AN EMBEDDED SYSTEM TO ASSESS THE AUTOMOTIVE SHOCK ABSORBER ...€¦ · shock absorber test within...

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An embedded system to assess the automotive shock absorber condition under vehicle operation P.J.C. Ventura, C.D.H. Ferreira, C. F. C. S. Neves Escola Superior de Tecnologia e Gestão Instituto Politécnico de Leiria Leiria, PORTUGAL R. M. P. Morais, A.L.G. Valente, M.J.C.S.Reis Centre for the Research and Technology of Agro- Environment and Biological Sciences CITAB/UTAD Vila Real, PORTUGAL Abstract— The automotive suspension plays a crucial role in vehicle safety and driving comfort. One of the most important components in vehicle suspensions is the damper (or shock absorber). Because there is no precise method to perform shock absorber test within the vehicle, an embedded autonomous system, powered by the energy harvested from the shock absorber itself, capable of monitoring shock absorber parameters and transmitting these values throughout a wireless interface to the vehicle central diagnostic unit, is presented. Such a device will permit the shock absorber condition assessment under vehicle operation, which to our best knowledge is considered a breakthrough in vehicle safety. I. INTRODUCTION Automotive suspension is designed to provide a satisfactory compromise between safety and ride comfort experienced by the vehicle occupants. Even if isolation from road vibration is definitely an important aim, maintaining the contact between vehicle wheels and road surface is far more crucial, as vehicle control and stability relies totally upon it. As mechanical systems, suspension components are subjected to wear gradually, and shock absorbers in particular due to its physical concept are more prone to wear, and suffer other damages like oil leak, adversely affecting vehicle comfort, drivability and safety, resulting in excessive vehicle oscillations as a response to road disturbances, instability and longer breaking distances. To assess the condition of shock absorber, two methods are usually employed: dynamometer testers, witch uses an electro-hydraulic cylinder actuator or an electric motor, together with a crank mechanism, to run the shock absorber (that must be removed from the vehicle) through different frequencies and measure the resultant force as a function of velocity, giving accurate results regarding the shock absorber condition [1-3]. On the other hand, ground suspension testers (for which shock absorber removal is not necessary) apply a shaking displacement to the tire at different frequencies (typically varying from 25 Hz down 0 Hz with constant amplitude close to 3 mm), measuring the tire contact force with the platform. The result is a good measure of the suspension system condition, but is not a sufficient indicator of the shock absorber condition, although some authors stated that is possible to evaluate shock absorber performance through phase-angle analysis [4, 5]. Currently there is no precise method to perform shock absorber test within the vehicle to verify their condition, in real time operation. II. ASSESSEMENT METHODS A theoretical analysis of vehicle suspension and of shock absorber internal pressures was performed to determine damping force as a function of shock absorber parameters and input (road) excitation. Two methods for assessing shock absorber status were studied and evaluated. The first method is based on sprung and unsprung mass acceleration measurements and consists in the determination of transmissibility (ratio between variables over the frequency spectrum) as a function of the damping coefficient. As the above concept requires the use of two measuring points, the second method was developed in order to measure shock absorber internal pressure and the input acceleration to assess shock absorber condition using only one sensing device. A. Assessment method 1(mass acceleration) A simplified linear model of a quarter vehicle, as shown in Fig. 1, was considered to study analytically the behavior of the suspension. The suspension system at each wheel was reduced to one spring and to one parallel shock absorber, connecting the sprung to the unsprung masses. The unsprung mass, associated with the wheel, has between itself and the road a spring related to the tire. It should be noted that suspension stiffness and damping coefficients were treated as constants independent of the excitation amplitude and frequency (1) 1-4244-2581-5/08/$20.00 ©2008 IEEE 1210 IEEE SENSORS 2008 Conference

Transcript of AN EMBEDDED SYSTEM TO ASSESS THE AUTOMOTIVE SHOCK ABSORBER ...€¦ · shock absorber test within...

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An embedded system to assess the automotive shock absorber condition under vehicle operation

P.J.C. Ventura, C.D.H. Ferreira, C. F. C. S. Neves Escola Superior de Tecnologia e Gestão

Instituto Politécnico de Leiria Leiria, PORTUGAL

R. M. P. Morais, A.L.G. Valente, M.J.C.S.Reis Centre for the Research and Technology of Agro-

Environment and Biological Sciences CITAB/UTAD

Vila Real, PORTUGAL

Abstract— The automotive suspension plays a crucial role in vehicle safety and driving comfort. One of the most important components in vehicle suspensions is the damper (or shock absorber). Because there is no precise method to perform shock absorber test within the vehicle, an embedded autonomous system, powered by the energy harvested from the shock absorber itself, capable of monitoring shock absorber parameters and transmitting these values throughout a wireless interface to the vehicle central diagnostic unit, is presented. Such a device will permit the shock absorber condition assessment under vehicle operation, which to our best knowledge is considered a breakthrough in vehicle safety.

