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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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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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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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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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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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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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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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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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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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
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(2012)
[ 5] LinXu, “Structure Designs and Evaluation of Performance Simulation of Hydraulic
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World Congress and Exhibition, Michigan, 0760 ( 2011)
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