This is only a preview of the September 2005 issue of Silicon Chip. You can view 36 of the 112 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 "Build Your Own Seismograph":
Items relevant to "Bilge Sniffer":
Items relevant to "VoIP Analog Phone Adaptor":
Articles in this series:
Items relevant to "PICAXE In Schools, Pt.4":
Purchase a printed copy of this issue for $10.00. |
Car cruise controls that use radar to
maintain pace with the car in front!
By
Julian Edgar
Adaptive Cruise
Control Systems
Cruise control systems have been available in cars for many years.
However, a new type of cruise control is now being fitted. It’s called
“Adaptive Cruise Control” and it uses radar to maintain a safe
distance to the car in front, even if that car’s speed changes.
O
n the road, it’s a brilliant innovation that improves safety,
reduces fatigue and adds convenience. But how does it work?
Intelligent cars
The last decade has seen the widespread introduction of systems than
enhance car intelligence. Anti-Lock
Braking (ABS) and Electronic Stability
Control (ESP) give the car the ability to
8 Silicon Chip
act in ways not specifically requested
by the driver – for example, to release
the brakes momentarily to prevent
wheel lock-up or to reduce throttle
opening if the car is sliding. Adaptive
Cruise Control is another step on that
road to enhanced intelligence – Fig.1
shows where it is on the path that leads
to full collision avoidance.
The presence of systems like ABS
and ESP means that many of the input
signals needed by Adaptive Cruise
Control are already available. These
include:
• vehicle speed
• vehicle lateral acceleration
• driver accelerator input
• driver steering input
• driver brake input
However, not present is the most
critical of inputs – a forward-looking
sensor.
siliconchip.com.au
Product
Collision Avoidance
Collision Warning
Lane/Road Departure
Forward Collision Warning
Adaptive Cruise Control
Cruise Control
Future
System Functionality
Complete 360° vehicle coverage. Braking and
active steering to avoid object. Lane keeping.
Improved all-vehicle coverage (forward, side,
rear) with full alert function.
Partial all-vehicle coverage, with lane/road
departure alerts. Vision required.
Identify stopped objects. Provide warning. Provide “Braketo-Stop” and “Low Speed Cruise/Stop and Go” ACC
capabilities. Vision required for advance alert features.
Provide throttle control with limited braking to maintain timedheadway distance. No stopped object identification and no
warning.
Drive controlled system. No dynamics.
Today
Fig.1: Adaptive Cruise Control is seen by many as the first evolutionary step towards intelligent cars that actively avoid
crashes. [Delphi]
Forward-looking sensor
characteristics
An Adaptive Cruise Control (ACC)
forward-looking sensor must meet certain strict design requirements.
1: Range – In order that an appropriate following distance can be
maintained, the sensor must be capable of working over a specific range.
For example, if the following distance
is defined in terms of time gap between
the two vehicles, a 2-second gap at
160km/h will require the distance between the vehicles to be about 90 metres.
However, in order that the sensor
can maintain continuous control, the
actual required sensor range will be
about 10% greater than this. So if the
maximum speed required of the ACC
is 160km/h, a sensor range of about 100
metres is the minimum requirement.
This diagram shows how Adaptive Cruise Control works from a driver’s perspective. In the first image the blue car is travelling
at 115km/h and the green car behind it has its cruise control set to 140km/h. As the green car draws close to the blue car,
the Adaptive Cruise Control radar senses the blue car’s proximity and automatically slows the green car until it maintains a
constant, safe gap. When the blue car turns off, the green car smoothly resumes its 140km/h cruise. (You can tell these diagrams
are from Germany – sitting at 140km/h on cruise control in Australia? Other than in the Northern Territory, we wish!) [Bosch]
siliconchip.com.au
September 2005 9
Control units for engine,
transmission and ESP
Instrument cluster with
DISTRONIC display
Proximity sensor
(radar aerial)
Adaptive Cruise Control
interfaces with existing car
systems like Electronic Stability
Control (ESP) and Anti-Lock
Braking (ABS). An electronic
throttle is also normally
used on cars equipped with
this type of cruise control.
[DaimlerChrysler]
Cruise
control
lever
Control unit for
DISTRONIC
2. Closing Rate – the sensor must
be able to rapidly detect that the car
ahead is being closed upon. If the
sensor is slow to react, a greater range
will be required otherwise the following car will draw too close before
throttle reduction or braking occurs.
The magnitude of permitted braking will also affect this requirement;
if the car is permitted to brake hard
then the sensor can be slower to react.
