In the fourth part of our Jetson RACECAR build, we are going to discuss platform mounting where the computer, sensors and other components will be placed. Also, we’re going to talk about which sensors we might use in building the project. Looky here:
There are a few areas that we are going to explore on this project. The first, as we talked about earlier, is to recreate the MIT RACECAR that has been constructed for the 2016 season. In the coming weeks, MIT will be releasing the drawings, documentation and software for their robot. The bill of materials (BOM) for the parts is ~ $3500 USD give or take which includes a few of custom laser cut and 3D printed parts.
The second area is an exploration of how to take the overall design of the RACECAR and reduce it to more minimal experience. The purpose of this is two fold, a curiosity as to determine which sensors, mechanisms and the amount of computational capability are actually necessary to get good performance from the car. The other part is to put a different budget constraint on the project, let’s say around $1500 USD, to make it more accessible.
Another exploration is to figure out what is actually needed to be able to build one of these robots. Ideally the robot would be assembled with a minimum of fuss with the right parts as a weekend project. The actual mechanism itself isn’t that complicated, so things should be fairly straight forward.
Since we’re a little ahead of the MIT release, this is a good time to examine some different alternatives of how we might put together a minimalist Jetson RACECAR of our own. With that said, I do have some information about the 2016 MIT RACECAR, so that should be useful.
Using the TRAXXAS Rally as a base, a platform is mounted above the chassis to hold the computational, electronic and sensing devices of the robot. Here are three ways to accomplish that task:
- Use standoffs. There are several M3 machine screws which hold the Nerf bars to the chassis. These screw holes would act as mounting points for 4 standoffs spread around the car, with the platform then attached to the standoffs. This is how the 2015 MIT RACECAR was assembled.
- Mount the platform on modified TRAXXAS body mounting posts. Many parts on TRAXXAS cars are interchangeable and are used between different models. The platform could be attached to carefully selected body mounts with little modification.
- Screw the platform directly into the cars suspension towers. This is probably the easiest, but requires relatively accurate aligned holes between the chassis and the platform.
For initial exploration, let’s mount the platform onto TRAXXAS body mounting posts. In the video, a 3/16″ thick, 9.5″ x 17.5″ impact resistant acrylic platform was used, mainly because it was left over from another project. As is typical in most of these types of projects, we’ll build a quick mock up using available resources before investing in the final design elements.
As you have probably already guessed, one of the important aspects of this design is impact resistance. This is because of the inevitable crashes that occur when testing out new steering and path planning algorithms. The electronics on the platform are expensive, and need protection when things go a little wrong. One of the findings from last years MIT car was that using 1/4″ ABS provided better impact resistance than the more brittle acrylic. Both are relatively easy to machine. We will be using 1/4″ ABS when we get past the mock up stage.
In the video, two Traxxas 6815R Body Mounts were used along with some Hex Socket Button Head Screws, 3x8mm. A body mount was attached to the front suspension tower, the other was attached to the rear suspension tower after removing the stock body mounts.
As noted in the video, the platform is not level. We’ll plan on adding some spacers to the front body mount to level the platform. Holes in the body mount will serve as mounting points for the platform. We’ll also add a second platform on top of the first using standoffs to act as a roll cage structure in case things get a little upside down.
Computers and Power
Either a NVIDIA Jetson TK1 Development Kit or a NVIDIA Jetson TX1 Development Kit can be used. There are slight variations that will be needed for wiring and mechanicals depending on the Jetson being used. In this series, we’ll probably make up a couple of different configurations of platforms for comparison, but will use a Jetson TK1 for the first mock up which will probably get beaten up a little. Platform mounting of the Jetsons will be on standoffs.
For power, a 3S Lipo battery will be used to power the Jetson. In the first, simplest version of the Jetson RACECAR we will power the Jetson with the battery, and any peripherals will draw power from the Jetson. This should be adequate for one USB camera and interfacing with few I2C devices.
On the 2016 MIT RACECAR there are three main sensors, an Stereolabs ZED camera, a Occipital Structure Sensor, and a Hokuyo LIDAR. As mentioned in the video, the Hokuyo LIDAR is relatively expensive; that part alone doubles the price of the car. With that in mind, one of the interesting experiments to perform is to determine how much better the robot car performs with the LIDAR installed versus without.
Remembering that the MIT RACECAR challenges are held indoors, outdoor performance of the sensors is an unknown. Typically IR based sensors like the Structure Sensor have issues with sunlight (the IR emitted by the sun tends to blind the sensor), so the ZED feels like a natural candidate for outdoor use. I happen to live in Southern California during a drought, and there is a park nearby which looks like a good place for working with the RACECAR. That being the case, the ZED will be the first sensor with which we will experiment.
One issue is that the car is basically blind to objects less than a foot or two away from the car. In the video I proposed using a LIDAR-Lite sensor. There is an issue with this selection in that Garmin acquired the company that is manufacturing the device. During the acquisition, the LIDAR-Lite became unavailable, but should become available soon. Ultrasonic sensors, or a TeraRanger One may be worth exploring as a replacement.
Electronic Speed Control (ESC)
On the MIT RACECAR, an open source ESC is being used which allows better control of the car motor. As the course that the MIT RACECAR is used in is about control theory, it makes sense that a different ESC is being used. My current thought on the stock ESC is that it’s a major pain in the ass. Even if you gear the car down by physically swapping gears in the differential, and get the car going slower, you ultimately don’t have the full range of control over it that you want. Every movie I’ve seen ends up with the robots being out of control and either killing everyone or taking over the world, except for Star Wars where the robots are pussies. To start the project with the robot car out of control seems like a bad idea. With that said, the first mockup will attempt to interface with the stock ESC and steering servo with a PWM driver.
The next bit here is to actually assemble the parts, load some software on the car, and get some action going.