DE4131076A1 - Active suspension for vehicle - has separate control valves for each side of vehicle - Google Patents

Abstract

To reduce the number of control valves, each side of the vehicle is fitted with a separate control valve (21). The suspensoin units nearest the engine have a larger thrust area than those at the other end of the vehicle, providing more support for the engine. The two control circuits are supplied via a pressure control circuit. The control system provides a self levelling adjustment. The control valves (21) are solenoid (12) operated from the processor control (38) and the pressure control system (11) has control valves (A) and a controlled bypass (15, 16). The suspension control incorporates acceleration detectors (45) to correct the suspension when braking, etc. ADVANTAGE - Simple control system, minimum of control valves.

Description

The invention relates generally to an active suspension, in particular a suspension, which the rolling movement of a Vehicle can regulate.

Active suspensions where pressurized hydraulic fluid alternatively to hydraulic Servo devices can be supplied are provided the rolling motion of a vehicle with others undesirable phenomena such as insufficient ground clearance, front or to prevent heavy loads or diving. Even so if the arrangement is limited to regulating the roll motion is required (i.e. the inclined position) in the conventional Arrangements still each one with a respective one Hydraulic cylinders connected to the vehicle wheel have their own Pressure control valve. A separate one for each vehicle wheel However, providing the pressure control valve increases the cost as well Weight of the suspension and makes it more complicated.

It is an object of the invention to have an active suspension to provide which only two control valves to regulate the pressure of all four hydraulically operated servo devices, which with the respective front or rear vehicle wheel are connected.

In short, this task is accomplished through an arrangement where the one directed into the cylinders on the left side Pressure is regulated by a first valve and that in the Right side cylinder passed pressure from a second Valve is regulated. Those cylinders that are on the engine-side end of the vehicle are arranged larger pressurized work surface than the others.

In particular, the invention is that a variable Suspension with the following features is provided: one front left and right hydraulically operated Servo device, which are suitable for an active  Connection between the left and right front wheel of the Create vehicle with the chassis of the vehicle; one rear left and right hydraulically operated Servo device, which are suitable for an active Connection between the left and right rear wheel of the To create vehicle with the chassis of the vehicle, the rear left and right hydraulically operated servo devices a smaller pressurized work surface than that Front left and right hydraulically operated servo devices Has; a source of fluid for providing pressure standing hydraulic fluid; a first pressure control valve, which is in the hydraulic fluid path between the Fluid source and the front left and rear left hydraulically operated servo is arranged; and a second pressure control valve, which in Hydraulic fluid path between the fluid source and the front right and the rear right hydraulically actuated servo device is arranged.

According to another embodiment, the invention is in one Suspension for a vehicle with a first and a second End, and one near the first end of the vehicle arranged engine, the suspension is provided with: a first and a second hydraulically operated Servo device, which are suitable for an active Connection between a first and second vehicle wheel with to create the chassis of the vehicle, the first and second vehicle wheel arranged at the first end of the vehicle are; a third and fourth hydraulically operated Servo device, which are suitable for an active Connection between a third and fourth vehicle wheel with to create the chassis of the vehicle, the third and fourth hydraulically operated servo device a lower one pressurized work surface as the first and second have hydraulically operated servo, and the third and the fourth vehicle wheel at the second end of the vehicle are arranged; a first pressure control valve, which in Hydraulic fluid path between a fluid source for  pressurized hydraulic fluid and the first and third hydraulically operated servo device is arranged; and a second pressure control valve, which in the Hydraulic fluid path between the fluid source and the second and fourth hydraulically operated servo devices is arranged.

The invention is more preferred in the following Exemplary embodiments with reference to the drawing explained. In the drawing shows

Figure 1 is a schematic representation of the active suspension with an arrangement for providing the hydraulic fluid.

FIG. 2 is a graph which, based on the output pressure and the output current, represents the characteristic of the pressure control valves according to the embodiment according to FIG. 1;

Fig. 3 is a model illustrating the charge of the rolling motion of the vehicle parameter;

Fig. 4 is a block diagram with a control loop of the aforementioned embodiment;

Fig. 5 is a flow chart showing the individual steps which characterize the control program as it runs in the schematically illustrated in Figure 4 microprocessor. and

Fig. 6 is a diagram showing the relationship between lateral acceleration and roll angle.

