This is only a preview of the January 2002 issue of Silicon Chip. You can view 30 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Computer Tips":
Items relevant to "Touch And/Or Remote-Controlled Light Dimmer; Pt.1":
Items relevant to "A Cheap 'n' Easy Motorbike Alarm":
Items relevant to "100W RMS/Channel Stereo Amplifier; Pt.3":
Items relevant to "Build A Raucous Alarm":
Purchase a printed copy of this issue for $10.00. |
Swapping hoses and pumps for electric
motors and electronic control . . .
Electric
Power Steering
By Julian Edgar
T
he conventional hydraulicallyassisted power steering used in
most cars is soon to be replaced
with electric power steering. Already,
many manufacturers are using electronically controlled hydraulic systems, while some car manufacturers
have recently introduced purely electric systems to their mass production
vehicles.
In addition to reducing parasitic
loads, full electric power steering
allows steering responsiveness to be
automatically varied depending on
speed, road conditions – and even the
driver’s ability!
Hydraulic Power-Assisted
Steering
Hydraulic Power Assisted Steering
(HYPAS) has been used in automotive
applications for about 50 years. The
systems use an engine-driven hydraulic pump, a control valve, steering
cylinder and connecting hydraulic
hoses. The pump is usually of a vane
design with an integrated internal
bypass. It is sized so that, even at
idle rpm, it delivers enough oil flow
to provide a suitable degree of power
assistance.
The control valve uses a flexible
torque-measuring device (such as
a torsion bar, spiral spring or leaf
spring) to convert the steering torque
into a small control movement. This
movement is transferred to a valve
that regulates fluid flow to the power
assistance mechanism.
In rack and pinion steering, a double-ended hydraulic ram mounted
parallel to the rack (within the rack as14 Silicon Chip
sembly) is used, while recirculating ball
systems incorporate the mechanism
into the steering box. Fig. 1 shows an
example of a traditional HYPAS recirculating ball steering system. Note that
in this particular system, the fluid reservoir is incorporated into the pump.
A major problem with simple
HYPAS systems is that the assistance
level is not reduced at high speeds,
resulting in a lack of steering feel.
American cars of the 1950s and 1960s
were particularly noteworthy for their
feather-light steering effort during
parking, a trait which resulted in extreme vagueness at high speeds.
To overcome this problem, most
HYPAS systems of the last few decades have incorporated mechanisms
that reduce steering assistance, either
as engine speed increases or (less
frequently) as road speed increases.
The reason that engine speed was
more commonly used as the control
parameter to reduce steering effort is
that such a system can remain purely
hydraulic, whereas using road speed
as the control variable requires the use
of an electronic system.
Electronically-Controlled
HYPAS
The introduction of electronic
speedometers – and subsequently, full
engine management – meant that an
electronic road speed signal became
available, allowing the widespread use
of electronically-controlled HYPAS
systems. These vary steering effort
depending on road speed and also, in
some cases, other parameters.
A number of different hydraulic
approaches to regulating steering assistance are used. These are:
1.Flow Control
A solenoid valve is located on the
discharge port of the hydraulic pump.
Electronic control is used to control
the solenoid valve opening, thus
regulating the fluid flow. The flow is
reduced at high road speeds, decreasing the degree of assistance provided.
2.Cylinder Bypass
A solenoid valve and associated
bypass line is located between the two
Fig.1: traditional Hydraulic Power Assisted
Steering (HYPAS) systems use an engine-driven hydraulic pump, fluid reservoir, connecting
hoses and a hydraulic steering box or rack.
[Nissan]
www.siliconchip.com.au
chambers of the hydraulic cylinder,
allowing a reduction of the pressure
differential. The solenoid valve opening is controlled electronically, its
opening greater at high road speeds.
This reduces the degree of assistance
that is provided.
3.Hydraulic Reaction Force
A hydraulic force is enabled that
works against the power assistance. As
speed increases, the reaction force is
increased. Since fluid flow to the
power cylinder is not affected,
the steering response rate
can remain high without
reductions occurring in
feel.
