INTRODUCTION
Traditionally automotive
suspension designs have been compromise between the three conflicting
criteria’s namely road handling, load carrying, and passenger comfort. The
suspension system must support the vehicle, provide directional control using
handling maneuvers and provide effective isolation of passengers and load
disturbance. Good ride comfort requires a soft suspension, where as insensitivity
to apply loads require stiff suspension. Good handling requires a suspension
setting somewhere between it. Due to these conflicting demands, suspension
design has to be something that can compromise of these two problems.
Active suspension system has the
ability to response to the vertical changes in the road input. The damper or
spring is interceding by the force actuator. This force actuator has it own
task which is to add or dissipate energy from the system. The force actuator is
control by various types of controller determine by the designer. The correct
control strategy will give better compromise between comfort and vehicle
stability. Therefore active suspension system offer better riding comfort and vehicle
handling to the passengers. Figure 1.3 shows simple block diagram to explain
how the active suspension can achieve better performance. Figure
describe basic component of
active suspension. In this type of suspension the controller can modify the
system dynamics by activating the actuators.
All these three types of
suspension systems have it own advantages and disadvantages. However
researchers are focus on the active car suspension and it is because the
performance obtained is better than the other two types of suspension systems
as mentioned before. For example the passive suspension system the design is
fix depend on the goal of the suspension. The passive suspension is an open
loop control system. It doesn’t have any feedback signal to correct the error.
It means that the suspension system will not give optimal ride comfort. In
other side which is active suspension, it has that ability to give ride
comfort. This is happen by having force actuator control by the controller. The
active suspension system is a close loop control system. It will correct the
error and gave the output to the desired level. In this project observation
will be made at the vertical acceleration of the vehicle body called sprung
mass and tire deflection. By using the right control strategy the ride quality
and handling performance can be optimize. Therefore, in this project there
will be modeling for active and passive suspension
only.
· Active Suspension
Ø
COMPONENTS
1.
A Computer or an
electronic control unit (ECU)
Short for Electronic
Control Unit, the ECU is a name given to a device that
controls one or more electrical systems in a vehicle. It operates much like the
BIOS does in a computer. The ECU provides instructions for various electrical
systems, instructing them on what to do and how to operate. Below are two
pictures of what an ECU might look like, depending on the vehicle.
There are a number of different types of ECUs,
including an Engine Control Module (ECM), Powertrain Control Module (PCM),
Brake Control Module (BCM), General Electric Module (GEM) and others. Newer
vehicles can have as many as 80 ECUs and due to their increasing complexity,
the programming involved in developing the ECUs is becoming more challenging to
maintain.
2.
SENSORS
Linear acceleration sensors, also called G-force sensors, are
devices that measure Acceleration caused by movement, vibration, collision,
etc. All acceleration sensors operate based on a simple principle in which
Newton's second law of motion is applied to a spring-mass system. A mass is
connected to the base of the acceleration sensor through an equivalent spring.
Since the force between the mass and base is proportional to the acceleration
of the mass and the relative distance between them has a linear relationship
with the force due to the spring, the acceleration can be calculated from a
measurement of the relative position of the mass or force on the spring as it varies
with time. Generally, the most common types of acceleration sensors include:
piezoelectric, piezoresistive, variable capacitance and variable reluctance.
Automotive Applications of Acceleration Sensors:
- Collision detection and airbag
deployment: To measure intensity of collision and signal to initiate
airbag deployment.
- Electronics stability programs
and control: Measures acceleration along various axes, (e.g. forward,
braking and cornering accelerations, to compute relative movements and
regulate them).
- Antilock braking systems.
- Active suspension systems:
Measures longitudinal and lateral accelerations as well as vehicle roll
characteristics to change damper characteristics accordingly.
- Hill descent/hold control:
Measures vehicle inclination and speed to regulate system.
- Monitoring Noise,Vibration and
Harshness.
- Vehicle navigation systems to
determining vehicle location, speed, etc.
3.
ACTUATOR OR
SERVO
A servomechanism,
sometimes shortened to servo, is an automatic device that uses error-sensing negative
feedback to correct the performance of
a mechanism and is defined by its function. It usually includes a built-in encoder. A servomechanism is sometimes called a heterostat since
it controls a system's behavior by means of heterostasis. The term correctly applies
only to systems where the feedback or error-correction signals help control mechanical
position, speed or other parameters. For example, an automotive power
window control is not a
servomechanism, as there is no automatic feedback that controls position—the
operator does this by observation. By contrast a car's cruise
control uses closed
loop feedback, which classifies it as a
servomechanism.
Uses
Position control
A
common type of servo provides position control. Servos are commonly
electrical or partially electronic in nature, using an electric
motor as the primary means of
creating mechanical force.
Other types of servos use hydraulics, pneumatics, or magnetic principles. Servos operate on the principle of
negative feedback, where the control input is compared to the actual position
of the mechanical system as measured by some sort oftransducer at the output. Any difference between the actual and
wanted values (an "error signal") is amplified (and converted) and
used to drive the system in the direction necessary to reduce or eliminate the
error. This procedure is one widely used application of control
theory.
