. Consequently, the pump 16 and reservoir 14 are blocked from being in fluid communication with the actuator 22 in this position. In this position, the hydraulic flow between the two chambers of the actuator 22 is controlled in such a manner as to allow the same amount of fluid to flow back to the hydrostatic steering circuit 20 as was supplied by the circuit 20 so that the movement rate of the steering input S in relation to the movement of the actuator 22 (e.g. the number of turns of rotation of the steering wheel W per degree change in the position of the wheels 25) does not change when the steering wheel W is released, thus enabling the steering wheel W to return to a neutral position when a force F is placed on the actuator 22 via the steered wheels 25.

Electronic Control System

[003.9] As described above, the steering system 10 can be placed in various operational modes. An electronic control system can be provided that monitors, initiates, and controls the initiation of the various modes. In one embodiment, an electronic controller 50 monitors various sensors and operating parameters of the steering system 10 to configure the steering system 10 into the most appropriate mode of operation and to control the electrohydraulic steering circuit 18 in the most appropriate manner.

[0040] Referring to Figure 1, the electronic controller 50 is schematically shown as including a processor 50A and a non-transient storage medium or memory 50B, such as RAM, flash drive or a hard drive. Memory 50B is for storing executable code, the operating parameters, and potential inputs from an operator interface, while processor 50A is for executing the code. Electronic controller 50 is configured to be connected to a number of inputs and outputs that may be used for implementing the steering circuit operational modes. For example, the electronic controller 50 can receive a steering wheel W or steering input S position sensor XI , a steered wheel 25 or actuator position sensor X2, a valve position sensor X3 in the electrohydraulic steering circuit 18, and various pressure sensors P arranged throughout the electrohydraulic steering circuit 18. One skilled in the art will understand that many other inputs are possible. For example, measured engine speed may be provide as a direct input into the electronic controller 50 or may be received from another portion of the control system via a control area network (CAN). The measured pump displacement, for example via a displacement feedback sensor, may also be provided. In one embodiment, the electronic controller 50 is configured to include all required operational inputs for the various circuits 12, 18, 20.

[0041] Examples of outputs from the controller 50 are actuator(s) 64 for operating valves associated with the electrohydraulic steering circuit 18 and a pump output signal(s) for activating/ deactivating the pump and/or controlling the output of the pump. Other outputs are possible as well. In one embodiment, the electronic controller 50 is configured to include all required operational outputs for the various circuits 12, 18, 20.

[0042] The electronic controller 50 may also include a number of maps or algorithms to correlate the inputs and outputs of the controller 50. For example, the controller 50 may include an algorithm to control the position of the valve assembly 28 based on the calculated flow rate through the valve assembly 30 to achieve a total desired flow rate at the actuator 22 (and/or flow through the valve assembly 28) based on the speed of the vehicle, the position of the actuator 22, and/or the position of the steering input S. Also, the controller 50 can simultaneously calculate and compare the flow rate through the hydrostatic steering circuit 20, the flow rate through the electrohydraulic steering circuit 18, and the flow rate into and out of the actuator 22 to identify if a fault condition exists and to safely shut the system down. An operator may also use a user interface in the control system to identify the conditions during which variable ratio steering should be engaged, in addition to the degree to which the variable ratio steering is implemented. For example, the operator may make a selection from any number of predefined or definable variable ratio steering maps that define the operation of the electrohydraulic steering circuit 18 with respect to the hydraulic steering circuit 20 and/or operating parameters of the vehicle.

[0043] The electronic controller 50 may also store a number of predefined and/or configurable parameters and offsets for determining when each of the modes is to be initiated and/or terminated. As used herein, the term "configurable" refers to a parameter or offset value that can either be selected in the controller (i.e. via a dipswitch) or that can be adjusted within the controller.

Electrohvdraulic Steering Circuit Example EmhnHiments

[0044] With reference to Figures 5-10, examples of valve assembly 28 are shown that are configured to operate as described above. As the examples illustrate, the above described operation for the valve assembly 28 may be accomplished through a single valve or through multiple individual valves. Also, it is noted that the descriptions relating to components in each of the examples are fully applicable to similar components of the other examples unless otherwise stated.

