Development of a Novel Displacement-Sensitive Magnetorheological Damper for Vehicle Suspension with High-Speed Stability and Impact Resistance
Abstract
With the continuous improvement of people's requirements for ride comfort, variable damping coefficient dampers are more and more widely used in automobile suspensions, among which the most widely used are magnetorheological dampers that adjust the damping coefficient by adjusting the magnetic field strength and CDC dampers that adjust the damping coefficient by adjusting the size of the runner. However, because they have a delay of tens to hundreds of milliseconds in the adjustment of the damping coefficient, the suppression effect on high-frequency vibrations, especially shocks, is poor. In this paper, a new type of displacement-sensitive magnetorheological damper is proposed, which is controlled by the piston position and the current size, and the adjustment of the damping coefficient can achieve zero delay and ensure the adjustable range of the damping coefficient. The new displacement-sensitive magnetorheological damper is significantly better than the traditional variable damping coefficient damper for controlling the high-frequency vibration and impact of the vehicle. At the same time, this paper also verifies the obvious advantages of the displacement-sensitive magnetorheological damper in isolating high-frequency vibration and shock resistance compared with the traditional variable damping coefficient damper through comparative experiments.
1. Introduction
The shock absorber is an important part of the car, which directly affects the ride and driving safety of the car.
Introduction to magnetorheological dampers
The magnetorheological damper is composed of an outer cylinder, a piston, an excitation coil, a magnetorheological fluid, a piston rod, etc., of which the magnetorheological fluid is a new type of intelligent material composed of non-magnetic carrier base fluid (such as mineral oil, silicone oil, synthetic hydrocarbon oil, etc.), high permeability ferromagnetic tiny particles and anti-sedimentation surfactants. The viscosity, yield strength and other characteristics of the magnetorheological fluid can be changed by adjusting the magnetic field strength of the environment in which the magnetorheological fluid is located. Magnetorheological fluids have been studied by a large number of scholars because of their fast response time, reversible phase, low power consumption, and easy control of yield stress. The magnetorheological damper is to change the magnetic field intensity generated by the coil by changing the current of the excitation coil, so as to adjust the damping coefficient of the magnetorheological damper. From the perspective of application, magnetorheological fluids are mainly used in automobile main suspension systems [1-3], driver's seat suspension [4, 5], and shock protection [6]. 7], bridge cable vibration [89], flexible structure vibration control [10] and other components or systems of vibration control.
Among the many applications, one of the commercial products is magnetorheological dampers for semi-active suspension systems in vehicles [1, 1]. This directly indicates that the research on magnetorheological shock absorbers for automobiles is the most positive and will have a positive impact on improving the performance of automotive suspension systems. Many research teams have investigated semi-active control algorithms for quarter-car, half-car, or full-car models with MR dampers.
In terms of improving the dynamic range of magnetorheological dampers, some researchers have proposed to optimize the design of magnetorheological valves by finite element method without considering the response time of magnetorheological dampers to maximize the static range of the damper. Lee et al. [1, 2] proposed a method to optimize the dynamic range of the magnetorheological damper, but the response time of the proposed magnetorheological valve was between 28 ~ 125 [ms].
In order to effectively control the vehicle's suspension system, the response time of the magnetorheological damper should be less than about 1/5 to 1/10 of the highest frequency to be controlled [13]. There are two critical resonance frequencies in a vehicle's suspension system: the body mode and the wheel mode. In general, the frequency of the body mode is around 1.5-2.0 [Hz], and the resonance frequency of the wheel mode is around 15-20 [Hz]. Therefore, for higher ride comfort and road stability, the response time of the MR damper should be less than 10 ms if the criterion chosen is 1/5 of the maximum frequency to be controlled, 20 [Hz]. In response to shocks, the requirements for response time are higher.
If the response time is not fast enough for the state of the shock absorber to react to changes in the input in a timely manner, the control efficiency will be reduced [13, 14].
