Vehicle Design

The original idea of our project was to design, construct and program a Mindstorms robot that would stream video and take commands from a mobile phone or handheld computer. Initially, the robot that we have come to refer to as Alessa, was intended to be a Mars vehicle. The very first designs were based around that theme and included several wheels instead of the ubiquitous set of four. However, the Mars prototype never left the drawing board, and we abandoned the idea in favor of a more RC car-like construction with 4-wheel drive. This model did get real, at least partially, and had more in common with its RC counterparts than we originally had imagined. Everything from its wheels and the accompanying suspension, to the transmission and differentials were all clones of your average RC car, and as such also very much like a real car. To make a long story short, it was flat out boring. Not only was the chassis finished within a couple of days ? we had also very little to do with the purely technical aspects of the vehicle since most of it had been copied from ready-built Lego models and RC cars. Also, its transmission was far from perfect and left much to desire. It was painfully slow and involved worm gears that had a spongy effect on the vehicle?s propulsion, much like the motors were connected with rubber bands. To top it off, we hadn?t been very cautious in the building process, having missed out on important safety features as a slip wheel, which would prevent gears from cracking and motors from overheat if the vehicle would encounter an obstacle too tough for it to get over. The worm gears did provide good torque and much power, but the over-engineered gearing absorbed too much of it, leaving us with a rubber band vehicle.

Next model is not that easy to describe, since we had a lot to do coming up with a fresh design to replace the discarded RC car. Much of our time went into brainstorming and researching various aspects of building with Mindstorms, and what we created with the bricks during this period is not worth describing, apart from a few neat inventions that we decided to keep for the final version of Alessa. Among these is a pneumatic compressor that, whilst not necessary for the robot?s core functions to work, is a very advanced technical solution that gives the robot the ability to raise and lower its wheels ? in other words an adjustable suspension. This compressor can obviously connect to any pneumatic device and do pretty much anything with it, but because of limitations of the on-board computer (the RCX), we have no matters of controlling additional features due to the lack of channels. Steering, drive and suspension occupy the total of three channels. For the pneumatics to work, two motors are working two large pumps simultaneously in order to obtain enough air pressure for the pistons to extend and retract, which we?ll cover in a second.

This was how the present Alessa was created ? around a pneumatic compressor. The pneumatics turned out to be such a cool feature that we decided to base our product around it and integrate motors, transmissions and other technical features that we liked. Continuing to describe the pneumatics, we had to come up with a way of remotely controlling the airflow. The standard parts only allow for manual input and there are no means of controlling them electronically by default. After a few days of testing, we concluded that a worm-geared valve run by a separate motor would fit the bill. Hooked up to the RCX, it can be operated with on-board buttons, irDA or firmware. The compressor?s undoubtedly coolest feature is its ability to automatically switch off when a desired level of pressure has been reached. This works explicitly using mechanics and no programming whatsoever is involved. A large pneumatic cylinder is hold shut tight by a spring-loaded mechanism. Connected to it is a power switch that is switched on when the cylinder is contracted. As pressure rises, the spring- mechanism slowly stretches and will eventually have reached to a point where the power switch cuts the power. Hence, the compressor stops to prevent the motors from stalling. When the pneumatic system is activated, provided it?s used heavily enough, the compressor will begin pumping again and repeat the whole process continuously as needed.

With the compressor and valve working, our next assignment was to construct a suspension that, as aforementioned, would make use of pneumatics. Beginning with the front suspension, we ran into problems creating a steering mechanism that was sufficiently powerful enough to turn the large tires, yet soft enough not to crash the motor if the tires were to be obstructed and not be able to turn. This was going to be done not by geared rods, but by a vertical axle in between the wheels that would rotate the connected bars in order to turn the wheels. Connected to the axle is a gear that is driven by a worm gear, which subsequently is driven by a transmission belt that is precisely tense enough to turn the wheels, but will slip if strained too much. Also connected to the vertical axle is a touch sensor that provides the RCX with information about wheel positions and how to turn them. The result is a mechanism that works like a charm given the use of low-powered motors. Part of the challenge was to make the wheels able to travel up and down using the pneumatics and still be able to turn in both fully extended and fully contracted positions. We did it.

The front wheels, however, do not rotate by themselves as we decided to leave out any means of transmission. The reason for this was to let the front of Alessa concentrate solely on steering without having to bother with propulsion. Using this idea, the vehicle obviously has to be rear-wheel driven. Experiences from previous prototypes told us that long drive shafts and inaccurate worm gears were not the way to go, so we redesigned the whole system from scratch and ended up with two engines mounted behind the wheels connected through a vast array of gears, including slip wheels. The engines travel up and down with the wheels themselves when the suspension is activated, which eliminates the need for drive shafts. And since we have two motors, there is no need for a differential that would drain power.

All this technology is a real battery hog and to top it off, we have two high-powered lights composed of clustered LEDs that want a lot of electricity. These lights are mounted like on a typical 80s sport car, in a way that people refer to as frog eyes. When the valve is turned, these lights pop up along with the suspension. The pneumatics that power the light mechanism double as an automated power switch that will turn on or off the power for the LEDs. This power switch is hooked up to a 9V battery pack that also provides power to the compressor. That said, Alessa uses one single battery pack for its unique features. This excludes steering and propulsion, which is powered by the RCX, however both are equally important for the machine to work properly. Battery power is monitored by the on-board Tungsten T handheld of which main task is to ? simply put - convert Bluetooth signals from a mobile phone to irDA utilized by the RCX. Please refer to the Client-Server Lego Communication section of this document for more information regarding the handheld and it?s purpose.

Obviously, Alessa is nothing that has been built over-night. Although briefly described, all of its functions have been thoroughly tested, modified and rebuilt several times to achieve a near-perfect result. Also, the whole process of constructing a chassis large enough to carry all necessities aboard, yet rigid enough not to collapse, have been a complicated task despite not being covered in detail in this document. It is important to understand that this description of the creation of Alessa is scaled down to concentrate on the most vital parts, leaving single nuts and bolds uncovered.

Pneumatics, motors, steering systems, automated valves, pressure meters and popup lights aside, Alessa?s perhaps most astonishing feature is its ability to stream and record video. A camera lens is mounted in the front between the lights, which is the perfect spot to look through. The lights will lit up the environment enough to see in completely black environments and the camera is mounted in a position that allows the viewer to get a good overview of the terrain ahead, but still being able to see the ground in order to navigate without a hassle. In the middle of Alessa, the camera?s circuit board and the accompanying LCD display is mounted, and an antenna is to provide good reception is right nearby. The camera is connected to the very same power source as the LED lights, which means the camera and lights will be activated and deactivated at the same time. Please note that the power source is, as said earlier, not the RCX but an external 9V battery pack.

Regarding the vehicles design, not much effort was put into making Alessa prom queen of the year. Keeping deadline has been of outmost priority and cosmetic details have been left out due to the lack of time. The other reason is that we are quite fond of the bony and industrial look of the machine, seeing that it reveals the technically detailed innards for spectators to behold. In addition, leaving out unnecessary parts accommodates easy access for repairs and tuning.

Links A source of miscellaneous ways to overcome the limitations of Mindstorms Various sources of inspirations, including the motorized pneumatic valve Kudos to JP Brown for the idea of a mechanic pressure sensor Still active, this place is worth a visit
Need a certain part? Check it out here
Visit for a lot of custom models, building instructions and more
The biggest network of them all - our most used resource Logo  
Page viewed times mobileRobotics is powered by Source Forge