I. INTRODUCTION Automotive suspension is designed to provide a

satisfactory compromise between safety and ride comfort experienced by the vehicle occupants. Even if isolation from road vibration is definitely an important aim, maintaining the contact between vehicle wheels and road surface is far more crucial, as vehicle control and stability relies totally upon it.

As mechanical systems, suspension components are subjected to wear gradually, and shock absorbers in particular due to its physical concept are more prone to wear, and suffer other damages like oil leak, adversely affecting vehicle comfort, drivability and safety, resulting in excessive vehicle oscillations as a response to road disturbances, instability and longer breaking distances.

To assess the condition of shock absorber, two methods are usually employed: dynamometer testers, witch uses an electro-hydraulic cylinder actuator or an electric motor, together with a crank mechanism, to run the shock absorber (that must be removed from the vehicle) through different frequencies and measure the resultant force as a function of velocity, giving accurate results regarding the shock absorber condition [1-3]. On the other hand, ground suspension testers (for which shock absorber removal is not necessary) apply a shaking displacement to the tire at different frequencies (typically varying from 25 Hz down 0 Hz with constant amplitude close to 3 mm), measuring the tire contact force

with the platform. The result is a good measure of the suspension system condition, but is not a sufficient indicator of the shock absorber condition, although some authors stated that is possible to evaluate shock absorber performance through phase-angle analysis [4, 5]. Currently there is no precise method to perform shock absorber test within the vehicle to verify their condition, in real time operation.

II. ASSESSEMENT METHODS A theoretical analysis of vehicle suspension and of shock

absorber internal pressures was performed to determine damping force as a function of shock absorber parameters and input (road) excitation. Two methods for assessing shock absorber status were studied and evaluated. The first method is based on sprung and unsprung mass acceleration measurements and consists in the determination of transmissibility (ratio between variables over the frequency spectrum) as a function of the damping coefficient. As the above concept requires the use of two measuring points, the second method was developed in order to measure shock absorber internal pressure and the input acceleration to assess shock absorber condition using only one sensing device.

A. Assessment method 1(mass acceleration) A simplified linear model of a quarter vehicle, as shown

in Fig. 1, was considered to study analytically the behavior of the suspension. The suspension system at each wheel was reduced to one spring and to one parallel shock absorber, connecting the sprung to the unsprung masses. The unsprung mass, associated with the wheel, has between itself and the road a spring related to the tire.

It should be noted that suspension stiffness and damping coefficients were treated as constants independent of the excitation amplitude and frequency

(1)

1-4244-2581-5/08/$20.00 ©2008 IEEE 1210 IEEE SENSORS 2008 Conference

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Solving the above equations in the frequency domain it is possible to analyse the transmissibility, output to input magnitude ratio, for sprung and unsprung accelerations as function of road excitation in the frequency range 0 Hz to 25 Hz.

Figure 1. One quarter of vehicle suspension system

B. Assessment method 2 (chambers pressure) Different models for describing shock absorber internal

working principles are present in literature. Most of this models were developed to study particular features of the shock absorbers, e.g. the dynamic characteristics for displacement-sensitive shock absorbers [6, 7], thermal models or heat transfer effects [8, 9], among others. Most of the proposed models require a detailed set of the shock absorber parameters, or were obtained as a function of results from dynamometer tests. Therefore, a simple model which correlates damping force with its physical characteristics, internal pressures, and oil temperature was developed.

Considering the basic schematic of an internal dual-tube shock absorber as presented in Fig. 2, it can be reduced to four main valves, two in the piston and two separating the compression chamber from the outer reservoir. Therefore, in the compression stroke the oil flows from the compression chamber to the extension chamber, and due to the difference of volumes between the two inner chambers the remaining oils is forced to flow to the reservoir. During shock absorber extension, oil is compressed from the extension chamber to the compression chamber, and as the oil coming from the extension chamber hasn’t enough volume to fill the bigger compression chamber, some oil will flow from the reservoir to the compression chamber.

