Hardware of an Autonomous Ground Vehicle
From CajunBot Wiki
Both of Team CajunBot's vehicles, CajunBot and CajunBot-II, have a very rugged design and are capable of traveling in offroad terrain. The domain the vehicle will be occupying may influence your platform choice.
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[edit] Hardware Devices
To operate an autonomous ground vehicle takes several different types of hardware devices working together. Below is an overview of the types of hardware devices the CajunBot uses and how they are used.
[edit] GPS Unit
A high grade differential GPS unit is needed so that the vehicle can know precisely its location on the surface of the Earth. Our GPS unit (CNAV 2050) is augmented with a differential correction signal so that position can be known to within a few centimeters.
For the vehicle to stay precisely within the lane and to identify obstacles correctly, the position of the vehicle needs to be accurately and quickly determined.
GPS has its limitations, however. If the vehicle were to go nearby to large building or pass under trees, the signal can be interrupted. Since it takes some time to reacquire the signal and start performing calculations, this can be a serious problem for a vehicle traveling on its own.
To learn more about GPS, go to
To learn more about differential GPS, please visit
[edit] Inertial Navigation System (INS) Unit
The INS Unit also plays a pivotal role in helping CajunBot determine is location. We use the Oxford RT3102 as our INS.
An INS unit takes input from various sources:
- GPS signal
- Accelerometers
- Wheel encoders
The INS unit uses data from these various sources produce data that not only includes position but also velocity and acceleration.
The more important use of the INS system is that of a continuous source of GPS information. As mentioned in the GPS section, various conditions may interrupt a GPS signal. The INS unit uses a method (such as a Kalman filter) to estimate based on the last known position and velocity and acceleration, the current position of the vehicle. By using the INS unit, the position information is continous, but may become inaccurate and degrade if the GPS unit is not receiving signal.
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[edit] LIDAR
LIDAR, which is a type of sensor that uses a scanning laser beam to detect obstacles, is the primary sensor that CajunBot uses to "see" obstacles within a close range (typically about 40m). To get a more complete view of the world around it, the vehicle has LIDAR sensors mounted in various locations: front, back, sides, low, high.
We have several SICK LMS LIDARs as well as two Ibeo Alasca LIDARs. For the SICK devices, we have a Digi Portserver TS8 to convert the serial packets from RS-232 to Ethernet.
The LIDAR device provides as data a full scan of the points that the beam has hit. So for one revolution, we would have a set of returns containing the angle and distance to some point. By pre-measuring the x-y-z offset of the physical device from the INS unit, we can transform the distance and range measurements into world-referenced coordinates in x-y-z. This is very useful as it allows the actual type of device and its location to be abstracted into a world frame. By positioning several LIDAR units around the vehicle, we transform their data into a common frame and operate on the entire set of data as a whole.
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[edit] RADAR
RADAR, a key technology in World War II, remains today a very important technology. This type of sensor uses radio waves to detect the range to distant obstacles. This sensor is the primary sensor that CajunBot uses to detect obstacles at longer ranges than LIDAR can be reliably used (typically greater than 40m-80m). CajunBot uses RADAR to supplement LIDAR and detect vehicles traveling at higher speeds from a long range. RADAR is especially useful when detecting thru traffic at an intersection.
We have two Eaton Vorad RADARs.
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[edit] Camera Vision
Camera vision can be used to supplement incomplete map information about the current roads. It may be that the map of GPS points used to navigate a street is out of date and that the street has been reconstructed slightly. In this case, camera vision can be used to communicate the boundaries of the road to the software so that it may adapt to its surroundings.
The camera has software attached to it that works by detecting the contrast between the road and painted lines, and attempts to estimate the distance and angle to those lines.
We use the Iteris LDW system to provide us with the lane detection data.
[edit] Power Source
Having all of that extra hardware means that you need to give it that much more power. To handle the extra load that is put on the electrical system, we have used two different approaches. Originally we had attached an external electric generator that powered all of the devices. After a much needed hardware upgrade, a second alternator was installed to provide the extra power that the computer systems and sensors needed.
Power related hardware that we use:
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[edit] Steering & Throttle Control Systems
The vehicle's steering & throttle control systems allow the computers to send a digital signal of (gas/break), (turn left/turn right) which is then converted into an analog signal which, in turn, make a motor activate or deactivate. The system used was originally designed for handicapped individuals to be able to drive their vehicle. Now with a little programming and hardware, a computer can specify how much the accelerator should be depressed or when the break should be applied.
Using the other sensors of the vehicle as its eyes and ears, programs on the vehicle calculate how fast the vehicle should travel and how it should navigate the terrain. In the end, it boils down to a few small motors that will turn the steering wheel, apply break, and/or press that gas pedal.
Steering control hardware used:
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[edit] Electronic Control Box
Ragin'Bot houses many devices to enable sensor processing and autonomous vehicle control. The E-box is a custom electronics box that houses emergency stop control hardware and provides a central connection for all vehicle control hardware. The E-box also combines the various time pulses from GPS together into the same signal stream.
[edit] Emergency Stop & Watchdog Devices
Safety is a very important concern, especially when you have a full size street legal vehicle attempting to drive itself. Aside from the many procedures we have to ensure safety when working with the vehicle, there are some hardware devices that help us:
The first device is a steering watchdog device. This device serves as an intermediary between the steering signal coming from the computer to the signal going to the steering motors. It is common that computer programs crash and the programs that control CajunBot have been known to encounter unrecoverable errors before. To decrease the risk of harm, the watchdog unit continuously monitors the signal coming from the computer. When that signal is interrupted, for more than a fraction of a second, the watchdog unit is preprogrammed to send a signal to steering motors to straighten the wheel bring the vehicle to a stop. This watchdog device can even be triggered in instances where the steering program is taking to long to generate a signal.
The second safety device is that of a hardware remote emergency stop ("E-Stop"). This device is able to remotely override any software controlling the vehicle to:
- Remotely kill the engine of the vehicle.
- Remotely instruct the vehicle software to "pause".
- Remotely instruct the vehicle software to continue from "pause" mode.
Additionally, there are physical buttons on either side of the vehicle that will immediately kill the engine of the vehicle when pressed.
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[edit] Computer Systems
The computer systems of the vehicle consist of EPIC form factor computer boards with flash storage and pentium mobile chips operating around 1.6GHz. The computers are not the "latest and greatest" but server as power efficient small form factor processing centers for the various programs that control the operations of the vehicle. In general the setup uses a logging machine, a steering control and related machine, a computer vision and path planning machine, and a time synchronization machine. If necessary, the programs that control the vehicle could be run on the same computer, or be spread out across several different systems and communicate over high speed networking routers. This scalability is one of the strengths of the CajunBot software design.
[edit] Networking
The vehicle is equipped with a high speed networking switch that allows multiple private networks. Since network latency is very critical, the networking switch must be able to handle all of the traffic on the network. The switch allows other devices to be physically connected but logically on a separate network, so network traffic doesn't become the bottleneck between devices.
The switch that we use is the Dell Powerconnect 5324.
There is also a wifi access point that is set up on the vehicle that allows remote broadcasting of real time data.
A cellular data card connects to a phone network and allows the vehicle to both broadcast a sample of its data in real time over the internet, as well as receive highly accurate differential corrections from a nearby base station.
[edit] Tour Navigation
- Hardware of an Autonomous Ground Vehicle
- Hardware-Software Connection
- Build and Test Cycle
- CBSystem Software Architecture & Data Flow
- Taking the plunge: Getting CBSystem
- Using CBSystem
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