Active suspension system design for smart cars

Active suspension is a type of automotive suspension on a vehicle. It uses an on-board system to control the vertical movement of the wheels relative to the chassis or body, rather than passive suspension provided by large springs, where movement is entirely determined by the road surface. Active suspensions fall into two categories: true active suspension and adaptive or semi-active suspension. Semi-adaptive suspension only changes the stiffness of the shock absorbers to adapt to changing road or dynamic conditions, while active suspension uses some type of actuator to independently raise and lower the chassis at each wheel. The active suspension system provides better ride comfort and vehicle stability. But its cost is too high compared to semi-active or passive suspension systems.

However, in terms of the development of increasingly intelligent

Therefore, this series of articles will explain the application theory of magic carpet active suspension in two different directions. These include the design principles of active suspension itself and the design of intelligent sensing road preview principles and performance indicators. This article will introduce the functional principle design of active suspension in detail.

Active suspension design principles

Active suspension systems operate between the sprung and unsprung masses of the vehicle. It minimizes vertical acceleration and vehicle vibrations caused by road and vehicle dynamics, improving vehicle handling and stability. Active suspension systems include hydraulic circuits, sensors and control systems . Most active suspensions that have reached the hardware development and production stages use some form of electro-hydraulic actuator.

Figure 1 Active suspension frame

The quarter vehicle model is shown in Figure 2 below. In terms of the time it takes to reach driving steady state, both active suspension and passive suspension can do it, but the impact of active suspension is obviously much smaller than that of passive suspension, which will achieve a better balance between ride comfort and vehicle stability. Good compromise.

Figure 2 Acceleration frequency response of active and passive suspension

There are currently two recognized forms of active suspension. The first is a fast active suspension or high-bandwidth system (HB), often called fully active. The second is a slow active suspension or low bandwidth system (LB). In active suspension, passive dampers and springs are replaced by force actuators, as shown in Figure 3 below.

Figure 3 Composition and design principles of different active suspension systems

Fully active suspension systems (high bandwidth), also known as high bandwidth, place the actuator between the sprung and unsprung masses. The primary function of a high-bandwidth system is to control the system over its full bandwidth. Specifically, this means it's designed to enhance suspension response around rattle spatial frequencies (from 10 to 12 Hz) and tire jump frequencies (from 3 to 4 Hz).

In the early days, an active suspension system was also established using an electro-hydraulic servo system (EHS), and a pressure control valve was applied for the first time. The suspension is controlled by a microprocessor and acceleration sensors, and the system emphasizes skyhook dampers, which reduce body vibrations compared to conventional low-frequency suspensions by applying an active damping force to the body relative to its absolute speed. . Hydraulic systems have passive damping characteristics that depend on the excitation frequency of the road input. Enhancements in these characteristics reduce vibrations generated by high-frequency road inputs.

The hardware used in active suspension systems ranges from simple swing dampers, semi-active dampers, low-bandwidth/soft active suspensions to high-bandwidth/rigid active suspensions. Adaptive and semi-active setups are effective means of improving straight-line driving and handling transients, although they don't offer as much improvement in driving as active suspensions.

Slow active suspension system (low bandwidth) is suitable for low bandwidth operation. In this system, the actuator is placed in series with a spring and/or damper. Slow active suspension systems (operating at a low bandwidth of less than 3 Hz) are designed to implement suspension control strategies in the lower frequency range, especially around the rattling spatial frequencies. At higher frequencies, the actuator effectively locks so the wheel hop motion is passively controlled. Low-bandwidth systems can achieve significant reductions in body roll and pitch during maneuvers with lower energy consumption compared to high-bandwidth systems. To provide suspension action beyond the controlled bandwidth, the actuator must be mounted in series with a conventional spring, which in turn reduces the energy requirements of the system.

There are currently two main forms of low-bandwidth systems, as shown in Figure 3(b)(c) above. One form is that the actuator is connected in series with the road spring and has a separate passive damper (LB1), and the other form is The actuator is in the spring and damper series (LB2). In slow-speed active suspensions, passive springs provide the required isolation at high frequencies, while actuators provide vibration control at low frequencies (typically below 3 Hz).