Referring to FIG. 1, a fluid source FS is connected to supply hydraulic fluid with a first drive unit (for example, an internal combustion engine) via a drive shaft 2 a, wherein the fluid source is provided with: an oil pump 1, an oil tank 3, a check valve 4, a pressure line 5 , an oil cooler 6 , through which oil is returned to the oil tank 3 by means of the return line 7 , a reservoir 8 , an oil filter 9 and a non-return bypass valve 10 , the aforementioned elements being connected to one another in the manner shown.

A pressure control unit 11 and a self-protection valve 12 are arranged in the pressure line 5 and the return line 7 between the liquid source FS and the left and right pressure control valves 13 L and 13 R, respectively.

The pressure control unit 11 has: a check valve 14 which is arranged in the pressure line 5 , a pressure relief valve 15 which connects the pressure line 5 to the return line 7 and regulates the pressure in the pressure line to a predetermined value PL Kg / cm ³ ; a pressure-sensitive check valve 16 which reacts to an operating pressure Pp arising downstream of the self-protection valve 12 and which is arranged in the pressure line 5 , and an electromagnetic valve 17 and a throttle bore 18 which are arranged parallel to one another between the check valve 14 and the self-protection valve 12 .

If the check valve 16 reacting to the actuation pressure is subjected to a pressure exceeding a predetermined dimension PN, the effect of the check valve is canceled and the check valve is thus opened, whereby hydraulic fluid can flow through. When the pressure falls below the PN value, the check valve closes and its one-way effect is restored.

The self-protection valve 12 and the electromagnetic valve 17 are supplied with the control signals CS1 and CS2 by the control unit 38 . As will become clear further below, the signal CS2 is transmitted to the electromagnetic valve 17 a few seconds later than the control signal CS1 to the self-protection valve 12 .

The self-protection valve is a spring branch valve and has four openings P, R, A, B. The opening P is connected to the pressure line via the one check valve 14 and the throttle arrangement 17 , 18 parallel to it, the opening R is connected to the opening 16 i of the pressure-sensitive check valve 16 in connection, the opening A communicates with the inlet openings 21 i of the pressure regulating valves 13 L and 13 R, and opening B communicates with the return openings 21 o thereof. A magnet 12 a of the self-protection valve 12 is arranged such that it can receive the control signal CS1. When the control signal CS1 assumes a small or an "off" value, the return spring 12 b moves a valve slide into its normal position, in which the connection between the openings P and R is blocked and a connection between the openings A and B is established . Otherwise, when the control signal CS1 takes an "on" value, the valve spool against the force of the return spring 12 b in its position pressed branch in which communication between the ports P and A and B & R is produced. If the connection between the openings B and R is interrupted, hydraulic fluid is pressed through a fixed throttle bore 12 c, which is intended to form a bypass for the two openings.

The pressure control valves 13 L and 13 R have valve spools which are provided to either establish a connection between the feed and return opening 21 i or 21 o with the pressure supply openings 21 c, or to shut off these connections. Both ends of each of the valve spools are exposed to an actuation pressure which is derived from the pressure at the supply and control openings 21 c and 21 i. Magnets 21 s are provided for electrically controlling the slide-in control valves, for which purpose a force is optionally applied to the respective valve slide of the valves, which regulates the supply pressure.

With the above pressure control valve arrangement, the supply openings 21 i are connected to the opening A of the self-protection valve 12 and the return openings 21 o are connected to the same opening B. The supply openings 21 c of the pressure control valves 13 L and 13 R are connected to the pressure chambers L of the front left and rear left and front right and rear right hydraulic cylinders 19 FL and 19 RL or 19 FR and 19 RR.

FIG. 2 shows the manner in which the control currents IL and IR act on the magnets 21 s of the pressure control valves 13 L and 13 R and thus regulate the supply pressure output by them. As can be seen, a minimum current Imin is provided which generates a minimum output pressure Pmin close to zero. As the strength of the current increases, the pressure increases proportionally according to a predetermined proportionality constant K.

A first accumulator 31 is connected to a return line arrangement 30 which connects the return openings 21 o of the pressure control valves 13 L and 13 R to the opening B of the self-protection valve 12 . This accumulator is provided to absorb the pressure that arises between the aforementioned openings in the return line arrangement 30 .

A second memory 32 is connected to the supply openings 21 i of the pressure regulating valves 13 L and 13 R and the opening A of the self-protection valve. This memory is intended to store the pressure and to serve as a buffer for periods in which there is a high supply requirement.