In their electronically-controlled
H Y PA S s y s t e m ,
Hyundai use an
ECU equipped with
an 8-bit microprocessor. Two major inputs – vehicle speed and
steering angular velocity – are
used. From these inputs the ECU
determines the driving conditions
and via a 3-dimensional look-up map,
provides the appropriate current flow
to a hydraulic solenoid valve.
Three different driving conditions
are recognised:
Parking – maximum current is supplied to the solenoid valve, resulting
in maximum steering assistance.
High Speed – minimum current
is supplied to the solenoid valve,
resulting in minimum steering
assistance.
Evasive Steering – a large and sudden steering input causes the ECU
to supply a current to the solenoid
proportional to the angular veloc-
·
·
·
Fig.2: electronically-controlled HYPAS
uses the inputs from
both a road speed
sensor and steering angle sensor.
[Mazda]
ity of the steering
input.
The control algorithm used in the
system is as follows:
IS = IV + IAW + IA + IT; where
IS = Solenoid actuating current
IV = Current according to vehicle
speed
IAW = Current according to steering
angle velocity
IA = Current according to steering
angle
IT = Current according to time
The purpose of IA is to prevent the
driver from experiencing an excessive steering holding force on banked
roads. This current is increased in
proportion to steering input angle.
IT provides additional assistance in
situations where the vehicle enters
a corner that follows a long straight
driven at high speed.
ECU
ANGULAR VELOCITY
SENSOR
CALCULATION OF
ANGULAR VELOCITY
CPU
VEHICLE SPEED
SENSOR
(FROM SPEEDOMETER)
CALCULATION OF
VEHICLE SPEED
ENGINE SPEED
CALCULATION OF
ENGINE SPEED
BASIC
CONTROL
MAP
POWER
CIRCUIT
CONTROL
VOLTAGE
PUMP
MOTOR
GENERATION OF
HYDRAULIC
PRESSURE
MONITORING OF
MOTOR CURRENT
Fig.3: a schematic diagram of a Honda hybrid HYPAS control system. The hydraulic pump speed is controlled on the basis
of inputs from steering wheel movement and engine and road speed. [Automotive Electronics Handbook]
www.siliconchip.com.au
January 2002 15
Fig.4: this General Motors hybrid
HYPAS system senses motor current
to determine the actual steering
loads and so the degree of assistance that needs to be provided. The
3-phase brushless DC motor (12) is
supplied power by the Motor Power
Circuit. The EHPS control provides
a duty cycle control to the Motor
Power Circuit in response to input
signals from the motor angle sensor,
motor current sensor and battery
current sensor, as well as operating
system voltage. The temperature of
the hydraulic fluid is also measured.
The scaler is used on the motor current input to allow high resolution at
low values without requiring a more
costly A/D converter.
[General Motors Corporation]
TF
TEMP
IMH
SCALER
IML
EHPS
CONTROL
ANGLE
DUTY
MOTOR
POWER
CIRCUIT
BATTERY
12
CURRENT
CURRENT
IB
IM
Fig.2 shows the layout of a Mazda
HYPAS system where the degree of
assistance is based on road speed and
steering angle.
Hybrid Hydraulic/Electric
Power Steering Systems
Hybrid HYPAS systems use an
electric motor to drive the hydraulic
pump, rather than having the pump
driven directly by the engine. This
approach allows the steering effort
to be easily controlled by varying the
pump speed. While the efficiency of
such an approach is actually lower
than a conventional belt-driven pump,
because flow can be better matched
to actual requirements, the overall
parasitic power loss is reduced. Fuel
economy savings of up to 0.2 litres/100
km are claimed to be possible by taking
this approach.
The control approach that is taken
can be of three types:
Driving Mode – where driving
conditions (such as city, country,
highway, etc) are automatically
judged with appropriate levels of
assistance then provided;
Steering Wheel Input Mode – where
·
·
Features
Benefits
Engine independence
Reduced engine power drain
Improved fuel economy and acceleration
Instant-on power steering
Assistance available even should the engine stall
Elimination of pump, hoses, Simplified packaging
fluid, drivebelt and pulley
Environmental compatibility
Reduced mass
Modular design and
integrated controller
Reduced assembly time
Design and packaging flexibility
Multi vehicle use
Design and packaging flexibility
Software tuning
Wide assistance range
In-vehicle laptop PC tuning
Tuning process reduced from months to hours
Cost-effective advanced
features
Variable effort steering
Assisted return to centre
Steering damping capability
Fig.5: some of the possible benefits of using Electric Power-Assisted Steering
(EPAS) systems in place of traditional hydraulic power steering. [Delphi]
16 Silicon Chip
the angular velocity of the steering
wheel movement is used to determine the degree of assistance
required.
