NUsolar

Northwestern's Solar Car Team

SC6: Our Car

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Overview

NU Solar’s 6th generation solar car is made up of a composite monocoque chassis and airfoil shaped body, with a steel roll cage.  The structural parts of the body are comprised of carbon fiber/Nomex sandwich glued together with high strength epoxy. Several steel and aluminum plates reinforce the steering and suspension system, which bears high amounts of stress.

Just as our other past cars, SC6 is a fully functional electric vehicle which can be legally driven on any road, although we don’t plan on taking it out on the highway just yet. It is equipped with headlights, turn signals, and a rear-view camera system for driving in reverse.

 

Structural Systems

Frame

SC6 is designed with a monocoque structural approach, meaning that the structural support of the car (the ‘frame’ which supports the various other important elements of the car such as the wheels, motor, and batteries) is also its external skin. The body of the car is a carbon-fiber reinforced polymer composite material. We baked the composite in an extremely large “oven for ovens” provided by Precision Quincy (they use the “meta-oven” to bake paint onto kitchen ovens).

Roll cage

Our roll cage is built out of 1.25″ steel tubing with a 0.06 wall-thickness. It is attached to the composite frame of the car with panel grommets, courtesy of The Young Engineers. The cage can sustain up to 62,506psi of stress with no side effects other than some minor plastic deformation. Based on our calculations, this is a high enough threshold to ensure that our driver will be safe in the event of a crash or a rollover.

Canopy

David is redesigning the entire canopy and hatch system to improve driver visibility and aerodynamics. Currently the cannopy is a single molded sheet of plastic which is embedded into the top frame of the car. This section of the frame is hinged to flip up towards the back so that the driver can quickly enter and exit the car. The canopy is transparent in all directions, providing the greatest visiblity for the driver. To improve visibility, a small camera at the back of the car acts as a rear-view mirror and transmits a live video stream to a screen inside the cockpit.

Dynamics

Motor

Our motor is a 96V NGM brushless DC motor. It uses a variable gap mechanism (an Air Gap) to change the torque constant dynamically, allowing us to adjust the car’s performance and power consumption for various external conditions. The motor is extremely efficient in transforming the electrical power from the solar cells and batteries to the mechanical energy necessary to drive the car forward. At 500 W power output, it reaches 95% efficiency. We are currently in the process of building a dynamometer to allow us to gather more specific data on our motor’s power consumption and vehicle efficiency.

Wheels

SC6 uses a three-wheel design. The two front wheels are connected directly to the steering wheel by a mechanical link, and have a turning radius of about 6 ft. They are suspended from two mounting plates, both of which we had custom machined, which are attached directly to the outer frame of the car. The frame extends down around the wheel, protecting it and the internal systems from any road debris. Our real wheel is attached to our DC motor and is mounted on a dangling suspension system attached to the rollcage. We use treaded tires with either aluminum or carbon-fiber rims.

Electrical Systems

Solar Cells

Arguably the most important part of a solar car are the cells which provide power. Our car’s array covers 95% of the top projection of SC6, with a surface area of about 6.9 meters. The array is electrically split into three parts, each of which is wired to a Maximum Point Power Tracker which take the ~80V supplied by the solar cells and convert it into 120V to send to the batteries (see below). The 468 solar cells which comprise the array are manufactured by the Sun Power corporation and encapsulated by SunCat Solar. These cells represent some of the best solar cell technology currently available (efficiency > 20%). During the Rayce, we are only given a few hours at the end of each day during which we can charge the batteries while the car is not running, so a higher rate of energy collection means we can gather more power and perform better.

Batteries

SC6 uses Lithium-Ion battery technology. Li-Ion batteries have a high output-to-weight ratio, so car is both well-powered and light. Our battery box contains 32 modules connected in series, each with 13 3.7V LG 18650 Li-Ion batteries connected in parallel by nickle plates. They receive power from the solar array through the MPPT’s at 120V and supply power to all the car’s motor, light, and telemetry systems. Each module is actively motitored by the BPS (see Telemetry), which will shut the car down if it detects any anomalies in temperature, voltage, or current. As an additional safety feature, each module is encased in a specially formulated wax which has a melting point slightly less than the combustion point of the batteries. Should the batteries begin to overheat, this wax will wick away excess heat by melting, hopefully giving us a few extra seconds to shut down the car should the BPS fail to do so for some reason.

Telemetry and Computing

Several computer systems control SC6. These are connected to each other by a CAN bus. The BPS, or battery protection system, monitors voltage, temperature, and current for each of the 32 battery modules and is responsible for shuting the car down should anything go wrong. It also relays telemetric data to the other systems over the CAN bus, as well as to a remote laptop over a cellular network. The driver controls circuit is responsible for controling the speed of the vehicle, as well as the lights and turn signals. The primary interface to this circuit is a panel of buttons built into our steering wheel, which the driver uses to send various commands.

We program our circuits in C and C++ and run the code on PIC32 chips.

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