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Jaycar Suns
shatters ra
On 16th January this year, the Jaycar
Sunswift III solar car rolled into Sydney
after five and a half days on the road from
Perth, shattering the previous West-East
Transcontinental Record by three days.
The UNSW Solar Racing Team is to be
congratulated for their tremendous
achievement.
8 Silicon
iliconCChip
hip
siliconchip.com.au
swift III
ace record
by LEO SIMPSON
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May 2007 9
D
espite cloudy weather for the first two days of the record attempt the team was still able to push through
for the fastest ever time. This record is unlikely
to ever be bettered since the technical regulations have
changed for the next solar car race, requiring the vehicles
to be more like conventional cars.
The adventurer Hans Tholstrup did the original westeast Australian solar car trip in 1983, in his solar car, the
Quiet Achiever.
Subsequently, in 1987, the World Solar Challenge invited
bright young engineers and scientists from around the world
to pursue the ideals of sustainable transport.
The ultimate challenge is to design and build a car capable of travelling across the Australian Continent on the
power of sunlight and prove it by undertaking the 3000km
journey in the spirit of friendly competition against others
with the same goal. Over the last 20 years more than 300
solar car teams from around the globe have competed in
the race from Darwin to Adelaide.
In effect, by breaking the West-East Transcontinental
record, the Jaycar Sunswift has written the last chapter for
solar race cars as we know them.
Why? Because this year’s Panasonic World Solar Challenge will be run with more conventionally shaped cars,
ending the reign of cars which are shaped like a credit card
and not much thicker.
Jaycar Sunswift probably also marks the end of evolution of existing solar car electronics, although this remains
to be seen.
Suffice to say, it is the end of development of cars
measuring 6m x 2m and with driver lying supine in a
mobile sauna.
Evolution has been the key word in solar car development over the last 20 years. Their overall design has
changed relatively little while their overall efficiency has
approached 100% but never quite got there – the classic
asymptote.
It was time for a change.
Ultimately, Sunswift is a show-case of the best in solar
car technology, as this story demonstrate s.
Dave Snowdon, from the University of NSW, designed
the electrical system in Sunswift III over the last five years
or so. The major components are the motor, motor control-
Here’s the Jaycar Sunswift III, tailed by support vehicles,
on Sydney’s M5 motorway, nearing the end of its epic
journey. They didn’t tell us if they had to pay the toll . . .
10 Silicon Chip
If you’re looking for comfort in the drive from Perth to
Sydney, we’re betting that the Jaycar Sunswift III is not the
best way to go. But what fantastic fuel economy: 0l/100km!
ler, solar array, maximum power point trackers, batteries
and telemetry/control system.
Pancake motor
The car’s motor is built into one of the rear wheels
in order to save the losses in a transmission if it were
used. The downside of that is more rapid tyre wear in the
driven wheel but the extra weight of dual wheel drive
was thereby avoided. The motor itself is approximately
98% efficient.
The electrical design of the motor was produced by the
CSIRO for the Aurora solar car. It is essentially a brushless
AC motor with a central stator (containing the windings)
and an outer rotor (containing the magnets). In effect, it is
like an inside/out synchronous motor and long-time tape
recording enthusiasts would recognise this as being similar
in construction to a Papst motor.
The differences are many. For a start, the motor is very
thin, allowing it to fit inside a thin wheel hub and streamlined fairing, for minimum wind resistance.
The mechanical design of the motor was done by a thesis
student and involved significant analysis. There is roughly
6kN force between the two rings of magnets when the mo-
And speaking of support vehicles, here’s the inside of one.
It has both voice and telemetry contact with the solar car
by radio, GPS and (just in view at left) laptop diagnostics.
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Back in the workshop at UNSW, Simon Li, now Electrical
Team Leader, shows us the driver’s position. It’s almost
supine, with just the head poking up through the solar cell
“lid” into the clear head guard. The two joysticks (one in
each hand) control the vehicle’s direction and speed.
tor is assembled but the motor casing must also take all the
cornering loads, etc.
Motor controller
The motor controller fills the normal role of the commutator. Three Hall Effect sensors allow it to sense the
position of the magnets relative to the windings and then
switch the motor currents accordingly. The controller was
designed by Tritium Pty Ltd, who originally worked on the
University of Queensland’s Sunshark solar car project. The
UNSW team worked closely with Tritium while they were
developing the controller and added a CAN interface and
other improvements.
The controller is about 99% efficient. It consists of three
Mosfet half bridges (one for each phase of the motor) and
a Digital Signal Processor. (Readers wanting to understand
bridge motor drive should refer to the DC Speed Control
article elsewhere in this issue).
The motor controller pulse width modulates the bottom
Mosfets in the half bridges in order to regulate the motor
current and to control the speed. The motor controller also
provides regenerative braking (ie, turning the motor into
a generator, to charge the batteries while slowing the car
down) by switching the phases in the appropriate order.
The motor controller can handle up to 100A through the
phases at up to 170V DC.