[0045] Referring to Figure 5, a valve assembly 28 is shown including a proportional control valve 29, a pair of isolation valves 90, 92, and a plurality of pressure sensors P. The isolation valve 90, 92, which may be manual or automatically operated, allow for the electrohydraulic steering circuit 18 to be isolated from the hydrostatic steering circuit 20 in those cases where a system fault or failure has occurred and the vehicle must be steered by the hydrostatic steering circuit alone. The pressure sensors P provide inputs to controller 50, as discussed previously.

[0046] As shown, the control valve 29 is disposed in the first flow path 24 between the ports 116/118 and the ports 112/114. In this example, the control valve 29 is a five- position, four-way valve. In the embodiment shown, the first proportional valve 29 is a spool and sleeve type valve including first through fifth positions 29-1 to 29-5. As shown, control valve 29 is also provided with a position sensor X3. [0047] In the subject embodiment, positions 29-2 and 29-3 are closed positions. It will be understood that the term "closed position" refers to a position in which fluid communication between the ports 116/118 and the ports 112/114 is blocked by the valve 29. In other words, with the control valve 29 in the closed position, fluid from the pump 16 cannot be communicated through the first flow path 24 to the fluid actuator 22 via the first and second actuator outlets 112, 114. In the exemplary embodiment shown in FIG. 5, ports 112, 114, 116, 118 are each individually blocked when the control valve 29 is in the closed position 29-2 or 29-3.

[0048] In the subject embodiment, the left turn position 29-1 places ports 112 and 116 in fluid communication with each other and ports 114 and 118 in fluid communication with each other. This position allows for pumped fluid to be communicated from pump 16 to the first end 46 of fluid actuator 22 and then returned to the reservoir 14 via ports 114 and 116. Likewise, the right turn position 29-5 places ports 112 and 118 in fluid communication with each other and ports 114 and 116 in fluid communication with each other. This position allows for pumped fluid to be communicated from pump 16 to the second end 46 of fluid actuator 22 and then returned to the reservoir 14 via ports 112 and 118. The third position 29-3 blocks ports 116 and 118 off while placing ports 112 and 114 in fluid communication with each other.

[0049] The first proportional valve 29 is moved between the right turn position 29- 1 and the left turn position 29-5 through the operation of an electromagnetic actuator 64. In the subject embodiment, the electromagnetic actuator 64 is a proportionally controlled voice coil. Alternatively, a pilot operated actuator(s) can be used in the place of electromagnetic actuator 64. To actuate the control valve 29, a signal, such as a pulse width modulation (PWM) voltage, is supplied from the controller SO to the

electromagnetic actuator 64. In one aspect of the present disclosure, the controller 50 receives information from a Global Positioning System (GPS) receiver related to the location, direction, and speed of the vehicle. The controller then transmits signals to the electromagnetic actuator 64 in order to control the position of the control valve 29.

Alternatively, the controller 50 can transmit signals to the electromagnetic actuator 64 based on other types of signals, such as a signal from a joystick. [0050] When the hydraulic circuit 12 is in the electrohydraulic steering only mode, the control valve 29 is operated between the first and second positions 29-1 and 29-2 for left hand steering and between the fourth and sixth positions 29-4 and 29-5 for right hand steering to attain the desired steering effect at the actuator 22. In the hybrid mode of operation, the control valve 29 is controlled similarly for the variable ratio steering mode, but based on other control parameters, as previously described. In the load reaction mode of the hybrid operational mode, the control valve 29 is placed in the third position 29-3.

[0051] Referring to Figure 6, another example is shown in which the valve assembly 28 includes a control valve 31 and a control valve 33, in addition to valves 90, 92 located adjacent ports 112, 114 respectively. As shown, control valve 31 is in series with control valve 33 with control valve 31 being nearest ports 112, 114 and control valve 33 being nearest ports 116, 118. Accordingly, hydraulic fluid must pass through both control valves 31, 33 in order to flow between ports 116/118 and ports 112/114.

