WO1998043866A1

WO1998043866A1 - Method for controlling electrohydraulic power steering

Info

Publication number: WO1998043866A1
Application number: PCT/FR1998/000628
Authority: WO
Inventors: Eric Ouvry; Philippe Saint-Martin
Original assignee: Renault
Priority date: 1997-03-28
Filing date: 1998-03-27
Publication date: 1998-10-08
Also published as: FR2761329B1; FR2761329A1; EP0969999A1

Abstract

The invention concerns a method for controlling a motor vehicle power steering, consisting of an electropump unit (9) electronically controlled and comprising a hydraulic circuit with flexible pipes (5), such that, during a transitory phase, when the steering is stressed and the unit (9) serviceable voltage (Um) shifts from a stand-by value (Umin) to a maximum value (Umax), and when the pipes (5) are inflated this serviceable voltage (Um) whether progressively or not decreases to an optimal value (Uop) providing the steering power up to a maximum speed threshold (Φ1) of the motor vehicle steering wheel, before resuming its stand-by value (Umin) when the vehicle is no longer stressed.

Description

METHOD FOR CONTROLLING AN ELECTROHYDRAULIC POWER STEERING

The invention relates to a method for controlling an electronically controlled electrohydraulic power steering, for a motor vehicle, intended to reduce the electrical consumption of the electric pump group when steering the vehicle.

There are currently three types of motor vehicle steering assistance, of different technologies.

First of all, conventional hydraulic steering systems are composed of a hydraulic valve, a hydraulic pump driven by the vehicle engine via a belt, hoses and a coupled double-acting cylinder at the steering rack. The drive of the pump by the heat engine allows the mounting of such a system on heavy vehicles, such as trucks.

However, this solution has several drawbacks, in particular, a significant overconsumption of the vehicle due to the fact that the pump is always driven by the heat engine.

In addition, the assembly of hydraulic hoses and the mechanical integration of the system, which is not modular, are complex. The enrichment of the service, to obtain a direction with variable assistance (DAV) according to the speed of the vehicle, leads to complicated and expensive solutions.

There are also electric power steering systems, composed of an electric motor, generally a clutch, a reduction gear, a torque sensor, electronic circuits for piloting and controlling the electric motor. The latter allow a reduction in the overall consumption of the vehicle, since the system consumes only when the driver requests steering, and also makes it possible to easily offer variable-assist, inexpensive steering.

These systems essentially have two drawbacks due first of all to the difficulty of integrating them on any vehicle, given their non-modularity, and then to the degradation of the steering wheel reminder.

The invention relates to the third type of hydraulic steering assistance using an electric pump group. Such a system comprises the same hydraulic elements as a conventional hydraulic steering, as shown in FIG. 1, that is to say a rotary valve 1 mounted on the steering column 2 leading to the steering wheel, a rack-and-pinion steering 3, coupled to an assistance cylinder 4, pipes 5 for supplying oil under high pressure to the rotary valve and pipes 6 for returning oil under low pressure to the pump reservoir 7, geared or paddle , for example, but this is driven by an electric motor 8 powered by the vehicle's electrical network, and constituting an electric pump group with the pump.

The main advantages of this system lie in the ease of integration of the electric pump group under the engine hood of vehicles, in the possibility of controlling the electric pump group to reduce its electrical consumption and to provide variable assistance (D.A.V.).

The French patent application published under No. 2,720,447 in the name of MERCEDES-BENZ relates to a strategy for controlling the electropump group of a hydraulic power steering, making it possible to reduce electrical consumption from information internal to the engine such as its internal resistance or the drop in electrical voltage in the motor. This solution makes it possible to reduce the power consumed in a straight line when the steering is not requested by the driver, but has the disadvantage of not reducing the power consumed during steering operations for the vehicle.

