Test Drive #2 Video!
More Videos
See video of our first test drive, including some from on-board cameras. More videos, including some "behind-the-scenes" looks at the build, can be found on MIT TechTV.
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Contents:
Project Updates
The Team
The Concept
The Components
The Circuit
Vehicle Specifications
Technical Notes and Data
Links
Yes, ultracapacitors, boat batteries, and large electric motors can be dangerous. Build at your own risk!
| 6/7/09: Check out some pictures from our trip to the 2009 International Conference and Exhibition on Ecological Vehicles and Renewable Energies (EVER '09) in Monaco this spring. We were awarded "best student paper in ecological vehicles" for our work on the Cap Kart, and got to see many examples of cutting-edge EV technology, including two Tesla Roasters. Also, video of some more electric karts at the e-Kart Challenge in Tours, France. We will be resuming test drives soon, as well as starting a new project, with a higher power to weight ratio... Check back soon for more information. |  |
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The ad-hoc team that came together in 2007 to create a DIY self-balancing scooter is back for seconds with an official new title: the Edgerton Center Summer Engineering Workshop. It is a collaboration of MIT students and high school students from FIRST Robotics teams: 97 (Cambridge Rindge and Latin School), 2043 (John D. O'Bryant School of Math and Science in Roxbury, MA) and 2349 (Wayland HS). Projects and teams like this are made possible by generous support from sponsors interested in furthering engineering education. If you or your company is interested in getting involved, please contact us. |
This year, the Summer Engineering Workshop team was interested in making an electric go-kart. (Some kind of obsession with things you can ride...) But to make things more interesting, we decided to add a 110F ultracapacitor into the mix. Capacitors store electrical potential energy; much less than a battery, but you can charge and discharge them more quickly. Research into automotive applications of capacitors (in hybrids, for example) has led to development of ultracapacitors with increasing energy density and decreasing price.
Research is research. We were also interested in having some fun. So, we wanted to see if we could very simply integrate an ultracapacitor into a stylish electric go-kart. One of the simplest ways to do so is to use it exclusively to store energy recaptured from the kart as it brakes. Then, put this energy at the driver's fingertips in the form of a power "assist," or, less formally, boost! While it's likely not the best solution for full-size autos, we wanted to make a tangible demonstration that the technology is closer to reality than many might think.
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The more obsessive members of the team insisted on a racing chassis on which to build our electric kart, so we acquired a used Haase racing kart, minus the engine. It was perfect for the job: wide enough to fit the batteries, low enough to stay stable, strong enough to take the extra weight, and efficient enough to utilize the power. |
| We were initially surprised to find that capacitors this size are already commercially available. Maxwell Technologies is one of a handful of manufacturers of ultracapacitor modules designed specifically for automotive use. Our 110F, 16V module stores about 14kJ when fully charged. While this is not much more than a AA battery, it's the equivalent in potential energy of the go-kart starting at the top of a 20-foot hill, and can be released in seconds. |  |
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No fancy lithium-ion batteries, unfortunately. They were a bit out of our price range. Instead, we use sealed lead-acid marine batteries, a staple of electric go-karts. Three 79Ah SeaVolt AGM batteries, weighing 53lbs each, are wired in series to supply 36V for primary power. They can handle bursts of hundreds of amps and can be fully charged in about 4 hours. |
| The motor we chose is a SepEx DC motor from D&D Motor Systems. It has no magnets; instead, field coils are energized to create the magnetic field. Unlike a permanent magnet motor, the strength of the magnetic field can be varied. This makes a sort-of electronic transmission, trading torque for speed with no gearing. It also allows for the terminals of the motor to be short-circuited across an empty capacitor while the field is used to control braking force. |  |
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No commercial motor controller fit our needs perfectly, so we built our own 300A-peak MOSFET-based controller with components cheap enough to be considered consumable (and many were consumed in the process). It controls both the armature and the field coils of the separately excited motor. The system is drive-by-wireless: pedal sensors and other instrumentation interact with the motor controller wirelessly, with failsafes, to avoid electrical noise. |
| The software for the operation of the kart itself is pretty simple. But to make sure that we capture as much information as possible, a custom radio and telemetry system transmits data such as speed, capacitor voltage, and motor current to a computer, which logs it for later analysis. The inexpensive radio system is based on 2.4GHz ZigBee modules from Digi. |  |
This is the primary power circuit implemented on the Cap Kart. It combines batteries and an ultracapacitor module in a simple series configuration, as opposed to placing them in parallel. Two sets of MOSFETs make a high-power DC-DC converter, which is controlled to set the current through the motor. A separate contactor reconfigures the circuit for boosting and regenerative braking with the ultracapacitor. More information is available in the technical documentation.
