Unequal weight distribution levelling control

(Fig 10.124(a and b)) Uneven weight distribution is automatically compensated for by the levelling control

(a) Heavy load without height level control

(b) Heavy load with height level control

(a) Heavy load without height level control

(b) Heavy load with height level control

Increased wheel load

(c) Cornering without anti-roll control

Reduced wheel load (+ W)

Increased wheel load

(c) Cornering without anti-roll control

Reduced wheel load (+ W)

(d) Cornering with anti-roll control

(d) Cornering with anti-roll control

Fig. 10.124(a-g) Active self-levelling hydraulic/coil spring suspension

Fig. 10.124 contd

valve which allows fluid to enter or exit from the individual strut actuators. Figure 10.124(a) shows the body height if there was no level control or if the self-levelling height is set to a low level, whereas Fig. 10.124(b) shows the suspension level raised against the compressed road springs to compensate for the heavy load. Longitudinal levelling also takes place between the front and rear suspension strut actuators when, for instance, all the passenger seats are occupied and the boot is full of luggage.

Body roll control (Fig. 10.124(c and d)) When the car is negotiating a corner the body tends to tilt so that the inner and outer wheel loads are reduced and increased, respectively. This lateral load transfer shown in Fig. 10.124(c) compresses the outer springs and expands the inner springs thus causing the body to roll and to become uncomfortable for the driver and passengers. To compensate for the

weight transfer, fluid is pumped or released into the outer strut actuators via the levelling control valve until it has lifted the body on the outside to the same height as the inside (see Fig. 10.124(d)). Usually a small angular roll is deliberately allowed to provide the driver with a sense of caution.

Anti-dive control (Fig. 10.124(e and f)) If the car is braked rapidly there is a tendency for the body to pitch forwards, that is, the front of the body temperately dives downwards and the rear lifts (see Fig. 10.124(e)); the dive experienced is due to the longitudinal weight transfer since the body mass wants to continue moving forwards but the road wheels and the unsprung suspension mass are being retarded by the action of the braking force. To overcome this inherent deficiency in the suspension design which occurs usually when soft springs are used, fluid is rapidly transferred into both of the front strut actuator cylinders (see Fig. 10.124(f)), thereby correcting the front to rear tilt of the body over the braking period and then releasing the excess fluid from the front actuators when normal driving resumes.

Anti-squat control (Fig. 10.124(g and h)) If a car is accelerated rapidly, particularly when pulling away from a standstill, there is a proneness for the body due to its inertia to hold back whereas the propelled wheels and unsprung part of suspension tend to move ahead of the interlinked body. This results in the body tilting backwards so that it squats heavily on the rear axles and wheels (see Fig. 10.124(g)). To correct this ungainly stance when the car is being accelerated, fluid is quickly displaced from the accumulator and pump through the open levelling control valve into the rear strut actuator cylinders; this levels the body longitudinally (see Fig. 10.124(h)). Once the acceleration sensor detects a reduction in acceleration, the electronic-control unit signals the levelling control valve to return the excess fluid trapped in the rear actuator cylinders back to the reservoir so that under steady driving conditions the body remains parallel to the road.

10.17.3 Semi-active controlled hydro/gas suspension (Fig. 10.125(a and b)) Most of the self-levelling suspension layouts are only designed to achieve semi-active suspension control as there is a cost factor and it might not be justified to produce a near perfect suspension since there is always some inherent deficiencies due to other factors built into the body/suspension/ transmission design. One version of a semi-active suspension is shown in Fig. 10.125(a, b) here, instead of using steel coil or leaf springs a hydro/ gas spring is employed. These spring units basically consist of two hemispherical chambers separated by a flexible diaphragm; the outer sealed chamber is filled with pressurized nitrogen gas which acts as a spring media whereas the underside chamber is connected through the fluid to the height adjusting strut actuators and the supply pump and accumulator. There are three springs for both front and rear pair of suspensions. The system is designed to give two spring rates, these are 'sports' (stiff springing) and 'comfort' (light springing) and there are two damping modes, firm and soft.

Wheel deflection is absorbed through the height actuator cylinder and piston ram so that as a wheel goes over a bump or pot hole, the suspension swing-arm will tilt correspondingly up or down. An upward movement of a swing-arm and ram will displace fluid into the chamber underneath the diaphragm thereby causing it to compress the nitrogen gas. If however the wheel goes into a pot hole the downward movement of the swing-arm and ram causes the effective cylinder space to increase, this reduces the fluid pressure and permits the hydrogen gas to expand by pushing down the diaphragm, and fluid will therefore be displaced back into the height actuator cylinder.

Height and levelling control (Fig. 10.125(a and b)) With the engine running fluid is pumped into the accumulator and into all four height actuators and

Sf ride Restriction third spring^ damper

Left hand hydro/gas spring

Height actuator

Right hand hydro/gas spring

Sf ride Restriction third spring^ damper

Left hand hydro/gas spring

Height actuator

Right hand hydro/gas spring

(a) Soft ride interlinked three spring suspension mode

Fig. 10.125(a and b) Semi-active hydro/gas suspension

(a) Soft ride interlinked three spring suspension mode

Fig. 10.125(a and b) Semi-active hydro/gas suspension

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fl, spv

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(b) Firm ride independent two spring suspension mode into each lower spring unit chamber until the predetermined body height setting is reached, at which point the levelling control valve blocks any further supply of fluid to the system. If the load changes due to more or less passengers and luggage, the levelling control valve will automatically permit fluid to enter or leave the spring/actuator system to maintain the optimum programmed body height setting.

