by Damián
Introduction
One of challenges that were part of the High School Robot Competition (Vandals Robotic Challenge) was to follow a line. After running into several problems to be able to find a robot or a kit with capability to follow a line, we needed to design, build and test our own kit. The final result is the O.A.R. Sensors Board.
Electrical Design
You can find the board schematic in this file: boardschematic.pdf
The sensors board uses four IR transmitter and receiver sensors (CNY70). Both emitter and transmitter are embedded in the same package. The polarization of the emitter diode is achieved by using a 100 Ohms resistor. The photo-transistor is polarized with a 43 Ohms resistor. The collector of the transistor switches from a value close to Gnd when it receives the wave that the diode sent almost as it was. If the wave amplitude is reduced, for instance when the sent wave is reflected in a black line, the collector reaches a voltage close to Vdd. The Schmitt-trigger inverters placed at the collector of the transistor discretize that voltage signal, converting it into digital values readable by a microcontroller.
First Prototype
The first prototype include in it two sensors. The idea was to be able to simply follow a line with them. All the design was soldered in a stripboard. The Schmitt-trigger inverters IC provides 6 gates in it. Therefore due to the simplicity of the board and the availability to upgrade it, the second prototype included four sensors.
Second Prototype
This one included four sensors. Two of the sensors are placed on the front and the two others on the sides. They were also aligned at different heights with the idea to be able to make it more flexible. This prototype was also build in a stripboard.
Final Prototype
The final prototype is the result of creating a printed circuit board (PCB) for the second prototype.
Cadence Layout Description
This is the cadence layout for the design. LS, LF, RF, RS are the four sensors. The rectangles below them are the resistors for the polarization. The rectangle in the middle corresponds with the inverters. In the bottom appears the 6 pins header to plug in a cable. The yellow part is the limit of the sensors board. The drill chart indicates all the different sizes of the holes. There are four corresponding with (top-bottom): resistors, inverters IC and CNY70 sensors, 6 pins header and screw holes.
Sensors Board Oscilloscope Capture
This picture shows the transition from gnd to Vdd of the output of the sensors board.