[0052] As shown, control valve 31 is a spool and sleeve type two-position, four-way proportional control valve that is actuated by an actuator 64, such as a solenoid actuator or voice coil. As shown, control valve 31 is also provided with a position sensor X3. In a first position 31-1, the control valve 31 places the ports 112, 114 in fluid communication with each other while blocking flow from control valve 33, and thus from ports 116, 118. In the second position 31 -2, the control valve 31 allows fluid to pass from control valve 33 to the ports 112, 114, thus allowing the control valve 33 to control flow between ports 1 12/1 14 and ports 116/118. [0053] As shown, control valve 33 is a spool and sleeve type three-position, four-way proportional control valve that is actuated by an actuator 64, such as a solenoid actuator or voice coil. In the subject embodiment, a left turn position 33-1 places ports 112 and 116 in fluid communication with each other and ports 114 and 118 in fluid communication with each other. This position allows for pumped fluid to be communicated from pump 16 to the first end 46 of fluid actuator 22 and then returned to the reservoir 14 via ports 114 and 116. Likewise, a right turn position 33-3 places ports 112 and 118 in fluid communication with each other and ports 114 and 116 in fluid communication with each other. This position allows for pumped fluid to be communicated from pump 16 to the second end 46 of fluid actuator 22 and then returned to the reservoir 14 via ports 112 and 1 18. A neutral position 33-2 blocks ports 112, 114, 1 16, and 118.

[0054] When the hydraulic circuit 12 is in the electrohydraulic steering only mode, the control valve 31 is placed in the open second position 31-2 while the control valve 33 is operated between the first and second positions 33-1 and 33-2 for left hand steering and between the second and third positions 33-2 and 33-3 for right hand steering to attain the desired steering effect at the actuator 22. In the hybrid mode of operation, the control valve 31 remains in the open position 31-2 while control valve 33 is controlled similarly for the variable ratio steering mode, but based on other control parameters, as previously described. In the load reaction mode of the hybrid operational mode, the control valve 31 is placed in the second position 31-1 while the control valve 33 can be placed in the closed position 33-1.

[0055] Figure 7 shows the same components as that shown in Figure 6, but with the control valves 31 and 33 in reverse order and with the isolation valves 90, 92 nearest the ports 116, 118. In such an arrangement, the operation of the control valves 31 and 33 is the same for the electrohydraulic only steering mode and the variable ratio steering mode, but is different for the load reaction mode in that the control valve 33 must be held in either the first position 33-1 or the third position 33-3.

[0056] With reference to Figure 8, another example of the control valve assembly 28 is shown. Similar to the embodiment shown in Figure 6, the control valve assembly 28 includes a control valve 33 that controls the steering and variable ratio steering functions of the circuit 18 and a control valve 31 responsible for the load reaction functions.

Additionally, an isolation valve 90 is also provided. However, and in contrast to the example shown in Figure 6, the control valve 31 is provided as a three-position valve with a third position 31-3 that blocks flow between the control valve 33 and the ports 112, 114. The control valve 31 can be placed in the third position 31 -3 when hydrostatic steering only mode is desired.

[0057] With reference to Figure 9, another example of the control valve assembly 28 is shown. Similar to the embodiment shown in Figure 8, the control valve assembly 28 includes a control valve 33 that controls the steering and variable ratio steering functions of the circuit 18. However, in the example of Figure 9, no isolation valves 90, 92 are provided. Instead, an additional control valve 35 is furnished that is in direct

communication with the ports 112, 114 to provide this safety function. As shown, the control valve 35 is provided as a two-position, four-way valve with a first position 35-1 that blocks flow between the control valve 33 and the ports 112, 114 and a second position 35-2 that allows flow between the control valve 33 and the ports 112, 114 when the system is in the electrohydraulic steering only or the variable ratio steering modes. Another difference is that a control valve 37 is provided to perform the load reaction functions instead of a control valve 31. As shown, the control valve 35 is connected between control valves 33 and 37 and is provided as a two-position, two-way valve with a first position 37-1 that places port 112 in fluid communication with port 114 when the control valve 35 is in the second position 35-2 (i.e. the load reaction mode). When the control valve 37- 1 is in the second position 37-2, the flow between ports 112 and 114 is blocked.