Comfort (light springing) ride (Fig. 10.125(a)) For a comfortable ride under normal driving road conditions the spring loaded plunger is pulled outwards (downward). Both height actuators are interconnected with the fluid supply and the soft-ride third spring; the latter increases the volume of compressible gas by 50%, thus reducing the spring stiffness, thereby providing greater ride comfort. When the body rolls during cornering fluid is transferred from the outer to the inner height actuator cylinders but this is slowed down to some extent by the restrictor dampers.

Sports (stiff springing) ride (Fig. 10.125(b)) If the car is to be driven fast or is moving sharply around a bend the suspension system can be switched to sports (firm) ride by way of pushing the stiffening plunger inwards (upwards), see Fig. 10.125(b). This has the effect of blocking the third spring fluid movement from both adjacent right and left hand wheel springs. All three springs are now isolated from each other so that the vertical deflection of each wheel strut actuator is confined entirely to its own gas filled spring. Accordingly the spring rate for vertical piston ram movement is stiffened for each wheel suspension, and this also provides a degree of body roll control.

10.17.4 Semi-active hydro/gas electronic controlled suspension system (Fig. 10.126) The system shown in Fig. 10.126 illustrates a front double wishbone suspension and a rear semi-trailing swing-arm suspension, each wheel having its own levelling strut actuator and hydro/gas spring unit. A third soft ride hydro/gas unit is shared between the front pair of suspensions and similarly a third unit is installed for the rear suspension. An electronic-control unit microprocessor is incorporated in the system which takes in signals from the various sensors; this information is then processed and converted to electrical instructions to the various solenoid control valves. Note that some of the sensors shown in Fig. 10.126 may not always be included in a semi-active suspension system, the actual sensors chosen will depend upon the degree of control sophistication to be built into the suspension design. The sensors shown are listed as follows: height sensors, load sensors, steering sensor, longitudinal and lateral acceleration sensors, acceleration pedal sensor, brake pressure and pedal sensors and vehicle speed sensor. For instance a simple semi-active suspension can operate effectively with just four sensors such as height sensors, steering sensor, brake pressure sensor and vehicle speed sensor. The electronic-control unit output directs the energizing and de-energizing of the front and rear solenoid controlled levelling valves and the front and rear solenoid stiffening valves. There is a mode selector which enables the driver to put the suspension in either sports (firm) ride or comfort (soft) ride; however, when switched to comfort ride, if driving conditions become harsh the suspension automatically reverts to sports (stiff springing) ride, but it will eventually change back to comfort ride when normal driving conditions prevail.

Comfort (soft) ride mode (Fig. 10.126) When switched to comfort ride mode the electronic-control unit will supply current to energize both the front and rear solenoid controlled delivery valves incorporated in both of the levelling control valve units and also to supply current to energize the front and rear stiffening solenoid valve units (note Fig. 10.126 shows the front levelling control valve and the front stiffening valves open to fluid delivery). Fluid is now permitted to flow to each wheel strut actuator cylinder and its corresponding hydro/gas spring unit and in addition to the both front and rear soft ride third spring units until the preset body height level is reached. The appropriate sensors now signal the electronic-control unit to switch off the power supply to all the solenoid valves; the correct quantity of fluid is therefore contained in the four actuator cylinders for the conditions prevailing at the time. Should the body load be reduced such as when a passenger gets out, the electronic-control unit signals both the solenoid controlled return valves installed in the front and rear levelling control valve units to open (note Fig. 10.126 shows the rear levelling control return valve open for the exit of fluid), fluid will now return to the reservoir until the body height sensors signal that the correct body height has settled again to the manufacturer's setting, and at this point the solenoid return valves close. This cycle of events for delivery and releasing fluid to the spring and levelling system is continuous. When driving conditions change

Front soft ride hydro/gas spring unit

LHF hydro/gas spring

Front stiffening solenoid valve (open)

LHF hydro/gas spring

Front stiffening solenoid valve (open)

Pulley hydraulic pump

Reservoir

Rear levelling control solenoid valve

Fig. 10.126 Semi-active hydro/gas electronic controlled suspension

Front height and levelling ' strut actuator

Height sensor

Pulley hydraulic pump

Reservoir

Rear levelling control solenoid valve

Acceleration pedal sensor

Brake pedal sensor

Brake pressure sensor

Steering wheel sensor

Longitude acceleration sensor

Lateral acceleration sensor

Fig. 10.126 Semi-active hydro/gas electronic controlled suspension such as during rapid acceleration, hard braking, fast cornering or high speed driving, the front and rear solenoid controlled stiffening valves are signalled to de-energize; this permits the stiffening plunger valves to close, thereby isolating the front and rear interconnected three spring units from each other, thus automatically changing the suspension mode to sports (firm) ride (note Fig. 10.126 shows the rear stiffening solenoid valve closed as for sports (stiff springing) ride mode).

Sports (firm) ride mode (Fig. 10.126) If the driver switches the suspension to sports (stiff springing) ride mode the electronic-control unit will instruct the front and rear stiffening valve unit to de-energize and push in the plunger valve. As a result each wheel spring and strut actuator becomes isolated from the spring opposite it and also from the third stiffening spring unit. As a result the reduction in diaphragm area exposed to the nitrogen gas (now 50% less) increases the stiffness rate of each spring/actuator unit; it therefore provides a firm ride for good steering response, and the prevention of fluid moving between the right and left hand strut actuators increases the suspension's resistance to body roll.

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