1.1 IntroductionLately, the calling for equality between women and men inopportunities was spread at wide range.
According to theAmerican Trucking Associations; about 5.8 percent of the 3.4 million U.S. truckdrivers are women.
They increased worldwide up to 400,000 by the end of 2017 1. More women have taken the wheel day byday as women can drive trucks just as well as men, and there are all sorts ofinsurance industry data that show that females, especially compared with youngmen, are drivers 2. Truck driving is hard work as the road can be tiringand the body of pregnant woman is an extremely sophisticated dynamicsystem that is sensitive to whole-body vibrations in a sitting posture, so thesurrounding environment should be facilitatedand adapted to give the pregnant woman adequate level of comfort toherself and her fetus to save their life.Many researches have been assigned for vibration effectson sitting for non-pregnant persons. However, seated pregnant women are out ofinterest. Qassem and Othman model is the first analytical study of the effectsof vibrations on seated pregnant women.
Based on their earlier researchinvestigating horizontal and vertical vibration responses of seated humanmales, The pregnant woman mathematical model was investigated .They used the samephysical properties; spring constants and damping coefficients. Moreover, for apregnant model, it was assumed that the mass increase due to pregnancy wasdistributed on the thorax and abdomen at the same ratios with their masses. 3Ingeneral, Suspension systems are classified into three major categorizations;passive, active and semi-active.
Passive suspensions using oil dampers aresimple, reliable and cheap, but the performance has some limitations. Activeand semi-active suspensions have control algorithms which force the system toenhance the ride quality and damp vibrations to safety levels. Semi-activesuspension systems can enhance performance over a wide-ranging of frequencycompared with passive systems. The research ofsemi-active seat suspensions widely introduces advanced control approaches orapplies appropriate actuators to enhance the ride comfort considering loadvariation, actuator saturation, time delays, and reliability, etc.
A thirteendegrees–of– freedom (DOF) seat and pregnant woman body model is used where theseat consists of frame and cushion and the human body consists of head, neck,upper arm, lower arm, torso, Thoracic spine, thorax, diaphragm, abdomen, lumbarspine and pelvis. The control performance criteria are estimated by differentparameters such as seat frame distance, cushion displacement, fetusacceleration and head acceleration. The pregnant woman body is eleven DOF formore accuracy.
1.2 BackgroundThe semi-active control algorithm for seatsuspension system and pregnant woman model using MR damper is shown in Fig.1-1.It consists of two controllers; system and damper controllers.
The systemcontroller needs the dynamic outputs of the seat suspension to calculate the desireddamper force according to the control algorithm.Whereas, the damper controller computes the command voltage vapplied to the damper coil in order to track the actual damper force to the desired force . A brief explanation of the system and damper controllersconsidered in this thesis is provided in the following sub-sections. Fig. 1-1 Semi-active control algorithm for MR seat suspensionThere are 3main categories of suspension systems; Passive, Active and semi-active, asillustrated in Fig.1-2 (a)Passive Suspension System (b) Active Suspension System (c) semi-active Suspension System Fig. 1-2 categorization of suspensionsystems(a) Passivesuspension systems: the traditional linear springs and oil dampers are used.
Dueto their simple construction, reliable performance and cheap price, passivesuspension systems were used at wide range during the last decades. But,performance limitations are unavoidable. (b) Fully activesuspension systems: instead of conventional passive damper, the force of thedamper will be directly supplied to an electro-hydraulic actuator (activedevice) to generate the desired damping force. By comparing the conventionalpassive system with active suspension system, an active suspension can offerhigh control performance over a wide frequency range.
However, it is not costeffective for commercial application, since it requires a high-power supply,many sensors, and servo-valves. 4(c) Semi-activesuspension systems: uses semi-active dampers (MR and ER dampers) whoseforce is commanded indirectly through a controlled change in the properties ofthe dampers. Semi-active dampers can provide performance very similar to thefully 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 aspassive suspension in case of a system failure. 5-6.Here, we applied semi-active suspension system to thepregnant woman body and seat model using MR damper to enhance the ride quality.
