Development of a Novel Hydro-Pneumatic Active Suspension

Meeting chaired by Phil Cooke

Bo Gao

Research PhD student, Dept of Mechanical Engineering University of Bath

25 May 2005

1. Introduction to vehicle suspensions

Introduction: As one of the most important systems in a vehicle the suspension is a major focus of automotive engineers. Suspension is the term given to the system of shock absorbers and springs as well as linkages, which connect a vehicle to its wheels. Its main functions are:

Isolate vehicle body from road induced vibration
Maintain contact between tyre and road
Control body pitch and roll
Limit wheel movement
Support range of loads
Figures [1-2] show two typical suspensions in modern vehicle designs.

1 McPherson suspension.

2 Double-wishbone suspension
The first one is called McPherson suspension. The shock absorber (also known as damper) and the coil spring are attached to an A-arm at the bottom to the wheel and attached at the top to the vehicle body. The second one is a double-wishbone suspension. It has two parallel wishbone-shaped arms to locate the wheel. Each wishbone (or arm) has two mounting positions to the chassis and one at the wheel hub. The shock absorber and coil spring mount to the wishbones to control vertical movement.

Hydraulic shock absorbers are most often used in a vehicle suspension. It can dissipate the kinetic energy in the system and convert it into heat. [3]

 

3 Shock absorbers.
shows 3 types of hydraulic shock absorbers used in automotive applications. When the hydraulic oil flow through the damper valve in the shock absorber, damping force is generated. The more restrictive the damper valve is, the more damping force can be generated. [4]

 

4 Damped spring &damper system.
demonstrates the effects of a shock absorber on the body movement. It can reduce the oscillation of the suspension movement by dissipating the energy. It is customary to have less damping for compression motion than that of extension motion so that less force is transmitted to the vehicle when it encounters bump-type disturbance. By comparison, more damping is provided for rebound motion in order to quickly dissipate energy stored in the suspension system.

Vehicle Models

When we design or analyse a suspension system, we need a physical model. [6]

 

5 Springs.

 

6 Quarter car model
shows a simplified linear quarter car model. Each corner of the vehicle can be simplified as a body mass, a suspension (which is composed of a shock absorber / damper and a spring) and a tyre mass. It is assumed there is a spring in the tyre to model the compressibility of the tyre. The movement of the vehicle body is:

 

 

If we extend the quarter car model to the full vehicle, we can get a full vehicle model as shown in [7].

 

 

7 Full vehicle model.
Besides the vertical movement at each corner, the vehicle body also has

three rotational movements, the roll, pitch and yaw.

How to evaluate a suspension performance

Experienced drivers used to carry out subjective assessments to judge a suspension performance. They drive the car over all kinds of roads and judge the suspension by personal feelings, e.g. noise, vibration and harshness. However, people’s feelings are tricky and sometimes for the same car drivers may have different subjective judgements. Therefore, we need some objective standards to evaluate a suspension performance.

In vehicle suspension designs, there are 4 main criteria to objectively evaluate a suspension performance. The first is the body acceleration, which is used to assess ride comfort. Previous research has shown that the vehicle body acceleration is highly related to a passenger’s feeling about ride comfort. The second is the dynamic tyre deflection, which is connected with handling performance. When a tyre deflection is too big, it may lose contact with the road. In this case, the driver can lose control of the vehicle. The third is the suspension deflection which means the suspension should have enough working space. When the suspension deflection is too big, the wheel may hit the vehicle body and make the driving very harsh. The last one is the body attitude. We want to keep the vehicle body level all the time during the driving; therefore pitch, roll and warp angle are very important.

Since the body acceleration is related to the feeling of a human being, how will it affect a driver’s judgement? Modern vehicle tends to have a natural frequency of about 1.0-2.0Hz. The reason is that this frequency is very close to the way an adult is walking based on a pace of 30in and speed of 2.5-4mph. Previous medical researches have proved that most uncomfortable frequencies lie in 20-200Hz. For example, if people are exposed to 4-8Hz vertical vibrations, they will soon get fatigued. Human head and neck are sensitive to 18-20Hz vibrations and visceral region is sensitive to 5-7Hz

vibrations. Besides the high frequency vibrations, some low

frequency vibrations can also make people feel uncom-fortable. For example, sea sickness is related to the vibrations below 0.75Hz. Also, lateral and pitch movements can make people feel uncomfortable.

 

Trade-offs in suspension design

 

In the past years automotive engineers are trying every method to improve the suspension design. However, due to some inherent trade-offs in suspension itself, it is very difficult to design a perfect suspension. The best known is the compromise between ride and handling. For example, to have a comfortable ride, soft spring and damper are helpful; however, the soft spring and damper may result in excessive tyre movement and make the handing worse. On the other hand, to stiff the spring and damper for a good handling may make the suspension feel very harsh. Accordingly, most conventional passive suspensions may only satisfy the essential requirements and will compromise on some of the less important considerations. For example, luxury limousines tend to use soft suspensions to offer good ride comfort, while sports cars usually have stiff suspensions to achieve superior handling performance.