Assuming a maximum automatic
braking deceleration of 0.2G, a maximum closing rate of 50-65km/h and
Brake
booster
a minimum following distance of 2030m, a sensor range of 80-100 metres
is again a minimum requirement.
3. Field of View – the field of view
(FOV) of the sensor can be defined both
in terms of azimuth (left/right) and elevation (up/down) angles. The azimuth
FOV is important if the system is going
to be effective at working on curves.
As Fig.7 shows, beam width has a
major affect on the distance at which
a cornering car can be tracked. At a
speed of 90km/h the ACC following
distance will be about 50 metres.
100
A
1
2
Control on
centre console
3
B
SIGNAL AMPLITUDE
SIGNAL AMPLITUDE RATIO
10
1
Radar sensors
2/1
0.1
3/2
3/1
0.01
–
0
ANGLE
+
–8°
–4°
0
ANGLE
4°
8°
Fig.2: the antenna patterns of the Bosch Adaptive Cruise control radar sensor.
Three lobes with overlapping patterns are used, with the angular position of
the sensed object determined by comparing the signal amplitude ratios between
antenna pairs. (a) shows the antenna patterns with (1) being the left lobe, (2)
being the centre lobe and (3) the right lobe. In (b) the relationship between the
amplitude ratios and the angular position of the object are shown. [Bosch]
10 Silicon Chip
Assuming a minimum radius-ofcurvature of 300 metres, a minimum sensor FOV of 5° is required.
However, additional FOV is usually
needed to take into account mechanical or electrical misalignment of the
antenna – a point that we will return
to.
In addition to these three requirements, the sensor must be able to
withstand a temperature range of
-40 to +80°C, be proof against water
splashes and pressurised steam, be
immune to vehicle vibrations, resist
stone impacts and be as small as possible.
Two types of forward-looking sensor
have been developed – lidar (light detection and ranging) and radar (radio
detection and ranging). However, the
radar-based sensor is the most widely
used and it is this type of sensor that
will be covered here.
Two types of radar sensors are used
– those with stationary antennas and
those that mechanically sweep back
and forth.
US automotive components manufacturer Delphi has developed a
scanning sensor with a narrow 2°
beam-width. This beam is mechanically swept over a 15° detection region
and has an elevation FOV of 4°. As the
siliconchip.com.au
Range
2 – 120m
Detectable
relative speed
±50m/s
Angular range
±4°
Resolution
0.85m; 1.7m/s
Scanning rate
10Hz
Frequency range
76 – 77GHz
Mean power
transmission
Approx. 1mw
Bandwidth
Approx. 200MHz
Fig.3: the specifications of the radar
sensor used on Bosch Adaptive Cruise
Control systems. [Bosch]
antenna is scanned, over 40 individual
transmit/receive beams are executed
with each pass. Beam object data is
updated within 100ms.
However, much more common is
a sensor that has a fixed antenna.
The Bosch system (used by DaimlerChrsyler, BMW and Audi) uses this
approach.
The Bosch system uses a Frequency
Modulated Continuous Wave (FMCW)
output. Instead of timing the period
between transmission of the signal
and the echo, a FMCW radar system
compares the frequencies of the transmitted signal and its echo.
The output frequency is changed
at a rate of 200MHz per millisecond
and so the time interval between the
transmit and receive signals can be
Fig.4: a sectional view of the Bosch
Adaptive Cruise Control unit, which
incorporates both the radar and the
control circuitry. (1) Circuit board 1,
(2) Oscillator block, (3) Beam sources,
(4) Lens, (5) Lens heater contact, (6)
Circuit board 3, (7) Circuit board 2,
(8) Radar Transceiver. [Bosch]
siliconchip.com.au
Different car manufacturers give different names to Adaptive Cruise Control
Systems – DaimlerChrysler calls the system ‘Distronic’. This Distronic system is
mounted directly behind the Mercedes star in the grille. [DaimlerChrysler]
established by determining their frequency difference.
However, because the distance
between the transmitter and its target
may be changing, this differential
frequency information contains not
only the time interval component
but also the frequency shift (ie, Doppler component) . This ambiguity can
be resolved by the use of multiple
FMCW cycles using differing rates of
frequency change.
