1.1 it was assumed that the mass increase

1.1        
Introduction

Lately, the calling for equality between women and men in
opportunities was spread at wide range. According to the
American Trucking Associations; about 5.8 percent of the 3.4 million U.S. truck
drivers are women. They increased worldwide up to 400,000 by the end of 2017 1. More women have taken the wheel day by
day as women can drive trucks just as well as men, and there are all sorts of
insurance industry data that show that females, especially compared with young
men, are drivers 2. Truck driving is hard work as the road can be tiring
and the body of pregnant woman is an extremely sophisticated dynamic
system that is sensitive to whole-body vibrations in a sitting posture, so the
surrounding environment should be facilitated
and adapted to give the pregnant woman adequate level of comfort to
herself and her fetus to save their life.

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Many researches have been assigned for vibration effects
on sitting for non-pregnant persons. However, seated pregnant women are out of
interest. Qassem and Othman model is the first analytical study of the effects
of vibrations on seated pregnant women. Based on their earlier research
investigating horizontal and vertical vibration responses of seated human
males, The pregnant woman mathematical model was investigated .They used the same
physical properties; spring constants and damping coefficients. Moreover, for a
pregnant model, it was assumed that the mass increase due to pregnancy was
distributed on the thorax and abdomen at the same ratios with their masses. 3

In
general, Suspension systems are classified into three major categorizations;
passive, active and semi-active. Passive suspensions using oil dampers are
simple, reliable and cheap, but the performance has some limitations. Active
and semi-active suspensions have control algorithms which force the system to
enhance the ride quality and damp vibrations to safety levels.

Semi-active
suspension systems can enhance performance over a wide-ranging of frequency
compared with passive systems. The research of
semi-active seat suspensions widely introduces advanced control approaches or
applies appropriate actuators to enhance the ride comfort considering load
variation, actuator saturation, time delays, and reliability, etc.

A thirteen
degrees–of– freedom (DOF) seat and pregnant woman body model is used where the
seat consists of frame and cushion and the human body consists of head, neck,
upper arm, lower arm, torso, Thoracic spine, thorax, diaphragm, abdomen, lumbar
spine and pelvis. The control performance criteria are estimated by different
parameters such as seat frame distance, cushion displacement, fetus
acceleration and head acceleration. The pregnant woman body is eleven DOF for
more accuracy.

1.2 Background

The semi-active control algorithm for seat
suspension system and pregnant woman model using MR damper is shown in Fig.1-1.
It consists of two controllers; system and damper controllers. The system
controller needs the dynamic outputs of the seat suspension to calculate the desired
damper force

according to the control algorithm.

Whereas, the damper controller computes the command voltage v
applied to the damper coil in order to track the actual damper force

to the desired force

. A brief explanation of the system and damper controllers
considered in this thesis is provided in the following sub-sections.

Fig. 1-1 Semi-active control algorithm for MR seat suspension

There are 3
main categories of suspension systems; Passive, Active and semi-active, as
illustrated in Fig.1-2

(a)Passive Suspension System

(b) Active Suspension System

(c) semi-active Suspension System

Fig. 1-2 categorization of suspension
systems

(a)  
Passive
suspension systems: the traditional linear springs and oil dampers are used. Due
to their simple construction, reliable performance and cheap price, passive
suspension systems were used at wide range during the last decades. But,
performance limitations are unavoidable.

(b)  
Fully active
suspension systems: instead of conventional passive damper, the force of the
damper will be directly supplied to an electro-hydraulic actuator (active
device) to generate the desired damping force. By comparing the conventional
passive system with active suspension system, an active suspension can offer
high control performance over a wide frequency range. However, it is not cost
effective for commercial application, since it requires a high-power supply,
many sensors, and servo-valves. 4

(c)   
 Semi-active
suspension systems: uses semi-active dampers (MR and ER dampers) whose
force is commanded indirectly through a controlled change in the properties of
the dampers. Semi-active dampers can provide performance very similar to the
fully active suspension system and consume less power than the fully active,
which enhances the ride quality. 
Moreover, semi active suspension systems have the ability to work as
passive suspension in case of a system failure. 5-6.