Speed control
Speed
control via a governor is another type of servomechanism. The steam
engine uses mechanical governors;
another early application was to govern the speed of water
wheels. Prior to World War II the constant speed propeller was developed to control engine speed for maneuvering
aircraft. Fuel controls for gas
turbine engines employ either
hydromechanical or electronic governing.
Other
Positioning
servomechanisms were first used in military fire-control and marine
navigation equipment. Today
servomechanisms are used in automatic machine
tools, satellite-tracking antennas,
remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems inaircraft which use servos to actuate the aircraft's control
surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move
the lens, and thus adjust the focus. A modern hard
disk drive has a magnetic servo system
with sub-micrometre positioning accuracy. In industrial machines, servos are
used to perform complex motion, in many applications.
4.
ADJUSTABLE
SHOCKS AND SPRINGS
A shock absorber (in reality, a shock
"damper") is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by
converting the kinetic
energyof the shock into another form of energy (typically heat)
which is then dissipated. A shock absorber is a type of dashpot.
Description
Pneumatic and hydraulic shock absorbers are used in conjunction
with cushions and springs. An automobile shock absorber contains spring-loaded
check valves and orifices to control the flow of oil through an internal piston
(see below).[1]
One
design consideration, when designing or choosing a shock absorber, is where
that energy will go. In most shock absorbers, energy is converted to heat
inside the viscous fluid. In hydraulic cylinders,
the hydraulic fluid heats
up, while in air cylinders,
the hot air is usually exhausted to the atmosphere. In other types of shock
absorbers, such as electromagnetic types, the dissipated energy can be stored and used
later. In general terms, shock absorbers help cushion vehicles on uneven roads.
Vehicle suspension
In
a vehicle, shock absorbers reduce the effect of traveling over rough ground,
leading to improved ride
quality and vehicle handling.
While shock absorbers serve the purpose of limiting excessive suspension
movement, their intended sole purpose is to damp spring oscillations. Shock
absorbers use valving of oil and gasses to absorb excess energy from the
springs. Spring rates are chosen by the manufacturer based on the weight of the
vehicle, loaded and unloaded. Some people use shocks to modify spring rates but
this is not the correct use. Along with hysteresis in the tire itself, they damp the energy stored in the
motion of the unsprung weight up and down. Effective
wheel bounce damping may require tuning shocks to an optimal resistance.
Spring-based shock absorbers commonly use coil
springs or leaf
springs, though torsion
bars are used in torsional shocks
as well. Ideal springs alone, however, are not shock absorbers, as springs only
store and do not dissipate or absorb energy. Vehicles typically employ both
hydraulic shock absorbers and springs or torsion bars. In this combination,
"shock absorber" refers specifically to the hydraulic piston that
absorbs and dissipates vibration.
Features & Benefits
·
Some
shock absorbers allow tuning of the ride via control of the valve by a manual
adjustment provided at the shock absorber.
·
In
more expensive vehicles the valves may be remotely adjustable, offering the
driver control of the ride at will while the vehicle is operated.
·
The
ultimate control is provided by dynamic valve control via computer in response
to sensors, giving both a smooth ride and a firm suspension when needed.
which allow ride height
adjustment or even ride height control,
seen in some large trucks and luxury
sedans, including Lincoln and Land Rover automobiles.
·
Ride height control is especially
desirable in highway vehicles intended for occasional rough road use, as a
means of improving handling and reducing
aerodynamic drag by lowering the vehicle when operating on improved high speed
roads, as seen in the Tesla Model S.
Ø How it works
The Active Suspension consists of three
masses. Each mass slides along stainless steel shafts using linear bearings and
is supported by a set of springs. The upper mass is known as the sprung mass.
The middle mass corresponds to one of the vehicle’s tires, or the un-sprung
mass. The upper mass and Lower mass is connected to a controller as shown, also
the sensors and actuators are connected to the controller. Controller gathers
all the measures and control pressure control source to control the adjustable suspension
through the hydraulic or pneumatic valves. This system is optimizing the
various suspension performance parameters which include:
ü Ride Comfort -
is related to vehicle body motion sensed by the passengers. It can be measured
using either the accelerometer that is mounted on the top plate, or the encoder
(for a direct position measurement).
ü Suspension
Travel - refers to relative displacement between the vehicle body and the tire
and is constrained within an allowable range of motion. This can be measured
using the suspension encoder that is mounted on the capstan.
ü Road Handling -
is associated with the contact forces between the road surface and the vehicle
tires and depends on tire deflection. Tire deflection is the relative
displacement between the tire and the road and it can be measured using all the
encoders.
FEATURES
• Heavy-duty and robust machine
components
• Three high resolution encoders
used to measure positions of bottom and top masses as well as suspension
deflection
• 226 W MICROMO brushless DC
motor connected to capstan for active suspension control
• 70 W Magmotor brushed DC motor
connected to belt-drive mechanism for road actuation
• Adjustable weight and spring
stiffness
• Accelerometer measurements as
sensory input
• Responsive belt-drive mechanism
to simulate the road suface
• Accelerometer mounted on top
plate to measure vehicle body acceleration
• Multi-coloured masses for
distinction (vehicle body in blue, vehicle wheel in red and the road suface in
silver)
• Limit switch and protection
circuit
• Fully documented system models
and parameters
• Open architecture
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