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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 siliconchip.com.au 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. siliconchip.com.au 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 siliconchip.com.au 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 siliconchip.com.au 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. siliconchip.com.au