Figure 2. Model for a twin-tube shock absorber

Considering the oil as incompressible, then the oil flow rate in the four main valves could be expressed as a function

of the compression chamber area, AC, extension chamber area, AE, and the piston velocity, x , as follows:

(2)

Equation (3) is used to correlate the oil volumetric flow rate in the valves, Q, with the corresponding pressure drop, ΔP. Cd is the discharge coefficient, A0 is the valve orifice area and ρ is the oil density

(3)

The reservoir, which is half filled with air/gas is sealed from the atmosphere, so the equation of an ideal gas (4), where Pa is the pressure of the gas, Va is the volume of the gas, ma is the gas mass, R is the gas constant and T is the gas absolute temperature, may be used to calculate the gas pressure

(4)

The oil pressure in the reservoir is equal to the gas pressure, and therefore can be determined from (4) correcting the air volume as a function of volumetric oil flow rate and variation of the oil volume with temperature over time. So, the equation for the gas and reservoir oil pressure will become:

(5)

Knowing the oil pressure in the reservoir Pa and the pressure drop between chambers ΔP, it is possible to determine the pressure in the extension chamber, PE, and compression chamber, PC. Therefore, the damping force, FDamping, is obtained from

(6)

An adjustable shock absorber instrumented with accelerometers and pressure sensors (both in compression and extension chambers) was tested in a dynamometer to validate the above assumptions, with results presented in Fig. 3.

Figure 3. Acquired vs. simulated shock absorber internal pressures

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III. PROPOSED EMBEDDED SYSTEM Recent years witnessed a significant increase in

electronic diagnosis systems in vehicles. One aspect of paramount concern is security, judging by the arguments used by vehicle manufacturers. The ability to assess shock absorbers characteristics in real time and during their lifetime will make possible to alert the driver about the need for shock absorber maintenance or replacement, and thus enable a more comfortable and more secure driving.

A wireless solution can provide a very significant reduction of wiring in vehicles, with the advantage of cost reduction and weight, allowing even an increase of reliability and reduction of assistance needed during maintenance. However, conventional wireless solutions raise the problem of power supplying, requiring a wired supply connection, or the use of any internal energy source. When such power depends on an internal energy source placed during device production (for example a battery), after some time it will always be necessary to maintain the device (replacement or recharge of the energy source). This maintenance implies costs (training, equipment, direct costs for the customer), always involving potential reliability problems caused by maintenance procedure itself.

Harvesting available energy, e.g. in the form of mechanical vibration or temperature, and converting it into electricity, thus making the device completely independent in terms of power supply will be of great interest. Such a self powered wireless device (e.g. a sensor), except in case of damage or injury, will have the lifetime expectancy of the vehicle, becoming a maintenance free device.

With the ever reducing power requirements of both analog and digital electronics, power scavenging devices are becoming a realistic form of supplying energy to electronic systems [10]. The most available energy form in shock absorbers is mechanical vibration. Therefore, although several other forms of energy harvesting devices had already been proposed, vibration energy harvesters are the most suitable solution to power the proposed shock absorber monitoring device.

Vibration driven generators based on electromagnetic, electrostatic or piezoelectric technologies have been presented in literature. The operation principle is that the inertia of a proof mass causes it to move relative to a frame when the frame experiences acceleration and this movement can be used to generate energy by causing work to be done against a damping force realized by a magnetic or electric field, or by straining a piezoelectric material [11].

A piezoelectric generator is proposed as a power harvesting device for this application, it’s energy being stored in an external energy reservoir (e.g. a supercapacitor). As there is no way to determine when there is enough energy available, as it depends on vehicle motion, a system management unit is necessary to measure the stored energy, and decide when to start the sensing procedure, data conditioning and data transmission.

Figure 4. Block diagram of the proposed Automotive Shock Absorber Smart Sensor

The sensing module is being developed with MEMS (Micro-Electro-Mechanical Systems) sensors. The fabrication technology is based on CMOS processes, allowing batch fabrication of sensors with small dimension, high quality, sensitivity and reproducibility, already proven in many, including automotive, applications. The small size, overall feasibility and reduced cost per unit are key factors for their selection and successful use.

To promote interoperationality among third-party suppliers and manufacturers, the embedded system relies on the IEEE1451 smart sensor standard, enabling plug-and-play installation and calibration features. Such device can be a major improvement in vehicle safety without added complexity and cost.