The present invention proposes to optimize the hydraulic power delivered by the electric pump unit providing only the flow rate useful for steering assistance, by controlling the supply voltage of the electric pump group as a function of the stress on the steering by the driver and possibly other information such as the speed of the vehicle, for example, this supply voltage being reduced when the vehicle is traveling in a straight line. For this, the object of the invention is a method for controlling an electrohydraulic steering for a motor vehicle, constituted by an electropump group controlled electronically, and comprising a hydraulic circuit, part of which is produced by flexible pipes, characterized in that that the supply voltage U _m of the electric pump group is controlled as a function of the stresses of the direction, so that, during the transient phase of pressure build-up of the hydraulic circuit starting from the detection of a stress of the direction, by pressure passage P of the hydraulic circuit to a minimum value ^p _m i _n e ^a threshold value P ₁ greater than ^p i _n / l ^a supply voltage U _m of the electropump group passes a standby value U _m _ _n to a maximum value U _maχ ensuring the swelling of the pipes, and in that, after this transient phase, the supply voltage U _m decreases towards an optimal value U _OD corresp rippling at a flow rate from the hydraulic pump ensuring steering assistance up to a maximum speed Ω, of rotation of the vehicle's steering wheel, before resuming its standby value ^ min ^a P ^r ^ Ξ at the end of the request for The direction.

According to another characteristic, the control method according to the invention is such that the supply voltage of the electric pump unit goes from the maximum value to the optimal value after the voltage build-up phase of the hydraulic circuit, ie according to a voltage step , or according to a progressive decrease curve.

Other characteristics and advantages of the invention will appear on reading the description of an exemplary embodiment illustrated by the figures following which are, in addition to FIG. 1 already described, which is a diagram of a hydraulic supply system for electro-hydraulic power steering:

- Figure 2: the block diagram of an electropump group controlled for electrohydraulic power steering;

- Figure 3: characteristics of the pump unit relating to the flow as a function of pressure, for two supply voltages;

- Figure 4: a characteristic of the pump unit relating to the assistance pressure as a function of the hydraulic flow;

- Figure 5a: the variation curve of the supply voltage of a controlled electric pump group, for electro-hydraulic power steering during a steering operation of the vehicle;

- Figure 5b: the pressure variation curve in the hydraulic circuit of an electric pump group controlled during a steering operation of the vehicle;

- Figure 6a: the variation curve of the supply voltage of a power steering electric pump group controlled according to the invention, during a steering operation of the vehicle;

- Figure 6b: the pressure variation curve in the hydraulic circuit of an electric pump group, controlled according to the invention;

- Figure 7: the variation curve of the power consumed, on a turning profile, without and with the method according to the invention.

The diagram in FIG. 2 shows an electropump group 9 of electro-hydraulic power steering, controlled by an electronic circuit 10 taking into account one or more additional information, such as the speed of the vehicle V _v , and supplied electrically by the vehicle electrical network delivering a voltage U _al ^ _m . This electric pump group is connected to a reservoir 11 containing oil which it sends to a valve 12, the latter distributing the oil to a jack 13.

A first solution currently proposed for reducing the electric consumption of the electric pump group consists in measuring the current I consumed by the motor of the group, this current being the image of the pressure in the hydraulic circuit, therefore of the stress on the steering by the driver. . The detection of variations in this current and their comparison with triggering thresholds make it possible to manage the transition from a state of low power to a state of high power and vice versa, obtained by modulation of the supply voltage across the terminals of the electric motor. The trigger thresholds can be self-adapting so as to take into account the slow variations of the steering assistance system, due to the viscosity of the oil, the temperature of the oil and of the engine, etc. ...

A second solution consists in supplementing the previous driving mode by taking into account one or more additional pieces of information, such as the speed of the vehicle.

FIG. 3 is an example of characteristics of the electric pump group relating to variations in the flow rate Q as a function of the oil pressure p for a low power P ^ delivered and for a high power P _e corresponding to two supply voltages of the electric pump group , a minimum tension U _mj _ when the wheels are in line with the vehicle to reduce the response time of the process, when the steering is used on the one hand and to create a minimum pressure in the hydraulic circuit, the variations of which are measured on the other hand, and a maximum tension ϋ when the steering is requested. A request for steering by the driver results in the passage from an operating point A on the curve P _f of low power, corresponding to a displacement of the vehicle in a straight line, to an operating point at B on the curve P _e corresponding to a steering operation of the vehicle.