Here, we applied semi-active suspension system to the
pregnant woman body and seat model using MR damper to enhance the ride quality.

   During the last few decades, the ride
comfort attracted several researchers to investigate and apply different types
of controllers. For the first time applying three different types of
controllers to the pregnant woman seat model and comparing their results. The
applying controllers are a) Proportional-Integral-Derivative
(PID) controller b) Single neuron PID (SNPID) controller c)
Fractional Order PID(FOPID), applying genetic algorithm for optimization the
controller parameters and using fuzzy logic control (FLC) to adjust the
parameters of each controller on-line according to the error and the change of
error.

1.3 Aims and Objectives of this thesis

This
thesis aims to improve the pregnant woman comfort by dissipating the vibration
energy from the vehicle body to her body by using a semi- active seat
suspension system and suitable control strategies.

The
next section introduces a literature review which summarizing the
previous researches which published in the core areas related to this thesis.
It starts by the brief fundamentals of MR fluids and MR dampers are provided
and followed by a review of MR dampers and system controllers. And finally, the
previous research related to the semi-active vibration control of vehicle and
car seat suspension systems using MR dampers is reviewed.

1.4 Thesis Organization

The rest of the thesis
is organized as follows.

Chapter 2       This chapter considers
a description of the semi-active seat suspension with pregnant woman body
model, and describes the dynamic equations of motion that describe the model.

Chapter 3       Chapter three provides the full details
of the used advanced control strategies of semi-active system. That introduces
the PID, SNPID and FOPID controllers besides fuzzy logic control (FLC), and how
to use FLC to adjust controller’s parameters on-line.

Chapter 4       In this chapter, the simulation results
presented for different system controllers. The system performance criteria are examined in both time
and frequency domains to verify
the success of the proposed controllers.

Chapter 5       The chapter shows the performance
comparison for the outputs resulted by each proposed
controller.

Chapter
6      
Finally,
the conclusions summarizing all main end results
and the advantages of the proposed control strategies of this study. Also,
recommendations for future work in this research area are listed at the end of the
chapter.

1.5 Literature Review

The purpose of this section is to present a
comprehensive literature review summarizing the previous published researches
relevant to this thesis. Firstly, a short background is given on the behaviour
of magneto-rheological (MR) fluids and the design of MR dampers. Then a review of MR dampers controllers is
presented. Finally, previous research relating to the semi-active
vibration control of vehicle and car seat suspension systems using MR dampers
is reviewed, focusing on both system and damper controllers.

 

 

1.5.1
Magneto-rheological (MR) Fluids

The term “magneto-rheological” is derived from a combination
of two words; magneto, meaning magnetic, and rheo, Is the prefix
for the study of deformation of matter under applied stress. The
magneto-rheological (MR) fluids and the electro-rheological (ER) fluids are
used in the controllable fluid dampers instead of oil compared with
conventional dampers. The fluid viscosity properties in these dampers can be
altered dramatically by applying a magnetic field (MR) or an electric field
(ER). These fluids were first discovered by the inventor Willis Winslow, who
achieved a US patent regarding these fluids in 1947 7 and published a
scientific article in 1949 8.

Many successful applications for field of vibration control
used these fluids, whose yield stress can be controlled very precisely by
changing the field intensity to generate a continuously variable damping force.
The MR fluid damper is considered in our research for the pregnant woman seat
application. A micro-sized magnetically particles, such as iron particles, forms
an MR fluid. These particles are suspended in a carrier liquid such as mineral
oil, synthetic oil, glycol or water. A typical MR fluid contains nearly 20 to
40 percent by volume of relatively pure iron particles around 3 to 10 micron
diameter in size. A variety of proprietary additives, similar to those found in
commercial lubricants, is commonly added. These additives are intended to
discourage gravitational settling and promote particle suspension, and enhance
lubricity, modify the viscosity and inhibit wear.