The check valve 33 is intended to open at an excessively high pressure in the return line arrangement and to allow the pressure to escape into the line which connects the accumulator 33 to the openings A and 21 i.

Each of the cylinders 19 FL to 19 RR has a cylinder tube 19 a and a piston 19 c, which is provided with a through hole 19 b and is firmly connected to a piston rod 19 d. The pistons each limit a pressure chamber L within the respective cylinder tube.

The lower ends of the cylinder tubes 19 a are connected by means of suspension parts 42 to the vehicle wheels 41 FL to 41 RR, the upper ends of the piston rods 19 d being connected to chassis parts 43 .

As can be seen from the drawing, those are from pressure pressurized work surfaces on the top of the pistons smaller than that on the bottom of the piston. In this Embodiment is the difference of pressurized work surfaces on those with the front wheels connected piston Af; and those with the rear wheels connected piston Ar. In the present invention: Af <Ar. This is achieved by using the front wheels interacting piston rods have a different diameter than have the piston rods interacting with the rear wheels. This arrangement enables one of the front, with the Front wheels work together to generate force that cylinders is larger than that interacting with the rear wheels Cylinder.

This arrangement allows one through the front suspension larger rolling roll than the rear Suspension and thus allows countermeasures and one Stabilization of the steering characteristics and at the same time an additional rolling moment, which is required if the Engine is at the front end of the vehicle, though reduced the maximum hydraulic fluid pressure can be. The latter impact is reduced of course the load of the pump and thus the first Drive unit and thus reduces the fuel consumption of the Vehicle.

Fig. 3 shows a model in front view, which shows the effect of the lateral acceleration on the wheel suspension of the vehicle. This effect occurs equally with the front and rear suspension. In this model designate:
W is the mass of the chassis of the vehicle that acts on the suspension;
H is the height of the center of gravity of the chassis of the vehicle above the roll center of the suspension;
T the track gauge;
ε the track width between the longitudinal axes of the hydraulic cylinders at the height of the roll center; and
α denotes the lateral acceleration;
In addition, ΔP denotes the hydraulically induced rolling moment and
A is the pressurized working surface of the hydraulic cylinder.

The following applies:

If the indices f and r denote the front and rear suspension, the following also applies:
Wf <Wr,
εf _* Tf ≈ εr _* Tr
Hf ≈ Hr to keep the swaying low,
ΔPf must be approximately equal to ΔPr and Af <Ar

Each of the chambers L delimited by the hydraulic cylinders 19 FL to 19 RR is connected to a memory 20 b by means of a fixed throttle bore 20 a. This arrangement is intended to dampen or absorb relatively high-frequency pressure fluctuations (for example in the range of 10 Hz) in the resonance range of the suspension elements.

With each of the cylinders in each case acts a helical spring 20 c together and connected between the suspension and the chassis parts 42, 43rd

An acceleration sensor 45 is arranged in a predetermined position within the vehicle (for example in the region of the center of gravity of the vehicle) and is designed in such a way that it emits a signal which is characteristic of the lateral acceleration α. In this connection, it is assumed that the signal has a positive value in the case of a rightward lateral acceleration and a negative value in the case of a leftward lateral acceleration.

An error detection device 46 is provided to display various operations that are essential for the functioning of the system and to output a signal AS. In the event that an error is detected, the signal AS assumes the logical value "1", for example.

The control unit 38 shown in FIG. 4 is designed such that it can receive the output signals of the lateral acceleration sensor 45 and the error detection circuit 46 . The analog output signal of the lateral acceleration sensor 45 is transmitted to an input / output I / O 51 a of the microprocessor 51 by means of an analog-to-digital converter A / D 50 . The digital output signal of the error detection circuit 46 is sent immediately there. The microprocessor 51 also contains a central data processing unit CPU 51 b and a memory unit 51 c with a RAM and / or ROM or the like.

The central processing unit CPU processes the input data and sends the signals characterizing the pressures PL and PR via the input / output I / O to the digital / analog converters D / A 52 L and 52 R. The latter circuits convert the digital signals into currents IL and IR, which are transmitted to the magnets of the pressure control valves 13 L and 13 R. In addition, signals CS1 and CS2 are generated and transmitted to the self-protection valve 12 and the electromagnetic valve 17 by means of a driver circuit 54 and 55 .