During the last few decades, the ridecomfort attracted several researchers to investigate and apply different typesof controllers. For the first time applying three different types ofcontrollers to the pregnant woman seat model and comparing their results. Theapplying controllers are a) Proportional-Integral-Derivative(PID) controller b) Single neuron PID (SNPID) controller c)Fractional Order PID(FOPID), applying genetic algorithm for optimization thecontroller parameters and using fuzzy logic control (FLC) to adjust theparameters of each controller on-line according to the error and the change oferror. 1.3 Aims and Objectives of this thesis Thisthesis aims to improve the pregnant woman comfort by dissipating the vibrationenergy from the vehicle body to her body by using a semi- active seatsuspension system and suitable control strategies. Thenext section introduces a literature review which summarizing theprevious researches which published in the core areas related to this thesis.It starts by the brief fundamentals of MR fluids and MR dampers are providedand followed by a review of MR dampers and system controllers. And finally, theprevious research related to the semi-active vibration control of vehicle andcar seat suspension systems using MR dampers is reviewed.
1.4 Thesis OrganizationThe rest of the thesisis organized as follows.Chapter 2 This chapter considersa description of the semi-active seat suspension with pregnant woman bodymodel, and describes the dynamic equations of motion that describe the model.Chapter 3 Chapter three provides the full detailsof the used advanced control strategies of semi-active system. That introducesthe PID, SNPID and FOPID controllers besides fuzzy logic control (FLC), and howto use FLC to adjust controller’s parameters on-line. Chapter 4 In this chapter, the simulation resultspresented for different system controllers. The system performance criteria are examined in both timeand frequency domains to verifythe success of the proposed controllers.Chapter 5 The chapter shows the performancecomparison for the outputs resulted by each proposedcontroller.
Chapter6 Finally,the conclusions summarizing all main end resultsand 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 thechapter.1.5 Literature Review The purpose of this section is to present acomprehensive literature review summarizing the previous published researchesrelevant to this thesis. Firstly, a short background is given on the behaviourof magneto-rheological (MR) fluids and the design of MR dampers. Then a review of MR dampers controllers ispresented. Finally, previous research relating to the semi-activevibration control of vehicle and car seat suspension systems using MR dampersis reviewed, focusing on both system and damper controllers. 1.
5.1Magneto-rheological (MR) FluidsThe term “magneto-rheological” is derived from a combinationof two words; magneto, meaning magnetic, and rheo, Is the prefixfor the study of deformation of matter under applied stress. Themagneto-rheological (MR) fluids and the electro-rheological (ER) fluids areused in the controllable fluid dampers instead of oil compared withconventional dampers. The fluid viscosity properties in these dampers can bealtered dramatically by applying a magnetic field (MR) or an electric field(ER).
These fluids were first discovered by the inventor Willis Winslow, whoachieved a US patent regarding these fluids in 1947 7 and published ascientific article in 1949 8. Many successful applications for field of vibration controlused these fluids, whose yield stress can be controlled very precisely bychanging the field intensity to generate a continuously variable damping force.The MR fluid damper is considered in our research for the pregnant woman seatapplication. A micro-sized magnetically particles, such as iron particles, formsan MR fluid. These particles are suspended in a carrier liquid such as mineraloil, synthetic oil, glycol or water. A typical MR fluid contains nearly 20 to40 percent by volume of relatively pure iron particles around 3 to 10 microndiameter in size. A variety of proprietary additives, similar to those found incommercial lubricants, is commonly added. These additives are intended todiscourage gravitational settling and promote particle suspension, and enhancelubricity, modify the viscosity and inhibit wear.
MR fluids respond to applied magnetic field with a dramatic transformin rheological behaviour. In addition, MR fluids can reversibly andinstantaneously change from a free-flowing liquid to a semi-solid within a fewmilliseconds with controllable yield strength when subjected to a magneticfield 9. Fig 1-3 shows the states of activation MR fluid when applyingmagnetic field. Firstly, in the absence of applied magnetic field, the ferrousparticles are in an amorphous state. When a magnetic field is applied, theferrous particles begin to align along the flux path.
Finally, the particlesare re-arranged and forming particle chains in the fluid. Such chains resistand 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 fieldand it has been shown that this change can occur in less than 1 millisecond 10.
Fig. 1-3 Activation of MRfluid (MR fluid – Working principle) 1.5.
2 MR Fluid Dampers MR fluid dampers are semi-active controldevices, which utilize the advantages of MR fluids, are becoming popular insemi-active vehicle suspension applications and the modern industry. MR dampersconsist of piston, magnetic coil, accumulator, bearing and seal and reservoirfilled with MR fluid through the Orifice area as illustrated in Fig 1-4. Fig.
1-4 Cross-section oftypical MR fluid damper 11MR fluid flows from the high pressedchamber to the low pressured chamber in the piston head. The accumulatorcontains compressed gas which allows for a slight movement and the gas hasthree main usages:· Providing a level ofsoftening by the existence of an extra volume when the piston rod enters thehousing.· Suitable for applicationssubjected to thermal expansions. · Prevents gases from theformation of an empty space within an MR damper.