MR fluids respond to applied magnetic field with a dramatic transform
in rheological behaviour. In addition, MR fluids can reversibly and
instantaneously change from a free-flowing liquid to a semi-solid within a few
milliseconds with controllable yield strength when subjected to a magnetic
field 9. Fig 1-3 shows the states of activation MR fluid when applying
magnetic field. Firstly, in the absence of applied magnetic field, the ferrous
particles are in an amorphous state. When a magnetic field is applied, the
ferrous particles begin to align along the flux path. Finally, the particles
are re-arranged and forming particle chains in the fluid. Such chains resist
and restrict fluid movement. As a result, a yield stress develops in the fluid.
The degree of change is related to the strength of the applied magnetic field
and it has been shown that this change can occur in less than 1 millisecond 10.

 

Fig. 1-3 Activation of MR
fluid (MR fluid – Working principle)

 

1.5.2 MR Fluid Dampers

MR fluid dampers are semi-active control
devices, which utilize the advantages of MR fluids, are becoming popular in
semi-active vehicle suspension applications and the modern industry. MR dampers
consist of piston, magnetic coil, accumulator, bearing and seal and reservoir
filled with MR fluid through the Orifice area as illustrated in Fig 1-4.

Fig. 1-4 Cross-section of
typical MR fluid damper 11

MR fluid flows from the high pressed
chamber to the low pressured chamber in the piston head. The accumulator
contains compressed gas which allows for a slight movement and the gas has
three main usages:

·        
Providing a level of
softening by the existence of an extra volume when the piston rod enters the
housing.

·        
Suitable for applications
subjected to thermal expansions.

·        
Prevents gases from the
formation of an empty space within an MR damper.

The magnetic field generated at the area
where the coil exists inside an MR damper manipulate the characteristics of the
yield stress, As a results, the magnetic field determines the physical
properties of the MR damper and the maximum force can be generated by an MR
damper mainly depend of the type of the MR damper 11.  

 

The MR fluid dampers
which used in vehicle suspensions are classified into mono-tube and twin tube.
Mono-tube MR fluid dampers have only one reservoir and accumulator as shown in
Fig. 1-5 which is commonly used in car seat suspension 12. A twin tube MR
damper has two fluid reservoirs, one inside the other as illustrated in Fig. 1-6
that is commonly used in vehicle suspension 13. This type of damper has inner
and outer tubes. The inner tube guides the piston, in exactly the same way as
in the mono-tube damper. The volume enclosed by the inner tube is the inner
reservoir. The volume that is confined by the space between the inner and the
outer tubes is the outer reservoir. The inner reservoir is filled with MR fluid
so that no air pockets exist. An outer reservoir that is partially filled with
MR fluid serves as an accumulator by accommodating changes in volume due to
piston movement.

 

Fig. 1-5 Mono-tube MR
damper 12

Fig. 1-6 Schematic
configuration of the twin tube MR damper without accumulator 13

In other designs 14
the outer reservoir is completely filled and separated by a diaphragm from a
gas filled accumulator as shown in Fig 1-7.

Fig. 1-7 Schematic
illustration of the twin tube MR damper with accumulator 14

1.5.3
Semi-active vehicle seat suspension using MR fluid dampers

One of the most important systems to the ride comfort
experience of a commercial vehicle’s driver is seat suspension system. It has a
major role in offering the vehicle’s driver with a sufficient level of satisfaction.
The seat suspension system is the support mechanism interposed between the
cabinet floor and the seat frame; it consists of springs and dampers mounted in
parallel between the seat frame and the cabinet floor as shown in Fig. 1-8.

