An overview of the development of intelligent materials and active vibration isolation systems for vehicles

Intend to reduce the impact of external vibrations on the smoothness of automotive precision instruments and vehicles and to improve the operational accuracy of automotive precision instruments and the comfort of passenger vehicles, vibration isolation technologies are used and active control systems are applied to vibration isolation systems. The extensive use of smart materials in vibration isolation systems has enabled the design of vehicle structures based on smart materials to meet the suppression of vehicle vibrations well. This paper reviews the application of commonly used smart materials to the field of vehicle vibration damping, summarizes the configuration design of single-axis and multi-axis vibration isolation systems, and develops the thinking behind the application of active control technology to other carriers, pointing out the future research direction of current smart materials for vehicles.


Introduction
The key point of active vibration isolation technique is the application of active control system in vibration isolation, where the basic concept of active control systems is again a feedback system, using a combination of inertial sensors and spring-type components to detect the relative motion of objects, and then signal processing by signal servo controllers, the processed signal is then sent to the driver, the spring-type components are driven to finally achieve the results of feedback control.As long as the detection device has a high enough accuracy, it is possible to make micron-level vibrations equally applicable to the conditions of the feedback system, and thus to high-precision instruments.
As an important branch of active control technology, active vibration isolation technology is now a relatively mature field, as early as 1955 the American scientists put forward a research report on active vibration isolation technology.Compared with passive vibration isolation, active vibration isolation technique has the advantages of flexibility and high efficiency.New micro-vibration isolation systems based on intelligent materials are widely used, and their technical development depends on the development of signal processing, automatic control theory, microelectronics and computers.The study of active vibration isolation in vehicles began with suspension vibration control in rail vehicles and has now been extended to the study of suspension design in all types of vehicles.The literature [1] used neural network control for active control of train suspension systems; the literature [2] used fusion methods for active control of vehicle suspensions.The above studies confirm that non-linear control methods are the focus of research on suspension vibration isolation.This article gives a systematic overview of the application of three intelligent materials, namely piezoelectric materials, magnetorheological materials and shape-memory alloys, in the field of automotive vibration damping, points out the current challenges in the commercialization of smart materials for vehicles and analyses and prospects for future research directions.

Single axis vibration isolation systems
Single-axis vibration isolation system is mainly used to suppress the micro-vibration in one direction, and they are mainly divided into direct active and active-passive integrated types.The basic principle of the direct active type is to use force interactions to input signals into the actuator to generate actuating forces to counteract the disturbing forces caused by the micro-vibration source [3,4].
A kind of active passive micro vibration isolation system and its vibration transfer rate curve in the frequency domain [5] are shown in Figure 1 and Figure 2.  Of these: passive stiffness is used to suppress medium and high frequency vibration disturbances, and primary dynamics is used to isolate low frequency vibration disturbances [6][7][8].

Multipath vibration isolation systems
To solve the problem of multidimensional micro-vibration disturbances, researchers have designed multipath vibration isolation systems that enable the isolation of forces generated by multidimensional motion in space from the loaded platform.Most of the platform structures use parallel results [9].With the combined action of multiple actuators, the load platform isolates the multidimensional disturbances in space.For example, 3-axis micro-vibration isolation stages are designed, the objective is to isolate the vibrating object from two rotational disturbances and a translational disturbance [10], or three moving disturbances [11]; 6-paths vibration isolation stages are designed to isolate disturbances in any orientation in space, most typically in the 'cube' configuration of the Stewart vibration isolation system.The platform consists of six actuators with an overall compact structure, high output accuracy, high load carrying capacity and good dynamic characteristics [12].The Stewart vibration isolation stage designed by ABU et al [13] is shown in Figure 5.

Piezoelectric materials
Due to the application of the positive piezoelectric effect in signal pick-up, piezoelectric shunt damping technology is used in the passive control of automotive vibration noise.By using this principle, the current generated by the piezoelectric material is dissipated in the form of thermal energy, so that to convert mechanical energy into heat energy, thus achieving the suppression of mechanical vibration.In the literature [14], a 1:5 scale model was used to design different parallel shunt circuits for the excitation of vertical elastic vibrations in train carriages.