The motor controller outputs a square wave (ie, the phases
are either on or off, directly dependent on the state of the
Hall Effect sensors. This causes a small efficiency loss, since
the current is always at the same high value for a given
torque. A newer version of the controller will produce a
sinewave output which will have a higher peak but lower
RMS current, for the same torque, giving better efficiency.
Interestingly, those very high currents caused severe
problems with motor over-heating when climbing long
hills at low speeds. The high currents are partly a result
of the “coreless” construction of the motor, meaning that
it generates very little back-EMF at low speeds. So in spite
the motor being rated at up to 98% efficient, at low speeds
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Again back at UNSW, Yael Augarten (one of the Sunswift’s
drivers) shows the inside of the driver’s “cabin”. The
full racing harness is essential, just in case a large gust
manages to cause loss of control.
it is nowhere near that efficient and dissipates considerable
heat as a result. This problem was exacerbated by the very
poor ventilation inside the motor hub and wheel fairing –
streamlining has a definite downside here.
Solar array
Sunswift III’s solar array is made up of 1034 Sunpower
A-300 solar cells. These are back-side contact cells, which
means the electrical contacts don’t shade the front of the
cell. Their efficiency is between 20 and 21%. The cells have
been laser-cut to be rectangular so that they can be packed
tighter into the solar array. They were encapsulated into
thin, flexible panels by Hans Gochermann.
Electrically, the solar array is organised into six panels
consisting of more than 100 cells wired in series. This
Working on the inside-wheel motor. There is only one
wheel driven to save weight, albeit at the expense of tyre
wear. The motor is up to 98% efficient.
May 2007 11
Unlike most cars, hoisting the bonnet simply means taking
the solar-panel “lid” completely off.
Taking it off is not difficult, as this photo shows. Four
people can easily lift it – it’s unwieldy rather than heavy.
is necessary for several reasons, the main one being cell
matching. A solar cell is only able to pass as much current
as the light falling onto it will allow. Even if two identical cells are wired in series, if one is receiving less light
than the other, the optimum current will be close to the
optimum current for the cell receiving the least light. This
is particularly important in a curved array, where cells
which point in the same direction will receive about the
same amount of light.
The UNSW team did thorough simulations of the car
over the course of the World Solar Challenge (from Darwin
to Adelaide) to work out which parts of the car received
the same amount of light for most of the day and secondly,
where the best cells should be placed (since there is a spread
in efficiency, even within the same type of solar cell).
The other consideration when designing the solar array
electrically was the Maximum Power Point Tracker (MPPT)
voltage. The closer the solar array voltage to the battery pack
voltage, the higher the efficiency of the MPPT. The solar
cells have an open-circuit voltage of approximately 0.65V
and a maximum power voltage of approximately 0.55V.
Solar cells have a non-linear current-voltage (IV) curve.
That means that the solar cells will operate best at a particular voltage. The position and shape of the curves changes
dramatically with changing light conditions. By contrast,
the battery voltage varies according to its state of charge.
Therefore to get the maximum output from the solar array,
separate MPPTs are connected to each solar panel. The
MPPT is a boost converter which attempts to find the solar panel’s maximum power voltage (Vmp) and perform a
voltage conversion from that voltage to the battery voltage.
The MPPTs for the Sunswift are partly home-grown and
partly outsourced. The power section was manufactured
by the University in Biel, Switzerland, who sell MPPTs to
other solar car teams. The control section is home-grown,
based around a microcontroller and an FPGA (Field Programmable Gate Array). The FPGA is required to generate
the relatively complex timing signals required by the power
section. The power section uses a soft-switching boost
converter. “Soft switching” means that there is both zero
voltage and zero current across and through the transistors
when they switch. Two Mosfets are required for this topology and a third is required for synchronous rectification.
As a result, the boost converter is up to 99% efficient.
Believe it or not, this three-line alphanumeric display is
the only instrumentation the driver can see. As well as
current, voltage, power for the array, battery and motor, it
shows speed and motor temperature.
The driver does have rear vision, courtesy of the video
camera mounted behind his/her head and an LCD mounted
inside the vehicle. This photo also shows two battery packs
and some of the control equipment.
Maximum power point tracker
12 Silicon Chip
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The control section measures the input current and
voltage and output voltage. It runs an inner control loop
to maintain a given voltage on the solar panel (since the
voltage doesn’t change nearly as much with changing light
conditions), and an outer current loop to adjust that voltage
in order to find the maximum power point. It can adjust
it by one of several algorithms. The two most well-known
algorithms are “perturb and observe”(for hill-climbing) and
“fixed-percentage” which periodically stops the tracker,
measures the open-circuit voltage and takes a fixed percentage of that to be an estimate of Vmp).
The control section also connects to the car’s CAN network for telemetry.
Batteries and microcontrollers
Sunswift’s batteries consist of 200 prismatic lithiumpolymer cells. These are wired into modules of five in
parallel, and 40 of the modules are wired in series to give
a complete battery pack. This gives a pack which is right
on the World Solar Challenge’s 30kg limit. At 4.1V per cell,
the battery pack can be charged to 164V and discharged to
2.7V per cell (108V) when absolutely flat. The pack has a
capacity 40Ah and can drive the car approximately 300km
at 100km/h.