Steering Load Mode – where demand for power assistance is indicated by the counter-pressure of
the hydraulic fluid, sensed through
variations in the motor current load.
Fig.3 shows the processes followed
in one Steering Wheel Input Mode
system to calculate the appropriate
degree of assistance, while Fig.4 shows
a schematic diagram of a control system that uses the Steering Load Mode.
·
Electric Power-Assisted Steering
Electric Power-Assisted Steering
(EPAS) completely replaces the hydraulic system that hitherto has always been
associated with power steering. EPAS
systems assist driver effort by the use
of an electric motor which acts through
a reversible gearbox and also, in some
cases, an electromagnetic clutch. An
electronic control unit determines the
degree of assistance that is rendered.
EPAS has some significant advantages over any form of HYPAS, both
for the owner of the car and its manufacturer. The reduction in engine load
of an EPAS system (it can be as low
as 4W when the car is being driven
in a straight line) means that the fuel
economy of a car equipped with EPAS
is very similar to that of a car with no
form of power steering.
Analyses provided by manufacturers of EPAS systems indicate potential
www.siliconchip.com.au
Fig.6: the electronic control system for a Honda EPAS system.
[Automotive Electronics Handbook]
fuel savings of 4-8 per cent over cars
equipped with conventional HYPAS,
with the lighter mass of an EPAS
also having an impact. The independence of the system from engine
operation also means that should
the engine stall, steering assistance
does not vary.
(In a conventional HYPAS system,
a stalled engine immediately reduces
steering assistance to zero – a problem
if this occurs part way around a tightening corner!)
From a manufacturer’s perspective,
it has cost benefits. Using EPAS reduces assembly line time, allows easy
software tuning of the steering assistance characteristics to suit a variety
of cars (eg, a sports car or a limousine)
and has the potential to improve reliability – 53% of all power steering
warranty claims are from pump and
hose problems.
Environmental gains are also possible from the decreased production
and disposal of hydraulic fluid (world-
wide, an estimated 40 million litres
of power steering fluid was in use in
1995) and from the decreased requirement for the non-recyclable polymers
used in hydraulic hoses.
Fig.5 shows the range of benefits
potentially realisable from EPAS.
A number of EPAS systems are
currently in production or in the final
stages of prototyping.
The LucasVarity system uses a
brushless DC servo motor and gearbox
to develop a torque that varies from
GPS SYSTEM
RAM
SPEED SENSOR
STEERING SENSOR
BRAKE PEDAL SENSOR
INPUT
UNIT
CPU
OUTPUT
UNIT
SKILL
RATING
THROTTLE PEDAL SENSOR
YAW RATE SENSOR
RAM
ESTIMATING DEVICE
Fig.7: a recently patented Honda EPAS system actively calculates the driver’s ability and provides steering feel and weight
to match. Inputs to this system can include GPS navigation and yaw rate information, with the system comparing the
actual path taken by the vehicle with its computed target trajectory. [Honda]
www.siliconchip.com.au
January 2002 17
Fig.8: calculation
of the available
road friction is
carried out in the
the Honda active
EPAS system by
spectrumanalysing the
noise generated
by the tyres on
the road!
[Honda]
START
SPEED INPUT
transformer) techniques, with the twist
of a torsion bar converted to a slider
displacement. Other system inputs
include vehicle speed and battery
voltage.
Fig.6 shows the schematic diagram
of a Honda EPAS system.
Driver Skill Estimation!
SOUND PRESSURE
INPUT
FREQUENCY
ANALYSIS
EVALUATE
ROAD CONDITION
DRY, WET, SNOWY,
POWDERY SNOWY, AND ICY
One of the most interesting aspects
of EPAS is the ability that the manufacturer has to ‘tune’ the system’s
responsiveness. As indicated earlier,
this allows the easy software matching
of a single EPAS to applications as
diverse as a two-seater sports car or
luxury sedan but it also means that
system responsiveness can be made
to vary in different driving situations
in the one car.