Figure 6.
Normalized FRF around the mode natural frequency with virtual L-R circuits [14].A significant reduction in vibration was obtained.In the literature [15], the effect of three shunt circuit tuning frequencies on the vibration attenuation of a piezoelectric thin plate was using FEM Simulation Analysis to Reduce Radiation Noise due to Plate Vibration .The figures indicate that the vibration attenuation of the piezoelectric sheet is higher compared to that of conventional materials.The literature [16] investigated the vibration suppression effect of a piezoelectric shunt damper on a carbon fibre automotive control arm, and the results showed that the vibration damping of the piezoelectric shunt damper was essentially the same as that of a tuned mass damper.Figure 9 shows three typical piezoelectric shunt energy dissipation circuits.According to the inverse piezoelectric effect of piezoelectric materials applied to signal excitation, for automotive vibration noise can play an active control role, in the piezoelectric material polarization direction to apply external force, the piezoelectric material will produce mechanical deformation, remove the electric field when the deformation disappears, the same piezoelectric material attached to the controlled object, the deformation into force to suppress vibration and noise, or the use of external excitation on the vibration parts to suppress.In [18], piezoelectric patches were applied to the surface of a thin-walled body structure to simulate piezoelectric active vibration control and to analyse the effect of structural parameters on vibration suppression, and the results showed that the amplitude of vibration at the measured point was reduced significantly, and that a thicker substrate was beneficial in improving the vibration suppression effect at the same size.
Figure 10.Amplitude decay curves before and after vibration control [18].In [19], a piezo-ceramic patch was applied on the surface of the thin-wall body and its control methods, feedback proportional and proportional differential, were used for active vibration control.In  Calculated SPL spectra at the virtual point of driver's right ear in the cavity model [20].Literature [21] designed a piezoelectric laminated actuator for jitter control experiments on the squeal noise of automotive disc brakes.The result is that a piezoelectric ceramic vibration control can reduce the squeal noise of brake under random braking state and control its occurrence at around 60%.

Magnetorheological materials
Magnetorheological materials are mainly used to suppress vibrations caused by engines.The stiffness and damping of traditional passive reducers cannot be changed with the change of working conditions, so the working range is not wide enough and the damping effect is not stable.The magnetorheological material is modified to improve the shear yield stress and sink rate of the magnetorheological fluid magnetic particles, among other indicators.In the absence of an external magnetic field, the magnetorheological fluid is free flowing with low viscosity and no resistance; with an applied magnetic field, the microscopic particles in the magnetic fluid form a chain structure under the action of magnetic field because of their magnetism, impeding the free flow and thus achieving the damping effect [22], as shown in Figure 12 and Figure 13.The main function of the additive is to prevent the magnetic particles from settling and agglomerating over a long period of time without work, thus improving the performance of the magnetorheological fluid and extending its service life.Figure 13.Distribution of magnetic particles per unit space (Have a magnetic field).Therefore, the semi-active control mounts designed using magnetorheological materials have a wider range of use than passive control, and the control strategy is simpler, less costly and more reliable than active control.In the literature [23], a magnetorheological fluid-based engine mount, shown in Figure 14, was combined with a fuzzy control method to effectively reduce body acceleration and stresses transmitted to the frame, improving the low-frequency phase of engine mount vibration.A special one is the shear valve magnetorheological damper designed in the literature [24], as shown in Figure 15 and Figure 16.In addition, a fuzzy control algorithm is used to control the output force of the damper, which can effectively reduce the vibration of the source for vibration isolation.The results of simulation and experiment indicate that the smoothness of the vehicle is improved and the adaptability is high in the random condition.Although the passive suspension of the car has the advantage of lower cost and more stable suspension structure, the fixed stiffness and damping make it difficult to have a strong universal application.In today's society, where the subjective driver is the main vehicle service user, it is not possible to achieve good vehicle smoothness and handling stability at the same time.The active control suspension uses the reaction force generated by the active control system to suppress longitudinal vibrations and lateral body roll caused by different road conditions, but given the complexity of the device and the high cost disadvantage, the general availability is currently low.Semi-active vehicle suspensions based on magnetorheological materials consume less energy when operating and have a performance close to that of active suspensions, making them more frequently used and more acceptable to researchers or suspension manufacturers, taking into account the cost of the design.