[0058] With reference to Figure 10, yet another example embodiment of the control valve assembly 28 is shown, including a control valve 39, a control valve 41, a pair of isolation valves 90, 92, and a plurality of pressure sensors P. As shown, the control valve 41 is provided with an actuator 64 and position sensor X3. A connector 43 is also provided to connect the control valve 39 to the valve 41 such that movement of the valve 41 imparts a corresponding movement to the valve 39. However, it should be noted that valve 39 could be provided with an actuator 64 without departing from the concepts presented herein. As shown, the valves 39 and 41 are three-position, six-way spool and sleeve type valves. In the embodiment shown, valve 39 has positions 39-1, 39-2, and 39-3 while valve 41 has positions 41-1, 41-2, and 41-3. Because the valves 39 and 41 are connected together and are in a parallel arrangement, valve 39 is in position 39-1 when valve 41 is in position 41-1, valve 39 is in position 39-2 when valve 41 is in position 41-2, and valve 39 is in position 39-3 when valve 41 is in position 41-3.

[0059] As with valve 33, the combination of valves 39 and 41 perform electrohydraulic steering functions in addition to providing control for the variable ratio steering mode when steering is occurring through the hydrostatic steering circuit 20. When valve 39 is in position 39-1 and valve 41 is in position 40-1, port 112 is placed in fluid communication with port 116 and port 114 is placed in fluid communication with port 118 for left hand steering control. When valve 39 is in position 39-3 and valve 41 is in position 41-3, port 1 12 is placed in fluid communication with port 1 18 and port 114 is placed in fluid communication with port 116. When valve 39 is in position 39-2 and valve 41 is in position 41-2, flow to ports 116 and 118 is blocked while ports 112 and 114 are placed in fluid communication with each other for load reaction control.

Error Compensation

[0060] Two features that are sometimes important for certain applications, for example agricultural equipment applications to improve operator productivity, are fast steering response time and the ability to change the steering ratio. These features are implemented by maintaining a consistent mapping between the steering wheel W position (e.g. via position sensor XI) and the steered wheel position (e.g. via position sensor X3).

However, these features can be challenging to implement.

[0061] For example, the change in the steering ratio should be transparent to the operator, meaning that no jerk should be present when the steering ratio is changed.