The magnetic field generated at the areawhere the coil exists inside an MR damper manipulate the characteristics of theyield stress, As a results, the magnetic field determines the physicalproperties of the MR damper and the maximum force can be generated by an MRdamper mainly depend of the type of the MR damper 11. The MR fluid damperswhich 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 inFig.
1-5 which is commonly used in car seat suspension 12. A twin tube MRdamper has two fluid reservoirs, one inside the other as illustrated in Fig. 1-6that is commonly used in vehicle suspension 13. This type of damper has innerand outer tubes. The inner tube guides the piston, in exactly the same way asin the mono-tube damper. The volume enclosed by the inner tube is the innerreservoir. The volume that is confined by the space between the inner and theouter tubes is the outer reservoir.
The inner reservoir is filled with MR fluidso that no air pockets exist. An outer reservoir that is partially filled withMR fluid serves as an accumulator by accommodating changes in volume due topiston movement. Fig. 1-5 Mono-tube MRdamper 12 Fig. 1-6 Schematicconfiguration of the twin tube MR damper without accumulator 13In other designs 14the outer reservoir is completely filled and separated by a diaphragm from agas filled accumulator as shown in Fig 1-7. Fig.
1-7 Schematicillustration of the twin tube MR damper with accumulator 141.5.3Semi-active vehicle seat suspension using MR fluid dampersOne of the most important systems to the ride comfortexperience of a commercial vehicle’s driver is seat suspension system.
It has amajor role in offering the vehicle’s driver with a sufficient level of satisfaction.The seat suspension system is the support mechanism interposed between thecabinet floor and the seat frame; it consists of springs and dampers mounted inparallel between the seat frame and the cabinet floor as shown in Fig. 1-8. Fig. 1-8 Seat suspension system’s location withreference to vehicle suspension system and driver model. 15As Fig. 1-6 illustrates, the suspension system traditionally consistsof spring and damper mounted in parallel. The spring supportsthe weight of the seat and driver mass.
When the vehicle is subjected toexcitation due to a non- uniform road profile, the spring allows for the seatand body to move at a certain levels on top of the cabinet floor. The spring stores potential energy which is transformed into kinetic energy bythe body, while the damper is responsible for dissipating this kinetic energy 16. 1.5.
3.1 System ControllerSemi-activevibration control system is composed of two integrated controllers; systemcontroller and damper controller. System controller is proposed to estimate thedesired damping force by using the dynamic responses of the plant, while damper controlleris used to convert the gains of the system controller into a DC voltage tosupply it to the MR damper. The more advanced the systems controller would bethe better performance can be achieved since the damping force of the MR damperdirectly related to the supplied voltage 17.Inthis thesis, three different advanced controller strategies are used as thesystem controllers to evaluate the ride comfort under different road profilessuch as bump and random road excitations and the seat frame acceleration ( )is used as the error signal. Firstly, the PID controller isintroduced as the system controller. It is the feedback controller commonlyused in most process industries. It is robust and has simple structure; hence about90% of industrial loops use PID controllers 18.
The gains of PID controllercan be described as follows, the proportional gain ( )is responsible to reduce the error responses of the system to disturbances, theintegral 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 mimicsDarwin’s theory about evolution to offer a robust solution. It isused as optimization tool in order to find the best parameters of the PIDcontroller. However, these parametersare relative to a certain system operating condition (uncertain dynamics, timedelays and non-linearity…).
Hence it is necessary to automatically adjust thePID parameters for obtaining satisfactory response. The fuzzy control (FLC) is tuning the PIDparameters on-line according to the error and the change of error. Based on expertknowledge a FLC system transforms a linguistic control strategy into anautomatic control strategy 18. FLC has self tuning capabilities in set pointtracking performance where, FLC is one of the most successful applications of fuzzy settheory, introduced by L.A Zadeh in 1973 and applied (Mamdani 1974) in anattempt 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 controlalgorithm had been widely proposed such as single neuron PID (SNPID), and Back Propagation (BP) neuralnetwork, etc 21.
The single neuron PID control algorithm is proposed secondly as a systemcontroller. The advantages of the single neuron are the self-learning abilitieswhich can adapt to environmental changes 22. The SNPID controller keeps the simple structureof conventional PID controller in addition to that; it can online adjust itsparameters using neural network auto-learning. On the other hand it still can’tself-tune gain coefficient (K) and the learning process of weight (W) is notfast 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 PIDcontrollers can be enhanced by using the FOPID controllers. The FOPIDcontroller may be regarded as the general case of the conventional PIDcontroller.