Can we design a suspension with excellent ride and handling simultaneously? Yes, and the solution is the active suspension.

2. Active suspensions

Introduction: With the development of modern computers, electronics, hydraulics and control technologies, a new suspension system – the active suspension, provides a possible solution to the conflict requirements between ride and handling. An active suspension refers to a suspension system which uses a micro-computer and sensors in a feedback loop to improve the suspension performance. [8]

8 Arrangement of an active suspension
shows a typical arrangement of the active suspension. In general, it is composed of:

Sensors – various sensors are installed around the vehicle to monitor the vehicle conditions and driver activities.
Electronic control unit (ECU) – all the sensor signals are fed to a microcomputer, also known as ECU. With the aid of a programmed map memory, calculations are made as to what adjustment should be made to the suspension.
Actuators – the instructions from ECU are converted into electrical signals and directed to various actuators to control the suspension. Hydraulic actuators are most often used for their compact volume and light weight.
Categories of active suspensions

Depending on various hardware employed in active suspen-sions, they can be divided into four categories.

Semi-active suspensions

The term semi-active suspension is often used to refer to a controlled damper under closed-loop control, which means the control is realized by varying the damper’s damping rate

as shown in [9].

 

9 Semi-active suspension.

A semi-active suspension is only capable of

dissipating energy. According to different damper configur-ations, semi-active dampers can be classified into the following categories.

 

Dampers with controllable orifice: the damping force in a shock absorber is generated when the oil flow through the hydraulic orifices in the damper valve of the shock absorber. The smaller the orifice is, the larger damping force can be generated. Therefore, we can control the opening of the orifice to adjust the shocker’s damping force. Currently ZF Sachs (a German tier 1 company) offers a line of semi-active shock absorbers under the name of CDC (continuously damping control) as shown in [10].

 

10 Damper with controllable orifice

 

Dampers with controllable fluid: if the hydraulic orifice is fixed, we can vary the oil viscosity to control the damping force. The bigger the oil viscosity is, the larger damping force can be generated. ER (Electrorheological) or MR (Magneto-rheological) fluid can be used for this purpose. There are polarizable particles of a few microns in the oil. When electrical of magnetic filed is applied to the oil, the particles will be polarised and distributed in a sequential order as shown in [11].

 

11 Work principle of electro-heological & magnetorheological dampers: Particles in an MR/ER fluid left without & right with applied magnetic/electrical field.
As a result, the oil viscosity changes, depend-ing on the strength of the electrical/magnetic filed.

Semi-active suspensions have been successfully used in some vehicle models, e.g. Audi A8, Lancia Thesis and the new Opel / Vauxhall Astra.

Fully active suspensions: Different from semi-active suspensions, a fully active suspension does not change the damper characteristics, but add a force generator in parallel with the passive damper and spring as shown in [12]

 

12 Fully active suspension.

. Therefore, the suspension can not only dissipate energy, but also inject energy into the system. That is why we call it fully active suspension. Normally the power of the force generator is supplied by the engine; therefore, compared with semi-active suspensions, active suspensions have higher cost and power consumptions. But as a return, it has better perform-ance than semi-active ones. Depending on the response speed of the actuator, there are fast active and slow active suspensions. Slow active suspensions have low cost and power consumption, but the performance is not as good as fast active ones.

The applications of fully active suspension can be found on Toyota Soarer, Nissan Q45A and some Mercedes-Benz models, for example, the SL500.

Active anti roll bars: Anti-roll bar is used to reduce the vehicle roll movement and it works in the same way as a torsion bar. It connects the two wheels in the same axle. When the two wheels moves in different directions, torque will be generated in the anti-roll bar and prevents the two wheels from moving in opposite directions. An active anti-roll bar has a hydraulic actuator in series with the anti-roll bar and covert the hydraulic pressure into torsion. [13]

13 Active anti-roll bar.

shows the working principle of the active anti-roll bar. Though it can effectively reduce the body roll movement, in straight line driving the performance improvement is still limited. Some automotive manufacturers suggest combining the active anti-roll bar with a semi-active suspension, which will achieve the performance comparable to a fully active suspension but with lower cost and power consumption.

Applications of active anti-roll bar can be found on BMW 645Ci, new Land Rover and Peugeot 206 World Rally cars.

 

Linear electromagnetic motors: The use of linear electro-magnetic motors in suspension control is a fairly new approach developed in the past five years. Inside a linear motor are magnets and coils of wire. When electrical power is applied to the coils, the motor retracts and extends, creating motion between the wheel and car body. One of the key advantages of an electromagnetic approach is the speed of response. The linear electromagnetic motor responds quickly enough to counter the effects of bumps and potholes, maintaining a comfortable ride. The regenerative power amplifier used for the electromagnetic motor allows power to flow into the linear electromagnetic motor and also allow power to be returned from the motor. This technology significantly reduces the system power consumption. Bose has developed a prototype system based on linear electromagnetic motors as shown in [14]

 

 

14 Linear electromagnetic motors.
. However, it is argued that the cost and weight of the system make it destined to niche market only.