Using these techniques, the distance
to the target and whether the target is
The location of the Adaptive Cruise Control radar sensor on a BMW. [BMW]
September 2005 11
DETECTION SENSOR
(RADAR, LIDAR)
VEHICLE SENSOR
(YAW RATE, SPEED)
OBJECT
DETECTION
MULTI-TARGET
TRACKING
Fig.5: the signal processing architecture of a typical
Adaptive Cruise Control. Once the objects are detected,
tracking needs to occur. Both their paths and also the path
of the controlled vehicle are estimated, the input commands
of the driver are noted and the ACC controls the throttle
and/or brakes. [Delphi]
drawing closer or moving further away
can be established.
However, some spatial data is also
needed – is the target directly ahead
or to one side of the forward aim? If
the target’s radar reflective characteristics are known, the amplitude of the
signal echo depends on the angle at
which the signal is received by the
radar. However, when the reflective
characteristics of the target are unknown, a different approach needs
to be taken.
To determine the angle at which
the radar detects an object, three radar
lobes are transmitted and analysed.
The ratio of the signal amplitudes of
the three different lobes provides this
angular information.
Fig.2 shows the antenna lobe
patterns and how signal amplitude
ratios are used to resolve the angular
position of the targets. Fig.3 shows
the specifications of the Bosch ACC
radar sensor.
The Denso system used in Toyota/
Lexus models uses a more conventional type of radar. Distance is detected by measuring the time between
transmission and reception, while
relative speed is detected by the frequency shift (Doppler Effect) of the
reflected waves.
The angular position is detected by
the phase differences of the signals
received by multiple antennas. The
Denso unit also differs from the Bosch
design in that it has separate receiving
and transmitting antennas (although
all the antennas are mounted in the
one assembly).
The physical layout of the Bosch radar sensor is shown in Fig.4. The radar
and the ACC controller are integrated
into one housing.
The front of the unit features a
Fresnel lens that is used to focus the
three radar lobes. The lens is made
12 Silicon Chip
HMI ALERTS
(VISUAL, AUDIO)
PATH
ESTIMATION
ACC
CONTROL
ACTUATOR CONTROL
(THROTTLE/BRAKE)
HMI DRIVER COMMANDS
(TIMED HEADWAY, ETC)
from a special temperature and stoneresistant plastic which is formed as
part of the module casing. The lens
incorporates a heating element which
prevents it becoming coated in snow
or ice. According to Bosch, wet snow
has a great attenuating effect on the
radar signal.
In one iteration of the Bosch design,
the sensor assembly comprises three
circuit boards. The first consists of
the radar transceiver unit which is
mounted directly on a circuit board,
keeping interconnections as short as
possible and so reducing susceptibility to interference. Also on this board
is a digital signal processor, purposedeveloped 10-bit and 12-bit analog
to digital converter, SRAM and flash
memory.
On the second board is a 16-bit
microcontroller which performs the
necessary car speed control calculations. The third board contains the
driver modules to allow connection
to the car’s electrical and CAN bus
communications systems.
The module must be aligned in both
vertical and horizontal planes. In the
horizontal plane Bosch state that a
degree of accuracy of better than 0.3°
is required, while BMW put the figure
at 1° and Cadillac at 2°.
The BMW system requires the use
of a BMW service tool to perform the
alignment, while Cadillac systems
can be placed in an ‘alignment mode’
and then automatically aligned by being driven along with a road that has
stationary objects either side.
Apparently, the more stationary
objects (such as light poles, mail
boxes, etc) there are, the quicker the
alignment occurs.
How the system works
It is all very well to detect the
presence of cars in front but how is it
The complexity of the technology of Adaptive Cruise Control systems currently
limits them to expensive cars. However, expect a trickle-down to more humble
cars to occur in the near future. [DaimlerChrysler]
siliconchip.com.au
The instrument panel of a BMW using Adaptive Cruise Control: (1) the set
cruise control speed, (2) indication that the vehicle ahead is being tracked, (3)
indication of the driver requested car-to-car spacing, (4) an indication that the
cruise control system is on. [BMW]
determined whether the car is in your
lane or another? What about when
cornering? And what happens when
a car cuts into your lane?
Fig.5 shows the signal processing
architecture of a typical ACC. Once
the objects are detected, tracking of
them occurs. Both their paths and also
the path of the controlled vehicle are
estimated, the input commands of the
driver are noted and the ACC controls
the throttle and/or brakes.
In the Bosch FMCW system, positive detection of objects is carried
out by comparing consecutive radar
modulation cycles. If in the second
cycle the object is found where it
could be expected to be (on the basis
of its previously detected speed and
position) it is assumed to be the same
vehicle.
In other words, the object data is
filtered on the basis of historical information. Additional object tracking
functions are carried out where there
are multiple simultaneous echoes
from different distances, which can
be the case with large trucks.