IV. EXPERIMENTAL RESULTS To validate the above methods and to verify the

feasibility of the in-vehicle shock absorber analysis in a multi-road excitation scenario, several experiments were conducted. Essays were conducted using several shock absorbers in different known conditions, with three different vehicles driven on a 36 Km tour of differentiated road profiles. Data was acquired using a NI USB 6009 data acquisition card and signal conditioning electronics connected to a laptop running LabView.

Figure 5. Average acceleration-acceleration transmissibility for different condition shock absorbers

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Fig. 5 shows some results concerning method 1, where in the unsprung mass resonance frequency range (aprox. 12 Hz in the test vehicles), it is possible to evaluate the damping factor, minimizing vehicle load (MS) and spring constant (KS) influences, measuring the sprung and unsprung mass accelerations.

The second method was validated using twin-tube adjustable shock absorbers, adapted to measure it’s internal temperature and oil pressure in both cameras. The shock absorbers were installed on a VW Sharan rear axle, driven on the same test tour, the average results being presented in Fig. 6. Oil was drained to simulate oil leak causing a poor condition shock absorber. Aspects as vehicle load effect (Fig. 7) or tire pressure (not shown) were also tested and evaluated, and the method proved to be independent of these factors.

Figure 6. Acceleration-pressure transmissibility for different condition shock absorbers

Figure 7. Load effect for the acceleration-pressure transmissibility.

V. CONCLUSIONS This paper describes part of the work that is being carried

out to achieve a complete embedded smart shock absorber system. Two methods to assess shock absorber condition were discussed. Tests results showed that the presented models are valid representations of shock absorber behaviour when submitted to normal road excitation.

The experimental results showed also the possibility of deciding the need for shock absorber service or replacement, by comparison with reference parameters, even in a random multi-road scenario. The proposed self-powered embedded system, with measurement, signal conditioning and wireless communication capabilities, can easily be integrated in a shock absorber to enhance diagnosis and vehicle security.

To our best knowledge, this is the first time (patent pending) that such a system is proposed. This shock absorber monitoring device will be a breakthrough in vehicle comfort, drivability and overall safety.

REFERENCES [1] J. C. Dixon, The Shock Absorber Handbook, SAE, 1999 [2] M.D. Rao, S. Gruenberg, “Measurement of Equivalent Stiffness and

Damping of Shock Absorbers”, Experimental Techniques, Vol.26, No.2, 2002, pp.39-42.

[3] W. Schiehlen, B. Hu, “Spectral simulation and shock absorber identification”, International Journal of Non-Linear Mechanics, Vol. 38, Issue 2, March 2003, pp. 161-171

[4] A. Tsymberov, “An Improved Non-Intrusive Automotive Suspension Testing Apparatus with Means to Determine the Condition of the Dampers”, SAE Technical Papers Series #960735.

[5] H. Nozaki, Y. Inagaki, “Technology for measuring and diagnosing the damping force of shock absorbers and the constant of coil springs when mounted on a vehicle”, JSAE Review, Vol. 20, Issue 3, July 1999, pp. 413-419.

[6] C. Lee and B. Moon, “Simulation and experimental validation of vehicle dynamic characteristics for displacement-sensitive shock absorber using fluid-flow modelling”, Mechanical Systems and Signal Processing, Vol. 20, Issue 2, February 2006, pp. 373-388.

[7] C. Lee and B. Moon, “Study of the simulation model of a displacement-sensitive shock absorber of a vehicle by considering the fluid force”, Proc. IMechE Vol.219 Part D: J. Automobile Engineering, 2005, pp.965-975.

[8] J. Ramos, A. Rivas, J. Biera, G. Sacramento and J. Sala, “Development of a thermal model for automotive twin-tube shock absorbers”, Applied Thermal Engineering, Vol. 25, Issues 11-12, August 2005, pp. 1836-1853.

[9] Alexander Lion, Swenja Loose, “A Thermomechanically Coupled model for Automotive Shock Absorber: Theory, Experiments and Vehicle Simulation on test Tracks”, Vehicle System Dynamics, Vol.37, No.4, 2002, pp.241-261.

[10] S. P. Beeby, M. J. Tudor, and N. M. White, “Energy harvesting vibration sources for microsystems applications,” Measurement Science and Technology, vol. 17, no. 12, pp. R175–R195, 2006.

[11] S. Roundy, P. Wright, and J. Rabaey, “Energy Scavenging forWireless Sensor Networks with Special Focus on Vibrations”, Kluwer Academic Press, 2004

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