Active control applications on other carriers
Considering the climate, wind speed has a large influence on the sea surface, the actual sea surface driving conditions are more complex than the road surface conditions, the importance of the ship's vibration and resistance for, the occupants and the ship's own mechanical life is also self-evident.Here active vibration control of the ship's propulsion shaft system is analysed using active vibration isolation technology.
When the propeller is moving in an uneven flow field under the sea surface, the blades cut the flow field and generate impulsive cyclic stresses, while the combined forces formed by the thrust of the blades can also deviate from the axis and form bending moment stresses.All of these forces can cause vibration through the ship's propulsion shaft system and cause strong underwater low frequency noise.As the deformation of low frequency vibration damping devices under such conditions is in conflict with the alignment requirements of the propulsion shaft system, the passive vibration isolation technology of the thrust bearing is very difficult to achieve,furthermore, the research on the active vibration isolation of the propulsion shaft system of the ship is very important.
The literature [25] links the magnetic bearing with the original thrust bearing, defined as a hybrid active-passive bearing, using a closed-loop control strategy to regulate the magnetic bearing, with some reduction in the pulsating cyclic stress and longitudinal vibration of Thrust Seat.In recent years, a prototype electromagnetic thrust bearing principle has also been established at the Chinese Ship Research Institute with the aim of reducing transmission of the Longitudinal Vibration Line Spectrum from Axis System to Base.The main difficulties in its application to real ships are the high power consumption required to carry the weight of the shaft system and the static thrust, and the electromagnetic compatibility and shock resistance issues to be resolved [26].
In the literature [27], the passive vibration isolation effect of the resonance regulator was used to reduce the force amplitude required for active control between the shaft system and the bearing housing, while the inertia actuator was mounted on the transom endplate bulkhead and the thrust bearing thrust collar, as shown in Figure 17, and numerical calculations showed that this method could reduce the first three orders of longitudinal resonance in the paddle-shaft system.  .Performance of the system using tuned actuators,when an RC is implemented.Documents [28] examine how to control the longitudinal vibration of shaft systems by installing an electromagnetic inertia actuator on a shaft with ball bearings near the thrust bearings, building a 50 mm diameter shaft system test rig mounted in a 2 m diameter cylindrical housing as shown in Figure 19, and using the FxLMS algorithm for anti-saturation and reconstruction of interference for effective suppression of shaft system and casing vibration.In [29], the actuator was mounted symmetrically on either side of the thrust bearing casing, and the thrust bearing housing was subjected to a vertical vibration to 100 Hz.In [30],a ship shaft system rotor model was constructed to study the active control of transmission forces resulting from the transverse multi-frequency vibration of the shaft system .Because the vibration of the shell structure caused by the transverse excitation of the shaft system is very complex, there are strong coupling and difficult contact surface analysis, and its vibration control needs further research and experiments.

Challenges and outlook
From the above it is easy to see that at present, taking into account the cost of design, semi-active control is used more frequently at the commercial level and is more acceptable to manufacturers and consumers.As research into smart materials continues, more highly sophisticated active control platforms are being designed and their range of applications in the automotive sector can only increase with each passing day, however there are still many challenges to achieving true volume and commercialisation.However, smart materials and active vibration isolation technologies are still in the innovative research stage because of the different nature of the materials and the different algorithms of the active vibration isolation platform.
The researchers' own costs and profits are also part of the commercialisation process that needs to be reconciled.Even if the consumer's spending power is met, too low a profit and too high a cost of innovative research is ultimately a major obstacle to commercialisation, and even to the stagnation of innovation in the field of smart materials development and active control algorithms.
The complexity of the active control algorithm itself and the high application cost are also problems that need to be solved.Secondly, although most of the algorithm research can prove the effectiveness through simulation or test, the method of setting the parameters is still used in the empirical values and multiple attempts to get the optimal method, which is still lacking the theoretical basis for setting the parameters to be applied in the actual complex working conditions, which is also a problem of the current active control research.
In the case of smart materials, the temperature, stress, humidity and other influencing factors required by the actual working conditions are complex and have a high chance of conflicting with the working conditions required by the material.
The current process of applying smart materials and active control in all areas of vehicle design is being driven by new vehicle design concepts and goals, which provide researchers with more ways to do this, and design premises that are increasingly focused on the subjective wishes of the user, which also provide more ideas.Future research into smart materials and active control could start with the following.
Integrated material-function-structure development for carriers in combination with smart materials, optimising more passive control and transforming it into active or semi-active control, enabling it to meet the weight, safety, economic and functional requirements of carriers.
At the same time, we can start from the simplification of the algorithm, such as the simplification of the multi-channel control system to the parallel connection of the multi-channel control system, simplifying its algorithm, effectively reducing the amount of operations, improving the convergence speed and control accuracy, thus reducing the time cost of developing new algorithms [31].
The application of new material processing technologies to smart materials can change the inherent properties of the material and thus the role of smart materials in active control systems.At the same time the use of technologies such as 3D printing, which can break down the original working condition limitations by optimising the distribution of materials, can help in the testing of smart materials and active control systems, and can also consider customised designs for different working condition requirements.

Conclusion
This paper summarises the design of single-axis and multi-axis vibration isolation systems from the perspective of active vibration isolation in vehicles, reviews the widespread use of intelligent materials in vibration isolation systems to enable the design of smart materials-based automotive structures that are well suited to the suppression of vehicle vibrations, and illustrates the application of active control systems in marine propulsion shaft systems.The author's views on how to commercialise smart materials and active control and increase their widespread use are presented in the context of cost and innovation, from the point of view of business and consumer, smart materials and active control algorithms respectively.Finally, an outlook is given on the common problems encountered in experimental and practical research on smart materials and active control vibration isolation.