The MR damper response time is mainly composed of three parts, namely the current formation time, the magnetic field formation time, and the particle chain formation time. The time of magnetic field formation dominates the response time, which is mainly due to the influence of eddy currents. The conductor generates eddy currents in a changing magnetic field, and the magnetic field generated by the eddy currents is opposite to the desired magnetic field direction, forming a phenomenon that hinders the change of the magnetic field, and the magnitude of the eddy current is proportional to the rate of change of the generated magnetic field [15]. The higher the velocity controlled by the magnetic field, the more pronounced the eddy current effect will be.
Because the response of most of the existing magnetorheological dampers has a delay of tens of milliseconds, the suppression effect of the existing magnetorheological dampers on high-frequency vibration and impact is average.
Many scholars have adopted the method of eddy current suppression to obtain a magnetorheological damper with faster response speed in reducing the response delay of magnetorheological damper. Yoon et al. [16] reduced the influence of eddy currents on the formation of magnetic fields by using soft magnetic materials (SMCs) and processing the inner surface of the piston housing with many grooves, and achieved a magnetorheological damper with a response time of less than 7 ms.
The excitation current formation time is mainly caused by the inductance. Yang et al. [17] demonstrated the possibility of using a current controller to reduce the time response of the excitation current. This controller maintains an input voltage higher than the corresponding voltage calculated according to Ohm's law until the required current is reached in the circuit. The initial input voltage of Yang's experimental current driver is approximately 5 times higher than the corresponding voltage calculated according to Ohm's law. The time response is 300 ms when voltage control is employed, while when the proposed current controller is employed, the time response is reduced to 60 ms. Strecker et al. [18] used a current controller to reduce the delay of the inductor on the formation time of the current, and the piston used a ferrite material to reduce the effect of eddy currents on the formation of the magnetic field. Although the response time can be significantly reduced by using a ferrite material, the adjustable range of the damper output force is much reduced due to the low permeability of the ferrite.
Give an example
These studies reduced the response delay of the damper from tens of milliseconds to about ten milliseconds by using SMC material, and although the effect was obvious, the range of damping coefficients that can be adjusted by coil current was reduced due to the low relative permeability of SMC material.
High-performance magnetorheological dampers not only need to have a low response time, but also a large damping coefficient dynamic range and a low input power.
In order to reduce the response time of magnetorheological dampers and to maintain an adjustable range with a large damping factor, we took inspiration from displacement-sensitive dampers. Its damping coefficient is passively changed and can achieve zero delay.
Introduction to displacement-sensitive dampers
Displacement-sensitive shock absorbers (DSSAs), also known as stroke-dependent shock absorbers, are similar in structure to conventional passive shock absorbers.
However, DSSA has additional flow channels, such as displacement sensitive holes in the cylinder wall. Depending on the piston stroke, the DSSA can implement different damping factor modes. When the piston stroke is within the range of the displacement sensitive hole, the leakage occurs through the displacement sensitive hole. In this range, the damping force is lower than that of passive shock absorbers. On the other hand, when the piston stroke is outside the range of the displacement-sensitive bore, leakage through the bore is stopped. In this range, the damping force becomes greater due to leakage blockage. Due to the small piston stroke and small damping force, the ride comfort under paved road driving conditions is improved. Due to the large piston stroke, high vibration amplitude and large damping force, the driving safety of the vehicle is improved when driving on rough roads or bumpy roads. As a result, DSSA can maintain ride comfort and driving safety.
But the damping coefficient of the existing displacement-sensitive damper can only be switched between two gears, although the vibration isolation effect is better on the paved road surface, but when the rough road surface is driving and the piston stroke is large, the transition of the damping coefficient from low to high is not smooth, and the ride comfort is affected. The damping coefficient only switches between two gears, and the energy dissipation efficiency is also low.
In this paper, a new displacement-sensitive damper structure is proposed, which is combined with magnetorheological technology, and the damping coefficient of the displacement-sensitive magnetorheological damper is controlled by the position and the current magnitude, which realizes the adjustment of the damping coefficient with zero delay and ensures the adjustable range of the damping coefficient with a large size. The high-speed stability and impact resistance of the vehicle are greatly improved.