During such a steering maneuver by the driver requiring assistance from the management, a first phase, transient, occurs corresponding to a rise in pressure in the hydraulic circuit causing the hoses to swell, which causes an additional flow call to the one actually requested to provide the service. These flexible pipes are used to meet acoustic constraints aimed at reducing parasitic noise in motor vehicles. The electric pump unit is dimensioned so as to obtain, under the supply voltage U _al ^ _m , the maximum flow rate, that is to say the necessary flow rate added to the flow rate caused by the swelling of the pipes, making it possible to ensure correct behavior of the assistance. , without a wall effect, that is to say without reducing the level of assistance during the transient phases.

Outside these transient phases, either in steering when the steering is requested, once the pipes have been inflated, only the flow rate requested by the assistance cylinder is necessary, the supply of the flow corresponding to the swelling of the pipes thus inducing unnecessary additional motor power consumption.

Also, at a given pressure, the electric pump unit provides a flow which can be broken down into two terms:

- a useful flow corresponding to the flow required by the jack in a rigid hydraulic circuit,

- a flow rate necessary for the inflation of the flexible pipes at high pressure during the transient phases of pressure build-up in the hydraulic circuit.

On the characteristic of the electric pump group relating to variations in the assistance pressure P as a function of the flow rate Q, proportional to the speed of rotation Ω of the flywheel (FIG. 4), the operating point C corresponds to an electric pump group whose hydraulic circuit is rigid, supplied with a voltage U _Q , actually obtained after swelling of the flexible pipes, meeting the specifications established in terms of steering assistance and allowing driver assistance up to a maximum value Ω- ^ of the flywheel speed determined to provide a correct service to the customer and corresponding to a flow rate Q (Ω ₁ ).

The operating point D corresponds to a group electropump powered at a maximum voltage ^u max ^ ^u ^ P ^was equal supply pressure ^u alim Y ^ant ^has ^a standard hydraulic circuit, with one part by hoses meeting the same specifications than before. This point of operation allows assistance up to a maximum value Ω ₂ of the flywheel speed, greater than the previous value Ω-_, corresponding to a flow rate Q (Ω ₂ ).

The difference in flow rate (Ω ₂ ) -Q (Ω ₁ ) existing between the two operating points makes it possible to swell the pipes, during transitions, without any wall effect.

FIG. 5a is the curve of temporal variations of the supply voltage U of an electric pump unit, controlled during a request from the direction, and FIG. 5b is the curve of temporal variations of the pressure P of the oil in the steering assistance hydraulic circuit. In the absence of stress from the flywheel, the voltage U _m has a minimum value U _mj _ _n , which is not zero, serving as standby voltage. Due to a stress on the flywheel, the pressure P increases from its minimum value P _m i _n and its passage through a determined threshold value P ^, at an instant t- ^, causes the voltage U _{m to} rise at a maximum value U _maχ , greater than U _m ^ _n , intended to increase the pressure P to a maximum value P _max - The time interval separating the moment t-_ of detection of the steering wheel stress and the instant t ₂ corresponds to the transient phase of inflation of the pipes.

This maximum value U _maχ can be equal to the supply voltage U _alj _ _m of the electric _pump group. ^u _m i _n -

At the end of the driver's request for steering, defined at time t ₃ by the passage of pressure P to a threshold value P ₂ , lower than the maximum value P _max but greater than the first threshold P- ^ the pressure P decreases to its minimum value ^P min ^ the instant t ₄ . The decrease in the supply voltage U _m is only controlled at the instant t ₅ after t ₄ , after a delay starting at the instant t ₃ of detection of the stopping of the solicitation, in order to avoid a new pressure build-up phase in the hydraulic circuit in the event of further stress.