Referring to Figure 11, a graph 600 is provided that illustrates this phenomenon wherein the vertical axis 602 represents the desired steered wheel angle (which is a function of steering wheel W position input or steering input S) and the horizontal axis 604 represents the actual steering wheel input angle. As can be seen, a first steering ratio 606 is implemented at one range of operation while a second steering ratio 608 is implemented at another range of operation. When switching between the two steering ratios 606, 608, a jump 610 will necessarily occur. If this jump 610 is not adequately addressed, steered wheels would suddenly move very fast to track the new trajectory resulting in a significant jerk at the operator station. Sometime, the operator may also feel the jerk through the steering wheel W or steering input S. Accordingly, a compensation for the switch between the ratios 606, 608 must be implemented if the switch is to be transparent to the user with no jerk response. The same circumstance exists as well when switching between different steering modes (e.g. normal steering, variable ratio steering, load reaction steering, drift correct, etc.). [0062] Another challenge is that the response time between an input to the steering wheel W or steering input S and the movement of the wheels 25 via actuator 22 should be acceptable, meaning that the steering should start and stop responding as soon as the steering wheel W or steering input S starts or stops moving. Maintaining an adequate response time is further complicated by the circumstance that perfect position tracking of the steered wheel angle and steering wheel W angle is not possible. Referring to Figure 12, a graph 700 is provided that illustrates this phenomenon wherein the vertical axis 702 represents the desired steered wheel angle (i.e. the position of the steering wheel W or steering input S) and the horizontal axis 704 represents the time progression. As can be seen, a graph showing a desired steered wheel position curve 706 is offset from the actual steered wheel position 708. This offset results in a response time delay 710 between a movement at the steering wheel W or steering input S and a corresponding movement at the steered wheels 25. The position tracking error 712 is thus a result of the two reasons mentioned above - namely (i) the necessity to stop the steered wheel movement as soon as the steering wheel movement stops and (ii) the inability to track the desired steered wheel position perfectly. These errors, if not corrected, can actually cause the electrohydraulic steering circuit to drive the actuator 22, and thus the steered wheels 25, in the opposite direction momentarily to that being input to the hydraulic steering circuit 20 when the input direction is changed. Drift inherently caused by internal hydraulic leakages is another circumstance that introduces additional error. Accordingly, position tracking error compensation and drift correction algorithms can be implemented to improve overall performance. [0063] Referring to Figures 13 and 14, processes 1000 and 1100 are shown that together can be used to compensate for changes in the variable steering ratio, steering modes, position tracking errors, and drift related errors. As presented, process 1000 determines which compensation mode should be selected while process 1100 shows the actual implementation of the compensation modes. [0064] Referring to Figure 13, process 1000 starts at a step 1002 wherein the algorithm is initiated. At a step 1004, the current steered wheel position and the steering wheel W position are measured. At a step 1006, the position error is calculated by subtracting the current steered wheel position from the desired steered wheel position. At a step 1008, the current steering wheel W rotational rate (revolutions per minute, rpm) is calculated, for example through differentiation. At a step 1010, the calculated current steering wheel W rotational rate can be filtered, for example using a 2 hertz (Hz) low pass filter. At a step 1012 it is determined whether the steering wheel W rotational rate is below a predefined threshold value, for example below 0.S rpm. If the rotational rate is below the threshold value, then the drift control algorithm is enabled at step 1014 and a persistent variable for the current steered wheel position is updated at a step 1016. If the rotational rate is not less than the threshold value, then a position control algorithm is enabled and the drift control algorithm is disabled at a step 1018. The steering wheel W rotational rate can also be compared to a second threshold value to determine if the position error persistent variable should be updated. For example, at step 1020 it is determined whether the steering wheel W rotational rate is below a second predefined threshold value, for example below 0.2 rpm. If the rotational rate is below the threshold value, the persistent variable for the position error is updated at a step 1022. If the rotational rate is not below the second threshold value, the control can be looped back to step 1018. The process 1000 can be terminated at a step 1024.

[0065] Referring to Figure 14, process 1100 is shown in further detail, which may be initiated at step 1102. At a step 1104, the steering control mode (e.g. the hybrid steering mode) is initiated. At a step 1106, it is determined whether the drift control is enabled. This information is available from the process 1000. If the drift control is enabled, the process moves to step 1108 wherein the desired steered wheel position is set to equal the current position persistent variable from process 1000 which can be stored at step 1110. If the drift control is not enabled, the process moves to step 1112 wherein it is determined if the steering ratio has changed or if the steering mode has changed.

[0066] If a change has occurred, the position error is updated at step 1114. In performing the position error update, the current desired position is calculated as a function of the current steering ratio and the current steering wheel W angle. The old or previous desired position is calculated as a function of the previous or old steering ratio and the current steering wheel W angle. Once these calculations have been completed, the updated position error can be calculated as the position error persistent variable from process 100 plus the current desired position and minus the old or previous desired position. At a step 1116, the desired steered wheel position is calculated as a function of the current steering ratio and the current steering wheel W angle, from which the position error calculated at step 1114 is subtracted. Once calculated, the desired steered wheel position can be stored at step 1118.

[0067] If no change has occurred at step 1112, then the position error is updated to equal the position error persistent variable from process 1000 plus a value, for example a value of zero, at a step 1120. From step 1120, the process moves directly to step 1116, thereby bypassing step 1114. The algorithm can be terminated at a step 1122.

[0068] By utilizing the algorithms shown in processes 1000 and 1100, changes in the steering ratio and direction of the steering wheel W can be compensated and response times are minimized such that the changes are transparent to an operator.

[0069] Various modifications and alterations of this disclosure will become apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that the scope of this disclosure is not to be unduly limited to the illustrative embodiments set forth herein.