Challenges in active suspension

Since firstly introduced by Lotus Engineering in a racing car in 1980s, active suspensions have been under extensive investigations. However, till now they are still limited to niche market and there is still a lot of work to do before they can go to massive production. The challenges automotive engineers are facing now are:

Cost: Although active suspensions can provide excellent performance, the cost is still high, especially for lower end market, where most of the cars are sold. How to reduce the system cost while still maintaining reasonable performance is the number one task for active suspension engineers.

Power consumption: Active suspensions (except semi-active suspensions) need engine to provide power supply. As a result the fuel consumption will increase compared with passive suspensions. The power consumption figures quoted for the fully active suspension is of 5-10kW peak. This is similar to an air conditioning system. Efforts still need carrying on to reduce the power consumption.

Installation: The active suspension has to be compact and light weighted in order to be easily installed on different commercial production vehicles. The system must have easy access for necessary maintenance or replacement.

Reliability: The active suspension should be able to work effectively all the time. No customers would like to pay extra money on a high-tech system but end up with going to garages frequently.

3. The active suspension investigated

at the University of Bath

Introduction

Cooperated with Jaguar, Ford and TOKICO (USA) Inc, a prototype hydro-pneumatic active suspension has been investigated at the University of Bath. From individual components to overall system, extensive computer simu- lations and experimental measurements were carried out. It is hoped that this work could lead to further understandings and improved designs of a computer controlled active suspension. For confidentiaol considerations, no detailed information about the system is enclosed with this talk.

The active suspension system

15 The active suspension investigated in this research.
[15] shows the layout of the active suspension investigated in this research. It is a fully slow active suspension system. The power is supplied by a gear pump driven by the engine. The oil from the pump goes through a fail safe valve unit first. This unit has two main functions. The first is to shut off the system in case of emergency. In this circumstance the system will behave as a passive suspension. The second is to adjust the supply pressure. When the active suspension need not work, the fail safe valve unit will reduce the supply pressure level. It increases supply pressure only when it is necessary. In this way, the system power consumption is reduced. Then the oil goes to a flow control valve at each corner, which controls the flow to and from the gas strut (composed of a coil spring and a gas shock absorber) to adjust the suspension movement. The flow control valve is controlled by an ECU, which makes judgements upon receiving various sensor signals around the vehicle.

The prototype active suspension was implemented on a Jaguar S-Type saloon and extensive experiments carried out.

Performance of the active suspension

The active suspension was tested on a 4-poster road simulator at the University of Bath as shown in [16].

 

16 The 4-poster rig at the University of Bath.
The rig has 4 hydraulic actuators at each corner controlled by a computer system. Vertical inputs can be applied to each wheel of the vehicle to simulate road profiles. In the tests, we used single frequency bounce inputs, roll inputs, pitch inputs, and warp inputs as well as random road profiles representing typical bumpy mountain road and smooth highway road.

Compared with conventional road tests, the 4-poster test has some unbeatable advantages:

The 4-poster can accurately repeat road inputs, ruling out the difference between each test caused by the driver’s manoeuvre or the road difference, making test results more accurate and comparable.
The 4-poster can generate arbitrary inputs to the vehicle,
even the most severe inputs that may be experienced on real roads.

Because the 4-poster test can be performed in the laboratory without running on special test track, the test cost, time and risk are greatly reduced.
Despite its advantages, 4-poster tests cannot totally replace road tests for the following reasons:

Dynamic handling tests like steering and braking cannot be simulated on the 4-poster.
The 4-poster can only input vertical excitations to the vehicle; therefore it cannot simulate complex driving conditions where side forces are involved.
Due to the complex suspension geometry and tyre dynamics, the vehicle will have different responses for zero and nonzero forward speed even if it is subject to the same vertical excitations, which means 4-poster tests cannot fully reproduce the actual road tests.

Therefore, it would be a logical combination to examine both the road tests and the 4-poster tests. Extensive road tests were carried out in USA and UK.

The active suspension demonstrated satisfactory perform-ance in both tests. (17)

 

17 Frequency response of the active suspension
shows the general frequency response of the active suspension. The system could effectively reduce the body acceleration, the suspension deflection and the tyre deflection below 3Hz, although occasionally the active suspension had slightly worse performance. On a harsh road straight line driving, the active suspension reduced the body acceleration 20%, the suspension deflection 19.4% and the tyre deflection 23%. In the double lane change driving, the active suspension could achieve zero-roll control up to 0.5g lateral acceleration. In case of brake and acceleration. the anti-dive and anti-squat control was up to 0.5g longitudinal acceleration.

Efforts to reduce cost and power consumption

As already being mentioned, an active suspension should have reasonable cost and power consumption besides good performance in order to be commercially acceptable. In the system investigated in this research, great efforts were undertaken to make the system ready for commercial production:

Compact and light weight component design for material cost saving.
Use of low cost components, e.g. use of gear pump instead of piston pump, use of low cost flow control valve, and the selection of sensors.
Reduced power consumption by pump pressure control.

Bo Gao

(email [email protected] for copy with images included)