Thursday, July 20, 2006

Hardware

Libramentum can be split into three basic hardware systems. Build, sensors and peripherals, and proccessing.

The physical build of our robot was designed by a german engineering exchange student. Though my lab partner and I had decided we wanted two motors driving the ball at ninety degrees to each other - similar to a mouse ball - our engineering student redesigned Libramentum to have three rollers at 120 degrees to each other.
With two rollers (90 degrees) there would always be a particular direction at which one of the rollers was doing no work, and in fact the friction of the ball rubbing across the unused roller would seriously distort our balancing calculations. The three-roller design (120 degrees) solved the problem by requiring all rollers to be driving the ball at all times. Therefore even though there would still be friction involved, it would be a more constant variable and easier to compensate for in the calculations.
Libramentum is not a large robot. There are pictures of other balancing robots on the net, and some of them are even as tall as a person! But Libramentum will be able to be placed on a table and should stand (once built) about 200-300mm high. The diametre of the ball is only 100mm! Most of the robot will be machined from aluminium with some parts being left in their 'rapid-prototyping' states. Rapid prototyping is a big 3-D printer. You can make a design on computer and the printer will construct it out of special powder and glues for you to see and test the design before making it out of a more expensive material.

The sensors will be placed at various points on the robot to assess where it is and what attitude it is in. To do this we need to be able to calculate Libramentum's angle of tilt to earth (attitude) and its position relative to other objects, or to its start point. Attitude is calculated using a small piezo-gyroscope and an accelerometer. The gyroscope uses oscillating vibrations to detect its movement relative to where it started, and an accelerometer detects the rate at which it is moving relative to gravitational pull. Because the robot can fall over the ball in any of 360 degrees, we need to have two sensing systems (two each of the acceleromter and gyro) each at ninety degrees to each other. Each set gives us accurate fall rates in each direction (the X and Y directions) and we can combine the two rates to calculate the real direction of fall - which will be a combination of the X and Y vectors.
The 'station-keeping' sensor is simply an optical sensor taken straight out of an optical mouse. It is mounted over the ball and takes pictures of the surface of the ball. The sensor then compares two pictures and works out how far the ball has moved in both X and Y directions then feeds that information back to the proccessors. This isn't much help when the robot is standing still trying to balance, but when it will be moving from place to place it will be using its own momentum as an aid to balancing and at that point the optical sensor will be able to tell us how fast Libramentum is travelling across the floor or the desk.

As well as the sensors are the motors and these will be controlled by the processors based on the information from the sensors. Each motor will be controlled individually - forward or backward - at a variable speed to get Libramentum upright again. The motors are situated near the top of the ball and belts run down the arms to the rollers which are postioned just under the equator. The arms provide the double purpose of holding the robot on top of the ball, and also providing the rollers to drive it.

The third part of the hardware is the electronics and processing. This is made up of two electronic printed circuit boards similar to what you would find in a computer, and a power supply board. The power supply is a rechargeable battery system which can provide the current and voltage for all the electronics and also a seperate and more powerful supply for the motors. The motors can draw a lot of current and they also introduce a lot of electrical noise which can fry an electronic chip. We don't want that! So it is important to keep them seperate.
The two proccessing boards are the motherboard and the daughterboard. The motherboard takes care of the sensors and peripherals. It receives the inputs from one gyro/accelerometer pair and combines it, recieves inputs from the other pair and combines it, then it sends these values to the daughterboard.
The daughterboard takes these rates of movement from the X and Y axis, and calculates in which direction the robot is falling over the ball and at what speed. Then it calculates which motors need to be driven in which direction and at what speed in order to get the robot back up on top of the ball again. It sends these values back to the motherboard which ensures the motors are provided with the correct voltages to do what they have to. The proccess repeats, and should catch the robot beginning to fall before it gets very far from top and centre.
Added to this the motherboard will read inputs from the optical sensor and send these to the daughterboard. The daughter board will assess if the ball has to be moved in a particular direction and command the mothorboard to move the motors as required.

You can see there are a lot of very high speed calculations that need to be done. The motherboard has three large proccessors designed to handle the sensor inputs and motor outputs, and the daughterboard has an even larger proccessor to handle all the calculations. The idea for us is to get Libramentum balancing. We will design the robot so that different types of daughterboards can be interchanged. This means other students can try their hands at writing code that will be more efficient and more able to balance Libramentum and in the future - command her to move in a particular direction via radio control or even bluetooth. Our supervisor even wants to use an FPGA daughterboard which is a revolutionary type of electronic processor which will quite literally be able to program itself into balancing the robot.

But we have to get there first :-)

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