Fig. 1-8 Seat suspension system’s location with
reference to vehicle suspension system and driver model. 15

As Fig. 1-6 illustrates, the suspension system traditionally consists
of spring and damper mounted in parallel. The spring supports
the weight of the seat and driver mass. When the vehicle is subjected to
excitation due to a non- uniform road profile, the spring allows for the seat
and body to move at a certain levels on top of the cabinet floor. The spring stores potential energy which is transformed into kinetic energy by
the body, while the damper is responsible for dissipating this kinetic energy 16.

   1.5.3.1 System Controller

Semi-active
vibration control system is composed of two integrated controllers; system
controller and damper controller. System controller is proposed to estimate the
desired damping force

 by using the dynamic responses of the plant, while damper controller
is used to convert the gains of the system controller into a DC voltage to
supply it to the MR damper. The more advanced the systems controller would be
the better performance can be achieved since the damping force of the MR damper
directly related to the supplied voltage 17.

In
this thesis, three different advanced controller strategies are used as the
system controllers to evaluate the ride comfort under different road profiles
such as bump and random road excitations and the seat frame acceleration (

)
is used as the error signal.

   Firstly, the PID controller is
introduced as the system controller. It is the feedback controller commonly
used in most process industries. It is robust and has simple structure; hence about
90% of industrial loops use PID controllers 18. The gains of PID controller
can be described as follows, the proportional gain (

)
is responsible to reduce the error responses of the system to disturbances, the
integral gain (

)
is used to eliminate the steady-state error, and the derivative gain (

)
dampens the dynamic response and improves the system stability 19.

Genetic Algorithm (GA) is one of the optimization techniques, which mimics
Darwin’s theory about evolution to offer a robust solution. It is
used as optimization tool in order to find the best parameters of the PID
controller. However, these   parameters
are relative to a certain system operating condition (uncertain dynamics, time
delays and non-linearity…). Hence it is necessary to automatically adjust the
PID parameters for obtaining satisfactory response.

The fuzzy control (FLC) is tuning the PID
parameters on-line according to the error and the change of error. Based on expert
knowledge a FLC system transforms a linguistic control strategy into an
automatic control strategy 18. FLC has self tuning capabilities in set point
tracking performance where, FLC is one of the most successful applications of fuzzy set
theory, introduced by L.A Zadeh in 1973 and applied (Mamdani 1974) in an
attempt to control system that are structurally difficult to model. Since then,
FLC has been applied and has a great attention in many industrial applications 20.

Recently, many neurons control
algorithm had been widely proposed such as single neuron PID (SNPID), and Back Propagation (BP) neural
network, etc 21.
The single neuron PID control algorithm is proposed secondly as a system
controller. The advantages of the single neuron are the self-learning abilities
which can adapt to environmental changes 22.  

The SNPID controller keeps the simple structure
of conventional PID controller in addition to that; it can online adjust its
parameters using neural network auto-learning. On the other hand it still can’t
self-tune gain coefficient (K) and the learning process of weight (W) is not
fast enough. In order to overcome these disadvantages, FLC is used to improve the SNPID 21.

Finally, the fractional order PID (FOPID)
controller optimized by GA is used as system controller. The performance of PID
controllers can be enhanced by using the FOPID controllers. The FOPID
controller may be regarded as the general case of the conventional PID
controller. FOPID has two more extra tuning parameters. It
gives more flexibility for the design of a control system 23.  The FOPID controller, originally contributed
by Podlubny (1999), has better performance and robustness than the conventional
PID controller. This makes the FOPID controller more
suitable for the vibration control (Aldair and Wang, 2010; Zhihuan et al., 2014).A
FOPID controller was tuned using evolutionary optimization algorithm and
applied in a full vehicle nonlinear active suspension system by Aldair and Wang
(2010). Ammar A. Aldair et al (2010) proposed FOPID controller for full vehicle
nonlinear active suspension model including hydraulic actuators in order to
improve the ride comfort and road handling during various maneuvers: traveling,
braking and cornering. Zhihuan et al. (2014) presented a FOPID controller
optimized using chaotic non-dominated sorting genetic algorithm II for
hydraulic turbine regulating system. S Gad et al (2015) applied a FOPID
controller optimized using GA algorithm for 6 DOF Seat and human body model.