In this situation the multiple echoes
are combined so that the system sees
only one object.
Object selection occurs in this
manner:
1. The lateral position of the object
versus the predicted course of the ACC
system’s own vehicle is calculated.
2. A calculation is made of the
object’s “lane probability”, that is,
which lane the object is most likely
to be in.
3. Lane probability is a main input
into the next step, that of a “plausibility attribute”. Together with the
frequency and reliability of object
detection, this determines the degree
of plausibility that the detected vehicle
is in the same lane as the ACC car.
4. The object is selected as the target
only if the degree of plausibility is
sufficient. This plausibility is based
only on moving objects – ACC systems
ignore stationary objects when selecting targets.
The first step – that of locating the
object relative to the predicted course
of the ACC car – is most critical.
Fig.6: if the
trajectory
of the car
equipped with
Adaptive Cruise
Control cannot
be accurately preA
dicted, the system
will have problems
on corners. Here there
are three cars travelling
around a curve on a
multilane road. Car 3, the
car equipped with the ACC,
is at the bottom of the
diagram. Without an ability
to accurately model the
predicted course of the ACC
car, the system would expect to
follow course B and therefore
sense car 2 as being ahead of it
in its lane. However, the ACC
car will actually follow course
A and so must sense car 1 as
being ahead of it. [Bosch]
siliconchip.com.au
κ
B
1
2
d YC
d RANGE
2 α RANGE
3 ACC
ACC 1
Fig.7: the effective range
of the radar beam is much
reduced in corners. This
has implications for the
required radar beam width
and also for the behaviour
of the system should it
lose sight of the car ahead.
[Bosch]
September 2005 13
The radar sensor
jointly developed by Denso and Toyota
uses a different design to the Bosch unit. The transmitting
and receiving antennas are separate (although in the one
package) and use pulsed output transmissions. [Denso]
The Bosch Adaptive Cruise Control system incorporates
the radar and control circuits into the one enclosure.
Dominating the package is the Fresnel lens which focuses
the three beams of the radar. [Bosch]
Fig.6 shows three cars travelling
around a curve on a multi-lane road.
Car 3, equipped with ACC, is at the
bottom of the diagram. Without an
ability to accurately model the predicted course of the ACC car, the system
LEVEL 1
LEVEL 2
RADAR
DATA
would expect to follow Course B and
therefore would sense Car 2 as being
ahead of it in its lane. However, the
ACC car will actually follow course
A and so must sense car 1 as being
ahead of it.
WHEEL SPEED
SENSOR
YAW RATE
SENSOR
RADAR
OBJECT DETECTION
DETERMINATION OF
COURSE CURVATURE
LEVEL 3
OBJECT SELECTION
COURSE PREDICTION, TRACKING
LEVEL 4
ACC CONTROL
LEVEL 5
LINEAR SPEED CONTROL
LEVEL 6
OTHER
SENSORS
ENGINE MANAGEMENT
DRIVE TRAIN
ACTIVE BRAKE
INTERVENTION
Fig.8: the Bosch Adaptive Cruise Control uses the 6-level control sequence
shown here. The first level is the input of data from the radar, wheel-speed
sensors, yaw sensor and other sensors. The second level is to identify any
moving objects ahead of the car and assess their plausibility of being in the
same lane. Once this has been done, the system can calculate the projected
trajectory of the Adaptive Cruise Control car and track and predict the
course of other vehicles. A target vehicle is established and the required
acceleration calculated. The actuation system by which the car’s speed is
to be changed is selected (it can be throttle, brakes or transmission) and
then finally, this control is exerted. [Bosch]
14 Silicon Chip
Course prediction is based on the
“trajectory curvature”. That is, the
change in direction that the car is undergoing as a function of the distance
travelled. This is determined by sensors detecting steering angle, lateral
acceleration, yaw and the difference
in left/right wheel speeds.
The effect of crosswinds, road
camber and differences in wheel
diameters can all reduce trajectory
curvature prediction. Combining the
techniques reduces the probability
of error.
In addition, the ACC system can
use the current and past positions
of stationary and moving objects to
determine the projected course of the
car. This can be carried out by analysing the apparent lateral movement of
vehicles in front as they enter a bend
and analysing near-road stationary
objects.
Special logic is used in sharp bends.
If it is sensed that the car is negotiating
a sharp bend, a reduction is made in
the maximum permissible acceleration (note that in this context, acceleration also refers to deceleration), so
as to maintain vehicle stability.
