4324 Suspension control systems

Suspension control entails more than just regulation of the vertical movement of the wheels. The many factors that have to be taken into account include comfort for the occupants, roll, both longitudinal and lateral weight transfer, and the maintenance of contact pressure between the wheels and the ground consistent with good stability and handling.

In a fully active system there is a pump and hydraulic fluid reservoir and, generally, one hydraulic actuator and one control valve for each wheel or pair of wheels. There may also be one or more hudraulic accumulators, to supplement the rate of flow from the pump to cater of sudden deflections of the suspension. The control valves may be all in one unit. They execute commands from an on-board computer which is served with information by sensors. The computer may be programmed to make instantaneous responses to changes in road surface or equilibrium (speed, roll, brake dive, or acceleration squat) as indicated by the sensors or, in a simpler control arrangement, it may issue its commands periodically, to adjust the system to suit average conditions over periods of seconds or even minutes, depending on the precision of control required. Correction can be made also to under- or over-steer, by adjustment of the front: rear roll stiffness ratio, even automatically while the car is being driven.

Ideally, perhaps, sensors would detect the rise and fall of the ground in front of each wheel, so that the suspension could be made to deflect a precisely equal amount and thus keep the vehicle riding at a constant height above the ground. A speed sensor would be necessary, too, so that the computer could synchronise the movements of the suspension with the passage of the measured surface profile under the wheels. A further refinement might be a transducer to measure the actual response of the wheel so that, by taking into account hardness or softness of the surface, the dynamic loading on the tyres could be limited.

Such a system, however, is impracticable at the current state of the art, and it is simpler to use a pressure transducer in each hydraulic actuator to signal to the computer any increase or decrease in the load applied by the ground to the wheel. Responding virtually instantaneously to this signal, the control system can direct fluid to flow either into or out of the actuator, as necessary.

The sensors actually used may include one transducer on each axle to measure the variations in height of the sprung mass above it under varying loads, a speed sensor, a yaw detection gyroscope or steering motion transducer, or a lateral accelerometer for measuring tendency to roll, a longitudinal accelerometer to detect braking and acceleration forces, and an accelerometer

1

Front actuator

7

Yaw gyro

2

Accumulator

8

Longitudinal and lateral accelerometer

3

Control panel

9

Rear actuator

4

Hydraulic pump

10

Hub accelerometer

5

Steer angle transducer

11

Computer

6

Oil reservoir

12

Servo valve

-Hydraulic pipeline -------Transducer signal -----Control signal

-Hydraulic pipeline -------Transducer signal -----Control signal

Fig. 43.29 Schematic diagram of the Volvo Computer Suspension (CCS). A major advantage of such a system is that changes to the performance characteristics of its different components, which of course may effect those of others, can be made simply by changing the program on the computer, and the overall effect assessed immediately without having to modify a vehicle and take it on to the test track or strain guage on each hub for assessing the quality of the road surface. In some instances a degree of simplification has been obtained by using for the rear axle only one height sensor and one actuator, the motions of the two rear wheels being made interdependent through a hydraulic interconnection. Alternatives to some of the above-mentioned sensors might be transducers sensing the displacements of the throttle, brake pedal, steering gear and axle.

A typical fully active suspension system is illustrated in Fig. 43.29 and a semi-active one in Fig. 43.30. Neither, however, is incorporated even as a standard option in a quantity-produced vehicle. Since, if studied in relation to the basic principles already outlined, Figs 43.29 and 43.30 are self-explanatory, it is not proposed to describe them in detail here. However, some comments are necessary regarding Figs 43.31 and 43.32.

Figure 43.31 shows a single suspension unit in the AP system. An increase in the static load on the vehicle causes the lever actuating the spool-type control valve, on the right in the illustration, to be deflected upwards about its pivot. This of course is provided the relative movement between suspension

Pump Tank Control valves

Pump Tank Control valves

Fig. 43.30 With gas springs, the AP suspension system is of the semi-active type. As with a fully active suspension system, it has the advantage of a soft ride coupled with high roll stiffness. Unlike such a system, though, it does not allow the car to sink on its suspension when the engine is stopped. There are three gas springs, one for the rear axle and one each for the two front wheels

Fig. 43.31 A single AP suspension module. The pendulus offset mass is on one end of a bell-crank lever, the other end of which actuates the spool valve. The connection between the first-mentioned arm and the suspension arm is a link in which is incorporated a small coil spring and damper unit

arm and body is slow, so that the coil spring and damper in the linkage between the suspension arm and the lever are not compressed. The consequent upward deflection of the lever pulls the spool valve to the left causing it to direct hydraulic fluid into the hydraulically damped gas spring, thus extending it until the ride height returns to what it was before.

Rapid upward movements of the lever, on the other hand, are opposed by the inertia of the offset mass, so the coil spring and damper are compressed and there is little or no movement of the spool valve. In these circumstances the gas spring performs as in a conventional suspension system and, since little or no fluid motion is involved, the energy consumption by the engine-driven pump is correspondingly small. To obtain this effect, the coil spring and damper have to be tuned so that the force exerted on the offset pendulous mass gives it the same vertical acceleration as that imparted to the body of the vehicle by its gas spring and integral damper. This system has two major advantages. First, the body does not sink down on its suspension when it is switched off. Secondly, it does not need an electronic control.

The AP system can be tuned to take into account not only the lateral weight transfer but also the vertical deflection of the tyres during cornering. In some instances, however, especially for heavy vehicles, it may react too slowly. This can be overcome by arranging for a steering input, as shown in Fig. 43.32. A double-acting ram, actuated by the steering mechanism, transfers hydraulic fluid from one side to the other of the vehicle through the dampers in the links between the suspension arms and the levers actuating the spool valve. It does this in a direction such as to lift the lever on the outer and lower that on the inner side of the turn.

Fig. 43.32 The hydraulic arm is actuated by the steering gear, which moves it to displace fluid in either direction through the damper unit, according to which way the vehicle is steered

Further information on active suspension in general, including design calculation, can be obtained from two papers, by Sharp and Hassan, Proc. I. Mech.E., Vol. 200 D3, 1986, and Vol. 201 D2, 1987.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment