sibi seminar part 4

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Energy Harvesting By A Regenerative Shock Absorber 1 B.Tech In Mechanical Engineering Department of Mechanical Engineering CHAPTER-1 INTRODUCTION 1.1 Background: A shock absorber is a device used to absorb, damp and dissipate the energy of a shock. The shock absorber dissipates the vibration energy into heat energy. This heat energy is rejected to the atmosphere. The shock from a road bump is absorbed by springs and the energy is dissipated by shock absorber. Hence damper is a technically correct term for a shock absorber. When a piston forces a fluid in a cylinder to pass through some hole a high resistance to the movement of piston is developed, which provides the damping effect. This is the principle of shock absorber. The damping produced is proportional to the square of speed. In an automobile almost 74% of the energy stored in fuel is wasted as heat. Major energy losses are engine losses, idle and standby, braking losses, aerodynamic drag etc. Thus only 26% of the available fuel energy is used to drive the vehicle is to overcome the resistance from road friction. One important loss is the dissipation of vibration energy by shock absorbers in the vehicle suspension under the excitation of road irregularity and vehicle acceleration or deceleration. Regenerative shock absorbers can efficiently recover the vibration energy wasted in the vehicle suspension system. Energy harvest from the suspension has been studied formany years, and there are two common types of energy recovery shock absorbers: linear design and rotary design. Okada and Harada proposed an electromagnetic energy regeneration system using a linear motor as an actuator, and the project was confirmed by a simple experimental apparatus. Suda developed a hybrid suspension system, in which a linear DC generator was used to harvest energy from vibration. However, the linear motor is generally very expensive, and its efficiency was much lower than the rotary one’s as Gupta reported. And thus rotary design has attracted more and more attention due to its better application potential. Suda developed an energy recovery shock absorber using DC motor and ball screw mechanism, and the performance of the trial electromagnetic damper was examined through numerical simulation and road test by passenger car equipped with the proposed damper. Yu also built their energy recovery shock absorber using the same method, and the feasibility of vibration energy regeneration was confirmed by the bench test while the damping performance was poor for high-frequency

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Page 1: sibi seminar part 4

Energy Harvesting By A Regenerative Shock Absorber 1

B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-1

INTRODUCTION

1.1 Background:

A shock absorber is a device used to absorb, damp and dissipate the energy of a

shock. The shock absorber dissipates the vibration energy into heat energy. This heat energy

is rejected to the atmosphere. The shock from a road bump is absorbed by springs and the

energy is dissipated by shock absorber. Hence damper is a technically correct term for a

shock absorber. When a piston forces a fluid in a cylinder to pass through some hole a high

resistance to the movement of piston is developed, which provides the damping effect. This is

the principle of shock absorber. The damping produced is proportional to the square of speed.

In an automobile almost 74% of the energy stored in fuel is wasted as heat. Major energy

losses are engine losses, idle and standby, braking losses, aerodynamic drag etc. Thus only

26% of the available fuel energy is used to drive the vehicle is to overcome the resistance

from road friction. One important loss is the dissipation of vibration energy by shock

absorbers in the vehicle suspension under the excitation of road irregularity and vehicle

acceleration or deceleration. Regenerative shock absorbers can efficiently recover the

vibration energy wasted in the vehicle suspension system. Energy harvest from the

suspension has been studied formany years, and there are two common types of energy

recovery shock absorbers: linear design and rotary design. Okada and Harada proposed an

electromagnetic energy regeneration system using a linear motor as an actuator, and the

project was confirmed by a simple experimental apparatus. Suda developed a hybrid

suspension system, in which a linear DC generator was used to harvest energy from vibration.

However, the linear motor is generally very expensive, and its efficiency was much lower

than the rotary one’s as Gupta reported. And thus rotary design has attracted more and more

attention due to its better application potential. Suda developed an energy recovery shock

absorber using DC motor and ball screw mechanism, and the performance of the trial

electromagnetic damper was examined through numerical simulation and road test by

passenger car equipped with the proposed damper. Yu also built their energy recovery shock

absorber using the same method, and the feasibility of vibration energy regeneration was

confirmed by the bench test while the damping performance was poor for high-frequency

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

excitation. The major obstacle to the application of the program is that, the motor changes

rotation direction twice in each vibration cycle and this phenomenon seriously affects the

high frequency response of the system, as well as the energy recovery efficiency. Zuo

modeled and tested a regenerative shock absorber based on rack and pinion mechanism, and

the program was proved to be feasible by both simulation and test results. This design made

great progress by adding a mechanical rectifier mechanism, and the motor would not change

its rotation direction with the help of mechanical rectifier mechanism. However, this program

has its drawbacks. Firstly, the frequency-limit of the mechanical rectifier mechanism needs to

be concerned because the vehicle was often subjected to high-frequency excitation. Secondly,

the backlash would seriously affects the reliability and durability of the system, as gear

cracks even teeth broken may happen due to serious road impact. Prior to this study, it’s also

important to clarify how much energy is dissipated by the shock absorber when the car is

being driven. Segel analyzed the suspension system and found that a vehicle with four shock

absorbers would dissipate about 200 watts with a speed of 13.4 m/s. Browne tested the

energy dissipated by four shock absorbers of a vehicle and found that the energy-dissipated

power was about 40-60 watts. Zuo proposed an energy dissipation formula and calculated the

suspension system would dissipate 100-400 watts in B-C class road with a speed of 100

km/h.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-2

COMPONENTS

The components used in the development of HESA include the following while some

of them are briefly discussed in the subsections below:

Hydraulic cylinder:the piston of the hydraulic cylinder is driven to reciprocate under

external stimulus

Hydraulic rectifier:the oil flows through the accumulator for weakening fluctuations

Accumulators:Accumulator 1 is used to stabilize the flow to improve the working

efficiency of the hydraulic motor, and the function of accumulator 2 is to prevent

high-frequency distortion of the shock absorber, which consists with the principle that

filling the gas chamber of the twin-tube shock absorber with suitable-pressure gas

Hydraulic motor: The oil flow passed through the hydraulic motor continues to

work.

Generator:Generator produces electricity.

Pipelines:Pipelines carries the oil to the system.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-3

THE DESIGN OF HESA

HESA is composed of hydraulic cylinder, hydraulic rectifier, accumulators, hydraulic

motor, generator, pipelines and so on. Its working process is as follows: the piston of the

hydraulic cylinder is driven to reciprocate under external stimulus, the oil flows into and out

of the hydraulic rectifier from the specified export in the compression stroke or extension

stroke, and then the oil flows through the accumulator for weakening fluctuations, which

drives the hydraulic motor to generate electricity. The electric energy can charge a battery or

supply to the vehicle directly there are two accumulators fixed on both sides of the hydraulic

motor.

Accumulator 1 is used to stabilize the flow to improve the working efficiency of the

hydraulic motor, and the function of accumulator 2 is to prevent high-frequency distortion of

the shock absorber, which consists with the principle that filling the gas chamber of the twin-

tube shock absorber with suitable-pressure gas. This design is helpful to make this scheme

more practicable.

Piston reciprocates under external stimulus

High pressure oil flows into rectifier

Oil flows from rectifier to accumulator to stabilize fluctuations

Oil from the accumulator drive the motor to generate electricity

Figure 3.1 : - Exploded view of HESA [1]

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

Figure 3.2 - Schematic of the HESA [2]

Prototype is manufactured according to its working principle ,and the test bench is

built in accordance with the standard QC/T 545-1999 as shown in Figure 3.1.Hydraulic

cylinder, hydraulic rectifier, and accumulator 2 are integrated into the HESA. Accumulator 1,

hydraulic motor, generator, and its connecting pipes are arranged in the panel on top of the

bench. P1 and P2 represent the pressure of upper and lower chamber of the hydraulic cylinder,

respectively. P3 and P4 represent the pressure of inlet and outlet of the hydraulic motor,

respectively. During the test, if the hydraulic circuit contains a lot of air or entrapped gas, the

analysis of damping force may become very difficult, which may degrade the energy-

regenerative performance. If the connecting pipes or components leak oil, the hydraulic

pressure of the system may reduce gradually, resulting in greater possibility of cavitation,

which may create numerous pockets of oil bubble throughout the oil. A small increase of

pressure can easily turn the oil bubble back into liquid, with a severe slam shock, causing bad

noise and possible damage to the damper internals. Therefore, we have to check the pressure

drop of the hydraulic circuit before and after each test and install exhaust valve in the circuit.

The force-displacement loop is asymmetric on 𝑦-axis, and the damping forces on 𝑦-axis have

a big difference, which indicates that larger damping force is needed to circulate the oil when

changing direction of the damping force under larger accumulator opening pressure.

Amutation of damping force occurs every time when it changes direction. Mounting

clearance between the HESA prototype and the test bench is inevitable, so the opening and

response time of the spool of one way valve and accumulator will both enhance the damping

force sharply when its direction changes

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

Figure 3.3-Digital image for the test bench of HESA prototype [2]

The basic function of a shock absorber is to isolate vibration. As HESA is a kind of

shock absorber, we need to pay great attention to its characteristic of vibration isolation.

The arrangement of experiment is as follows. A sine wave excitation in displacement with

1.67Hz in frequency is executed on the HESA, and the displacement is increased gradually

until the damping force reaches the limit of the force sensor. Because of the cracking pressure

of accumulator 1 greatly affecting the characteristic of HESA, we test the characteristic of

HESA with two accumulators, 5 bars and 20bar sin cracking pressure. The reason of selecting

5bars, and 20 bars as the cracking pressure is that the pressure in gas chamber of twin-tube

shock absorber is always between 4 and 6 bars and of mono tube is always between 15 and

40 bars.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-4

OPTIMAL ENERGY RECOVERY ANALYSIS OF

HESA

The goal of optimal analysis of HESA is to recover energy as much as possible. For

example, it should be installed in the rear suspension of commercial vehicles when the

requirements of ride are not so serious. Besides that, if the real road excitation is random in

frequency and amplitude, it would be hard to clarify the influence of frequency and amplitude

on the energy recovery, so it is assumed in this section that the tire is subjected to harmonic

excitations. Under this assumption, the recyclable energy is analyzed and the optimal

conditions are derived based on the analysis.

4.1Harmonic Force Excitation

Assuming the tire is subjected to force excitation, the governing equations of the vibration

energy recovery system can be written as follows:

m2a2 + ceq(v2 − v1) + k2(x2 − x1) = 0 (1)

and

m1a1+ ceq(v1− v2) + k2(x1− x2)

(2)

+k1x1 = Fsin(t)

where m1and m2 are the wheel mass and 1/4 vehicle body mass respectively, k1 and k2 are the

tire stiffness andsuspension stiffness respectively, x1 and x2 are the tire displacement and

body displacement with respect to their equilibrium positions respectively, v1 and v2 are the

correspondingly speed of tire and body respectively, a1 and a2 are the correspondingly

acceleration of tire and body respectively, ceq is the equivalent damping coefficient of HESA,

F is the maximum excitation force, t is the time, and w is the excitation frequency. So the

steady-state response of the relative motion between sprung mass and the unsprung mass

would be

x2− x1= (X2− X1)sin(t −)

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The energy-dissipative power under harmonic forceexcitation can be written as

Pave = Ceqω2

2(X2-X1)

2

so the energy-recyclable power delivering to the electrical load is given by

4.2 Motion excitation

Assuming the tire is subjected to harmonic displacement excitation, the dynamic equations

can be written as:

m2a2 + ceq(v2 − v1) + k2(x2 − x1) = 0

where X0 is the maximum excitation amplitude.Similarly, the energy-dissipative power can

be expressed as Equation , and its energy-recyclable power, Phar1 meets

Phar1

Phar= (

X0k1

F)2

where the parameters F, X0 and k1 are all constant. It is obvious that the tire subjected to

harmonic displacement excitation has the same characteristic as subjected to harmonic force

excitation. Hence, we omit the analysis on harmonic displacement excitation. According to

the above work, the presence of the hydraulic rectifier is considered to be the reason of poor

performance in high-frequency excitation.However, the hydraulic rectifier cannot be

removed. If the hydraulic rectifier is removed, the oil will drive the motor from two sides,

and themotor will change its rotational direction twice in each excitation cycle. This

phenomenon seriously affects the efficiency of the motor, and the motor even stops rotating

during high-frequency excitation because of the inertia of the rotating parts, the backlash, and

friction.

Phar

=

Rload

ce P

ave

Rin +

Rload

ceq

m1a1 + ceq(v1 − v2) + k2(x1 − x2) +k1(x1 − X0 sin ɷt) = 0

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

4.3 Optimal damping ratio based on the maximum energy-recyclable

power

Figure 4.3- Energy-recyclable power at different excitation frequencies and load resistance

[1]

As to maximum energy-recyclable power, it’s easier to analyze it by load resistance than by

damping ratio of the suspension. But to the ride comfort, it’s more convenient to evaluate it

by damping ratio than by load resistance damping ratio. Along with excitation frequency

ranging from 0.3 to 30 Hz, the dependence of energy-recyclable power on excitation

frequency and load resistance can be illustrated in Figure 4.3. And presents the corresponding

contours, in which the black dotted line represents the optimal load resistance Ropt at different

excitation frequencies. There are two undamped natural frequencies of the dual-mass

system.As to maximum energy-recyclable power, it’s easier to analyze it by load resistance

than by damping ratio of the suspension. But to the ride comfort, it’s more convenient to

evaluate it by damping ratio than by load resistance. when excitation frequencies are the

natural frequencies.the maximum energy-recyclable power at the natural excitation

frequencies. Through numerical simulations and basic experiments, Suda and Shiiba

concluded that the harvest energy would drop as the load resistance decreased, while Okada

and Harada drew a conclusion that harvest energy would rise as the resistance decreased in

high excitation frequency range. We can get that the maximum energy-recyclable power

appears only when the load resistance is the optimal load resistance. It means that for any

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

4.4Optimal load resistance based on maximum energy recovery

Figure 4.4-Energy-recyclable power at different excitation frequencies and damping ratios[1]

excitation frequency, the energy-recyclable power first increases and then decreases as the

load resistance increases gradually, and the optimal load resistance is the critical value.

Stephen claimed that the maximum power (RNGload) wasdelivered to the load resistance

when its resistance is equal to the sum of the coil internal resistance (Rin) and the electrical

analogue of the mechanical damping coefficient.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-5

ACTIVE CONTROL EXPLORATIONOF HESA

The test bench is executed on HESA according to the standard of QC/T 545-1999,

and the excitation frequency and amplitude is 1.667 Hz and 50 mm respectively in

accordance with the standard. For the HESA trial prototype, the diameter of the inner

chamber and piston rod is 50 and20 mm respectively, and the working volume of hydraulic

motor is 11.0 cc/rev. By regulating the electric load to make the current zero in the circuit as

shown in Figure 7, we can get that the rotational speed of the motor is about2820 rev/min,

and the damping force is F ϵ[−2000,2000]N. Using theoretical calculations, it can be

determined whether the speed of the motor is reasonable or not. Within one cycle, the total

flow of the hydraulic cylinder is:

V= 100𝜋

4502 +

100𝜋

4(502 − 202) = 0.3623L

and the corresponding volume flow is:

Q = V × 1.667Hz = 36.13L/min

Table 5.1: Stable motor speed and damping force at different current values [1]

Compared with theoretical average speed, the average speed we get from the experiment is

about 2820 rev/min, and the 2.4% deviation is acceptable. By adjusting the electric load, the

current can be stabilized to 5 amp, and the speed of the motor declines rapidly to about 2510

rev/min correspondingly. However there is a break during the decreasing process of the

Current/amp

Rotational

speed/rev/min

Maximum

Dampingforce/N

0 2820 [-2000, 2000]

5 2510 [-3600, 3500]

10 2260 [-5800, 5700]

20 1900 [-8800, 8500]

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

rotational speed. That is to say, the speed drops suddenly and then returns to the stable speed

quickly as shown in Figure 5.2.