Secondly, as Fig.7 shows, the effective range of the radar beam is much
reduced in corners and so the ACC
modifies the allowable acceleration to
suit this reduced “visibility”.
Finally, if the target car disappears
from view, logic prevents the ACC
vehicle from suddenly speeding up.
The Bosch ACC uses the 6-level
control sequence shown in Fig.8. The
first level is the input of data from the
radar, wheel-speed sensors, yaw sensiliconchip.com.au
Driving with Adaptive
Cruise Control
In order that the car reacts adequately quickly to a changed situation but
at the same time avoiding uncomfortable braking or acceleration if it is not
essential, a non-linear control system
is employed. This causes changes in
relative speed (eg, a rapid closing
speed) to produce a greater reaction
than changes in distance.
On the road
We were able to spend some time with the Audi A8 4.2, a car that features
Bosch Adaptive Cruise Control. And what was it like? In a word, brilliant.
We didn’t have a chance to test it on tight, winding country roads but in freeway
conditions it was superb. Speed selection is available only in 10km/h increments – which is fine when you no longer need to ‘tap-up’ and ‘tap-down’ in
tiny increments, trying to maintain a constant gap to the car in front. As you
would expect with a system that maintains a constant time gap, at slow speeds
the Audi would creep up on the car in front and at higher speeds it would drop
back; all automatically, of course.
If the car ahead slowed abruptly, the Audi would automatically apply the brakes
– and if it was deemed by the system to be an emergency stop, an audible
alarm sounded and you were expected to brake. A green symbol showed on
the instrument display when the car in front was within the minimum safe distance – and this changed to red when driver braking was needed.
Describing the system in step-by-step detail makes it sound more cumbersome
than it really is. This is literally a set-and-forget system – on a drive from Sydney
to Canberra or Melbourne, it would be simply awesome.
If the price of the technology drops as it has for other car innovations, we’re
happy to go on record and say that in the foreseeable future – say, in 10 years
time – all cars with cruise control will have a radar proximity function. It just
works so well….
Most cars equipped with ACC
use a similar driver interface. The
selected cruise speed is shown by
an illuminated segment or LED on
the speedometer. The selected gap
spacing is shown diagrammatically
on a dot matrix or TFT display – for
example, by the spacing between two
car symbols.
When the ACC is tracking a car,
another symbol illuminates on the
dashboard display. In this way, the
requested and actual vehicle speeds,
the requested gap and the tracking
action of the ACC can all be quickly
and easily seen.
Current ACC systems are suitable
for use primarily on freeways and
open rural roads.
They will not brake a vehicle to
a standstill, even if the vehicle is
aimed straight at a roadside obstacle.
Furthermore, if the traffic ahead is
stopped, an alarm may sound but
again the vehicle will not be emergency braked.
Such collision avoidance systems
are in the pipeline but as was remarked
at the beginning of this story, ACC is
only the first step on that road.
However, it’s a pretty impressive
SC
step…
Which Cars?
sor and other sensors.
The second level is to identify any
moving objects ahead of the car and
assess their plausibility of being in
the same lane. In this step the data
from the other car system sensors is
assessed to determine the degree of
curvature of the road.
Once this has been done, the system
can calculate the projected trajectory
of the ACC car and track and predict
the course of other vehicles. A target
vehicle is established – normally it
will be the one calculated as being
ahead of the ACC car in the same lane.
However, this is not always the case: if
vehicles ahead of the ACC car (or the
siliconchip.com.au
ACC car itself) change lanes, a group
of several possible target vehicles can
be considered.
The next step is the calculation of
the required acceleration. The actuation system by which the car’s speed
is to be changed is selected (it can be
throttle, brakes or transmission) and
then finally, this control is applied.
The driver has control over two
functions: the set speed and the
distance to be maintained between
the ACC car and the car ahead. As
mentioned earlier, the distance is
set by means of a requested time gap
which is generally in the range of one
to two seconds.
Cars fitted with Adaptive Cruise
Control are currently limited to the
upper echelons. A brief world list
includes: Audi A8 (2004 - current),
BMW 7 Series (2003-current), BMW
5 Series (2004 - current), General
Motors Cadillac XLR (2003 - current), Nissan Infiniti Q45 (2003 - current), Jaguar XKR (2003 - current),
Lexus LS430 (2004- current), Lexus
GS430 - current), Mercedes-Benz
S Class (2000 - current), Mercedes
Benz CL Class (2000 - current),
Mercedes Benz E Class (2003 current), Mercedes Benz SL Class
(2003 - current).
September 2005 15
|