The solution proposed by the method according to the invention for reducing the electrical consumption of the electric pump group consists in controlling its supply voltage as a function of the demands of the flywheel, determined by parameters dimensioning the assistance such as the current consumed by the electric pump group, or any other information such as vehicle speed. Thus, during a steering operation of the vehicle, the hydraulic assistance circuit is in the oil pressure rise phase, and the method controls the supply voltage of the electric pump group to its maximum value, then reduces this supply voltage, when the hydraulic circuit is under pressure, at an optimal value making it possible to provide steering assistance up to a value of speed of rotation of the flywheel equal to Ω-_. Thus, the flow rate necessary for the inflation of the flexible pipes is only provided during the transient phase of inflation of said pipes, and only the flow rate necessary for the assistance of the management during the steering maneuver is provided after this transitional phase. FIG. 6a is the variation curve of the supply voltage U _m , when the method according to the invention is used, for a variation of pressure as a function of the requested stress, represented in FIG. 6b identical to that of the figure 5b. This variation can be carried out according to a step represented by dotted lines, or else according to a progressive decrease curve.

During the transient phase of pressure build-up of the hydraulic circuit, between the instants t ^ and t ₂ , the electric pump group is supplied with maximum voltage ^U m ~ ^u max 'thus allowing the swelling of the flexible pipes. Then, from the operating point D in FIG. 4, the supply voltage decreases, gradually or not, towards an optimal value U corresponding to a flow rate of the hydraulic pump which assures the assistance of the management up to a speed of rotation of the flywheel equal to the maximum value Ω ^, (operating point C in FIG. 4), previously mentioned. After a time delay starting at time t ₃ which corresponds to the end of the assistance request, the supply voltage U _m resumes its standby value U _mj _ _n at time t ₅ . This gain in supply voltage to the electric pump group, during steering maneuvers, makes it possible to reduce its consumption and therefore its average power consumed. The gain in power is represented by hatching in FIG. 6a.

According to a particular, nonlimiting example of carrying out the method according to the invention, the curve of progressive decrease in the supply voltage corresponds to the following equation E, corresponding to a low-pass filtering which aims to delay the decrease in the supply voltage U _m and thus ensure the flow required for swelling of the pipes. This low-pass filtering P _β relates to a term which depends on the one hand on the instantaneous current I supplied by the motor of the electric pump group, on the maximum current I "flowing through the motor when the oil pressure of the hydraulic circuit is maximum , the current I _r output by the engine at rest under standby voltage U _m in C _ι ^hen I ^e vehicle in a straight line, and on the other hand the maximum voltage _maχ which may be equal to the supply voltage U _a ii _m from the electric pump group - voltage from the electrical network for example - and the optimal voltage U:

^{E: u} m ^{= U} max ^"P B ^ m ^{" ∑ "} V * < ^U max ^{" U} op ⁾ 3

Thus, during a rapid request from the direction, in the event of an avoidance maneuver for example, the swelling of the pipes is ensured since at the start of the maneuver, the instantaneous current I of the motor being equal to the current I _r at at rest, the electric pump group is supplied with a maximum voltage U_ Hlα_-X,.

As the pressure rises in the hydraulic steering assistance circuit, the instantaneous current I in the motor increases and the term subject to filtering:

^(Χ m ^"I r / ^I ^M" V * ^(U max ^"U op) ^has ugmente.

At the end of the transient phase of inflation of the pipes, the supply voltage of the electropump group stabilizes at the optimal value U allowing total assistance from the management up to the threshold Ω- ^ of maximum flywheel speed. The time constant of the low-pass filtering must be greater than the hydraulic time constant of the circuit, of the order of a few hundred milliseconds for example.

According to an alternative embodiment of the invention, the respectively maximum and optimal values U _maχ and U of the supply voltage of the electric pump group can be a function of external information such as the speed of the vehicle.

According to another variant, the maximum current I _M which flows through the motor depending on the temperature, it can be recalculated periodically as a function of the stress on the steering.