Figure 1 .
Figure 1.Design of Active Passive Micro Vibration Isolation System.

Figure 2 .
Figure 2. Vibration Transmission Velocity Curve.Of these: passive stiffness is used to suppress medium and high frequency vibration disturbances, and primary dynamics is used to isolate low frequency vibration disturbances[6][7][8].

Figure 3 .
Figure 3. Schematic diagram of the platform.

Figure 4 .
Figure 4. Geometric configuration.The platform consists of six actuators with an overall compact structure, high output accuracy, high load carrying capacity and good dynamic characteristics[12].The Stewart vibration isolation stage designed by ABU et al[13] is shown in Figure5.

Figure 5 .
Figure 5. Physical view of Stewart vibration isolation platform.The platform consists of 1 voice coil motor and 1 force sensor per pivot chain and 12 acceleration sensors arranged in two groups on the base and moving platform.[13]

Figure 8 .
Figure 8. Plain sheet transfer characteristic curves[15].The figures indicate that the vibration attenuation of the piezoelectric sheet is higher compared to that of conventional materials.The literature[16] investigated the vibration suppression effect of a piezoelectric shunt damper on a carbon fibre automotive control arm, and the results showed that the vibration damping of the piezoelectric shunt damper was essentially the same as that of a tuned mass damper.Figure9shows three typical piezoelectric shunt energy dissipation circuits.[17]

[ 17 ]Figure 9 .
Figure 8. Plain sheet transfer characteristic curves[15].The figures indicate that the vibration attenuation of the piezoelectric sheet is higher compared to that of conventional materials.The literature[16] investigated the vibration suppression effect of a piezoelectric shunt damper on a carbon fibre automotive control arm, and the results showed that the vibration damping of the piezoelectric shunt damper was essentially the same as that of a tuned mass damper.Figure9shows three typical piezoelectric shunt energy dissipation circuits.[17] CONF-MSS 2023 Journal of Physics: Conference Series 2649 (2023) 012020the literature,[20] piezoelectric Ceramic Material and Fuzzy Controlare used to realize the active control of low frequency noise.Under the excitation of a random signal, the vibration amplitude of the front plate of a cavity structure simulating the in-vehicle environment was significantly reduced and the cavity noise was reduced by 6 dB.

Figure 11 .
Figure 11.Calculated SPL spectra at the virtual point of driver's right ear in the cavity model[20].Literature[21] designed a piezoelectric laminated actuator for jitter control experiments on the squeal noise of automotive disc brakes.The result is that a piezoelectric ceramic vibration control can reduce the squeal noise of brake under random braking state and control its occurrence at around 60%. Table1.Comparison of noise peak at drag condition[21].

Figure 12 .
Figure 12.Distribution of magnetic particles per unit space (No magnetic field).

Figure 16 .
Figure16.Shear valve magnetorheological damper designed within the literature (Physical images)[24].Although the passive suspension of the car has the advantage of lower cost and more stable suspension structure, the fixed stiffness and damping make it difficult to have a strong universal application.In today's society, where the subjective driver is the main vehicle service user, it is not possible to achieve good vehicle smoothness and handling stability at the same time.The active control suspension uses the reaction force generated by the active control system to suppress longitudinal vibrations and lateral body roll caused by different road conditions, but given the complexity of the device and the high cost disadvantage, the general availability is currently low.Semi-active vehicle suspensions based on magnetorheological materials consume less energy when operating and have a performance close to that of active suspensions, making them more frequently used and more acceptable

Figure 17 .
Figure 17.Schematic diagram of the installation position of the vibrator.

Figure 18
Figure18.Performance of the system using tuned actuators,when an RC is implemented.Documents[28] examine how to control the longitudinal vibration of shaft systems by installing an electromagnetic inertia actuator on a shaft with ball bearings near the thrust bearings, building a 50 mm diameter shaft system test rig mounted in a 2 m diameter cylindrical housing as shown in Figure19, and using the FxLMS algorithm for anti-saturation and reconstruction of interference for effective suppression of shaft system and casing vibration.In[29], the actuator was mounted symmetrically on either side of the thrust bearing casing, and the thrust bearing housing was subjected to a vertical vibration to 100 Hz.In[30],a ship shaft system rotor model was constructed to study the active control of transmission forces resulting from the transverse multi-frequency vibration of the shaft system .Because the vibration of the shell structure caused by the transverse excitation of the shaft system is very complex, there are strong coupling and difficult contact surface analysis, and its vibration control needs further research and experiments.

Figure 19 .
Figure 19.Schematic diagram of the ship propulsion shaft system test rig.