As
classical PID, The FOPID parameters are detected relative to a certain system operating condition.
Hence, the self-tuning fuzzy FOPID control by different techniques is used to
improve the pregnant woman comfort and reduce health risks of her. Mohamed.
A.Shamseldin et al (2016) compared between different
techniques of self-tuning FOPID control for Brushless DC motor.

The
first technique is the Fuzzy self tuning FOPID controller, where the fuzzy
logic control is used to adjusting the output of the FOPID controller on-line
without changing the values of the parameters of FOPID controller. The second technique uses the fuzzy logic
control to tune the proportional, integral and derivative parameters of FOPID
control, while the fractional order integral and derivative gains will be still
constant. The third technique investigates the performance of the FOPID control
through tuning the fractional order integral and derivative gains only using
the fuzzy logic control, while the proportional, integral and derivative gains
are constant. The forth technique, the fuzzy logic control will be tuned the
five parameters of FOPID controller parameters simultaneously on-line 24.This
comparison discovering the sensitive FOPID controller parameters, which will
give better performance when it will be tuned online.

  1.5.3.2 MR damper controllers

The damper controller estimates the DC voltage

 which applied to the MR damper’s coil for varying the damping coefficient
to track the actual force to the desired force. Here, the thesis introduces
exclusively with the damper controller.

The
most popular three types
of conventional damper controller are Heaviside step function (HSF),
Signum Function Method (SFM)
and Continuous State Controller (CSC).

HSF
controller is an “ON-OFF” damper controller where the applied voltage is either
zero or max value based on a comparison held between the estimated force
supplied by the system controller and the actual force measured by the sensor
placed in series with the MR-damper. The SFM is an improved method from the HSF
as it allows the applied voltage to switch between discrete values on certain
conditions; HSF and SFM are classified to be discontinuous damper controllers, In
this work (SFM) is introduced.

1.5.4 vehicle
Seat Suspension System for the pregnant woman driver

  Many
studies on vehicle suspension systems control considered a single degree of
freedom (SDOF) model for human body to facilitate the study. An example of SDOF
model is shown in fig.1- , which present a tow degrees of freedom seat and
human body model 25.

Fig 1-9 a tow degrees of freedom seat and human body
model 25.

But SDOF isn’t sufficient model and neglects
many dynamic properties of a human body, which is lead to a lack of accuracy in
the estimation of the ride quality. Multi degrees of freedom model allow
studying the vibration effect on specific parts of the body to evaluate the
ride comfort.

Fig 1-10 is an example for the Multi degrees
of freedom model. It is Six degrees of freedom seat and human body model 26 with four degrees of freedom of
a human body.

Fig 1-10 a Six degree of freedom
seat and human body model

 

In this thesis, we
are considering a thirteen degrees of freedom model. The model consists of:

  
I.  The two degrees of freedom (2DOF) seat suspension mechanical
system that represents the vehicle seat suspension model. It involves of a
lower mass (

) demonstrating the seat frame mass and an upper
mass (

) demonstrating the seat cushion mass. The excitation
due to the road is

, that represented here by bump and
random road excitations.

Fig. 1-11 Seat suspension
model

II.       
The eleven-DOF model
(Qassem et al., 1994; Qassem and Othman, 1996)) 27. Many details for the
pregnant woman body can be illustrated at Fig. 1-12. The pregnant woman body has divided into 11 DOF
for more accuracy to reduce health risks of the woman and her
fetus.

Fig. 1-12 Pregnant woman model
(11- DOF) 27

By combining the two previous models, the
seat and the pregnant woman driver model is resulted.