Claims

What is claimed is: 1. . A method of controlling a steering system for the wheels of a vehicle comprising:

(a) providing a first steering circuit in selective fluid communication with a fluid actuator; and

(b) providing a second steering circuit in selective fluid communication with the fluid actuator, the second steering circuit being arranged in parallel relation to the first steering circuit;

(c) operating the first steering circuit to provide a selected hydraulic fluid flow to the fluid actuator based on a steering input to the first steering circuit;

(d) operating the second steering circuit to selectively provide a first variable steering ratio and a second variable steering ratio at the actuator;

(e) calculating a position error between a desired position of the wheels and an actual position of the wheels; and

(f) incorporating the position error into a desired wheel position control algorithm to the second steering circuit when transitioning between the first and second variable steering ratios to minimize a jerk response at the steering input.

2. The method of claim 1, further including the step of implementing a drift control algorithm when the steering input is below a threshold value.

3. The method of claim 2, wherein the steering input is a steering wheel.

4. The method of claim 3, wherein the drift control algorithm is enabled when the

steering wheel is moving at less than about 0.5 revolutions per minute.

5. The method of claim 4, wherein the position control is disabled when the drift control algorithm is enabled.

6. The method of claim 1 , wherein the first steering circuit includes a hydrostatic control valve and the second steering circuit includes one or more electrohydraulic control valves.

7.

The method of claim 6, wherein the one or more electrohydraulic control valves are actuated by an electronic controller configured to implement the position control algorithm.

8. A method of controlling a steering system for the wheels of a vehicle comprising:

(c) operating the first steering circuit to selectively provide a first hydraulic fluid flow to the fluid actuator based on a steering input to the first steering circuit;

(d) operating the second steering circuit to selectively provide a cooperative

supplemental second hydraulic fluid flow to the actuator;

(f) incorporating the position error into a desired wheel position control

command to the second steering circuit when:

i. transitioning between a first variable steering ratio and a second variable steering ratio to minimize a jerk response at the steering input; or

ii. the steering input changes direction in order to prevent the second steering circuit from actuating the wheels in a direction that is opposite to the direction indicated by the steering input

9. The method of claim 8, further including the step of implementing a drift control algorithm when the steering input is below a threshold value.

10.

The method of claim 9, wherein the steering input is a steering wheel.

11. The method of claim 10, wherein the drift control algorithm is enabled when the steering wheel is moving at less than about 0.5 revolutions per minute.

12. The method of claim 11, wherein the position control is disabled when the drift control algorithm is enabled.

13. The method of claim 8, wherein the first steering circuit includes a hydrostatic control valve and the second steering circuit includes one or more electrohydraulic control valves.

14. The method of claim 13, wherein the one or more electrohydraulic control valves are actuated by an electronic controller configured to implement the position control algorithm.

15. A method of controlling a steering system for the wheels of a vehicle comprising:

(a) providing a hydraulic steering circuit in selective fluid communication with a fluid actuator; and

(b) providing an electrohydraulic steering circuit in selective fluid communication with the fluid actuator, the second steering circuit being arranged in parallel relation to the first steering circuit;

(c) operating the hydraulic circuit to selectively provide a first hydraulic fluid flow to the fluid actuator based on a steering wheel input to the hydraulic steering circuit;

(d) operating the electrohydraulic steering circuit to selectively provide a

cooperative supplemental second hydraulic fluid flow to the actuator;

(e) implementing a drift control algorithm when a rotational speed of the steering wheel is below a threshold value;

(f) implementing a desired steered wheel position control algorithm when the drift control algorithm is disabled, the position control algorithm including: i. calculating a position error between a desired position of the wheels and an actual position of the wheels to determine a desired steered wheel position and commanding the electrohydraulic steering circuit based on the calculated steered wheel position.

16. The method of claim 15, wherein the drift control algorithm is enabled when the steering wheel is moving at less than about 0.5 revolutions per minute.

17. The method of claim 16, wherein the position control is disabled when the drift control algorithm is enabled.

18. The method of claim IS, wherein the first steering circuit includes a hydrostatic control valve and the second steering circuit includes one or more electrohydraulic control valves.

19. The method of claim 18, wherein the one or more electrohydraulic control valves are actuated by an electronic controller configured to implement the position control algorithm.