GB2509186A

GB2509186A - Emergency steering mode of a vehicle hydraulic system having a first pump driven by a prime mover and a second pump driven by a ground engaging member

Info

Publication number: GB2509186A
Application number: GB201223535A
Authority: GB
Inventors: Noud Gielens
Original assignee: AGCO Netherlands BV
Current assignee: AGCO Netherlands BV
Priority date: 2012-12-21
Filing date: 2012-12-21
Publication date: 2014-06-25
Also published as: GB201223535D0

Abstract

A vehicle hydraulic system 10 comprises a first pump 12 driven by a prime mover 16, a second pump 14 driven by a ground engaging member, preferably a wheel 40, and a steering control system 34 having a hydraulic supply input. A distribution circuit 22 connects outputs of the first and second pumps 12,14 to the hydraulic supply input. The second pump 14 is a variable displacement pump having a piston stroke that can be varied to adjust the output flow volume. The piston stroke is controlled in response to the output of the first pump 12. The hydraulic system 10 is operable in an emergency steering mode in which the first pump 12 is isolated from the steering control system 34 by the distribution circuit and the steering control system 34 is supplied with pressurised fluid from the second pump 14.

Description

DESCRIPTION

VEHICLE HYDRAULIC SYSTEM WITH EMERGENCY STEERING MODE

FIELD OF INVENTION

The invention relates to hydraulic systems for vehicles having hydraulic power steering with an emergency steering mode of operation.

BACKO ROUND

Ever stringent safety legislation for vehicles used on public highways are, in some jurisdictions at least, demanding that vehicle manufacturers include an emergency steering mode to allow the vehicle fitted with power steering to be steered in the event of hydraulic pressure loss through engine or pump failure. The most common solution to this problem is to provide a dedicated electric motor to power the hydraulic pump from stored electrical energy. However, the provision of a dedicated motor, albeit rarely used, adds significant costs to production of the vehicle.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide an improved system for providing an emergency steering mode and at less cost than current systems.

According to the invention there is provided a vehicle hydraulic system comprising a first pump driven by a prime mover, a second pump driven by a ground engaging member, a steering control system having a hydraulic supply input, and a distribution.

circuit connecting outputs of the first and second pumps to the hydraulic supply input, the second pump being a variable displacement pump having a piston stroke that can be varied to adjust the output flow volume, wherein said piston stroke is controlled in response to the output of the first pump, and wherein the hydraulic system is operable in an emergency steering mode in which the first pump is isolated from the steering control system by the distribution circuit and the steering control system is supplied with pressurised fluid from the second pump.

By providing a variable displacement pump driven by a ground engaging member such as a wheel, the momentum of the vehicle can be exploited in an emergency situation to generate hydraulic pressure for the steering system. Advantageously, this avoids the need for an additional dedicated (and expensive) pump.

prevent unnecessary drag, and thus energy loss, during normal operating conditions the variable displacement pump is be set at a minimum piston stroke so as to provide no hydraulic flow. In response to failure of the engine or the main supply pump, the piston stroke of the emergency pump is increased so as to generate a back-up source of hydraulic pressure which is directed to the steering control system. Advantageously, the momentum of the vehicle delivers sufficient hydraulic pressure to allow the operator to steer the vehicle and maintain control as demanded by the aforementioned safety regulations. In the event of insufficient momentum (at low speeds), the problem of steering the vehicle is not an issue.

The hydraulic system preferably comprises a hydraulic actuator coupled to the second pump and operable to control the piston stroke, the actuator having a hydraulic connection to the output of the first pump. By providing a hydraulic connection between the output of the first pump and the piston stroke control actuator of the second pump the latter can be activated automatically in response to a drop in pressure at the first pump's output regardless of the reason.

Alternatively, an electric actuator may be employed to control the piston stroke of the second pump wherein a pressure sensor associated with the output of the first pump provides an electrical signal which is converted into a control signal for the adjustment of the piston stroke.

The hydraulic actuator preferably comprises biasing means to bias the control of the piston stroke to a maximum flow output in the absence of a hydraulic signal applied thereto. In this case, the control of the second pump is failsafe wherein the second pump automatically generates an emergency hydraulic supply pressure in the event of no controlling hydraulic signal being supplied to the hydraulic actuator. The biasing means may comprise a coil spring or pressurised pneumatic cylinder. A further advantage of providing the biasing means is that only a single hydraulic control line is required. In other words, the actuator is single acting.

The system may be provided with a pump regulating valve having an output connected to the hydraulic actuator, the regulating valve being controlled hydraulically from a signal derived from the output of the second pump. When in the emergency steering mode the second pump can be self-regulating with the assistance of the regulating valve wherein the hydraulic actuator, the second pump and the regulating valve act together in a negative feedback control loop in a known manner.

The first pump (or main pump or chassis pump) is preferably a variable displacement pump which may also have a regulating circuit to maintain a desired output pressure regardless of the flow demand.

The hydraulic system may further comprise a brake contra! system which is also supplied with pressurised fluid from the second pump in an emergency mode.

Advantageously, this permits the safe and correct operation of the brakes in the event of failure of the main hydraulic pump and/or the engine.

BRIEF DESCRIPTION OF DRAWINGS

Further advantages of the invention will become apparent from reading the following description of a specific embodiment with reference to the appended drawings in which:-Figure 1 is a simplified schematic diagram of a vehicle hydraulic system in accordance with an embodiment of the invention, and Figure 2 is a hydraulic circuit diagram of the hydraulic system of Figure 1.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

It should be understood that Figure 1 is a simplified version of the hydraulic circuit of Figure 2. The following description is made with reference to both Figures 1 and 2 although some of the components shown in Figure 2 are omitted from Figure 1 for simplicity.