Fig. 5 shows the profile of the individual control steps to the desired values PR, PL, CS1 and CS2, due to the lateral acceleration and the input to determine the error signal.

The mode of operation of the present exemplary embodiment is as follows: By switching the ignition switch of the vehicle into the “on” position, the control unit 38 is supplied with electrical current and the microprocessor starts the control program shown in FIG. 5. The first step of this program is designated 1001 and sets both control signals CS1 and CS2 to the value "on" and the pressure signals PL, PR equally to the value PN, which is chosen so that the vehicle is raised to a high standard level. When both control signals CS1 and CS2 are set to "on", the self-protection valve 12 assumes a position in which the openings PA and RB are each connected to one another and the electromagnetic valve is closed, forcing the hydraulic fluid through the throttle bore 18 flow.

In the next step, a timer 1002 is polled in order to determine whether a predetermined period of time since power on of the system has passed or not. This period of time is so chosen to respond to the actuation pressure Check valve take its fully open position can.

When this period of time has elapsed, the program jumps to step 1003, in which the control signal CS2 is set to its "off" value. As a result, the electromagnetic valve 17 assumes its open position and thereby allows the hydraulic fluid to bypass the fixed throttle 18 .

In step 1004, the A / D converter converts it Input signal and that from the lateral acceleration sensor incoming signal (i.e. signal α) is read in and in step 1005 used to derive the values for PL and PR. To the Derivation algorithms are used, the following Equations are sufficient:

PL = PN + Kα (1)

PL = PN - Kα (2)

In step 1006, the values PL and PR are output to the driver circuit 53 L and 53 R such that the control signals IL and IR assume suitable values for the magnets.

The output signal AS of the error detection device 46 is then read in. In step 1008, it is determined whether the value of AS is "1" or not. In the event that everything has been recognized as "normal" (AS = 0), the program jumps back to step 1004. Otherwise, ie in the event of an error, the program proceeds to step 1009, in which a signal is sent which sets the signal CS1 to its "off" value.

When the ignition switch is turned to its "off" position and the engine of the vehicle is stationary, the check valve 16 , which reacts to the actuation pressure, closes and, as a result, the pressure control valves 13 L and 13 R are in a closed control loop. This maintains the pressure in it. When the ignition switch is closed again, the control unit 38 is again supplied with electrical power and the program shown in FIG. 5 is started again.

As a result, the left and right pressures PL and PR are set to a predetermined value PN and the self-protection valve 12 is set to an open position while the electromagnetic valve 17 is in a closed position. It is believed that due to the ignition switch being closed, the engine starts and is in its idle state at the same time. In this state, the pump 1 is driven by the drive shaft 2 a and hydraulic fluid is fed into the pressure line. The pressure in the pressure line 5 rises rapidly and if a value PHO is exceeded, hydraulic fluid is passed through a check valve 14 and the fixed throttle bore 18 (the electromagnetic valve 17 is closed at this time) and the self-protection valve 12 to the pressure control valves 13 L and 13 R. . The throttle bore 18 prevents the pressure in the pressure line downstream of the throttle bore from building up too quickly and ensures that the control pressure output by the pressure regulating valves does not exceed a certain rate of change. Accordingly, in the event that the initial pressure in the hydraulic lines leading to the hydraulic cylinders is below the PN value, the pressure is gradually increased to the required level and shocks or changes noticeable by the passengers are avoided.

The above explanations are then of importance, if the engine of the vehicle has stopped for some time and the pressure in the chambers of the cylinders slow to one value due to inevitable leakage degraded below PN. When the engine started again pressure is gradually built up again and so is that Height level of the vehicle slowly restored.

According to this sequence of operations, the signal CS2 assumes its "off" value, as a result of which the electromagnetic valve assumes its "normal" state, in which it is fully open. Of course, this has the consequence that the throttle bore is bypassed and is therefore ineffective, since there is a continuous connection between the pressure control unit 11 and the pressure control valves 13 L, 13 R and so the control of the rolling movement of the vehicle can take place.

Furthermore, in connection with pressure control unit 11, the actuation pressure Pp, which is determined by the pressure prevailing at the opening A of the self-protection valve 12 , exceeds the value PN, the pressure-sensitive check valve 16 opens and hydraulic fluid can flow back from the pressure control valves 13 L, 13 R flow the oil tank 3 . In addition, when the pressure upstream of the check valve 14 in the pressure line exceeds a predetermined value PH, the pressure relief valve 15 opens and excess pressure is released. In this way, the value PH is maintained for the pressure immediately upstream of the check valve 14 .