| Weight (No Driver) | 350lbs (160kg) |
| Ground Clearance | 1" |
| Drive | RWD |
| Wheel Size | F:10"/R:11" |
| Gear Ratio | 3.6:1 |
| Battery Voltage | 36V |
| Battery Capacity | 79Ah (2.8kWh) |
| Ultracapacitor Voltage | 16V |
| Ultracapacitor Capacitance | 110F |
| Ultracapacitor Energy | 14kJ (3.9Wh) (19hp-s) |
| Peak Controller Current | 300A |
| Peak Mehanical Power | 10hp (7.5kW) |
| Peak Torque @ 0RPM | 40lbf-ft (54N-m) |
| Peak Acceleration | 0.6g |
| Top Speed (est.) | 35mph (56km/h) |
| Person-Hours to Build | 500 |
| Total Cost to Build | $6,000 |
Torque-Speed Curve:
Shaded areas: 0-12V ultracapacitor boost.
Design Notes:
Sometimes it's fun to go back and look at the early design notes from a project. Here are the minutes from our first two design meetings:
Design Meeting #1: June 21, 2008
Design Meeting #2: July 1, 2008
Technical Documentation:
Simple Modular Half Bridge Rev.1 - This is an updated version of our motor controller documentation. It describes a simple design for a modular half-bridge that can be used for high-current DC motor control. You probably only want to attempt to build this or something like it if you are genuinely interested in power electronics design not afraid to blow up a few MOSFETs along the way. For most EV needs, there are reasonably-priced off-the-shelf controllers such as the Alltrax line.
Raw Data:
This is raw data from our test drives and flywheel testing. The format is a comma-delimited text file with two header rows: The first header row specifies the date and start time, the second has the column headings.
Raw data from our first test-drive (5 runs). No ultracapacitor data.
Raw data from our second test-drive (5 runs). Includes two-speed and capacitor boost data, but no regen.
Raw data from the flywheel testing.
Raw data from the second set of flywheel tests, with improved regenerative braking.
Formatted Data:
This data has been processed and in most cases visualized. Each set highlights particular vehicle functions or other interesting test events.
Data highlight showing two-speed field-weaking mode and a MOSFET failure.
Data highlight showing the first test-drive of ultracapacitor boost.
Data highlight showing capacitor regenerative braking over two cycles (flywheel).
Data highlight showing capacitor regenerative braking from high speed (flywheel).
Data highlight showing capacitor boost to reduce battery current (flywheel).
Data highlight showing capacitor boost to increase top speed (flywheel).
Links:
Suppliers and Vendors:
D&D Motor Systems, Inc. - Motor source.
Maxwell Technologies via Tecate Group - Ultracapacitor source.
The Robot Marketplace - Misc. parts source.
EVParts - More great electric vehicle components.
West Marine - Battery source.
International Rectifier - Source of power MOSFETs for motor control.
Big Blue Saw - Custom-cut metal for mounting hardware, flywheel steel.
SolidWorks - Our favorite CAD software.
References:
Un support pédagogique pluritechnologique: le kart électrique - A paper (in French) outlining the design of an electric go-kart as an educational project.
4QD-TEC - One of the only sites on the internet that simply (and correctly) explains DC motor control and regenerative braking.
@ MIT:
MIT Electric Vehicle Team - With a stylin' electric Porsche.
Solar Electric Vehicle Team - "Tesseract" and "Eleanor"
Media Lab Smart Cities Group - Check out the City Car and Roboscooter in the Mobility division.
LEES Ultracapacitor - Approaching the energy density of batteries.
In the US:
Go Green Go-Kart Competition - Winston-Salem/Forsyth County Schools are hosting an alternative fuel go-kart competition for high school teams on May 20, 2009.
Much More in Europe:
E-Kart - A program and competition in France that promotes electric karts as an educational tool for universities.
Cranfield University Electric Go-Kart - A dual energy source kart that precedes ours.
Cranfield IEEE paper. - Technical description of the above.
Electric Green Racing - Awesome video from a dual energy source electric kart team from the University of Manchester.
More Electric Karts:
Mythbusters Electric Go-Kart - Featured in Popular Mechanics.
