Kevin M Muthuri
Group 10
Printed Circuit Board Layout Design
Introduction
The purpose of this project is to design an automated robotic waitress that is capable of traveling through given set points in order to serve food to various tables in some sort of restaurant. Proximity sensors as well as tilt sensors are used to detect any objects blocking the path of the robot or to indicate that the “waitress” has tipped over. Another sensor on the tray checks if the tray has been emptied and is ready to go back to the control center. The robot will communicate wirelessly through an RF interface to a control center to acquire the given path to travel, signal alerts, and also send its current status. By counting the number of rotations of the motor, a reasonable estimate of the robotic waitress’ position can be calculated. With such information, the control center has the ability to re-route the path of the robotic waitress through an algorithm if needed. An intuitive user interface will be provided on the control center computer. Such software will allow the user to check on the current status and position of the robot, load and re-load a new map for the restaurant, and also call the robotic waitress back to the control center at any point in time.
PCB Layout design consideration
There will be two main boards implemented. One such board (main board) will be mounted under the food tray right at the central section of the robot chassis while the other will be located at the control center. Due to the size of the robot, the main board size must be limited to a size of about is 4 inches by 3 inches if possible. Several of the components on the main board didn’t have suitable footprints in the existing libraries and therefore requiredcreating footprints for them. These components included the RF transmitters and receivers, the RF antennas and the crystal oscillator. Apart from the RF components, most of the other parts didn’t present many constraints when doing the layout. First, the RF transmitter and receiver had to be place on opposite ends of the board so as to reduce interference. No conductive items could be placed within 0.15inches of the receiver module’s top or sides.Also, the receiver module had to be kept away from interference sources such as high speed logic, switching power supplies and oscillators. The connection between the transmitter and receivermodules to their antenna required a trace that ran for less than 0.25" but longer distances could be covered using 50Ω coax or a 50Ω microstrip transmission line. As for the antennas, they required to be fed with traces less than or equal to 0.25". Nothing could be placed under or directly beside the antenna element. Also, the area under the antennas on all board layers had to be completely free of components, traces, or groundplanes. In addition, no components or traces could pass within .25" of the top, sides, front or back of the antennas. All the RF components required a groundplane under the modules to connect the appropriate pins using vias. As a result of all these constraints, a lot of space was taken up on the board to meet the requirements hence making the layout even more challenging.
As for the control center board; size was not much of a factor although the same constraints as above had to be taken into account with regard to the RF components. For both boards, the power and ground traces are 40mils while the other traces are all 12mils wide. Care is also taken to ensure that by-pass capacitors are close to their respective chips.
References
Motorolla AN1259
LinxTechnologies TXM-315/418-LC-S Datasheet
LinxTechnologies RXM-315/418-LC-S Datasheet
LinxTechnologies “SPLATCH” PLANAR ANTENNA Datasheet