When the vehicle is at a standstill or when driving straight ahead, it experiences little or no roll motion, and the output signal of the lateral acceleration sensor 45 is approximately zero. In accordance with the control program shown in FIG. 5, the strength of the signals IL and IR is set to the value IN which ensures that the pressure in the chamber L of the cylinders maintains the value PN. This ensures that the chassis of the vehicle is kept at the same level.

For example, when the vehicle begins a right turn, the rear left corner of the vehicle lowers, the vehicle compresses, and the roll begins. At this time, the acceleration sensor 45 sends out a positive value. According to this change, the values of the signals IL and IR are derived in step 1005 of the control program shown in FIG. 5 and transmitted to the pressure control valves. This means that the value of IL is raised above the value IN, while the value of IR is reduced below the value IN. As a result, the control pressure PCR transmitted to the chambers L of the cylinders 19 FR and 19 RR is reduced below the value PN, while the pressure PCL transmitted to the chambers L of the cylinders 19 FL and 19 RL is raised above the value PN. Increasing the pressure in the cylinders on the left side of the vehicle increases the resistance of the cylinders to compression, while reducing the pressure in the cylinders on the right side of the vehicle prevents further rebounding. As a result, an anti-roll effect is achieved.

Accordingly, when driving through a left turn, the reverse process takes place and a suitable anti-roll effect will achieved.

As shown in FIG. 6, in the case of cornering, the pressure in the cylinders located on the outside of the curve is increased above the value PN and the pressure on the inside of the curve is reduced in accordance with the strength of the lateral acceleration signal α. That is, if α = o, the pressure on both sides is regulated to ΔPo. As the value of α increases in one direction (e.g. in the positive direction), the pressure in the left cylinder is increased and decreased in the right cylinder. If -αoαo + αo, the maximum and minimum rolling moment ΔPmax, ΔPmax ′, ΔPmin, ΔPmin ′ can be generated, and any increase in -α or + α over the values -αo and + αo results in a further change in pressure.

Because the working area Af of the front cylinder is larger than that Working area Ar is the rear cylinder, that is from the front cylinders generated roll moment ΔPmax greater than that of rolling moment generated the rear cylinders ΔPmax '. The improves the understeer tendency and the steering stability and also provides an additional force to do so is required to roll at the front end of the Regulate vehicle where the vehicle due to the arrangement of the Motors is usually heavier.

With the help of this arrangement it is possible the front Regulate the cylinder of the suspension with the same maximum pressure like the rear cylinders of the suspension. That reduces that to the drive machine transferred pump load and thus reduced fuel consumption. The longevity of the system will thus increasing the weight and size of the Liquid source FS can be reduced.

When the ignition switch is in its "off" position and the motor stops and with it the pressure at the outlet of the pump drops rapidly to the ambient pressure, the supply of electrical power to the control unit nevertheless via a predetermined period after turning off the ignition switch maintained by an energy self-sufficiency arrangement. This enables the control of the roll movement and a gradual decrease in pressure in the line Pressure Ps. When the value of Ps reaches the threshold value PN  the check valve closes and closes the Return line and thus represents an arrangement of a closed Circuit, which prevents pressure loss.

Subsequently, when the energy self-sufficiency period has elapsed, the control signals CS1 and CS2 assume a low or an "off" level. As a result, the self-protection valve switches to a position in which the openings A and B are connected to one another and the connection between the openings P and R is interrupted. At this time, the pressure line is connected to the pressure-sensitive check valve 16 by means of the fixed throttle bore 12 c. If Ps <PN and at the same time the pressure-sensitive check valve 16 is still open, the excess pressure can be reduced by draining through the throttle bore until Ps = PN, at which point the draining is stopped.

In the event that the working pressure at the pressure control valves 13 L and 13 R is recognized as insufficient, which indicates a defective pressure line or the like, an error signal (AS = 1) is sent out by the error detection circuit 46 . On the basis of this signal, the signal CS1 is set to the value "off" (step 1009), and the self-protection valve switches to a position in which the openings AB are immediately connected. In this state, any excess pressure can escape through the first throttle bore up to Ps = PN.