Theoretical studies show that limited-bandwidth active systems perform similarly to fully active systems, but at lower cost and implementation complexity. These studies, based on a quarter car model, show that when components are assumed to be in idealized conditions and the vehicle is operating in straight-line driving conditions, the power requirements will be very modest. Some possible practical implementations of this system have been proposed through the use of hydropneumatic components, such as pneumatic springs with valves to control the air supply and exhaust.

Since the actuators only require a narrow bandwidth of 3-4 Hz, slow active suspension systems are much cheaper than fully active suspension systems which require wideband actuators. But active control still encompasses the normal range of body resonance frequencies in bounce, pitch and roll, as well as the frequency range of interest in terms of response to steering control. Therefore, slow active suspensions are a commercially viable alternative.

Main working mathematical model of active suspension system

1) Mathematical model of electro-hydraulic servo valve

Since the mathematical model of EHSV is described by 27 equations, for simplicity, the typical EHSV model is no longer used in the fully active suspension system model because the calculation and iteration process takes a long time. Therefore, it is important to find the equivalent transfer function of EHSV. To this end, the actuator displacement is calculated to further obtain the transient response of the input current . It is found that the step response of the spool displacement behaves like an over-damped second-order system, which can be described by the following transfer function of the second-order system.

In this transfer function, it is necessary to calculate the coefficients k, ωn and ξ of the corresponding representative transfer function. The gain value (k) is the spool displacement under steady-state conditions divided by the excitation current value of 10 mA. The values ​​of ωn and ξ can be  calculated by running a Simulink program.

2) Mathematical model of hydraulic and pneumatic suspension devices

The hydropneumatic suspension device scheme is shown in the figure below. A mathematical model of the system was developed by applying equations describing the dynamic behavior of the suspension units.

The following equation describes this system.

Among them, is a term that considers the impact of the compressibility of the piston cavity.

Figure 4. Schematic diagram of hydropneumatic suspension Figure 5. Damping system valve of hydropneumatic suspension device

The variable orifice area in the compression and rebound strokes of the damping system as shown in Figure 4 consists of four holes covered by a circular thin plate riveted at its center.

3) Quarter vehicle suspension parameters

The mathematical description of a quarter car model is as follows:

The damping coefficient (CS) of the shock absorber is calculated on the fly based on a proven damper simulation model. F(t) is the excitation acting on the wheel and caused by surface irregularities. If xo is the elevation of the surface profile, ox& represents the vertical speed of the tire at the point of contact with the ground, which is the slope of the road profile multiplied by the forward speed of the vehicle.

Active suspension system design

This article will explain in detail a typical active suspension working model as shown in the figure below, which can help readers quickly understand the working process of active suspension. The model mainly consists of electro-hydraulic servo valve, actuator, air spring, LVDT and controller . Electro-hydraulic drives are widely used in the design of active suspensions, and electro-hydraulic servo systems provide good control from the perspective of accuracy and speed.

(1) Hydraulic pump (2) Relief valve (3) Accumulator (4) EHSV (5) Accumulator (6) Throttle valve (7) Gas spring (8) Hydraulic actuator (9) LVDT (10 ) Tires (spring + shock absorber) (11) Fuel tank

Figure 6 Active suspension working model

The output control variables are selected to achieve the required dynamic response of the vehicle body, the control structure for measuring the feedback values ​​is constructed, and the measuring device is determined. The designed active suspension system operates in three modes; neutral mode, compression mode and rebound mode.

1) Neutral mode

When the vehicle is driving on a very flat road or when the vehicle is stopped, it means that there is no input displacement from the road surface and there is no relative movement between the body and the wheel assembly. The feedback current from the LVDT and accelerometer is zero, and the error signal  (ie) to the servo valve is zero. As shown in the figure (4) above, the valve core of the EHSV is in the neutral position at this time.