The batteries are connected to a custom-designed battery
monitoring system. The battery monitoring system is built
into the same PC boards which form the interconnection
between the cells. Each PC board has a microcontroller
which monitors four of the series modules. There 10 microcontrollers which communicate with a master via an
isolated serial bus. The microcontrollers themselves are
powered via linear regulators from the batteries they are
measuring. The battery monitoring system is important
because the cells are not necessarily identical and therefore do not charge and discharge the same way. Cells can
become out of balance at different states of charge.
The guys assured us they were not being (overly!) sexist
with “Yael’s First Drive” trophy . . . they admitted that just
about all novice drivers manage to damage tyres while
they get the hang of controlling the vehicle.
The car’s telemetry and control system consists of a
telemetry network and a control bus. The control bus is a
dedicated link between the driver controls and the motor
controller, and is a simple serial RS485 bus. The telemetry uses a CAN network, consisting of a large number of
microcontroller-based nodes spread throughout the car.
These nodes have dedicated jobs such as controlling the
front indicators, measuring the battery/array/motor current and interfacing with the battery monitoring system.
A CAN network links all these nodes, the MPPTs and the
motor controller.
All the data off the network, is sent in packet form by
wireless ethernet to the support car. The support car can
also send messages back to the telemetry network for con-
Apart from the tiny driver’s pod, the whole surface of the
vehicle is covered with solar panels. They are specially
made to be able to fix to curved surfaces and in total are
worth approximately $150,000
The low-profile and extremely low resistance Michelin
radial tyres were made specifically for the solar racers
but are now unfortunately not available. The team is now
looking for alternative tyres.
CAN and telemetry
siliconchip.com.au
May 2007 13
Here’s a close-up of one of the battery boxes. Every cell is
individually monitored to ensure that maximum power is
available from each one. Some of the connecting buses are
shown in the photo below (the side of the above photo) and
a couple of the lipol cells are showing signs of expansion.
in order to know exactly how far the car has gone and needs
to go. This is essential for race strategy.
Tilt sensor – measures the car’s angle up/down relative
to horizontal. This allows the strategy software to calculate
how much power the car would be using on a flat road,
and fit a model so that it is possible to a) work out whether
the car is using more or less power than it should and b)
what speed the car should be run at in order to reach the
destination (the course survey also gives the gradient and
overall rise).
One-wire temperature sensors – controls a network of
1-wire temperature sensors on the solar array.
MPPT - sends out panel current and voltage, as well
as several diagnostic values such as heatsink temperature and ambient temperature. Is configurable via the
network (can change the tracking algorithm, perform an
IV sweep, etc).
Motor controller - sends out lots of data. The main values
are the car’s speed, motor current, input current (which is
also measured in the negative sum), motor temperature,
motor controller temperature, low-voltage bus voltage, etc.
The motor controller can also be controlled via the network,
including modification of the cruise-control set point.
Driver display - a 40x2 character LCD panel allows the
driver to read what is going on, including speed. The driver
also tries to maintain limits on the motor currents in order
to avoid overheating and loss in efficiency. Furthermore,
should the wireless link fail, the driver can communicate
information displayed on the driver display back to the
support vehicle via CB radio.
Left-hand-side controls - while the driver’s right-handside controls interface directly with the motor controller via
RS485, the left hand side controls interface with the CAN
network and controls the indicators, hazards, horn, etc.
There are also other miscellaneous electronic devices
in the car, including the rear vision display and camera
and CB radio.
In summary, while the photos in this article show that
Sunswift is not much different mechanically speaking, from
many earlier World Solar Challenge vehicles, its overall
electronics and electric design is fiendishly complex.
What will the new solar race vehicles bring?
SC
Websites for further information:
www.sunswift.com
www.tip.csiro.au/Machines/success/sc.html
trol, configuration and maintenance purposes.
The entire network is isolated, giving the system fault
tolerance. The whole system is decentralised, meaning that
if one node fails, the rest of the system should continue
to operate.
Some of the most interesting CAN nodes in the car are
follows:
Negative sum – forms the negative star-point of the battery, motor and array. It measures the current from each
into the star-point using isolated Hall Effect sensors. It
also measures the battery pack voltage and integrates the
currents to give amp-hours. This forms the basis of battery
state of charge estimation.
GPS – measures the car’s position, altitude, etc. This allows the support car to do a look-up in the course database
14 Silicon Chip
for information on CSIRO motor design.
www.chuck-wright.com/SolarSprintPV/
SolarSprintPV.html for information on solar cells
www.wsc.org.au/2007 for World Solar Challenge
technical and event regulations
www.tritium.com.au/ for information on motor
controllers
Acknowledgement: Our thanks to David Snowdon,
Yael Augarten and other members of the UNSW
Solar Racing Team for their assistance in the
preparation of this article.
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