When this approach is taken, the
input by the driver of a certain amount
of steering lock does not always result
in the same degree of assistance –
should the ECU determine that such a
steering movement is not appropriate
for the conditions that the vehicle is
undergoing, the steering assistance
may be reduced or the steering input
even actively resisted!
As an indication of the far-reaching
implications of this, Honda has very
recently developed an EPAS system
that estimates the skill of the driver
and provides steering assistance to
match.
In the Honda system, a ‘driver skill
estimation device’ is used, as shown
in Fig.7. This device has inputs from:
a GPS system(!);
a vehicle speed sensor;
a steering sensor that provides
information on steering angular
speed, angular acceleration and
torque input;
a brake pedal sensor that detects
braking stroke, speed and force;
a throttle pedal sensor that detects
accelerator stroke and speed;
a yaw rate sensor;
a road friction estimate input.
The road friction estimate is deter-
mined by yet another system, with the
approach taken shown in Fig.8. Vehicle speed and a sound pressure signal
are gained from appropriate sensors,
with an audio frequency analysis of
this data then undertaken to determine
whether the road is dry, wet, snowy,
powdery snow or icy.
(Note that while the GPS and yaw
rate inputs are included in the Honda
patent of the system, Honda state that
the system can still work effectively
without them.)
The ‘driver skill estimation device’
analyses the actual path taken by the
vehicle and compares this with a
computed target trajectory.
Using this and data on the vehicle
wheelbase, the distance that the front
and rear wheels are from the vehicle
centre of gravity and other factors, the
system awards the driver an ability
that varies on five levels from “very
poor” to “very good”.
A very good driver is rewarded with
very little steering force resistance (the
driver gets what he or she asks for),
while a poor driver will encounter
steering that actively does not allow
major steering inputs to be made at
high speed.
According to Honda, this allows the
skilled driver to “positively control
the turning behaviour of the vehicle
so as to briskly manoeuvre the vehicle.
Conversely, if the vehicle operator
is not skilled, the control system produces a reaction which prevents the
vehicle operator from over-reacting to
the vehicle response, and [so] stabilises the vehicle.”
One wonders what happens when
a ‘very poor’ driver suddenly needs
to swerve around a child that runs
out onto the road. . . That they are a
poor driver becomes a self-fulfilling
prophecy, perhaps?
However, the Honda system does
provide a very strong indication of the
direction that EPAS systems can be
expected to follow in the future. SC
about 15Nm in a small car to 75Nm
in a large sedan. Other manufacturers,
such as TRW, use variable reluctance
motor designs. The electric motor that
is used requires low levels or ripple
and “cogging”.
LucasVarity achieve this by using
a three-phase inverter to vary motor
phase currents and so torque. Power mosfets are used to control the
switching and pulse width modulation
techniques are used.
Depending on the location of the
electric assist unit, drive can be transmitted to the steering mechanism by
a number of means. These are shown
in the table of Fig.9.
In the LucasVarity system, a dual-channel optical device is used to
sense steering input torque. Two optical discs are mounted 50mm apart
at either end of a torsion bar, which
is incorporated into the steering shaft.
Torque applied to the steering wheel
Method
Electric Assist Unit Location Power Transmission
causes a relative movement of the two
Pinion assist Under the dashboard on
Motor > worm gear > column
discs, with the angular offset optically
the steering column
shaft > pinion shaft
sensed.
On the steering rack input pinion
Motor > gear train > pinion shaft
Comparison of the two output sigRack
assist
On
the
steering
rack
Motor > ball screw > rack shaft
nals allows the calculation of steering
On a second pinion on
Motor > planetary geartrain >
torque, steering wheel angular velocthe steering rack
another shaft pinion > rack shaft
ity, and steering angle. Anther torque
sensor that can be used incorporates Fig.9: electric assist units can transmit drive in a variety of ways, depending on
LVDT (linear variable differential their physical location in the vehicle.
··
·
·
·
··
18 Silicon Chip
www.siliconchip.com.au
|