The hydraulic power steering control method according to the invention makes it possible to adapt the flow rate of the electric pump group to the flow rate called by the assistance cylinder as a function of the level of assistance required during each maneuver. The main advantage of this process is the energy saving brought compared to current processes which do not optimize the flow of the electric pump group during steering maneuvers. FIG. 7 shows the gain in power consumed, on a turning profile, between an electro-hydraulic power steering controlled without the method of the invention and a steering controlled with such a method. The solid line curve represents the variation of the instantaneous power without the process and the dashed line represents the variation of the instantaneous power optimized thanks to the process. Another advantage of this process is that it is applicable to hydraulic steering assisted by an electropump group controlled electronically without the addition of an additional sensor, to measure the vehicle speed or the steering angle for example, since it uses representative information of the motor load and which is already used to control it, information given by its internal resistance, the internal voltage drop or the current output.

Claims

1. Method for controlling an electrohydraulic steering of an electric pump group (9) electronically controlled as a function of the stresses of the steering, and comprising a hydraulic circuit, part of which is produced by flexible pipes (5), characterized in that , during the transient phase of pressure increase in the hydraulic circuit starting from the detection of a stress on the steering, by passing the pressure (P) of the hydraulic circuit with a non-zero minimum value (P _mj _ _n ) , at a threshold value (P. ^ greater than (P _m i _n ), the supply voltage (U _m ) of the electric pump unit (9) changes from a standby value (U _min ) not zero to a maximum value ( ^u _maχ ) ensuring the swelling of the pipes, and in that, after this transient phase, the supply voltage (U _m ) decreases to an optimal value (U _op ) corresponding to a flow rate of the hydraulic pump (7 ) providing management assistance up to a vi maximum tesse (Ω ^) of rotation of the vehicle steering wheel, before resuming its standby value ( ^u rαin) after the end of the steering request.

2. Control method according to claim 1, characterized in that the supply voltage (U _m ) of the electric pump group (9) goes from the maximum value

(U _maχ ) at the optimum value (U _op ), after the pressure build-up phase of the hydraulic circuit, according to a voltage step.

3. Control method according to claim 1, characterized in that the supply voltage (U _m ) of the electric pump group (9) gradually decreases from the maximum value (U _maχ ) to the optimal value (U), after the pressure build-up phase of the hydraulic circuit.

4. Control method according to claim 1, characterized in that the supply voltage (U _m ) of the electric pump unit (9) goes from the optimal value (U) to the standby value ( ^u _m in ^ after a time delay starting at the end of the steering load, detected when the pressure (P) of the hydraulic circuit changes from its maximum value (P _max ) to a threshold value (P ₂ ) less than ( ^p _max ) ^and greater than first threshold (P,).

5. Control method according to claim 1, characterized in that the maximum ( ^u _ma ) ^and optimal (U) values of the supply voltage (U _m ) of the electric pump unit (9) are determined according to external information such as vehicle speed (V _v ).

6. Control method according to one of claims 1 to 5, characterized in that the maximum value (U _max ) of the supply voltage (U _m ) of the electric pump unit (9) is equal to the supply voltage ( ^u _a ιi _m ) ^of the vehicle's electrical network.

7. Control method according to claim 3, characterized in that the decrease function of the supply voltage (U _m ) of the electric pump group (9) is that of a low-pass filtering, intended to delay the decrease of the voltage from the maximum value (U _maχ ) to the optimal value (U _op ) to ensure the swelling of the flexible pipes of the hydraulic circuit, the equation of which is: ^u m - ^u max ^"P BC ^ m - V ¹ !! ^{" X} r) * ( ^u max ^"u op>] U _m being the supply voltage the pump unit, ^u max ^is the maximum value of the supply voltage to ensure the swelling of hoses, ^u Uo _OPD ^e e ^t t ⁱ ant the optimum value of the supply voltage,

I _m being the instantaneous current supplied by the electric pump group,

I _M being the maximum current delivered when the oil pressure of the hydraulic circuit is maximum,

I being the current supplied when the electric pump group is supplied with standby voltage (U. mm

P _B expressing the first order filtering of the term

^[ d "m - ^I r ^IC M"^"x r ⁾ * ⁽ x ^u op ⁾