Although the embodiment described above is one Front engine, and therefore for this embodiment applies that Af <Ar, can in the case of a rear engine inverse ratio may be desirable, namely Af <Ar.

Although a difference in the working surface according to the above embodiment was achieved in that the diameters of the piston rods 19 d were different and the effective working surface on the upper side of the pistons was changed, the inner diameter of the cylinder tubes and the corresponding pistons can also be changed instead.

Because of the relatively simple basic principle of the control, the control unit 38 can also do without a microprocessor with hardware such as variable amplifiers, function generators and polarity reversal circuits.

Although the pressure control unit 11 of the above embodiment is provided with an electromagnetic valve 17 and a throttle bore 18 arranged in parallel thereto, alternatively both elements can be omitted and the self-protection valve 12 can be provided with a further throttle bore arranged in parallel and the arrangement can be designed such that for a Delay in power supply whose magnets are taken care of for a certain period after the ignition switch is turned on. As a further alternative, the electromagnetic valve 17 and the throttle bore 18 arranged parallel thereto can be arranged between the self-protection valve 12 and the two pressure control valves 13 L and 13 R.

The pump 1 can be driven by an electric motor instead of being driven directly by the vehicle engine.

Claims (5)

1. Variable suspension with:
a front left and right hydraulically operated servo device, which are adapted to provide an active connection between the left and right front wheel of the vehicle with the chassis of the vehicle;
a rear left and right hydraulically operated servo devices adapted to provide an active connection between the left and right rear wheels of the vehicle to the chassis of the vehicle, the rear left and right hydraulically operated servo devices having a smaller pressurized working surface than that has front left and right hydraulically operated servos;
a fluid source for providing pressurized hydraulic fluid;
a first pressure control valve disposed in the hydraulic fluid path between the fluid source and the front left and rear left hydraulically operated servos; and
a second pressure control valve located in the hydraulic fluid path between the fluid source and the front right and rear right hydraulically operated servos. 2. Variable suspension according to claim 1, with a pressure control unit which is arranged in the liquid path between the liquid source and the first and second liquid control valve, the pressure control unit being provided with:
an opening and closing electromagnetic valve which interacts with a pump;
a fixed throttle bore, which forms a bypass for the electromagnetic valve to be opened and closed, whereby hydraulic fluid supplied by the fluid source can flow even when the valve is closed;
a check valve arranged upstream of the electromagnetic valve to be opened and closed and the fixed throttle bore;
a pressure relief valve configured to open at a predetermined pressure and to drain hydraulic fluid from a location upstream of the check valve; and
a pressure-sensitive check valve, which reacts to the pressure developing downstream of the pressure control unit and controls an outflow channel through which hydraulic fluid can flow out of the first and second pressure control valves. 3. Variable suspension according to claim 1, with:
a sensor device for detecting the lateral acceleration;
a control device which is connected between and interacts with the sensor device and the first and second pressure control valve, the control device having a circuit arrangement which receives the output signal of the sensor device for detecting the lateral acceleration in order to regulate the first and second pressure control valve and so on regulate the pressure supplied to the front and rear left and front and rear right hydraulically operated servos. 4. Variable suspension according to claim 1, with:
an error detection circuit connected to and cooperating with the control unit; and
a self-protection valve, which is arranged between the liquid source and the first and second pressure control valve, the self-protection valve being designed such that it interrupts the connection between the liquid source and the first and second pressure control valve if an error message is issued by the error detection circuit. 5. Suspension for a vehicle having a first and a second end, and an engine disposed near the first end of the vehicle, the suspension being provided with:
first and second hydraulically actuated servo devices adapted to provide an active connection between a first and second vehicle wheel to the chassis of the vehicle, the first and second vehicle wheels being located at the first end of the vehicle;
a third and fourth hydraulically operated servo device, which are adapted to provide an active connection between a third and fourth vehicle wheel with the chassis of the vehicle, the third and fourth hydraulically operated servo device having a smaller pressurized working area than the first and second hydraulically have actuated servo, and the third and fourth vehicle wheels are arranged at the second end of the vehicle;
a first pressure control valve disposed in the hydraulic fluid path between a fluid source for pressurized hydraulic fluid and the first and third hydraulically operated servos; and
a second pressure control valve which is arranged in the hydraulic fluid path between the fluid source and the second and fourth hydraulically operated servo devices.