FOPID has two more extra tuning parameters. Itgives more flexibility for the design of a control system 23. The FOPID controller, originally contributedby Podlubny (1999), has better performance and robustness than the conventionalPID controller. This makes the FOPID controller moresuitable for the vibration control (Aldair and Wang, 2010; Zhihuan et al., 2014).
AFOPID controller was tuned using evolutionary optimization algorithm andapplied in a full vehicle nonlinear active suspension system by Aldair and Wang(2010). Ammar A. Aldair et al (2010) proposed FOPID controller for full vehiclenonlinear active suspension model including hydraulic actuators in order toimprove the ride comfort and road handling during various maneuvers: traveling,braking and cornering. Zhihuan et al. (2014) presented a FOPID controlleroptimized using chaotic non-dominated sorting genetic algorithm II forhydraulic turbine regulating system. S Gad et al (2015) applied a FOPIDcontroller optimized using GA algorithm for 6 DOF Seat and human body model. Asclassical 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 toimprove the pregnant woman comfort and reduce health risks of her.
Mohamed.A.Shamseldin et al (2016) compared between differenttechniques of self-tuning FOPID control for Brushless DC motor.
Thefirst technique is the Fuzzy self tuning FOPID controller, where the fuzzylogic control is used to adjusting the output of the FOPID controller on-linewithout changing the values of the parameters of FOPID controller. The second technique uses the fuzzy logiccontrol to tune the proportional, integral and derivative parameters of FOPIDcontrol, while the fractional order integral and derivative gains will be stillconstant. The third technique investigates the performance of the FOPID controlthrough tuning the fractional order integral and derivative gains only usingthe fuzzy logic control, while the proportional, integral and derivative gainsare constant. The forth technique, the fuzzy logic control will be tuned thefive parameters of FOPID controller parameters simultaneously on-line 24.Thiscomparison discovering the sensitive FOPID controller parameters, which willgive better performance when it will be tuned online. 1.5.
3.2 MR damper controllersThe damper controller estimates the DC voltage which applied to the MR damper’s coil for varying the damping coefficientto track the actual force to the desired force. Here, the thesis introducesexclusively with the damper controller. Themost popular three typesof conventional damper controller are Heaviside step function (HSF),Signum Function Method (SFM)and Continuous State Controller (CSC).
HSFcontroller is an “ON-OFF” damper controller where the applied voltage is eitherzero or max value based on a comparison held between the estimated forcesupplied by the system controller and the actual force measured by the sensorplaced in series with the MR-damper. The SFM is an improved method from the HSFas it allows the applied voltage to switch between discrete values on certainconditions; HSF and SFM are classified to be discontinuous damper controllers, Inthis work (SFM) is introduced.1.5.
4 vehicleSeat Suspension System for the pregnant woman driver Manystudies on vehicle suspension systems control considered a single degree offreedom (SDOF) model for human body to facilitate the study. An example of SDOFmodel is shown in fig.1- , which present a tow degrees of freedom seat andhuman body model 25. Fig 1-9 a tow degrees of freedom seat and human bodymodel 25.But SDOF isn’t sufficient model and neglectsmany dynamic properties of a human body, which is lead to a lack of accuracy inthe estimation of the ride quality. Multi degrees of freedom model allowstudying the vibration effect on specific parts of the body to evaluate theride comfort.
Fig 1-10 is an example for the Multi degreesof freedom model. It is Six degrees of freedom seat and human body model 26 with four degrees of freedom ofa human body. Fig 1-10 a Six degree of freedomseat and human body model In this thesis, weare considering a thirteen degrees of freedom model. The model consists of: I.
The two degrees of freedom (2DOF) seat suspension mechanicalsystem that represents the vehicle seat suspension model. It involves of alower mass ( ) demonstrating the seat frame mass and an uppermass ( ) demonstrating the seat cushion mass. The excitationdue to the road is , that represented here by bump andrandom road excitations. Fig. 1-11 Seat suspensionmodel II. The eleven-DOF model(Qassem et al.
, 1994; Qassem and Othman, 1996)) 27. Many details for thepregnant woman body can be illustrated at Fig. 1-12. The pregnant woman body has divided into 11 DOFfor more accuracy to reduce health risks of the woman and herfetus. Fig. 1-12 Pregnant woman model(11- DOF) 27By combining the two previous models, theseat and the pregnant woman driver model is resulted.