14-12-2012, 02:58 PM
Design of a Magnetorheological Brake System Based on Magnetic Circuit Optimization
Design of a Magnetorheological.pdf (Size: 1.76 MB / Downloads: 256)
Abstract
Conventional hydraulic brake (CHB) systems used in automotive industry have
several limitations and disadvantages such as the response delay, wear of braking pad,
requirement for auxiliary components (e.g. hydraulic pump, transfer pipes and brake
fluid reservoir) and increased overall weight due to the auxiliary components. In this
thesis, the development of a novel electromechanical brake (EMB) for automotive
applications is presented. Such brake employs mechanical components as well as
electrical components, resulting in more reliable and faster braking actuation. The
proposed electromagnetic brake is a magnetorheological (MR) brake.
The MR brake consists of multiple rotating disks immersed into an MR fluid
and an enclosed electromagnet. When current is applied to the electromagnet coil,
the MR fluid solidifies as its yield stress varies as a function of the magnetic field
applied by the electromagnet. This controllable yield stress produces shear friction
on the rotating disks, generating the braking torque. This type of braking system has
the following advantages: faster response, easy implementation of a new controller
or existing controllers (e.g. ABS, VSC, EPB, etc.), less maintenance requirements
since there is no material wear and lighter overall weight since it does not require the
auxiliary components used in CHBs.
Introduction
Automotive industry is changing everyday. Billions of dollars are invested in research
and development for building safer, cheaper and better performing vehicles. One
such investment is the “x by wire”topic which has been introduced to improve the
existing mechanical systems on automobiles. This term means that the mechanical
systems in the vehicles can be replaced by electromechanical systems that are able to
do the same task in a faster, more reliable and accurate way than the pure mechanical
systems.
This thesis work is concerned with the topic of braking in the vehicles. Nowadays,
conventional hydraulic brakes (CHB) are being used in order to provide the required
braking torque to stop a vehicle. This system involves the brake pedal, hydraulic
fluid, transfer pipes and brake actuators (disk and drum brakes). When the driver
presses on the brake pedal, the hydraulic brake fluid provides the pressure in the brake
actuators that squeezes the brake pads onto the rotor. The basic block diagram of
this type of brake is shown in Figure 1.1 (right).
Thesis Outline
In Chapter 2, the dynamic model of a typical passenger vehicle is introduced and the
braking torque requirements are calculated for different vehicles (i.e. a fully loaded
car, a sport motorbike and a scooter). After the required braking torque values
are obtained, the analytical model of an MRB is presented. The behavior of MR
fluid is modeled using the Bingham plastic model. Then, by using this model, the
total braking torque generated is analytically described in terms of the magnetic field
intensity applied and the viscosity of the fluid.
In Chapter 3, the design process of an MRB is explained in detail. The proposed
MRB is designed considering the design criteria such as the number of disks used,
the dimensional design parameters, the materials used and the configuration of the
magnetic circuit. There are also some additional practical considerations that are
included during the design process, e.g. sealing of the MR fluid, cooling the MRB and
the viscous torque generated within the MRB due to MR fluid viscosity. However, the
main focus of the design process is on magnetic circuit design and material selection.
Modeling of MR Brake
As mentioned in the previous chapter, a magnetorheological brake (MRB) is proposed
in this work as a possible substitute for CHBs. In this chapter, before modeling the
MRB, the vehicle dynamics are studied in order to calculate the required amount of
braking torque for stopping a vehicle. Then, required braking torque values of different
vehicles are calculated using the dynamic vehicle model. Finally, an analytical
model of the MRB itself, required to obtain the braking torque generation, is derived.
Vehicle Dynamics
In this work, the motion of a vehicle is described using the quarter vehicle model [39].
This model is needed to calculate the required braking torque that a brake should
provide. The basic assumption of this model is that the mass of the vehicle is divided
equally between four wheels. In Figure 2.1, a free body diagram of a wheel rotating
clockwise is shown. During braking, a torque is applied by the brake, Tb , and Fr,
Ff , Fn and FL are the rolling resistance force, the friction force, normal force and the transfer of weight caused by braking of the vehicle. Let’s denote I as the total mass
moment of inertia and ¨ is the angular acceleration of the vehicle. The radius of the
wheel is Rw and x is the distance traveled by the vehicle.