The hydraulic system described by way of example may be fitted to any self-propelled vehicle which employs hydraulic power steering. Therefore, the following description can be applied to any highway vehicle such as cars, utility vehicles and agricultural machines including tractors and self-propelled sprayers and harvesters.

Hydraulic system 10 includes a first variable displacement pump 12 and a second variable displacement pump 14. Both pumps are of the axial piston type with a controllable swashplate to adjust the piston stroke and thus the output flow at a given input drive speed.

First pump 12 is drivingly connected to an internal combustion engine 16 via a driving connection represented at 18 in Figure 1. It should be understood that alternative prime movers may be employed to drive first pump 12 such as an electric motor.

Furthermore, the mechanical connection between the engine 16 and the first pump 1 2nmay take a variety of different forms which are well known to the skilled person.

In practise different terms may be used for the first pump including chassis pump', main pump' or load-sensing pump'. However, regardless of the terms used, the first pump 12 serves to generate a source of pressurised hydraulic fluid for use by the various hydraulic systems present on the vehicle. These systems may include, by way of example only, systems for steering, braking and other auxiliary outputs depending on the type of vehicle.

Shown schematically in Figure 1, first pump 12 pressurises hydraulic fluid taken from a reservoir 20, the pressurised fluid being conveyed to a distribution circuit designated generally at 22. With reference to Figure 2, the first pump 12 has an associated regulating circuit designated generally at 24 and which serves to control the piston stroke by means of a hydraulic actuator 26 so as to maintain a working pressure at the output 28. The regulating circuit 24 operates with negative feedback adjusting the piston stroke of the pump 12 continuously and in a known manner.

The pressurised fluid from first pump 12 is conveyed by a check valve 30 and a hydraulic delivery line 32 to a steering control system designated generally at 34.

Referring back to Figure 1, the steering control system 34 has a steering wheel 36 mechanically connected thereto and serves to convert driver commands into a hydraulic steering signal conveyed to steering cylinder 38.

Second pump 14 is drivingly connected to a wheel 40 of the vehicle. The mechanism by which the second pump 14 is coupled to the wheel 40 may take various alternative constructions obvious to those skilled in the art. For example, the second pump 14 may be installed on the transfer case on the rear of the vehicle transmission. Alternatively, the pump may be connected directly to a driven axle in the final drive. In any case, it should be understood that the second pump 14 derives torque from the momentum of the vehicle via the ground engaging wheel 40.

A hydraulic actuator 42 is connected to an adjustment member of the second pump 14, the actuator serving to adjust the piston stroke of the pump 14 in response to hydraulic signals conveyed thereto. The actuator 42 includes a coil spring 44 which biases the piston stroke adjustment to a maximum output flow. Therefore, in the absence of any hydraulic signal, the second pump 14 delivers a maximum flow rate.

Second pump 14 has an associated regulating circuit designated generally at 46 which serves to generate hydraulic control signals for the control of the piston stroke via actuator 42. The regulating circuit 46 receives hydraulic pilot signals from the output of first pump 12 via line 48 and from the output of second pump 14 via pilot line 50.

During normal operation, the pressure generated by first pump 12 is sufficient to push the actuator 40 to the right against the force of spring 44 moving the swashplate of the pump 14 to an angle of around -2 degrees so that the output flow from pump 14 is negligible. Advantageously, this minimizes the drag placed upon the ground engaging wheel 40 and thus minimizes any increase in fuel consumption as a result of the pump 14.

In the event of failure of the engine 16 or pump 12 the operating pressure at output 28 and thus pilot line 48 falls to a level which causes the second pump 14 to stroke and thus generate a source of pressurised fluid. The regulating circuit 46 ensures that the output of second pump 14 is maintained at a working pressure which is sufficient to power the steering control system 34. Check valve 30 in the distribution circuit 22 serves to isolate the first pump from the steering control system 34 during the emergency steering mode. During the normal operation a further check valve 52 in the distribution circuit 22 isolates the second pump 14 from the steering control system 34.

The invention provides a cheaper solution to providing a back-up source of hydraulic fluid in the event of an emergency without an expensive electric pump and without significant increase in fuel consumption. Various modifications to the hydraulic circuit described herein will be obvious to a person skilled in the technical field of hydraulics and alternative components may be employed whilst remaining within the scope of the invention.

Claims

CLAIMS1. A vehicle hydraulic system comprising a first pump driven by a prime mover, a second pump driven by a ground engaging member, a steering control system having a hydraulic supply input, and a distribution circuit connecting outputs of the first and second pumps to the hydraulic supply input, the second pump being a variable displacement pump having a piston stroke that can be varied to adjust the output flow volume, wherein said piston stroke is controlled in response to the output of the first pump, and wherein the hydraulic system is operable in an emergency steering mode in which the first pump is isolated from the steering control system by the distribution circuit and the steering control system is supplied with pressurised fluid from the second pump.
2. A hydraulic system according to Claim 1, further comprising a hydraulic actuator coupled to the second pump and operable to control the piston stroke, the actuator having a hydraulic connection to the output of the first pump.
3. A hydraulic system according to Claim 2, wherein the actuator comprises biasing means to bias the control of the piston stroke to a maximum flow output in the absence of a hydraulic signal applied thereto.
4. A hydraulic system according to Claim 2 or 3, further comprisind a pump regulating valve having an output connected to the hydraulic actuator, the regulating valve being controlled hydraulically from a signal derived from the output of the second pump.
5. A hydraulic system according to any preceding claim, wherein the first pump is a variable displacement pump.
6. A hydraulic system according to any preceding claim, wherein the ground engaging member comprises a wheel.
7. A hydraulic system according to any preceding claim, further comprising a brake control system having a hydraulic supply input that is connected to the distribution circuit.