Fully active suspension system (call hydraulic fully active since it mostly uses a hydraulic actuator in vehicle industries, even though in a number of luggage pneumatic and electromagnetic actuators are used) fundamental ambition is to attain the performance of an perfect suspension by straight controlling the force or equivalent movement of the actuator. "By sensing the longitudinal and side accelerations of the vehicle, this suspension system has the potential of estimation the inertial loading generated by the a diversity of vehicle manoeuvres; and also providing spring forces to support the vehicle's heaviness, as well as different damping forces with admiration to the body and the tyre motion" [8]. Although there are various implementations of the hydraulic fully active suspension system, it often shares the same basic arrangement of components which are: -
lake - for storing the hydraulic fluid;
Hydraulic Pump - supply the hydraulic fluid at pressure to the system;
Cooler - for cooling the hydraulic fluid which generate heat while flowing through the system;
Accumulators - to decrease the pressure fluctuations and supply additional fluid for peak demands;
Hydraulic Actuator - on its own or double acting hydraulic piston located inside a strut which converts the fluid pressure supplied to a force which is transmitted between the body and wheels of the vehicle. To carry some of the vehicle's load (on some vehicles) a spring is mounted in parallel with the actuator. Due to the rapid relative movement between the vehicle and the wheel assembly, the hydraulic actuator is designed with some form of compliance (small accumulator within the hydraulic circuit or bushings at mounting point) build into it to help reduce load fluctuations.
Servo Valve - to organize the flow rate of the hydraulic fluid to each suspension strut assembly;
Sensors - these are placed at each wheel assembly (while in some cases two other sensors are placed on the vehicle body) to measure the relative positions of the wheels to the vehicle body, the wheels or vehicle body accelerations and transmitted loads between the vehicle and each wheel assembly;
Computer - receives inputs from the sensors and then generate control signals for the servo valves.
The representation of a car's suspension system is usually shown as quarter car models for simplicity in both construction and explanation. The diagrammatical representation shown in Figure 1.2 is the quarter car model of the fully active suspension system [6].
Figure 1.2: Shown diagrammatical representation of Fully active suspension system [9]
1.c SLOW ACTIVE SUSPENSION SYSTEM
The second main type of active systems is the slow active suspension system as know as active suspension system which implies (just like its name sake) that it has a low bandwidth (up to approximately 6 hertz) [6], with an aim to control the body mode to improve vehicle ride. This slow active system reverts to the conventional passive system above its upper frequency limit because it cannot control the wheel-hop mode better than the passive system. This type of system however, uses less power with simpler forms of actuations than a fully active system. On the other hand though, the fully active suspension still have more potential performance gains than the slow active suspension system, but its viability is much improved because the actuator is in series with a passive spring - which eliminates the problem of high frequency vibration transmissions. Figure 1.3 shows the quarter car model representation of a slow active suspension system [6].
Figure 1.3: Shown the quarter car model representation of a slow active suspension system [9]
The demand of better ride comfort and controllability of road vehicles has motivated many automotive industries to consider the use of show active suspension. Slow active suspension employs pneumatic or hydraulic actuators which in turn creates the desired force in suspension system as shown in Figure 1.3. The actuator is secured in parallel with spring and shock absorber. Active suspension require two accelerometers that mounted at sprung and unsprung mass, and a unit of displacement transducer to measure the motions of the body, suspension system and the unsprung mass [6]. This information is used by the online controller to command the actuator in order to provide the optimum target force. Slow active suspension may consume large amounts of energy in providing the control force. Therefore, in the of active suspension
system the power consumptions of actuator should also be considered as an important factor. In analysis of suspension system, there are varieties of performance criteria which need to be optimized. There are three performances criteria which should be considered carefully in designing a suspension system; namely, body acceleration, suspension travel and wheel deflection.
The performance of the system can be further improved by introducing the suitable controller into the active suspension system [10].
1.d SEMI-ACTIVE SUSPENSION
Semi-active suspension are often found on today's high-end production sports cars and feature damping control on all four corners. The control objective of semi-active suspension is to counteract for heave, roll and pitch. This control is achieved by pairing a microprocessor with four shock absorbers that have a continuously variable (and controllable) damping coefficient.
Semi-active suspensions provide real-time dissipation of energy, which is achieved through a mechanical device called an active damper. The active damper is installed in parallel with a conventional spring. The main feature of this system is the ability to adjust the damping of the suspension system, without any use of actuator. This type of system requires some form of measurement with a controller board in order to properly tune the damping force.
Figure: 1.4: schematic of a semi-active suspension system [10].
Semi-active suspension system is the third type of active systems which is some what similar to the passive suspension system with the passive damper being replaced by a controllable one. This controllable damper acts like an actuator but with limited capability, to produce a controlled force when dissipating energy and when supplying energy it switches to a national zero damping state. In comparison with the other two types of active suspension systems, the hardware and operational costs are relatively less for the semi-active suspension system. However though, for performance potential this type of suspension system operates closely to the fully active system under certain conditions. The semi-active suspension system is represented as a quarter car model in Figure 1.4 [6], [11].
There are some potential advantages of the active suspension system over the passive suspension system which are: the ability to improve vehicle body acceleration; minimize suspension deflection and tyre deformation; the ability to improve tyre grip to the road surface (which results in the improvement of braking, traction control and maneuverability of the vehicle); it generates desired forces which acts between the unsprung (wheel) and sprung (vehicle body) masses of the vehicle; providing good insulation of the vehicle sprung mass from the road surface disturbances without adverse effects and also creating low friction with no backlash (which results in high accuracy, high acceleration and velocity, sufficient force, satisfactory reliability and long lifetime). Although this type of suspension system shows such potential in creating what is called an ideal suspension, that is, allowing the vehicle's body to follow the road surface (profile) without any low frequency body bouncing and extra high frequency harshness; there are limitations or disadvantages to the applications of this system.
1.e ACTIVE SUSPENSION SYSTEM
Active suspension systems most often use hydraulic systems with a hydraulic actuator to carry out the practical implementations of the suspension; but the actuator is also its major limitation. This is because the actuator is expected to exert a prescribed force that is independent of its motion; but in reality, the exerted forces produced by any actuator are completely dependent on its motion along with the commanded forces. Hitherto, the difficulties in achieving isolation and good handling are still apparent to the realization of the actuator forces [13]. Other limitations to this system are: it requires an additional energy source to attain the required ride and handling performances; it relies largely on adequate hardware equipment; the maximum allowable displacement between the body and the suspension parts; the tyre deflection, overall system robustness, reliability, cost and power consumption and the misjudgement of sensors in detecting the road profile (for example; a pot hole in the road filled with water).
Figure: 1.5: Schematic of an Active suspension system [10]
With the implementation of development of modern computer, hydraulics actuator and control technologies, active suspension provides a possible solution to the conflict solution between ride and handling.
An active suspension defines to a suspension system which uses a micro-computer and sensors in a feedback loop to improve the suspension performance. [12]. The Figure 1.6 below shown typical feedbacks close loop Arrangement of an 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 [12].
On the other hand, the active suspension system can provide high control performance in wide frequency range. However, the active suspension requires high power consumption, many sensors and actuators such as servo-valve.
Figure 1.6: Shown a Feedback Close loop Arrangement of an active suspension [12]
The aim of this project is to Design and Build an Active Suspension System based on simulation using IPG CarMaker software by using a Proportional, Integral and Derivative (PID) Controller to control the transient response of the suspension system of its virtual vehicle, thus producing the characteristic performance features of an active suspension system and testing the design on test rig (real life simulation in lab). A simple test track will also be designed to test and simulate this system.