Figure 5.2 - Rotational speed of the motor [1]

The reason of this phenomenon is that, when the current is changed to 5 amp in a short time,

the oil pressure rises rapidly and accumulator 1 absorbs some oil accordingly, which results

in the reduction of oil flowing through the hydraulic motor and the decline of the rotational

speed of the motor. When accumulator 1 reaches its equilibrium position, the rotational speed

also reaches the balanced range. Along with this, the damping force increases as the pressure

rises, and it is in the range of [−3600,3500]N as shown in Figure 5.3.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

Figure 5.3- Damping force [1]

From Figure 5.3, it is also noticed that there is a break during the damping force changed its

direction. That is because the mounting clearance between the HESA prototype and the test

bench isn’t eliminated completely, and we can even hear the impact sound in high frequency

excitation.

The current drops from 5 to 0 amp some time later,and a speed break of the motor comes out

again. The speed of the motor increases sharply to the stable rotate speed around 2820

rev/min, and the damping force alsoreturns to its initial range. This phenomenon is believed

to result from pressure drop of the system, since accumulator 1 would release some oil

lubricating the motor during the process. When the current rises to 10 or 20 amp suddenly, a

break of the rotational speed appears again, and the break becomes more prominent if the

current value changes more greatly. Table 3 shows the stable motor speed and the damping

force at different current values. We can get from the test that the damping force can be

changed by adjusting the circuit current, which means the active control of HESA can be

achievedby adjusting the circuit current.

Actually, the control of the damping force of HESA is influenced by many factors, such as

the size of hydraulic cylinder and accumulator 1, pre-load of accumulator 1, parameters of the

rectifier and so on, so some more work need to be done to achieve active suspension based on

HESA. Firstly, hydraulic cylinder, accumulator 1 and hydraulic rectifier must match with

reference vehicle.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

Secondly, accumulator 1 should be matched with the road excitation to avoid dead zone

during the active control of HESA. Finally, the relationship between the current and damping

force should be clarified.

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

CHAPTER-6

CONCLUSION

This work presents one kind of HESA, and optimal energy recovery analysis and active

control of HESA are carried out based on a quarter-car model combined with simulation

results. Based on the discussion above, the following conclusions can be drawn:

1. For any excitation frequency, the energy-recyclable power first increases and then

decreases as the load resistance increases gradually,

2. The energy-recyclable power is more sensitive to excitation frequency than to

damping ratio.

3. The wheel resonant frequency is the ideal excitation frequency for suspension energy

recovery system.

4. Regenerated power helps to reduce the work of alternator.

5. Increased fuel efficiency of about 10%

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B.Tech In Mechanical Engineering Department of Mechanical Engineering

REFERENCES

[ 1] Zhigang Fang, Xuexun Guo, Lin Xu and Han Zhang “An Optimal Algorithm for

Energy Recovery of Hydraulic Electromagnetic Energy-Regenerative Shock

Absorber”, Appl. Math. Inf. Sci. 7, No. 6, 2207-2214 (2013)

[ 2] Zhigang Fang, Xuexun Guo, Lin Xu, and Han Zhang, “Experimental Study of

Damping and Energy RegenerationCharacteristics of a Hydraulic Electromagnetic

Shock Absorber”, Hindawi Publishing Corporation, Advances in Mechanical

Engineering,Volume 2013, Article ID 943528

[ 3] Zhongjie Li, Lei Zuo, George Luhrs, Liangjun Lin, and Yi-xian Qin,

“Electromagnetic Energy-Harvesting Shock Absorbers: Design, Modeling, and

Road Tests”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL.

62, NO. 3, MARCH 2013

[ 4] Zhigang Fang, XuexunGuo, LinXu, Energy Dissipation and Recovery of Vehicle

Shock Absorbers, SAE Commercial Vehicle Engineering Congress, Illinois, 2037

(2012)

[ 5] LinXu, “Structure Designs and Evaluation of Performance Simulation of Hydraulic

Transmission Electromagnetic Energy-regenerative Active Suspension”, SAE 2011

World Congress and Exhibition, Michigan, 0760 ( 2011)

[ 6] Lei Zuo, Brian Scully, Jurgen Shestani and Yu Zhou, “Design and characterization

of an electromagnetic energy harvester for vehicle suspensions”, IOP

PUBLISHING SMART MATERIALS AND STRUCTURES Smart Mater. Struct.

19 (2010) 045003