Proceedings of the Multi-Disciplinary Senior Design Conference Page 2

Project Number: P15311

PRinted Circuit Board Isolation Routing System rev. 2

Devon Monaco
Mechanical Engineering / Joseph Lee
Mechanical Engineering
Yevgeniy Parfilko
Mechanical Engineering / Thomas Bizon
Electrical Engineering / Emily Roberts-Sovie
Industrial Engineering
Kenny Ung
Electrical Engineering / Nathan Faulknor
Electrical Engineering

Proceedings of the Multi-Disciplinary Senior Design Conference Page 2

Abstract

Almost all engineering projects within the Multidisciplinary Senior Design program rely on some form of electronic controls that require a printed circuit board, or PCB. Traditionally, these custom boards are designed by students and sent off for fabrication by an external supplier - resulting in high costs and long lead time. Team P15311 introduces an integrated PCB router that gives students the ability to convert their design to prototype boards with very low cost and a few hours of process time.

The project introduces a standalone system that incorporates a CNC-controlled router, a networked computer equipped with a mounted monitor and keyboard, an equipment drawer, a moveable enclosure, and auxiliary systems for safe operation within a lab environment. The complete system is designed to accelerate and consolidate all steps of the board routing process, and includes user-friendly functions such as debris collection, board vacuum clamping, auto-home positioning, and live video feedback. Project teams that follow the recommended training would be able to learn and use the key features within a matter of hours.

Introduction

Often times, students do not have extensive experience with PCB layout design, but are required to develop and have fabricated a functioning board for the scope of their project. Coupling this inexperience with changing needs of projects, lead times of two or more weeks for board fabrication, and costs in the hundreds of dollars makes the PCB one of the most challenging aspects of an engineering project. The main objective of the P15311 team is to provide RIT students with the resource of a system that can reliably route the traces and through-holes needed for their one or two sided board layouts, while keeping cost and lead time low enough to allow for multiple design trials.

This revision of the project is a continuation of the P14311 senior design group [1] which was able to produce a base routing system composed of the trial Mach3 CNC software package, a Bosch hand router, a stepper motor gantry, and a first pass vacuum system. The previous revision made strides in the system power, controls, and basic software interfacing aspects of the project, but was unable to achieve features such as accurate board locating, board securing, video feedback, debris collection, acceptable safety limits, and a simplified user experience. These were areas that P15311 was able to improve on to create a complete and usable system.

Overview of the Board Fabrication Process

Board routing is a multistep process that requires a design, fabrication, and testing skillset. The layout of isolation traces must be designed in a CAD format, and converted to a CNC tool path. The tool path is uploaded to a microcontroller which moves a spindle and on a 3-axis gantry. The routing bit, which may be used for milling, drilling or rubout operations, traces out the geometry layer by layer, which may require tool changes in between process steps. Once the board is complete, it must be inspected for trace quality and electrical continuity.

Design Process

The first step that the team took in the design process was to understand the state of the machine as it was left by the previous MSD team. The team debugged and troubleshot the machine until it was in a partially working state and was able to make determinations on which subsystems could be left alone, which needed to be modified, which needed to be redesigned altogether, and what needed to be added as new functions.

Once the important design targets were indicated, the team was able to refine the customer requirements to set the scope of the project. Many of the team’s customer requirements were taken from the requirements of the previous team, as well as re-interviewing the customer and surveying RIT students and faculty. The focus of these requirements were to meet any requirements not met by the previous team and to add some additional desired features that were identified in the trial runs.

Figure 1. Customer Requirements Chart

Engineering requirements were pulled from the shortcomings of the previous team and also generated based on additional customer requirements uncovered. These requirements guided the design and test parameters of the team’s subsystems.

The team’s major constraints were identified as physical footprint, as the previous team already purchased a set enclosure; limitations with software, as affordable, standalone, and commercially available CNC programs were very few in number; and budget, which had originally been listed as $500. The team was able to renegotiate a final budget of $2000 in MSD I. Some assumptions that were made by the team in the design process are that the user already has their board layout designed, the user is inexperienced with this type of machine, and that the machine will be overseen and kept up by the lab manager, Jeff Lonneville.

Several brainstorming sessions were held in order to come up with solutions to the major design initiatives and evaluate those solutions. The team used a morphological chart in order to organize concepts in a coherent visual manner, and promote creativity. This allowed the team to easily come up with new ideas and share them with each other. The team used Pugh charts to evaluate the concepts drawn up in the morphological chart in regards to the debris removal system, board security, and auto-homing.

In order to analyze the system designs, each engineering specification was matched to a sub-system. Preliminary calculations were performed to determine theoretical holding force by the vacuum table, flow through the debris inlet nozzle, mechanics in the monitor mounts, and tool cutting speed and feed. Several tests were designed to ensure that the sub-systems met the desired requirement and then, pending successful trials, system level tests were designed as well. A more detailed description of these sub-systems and associated tests are included further in this paper and on the team’s edge website.

Redesigned Subsystems

Vacuum Table and Debris Collection

In order for the routing system to be viable, the two critical subsystems of debris collection and board security required major redesigns. These two systems were fighting opposing battles requiring both high flow and a high pressure seal with only a single vacuum. Combined with poor joints, poor sealing, and poor diameter reductions, this approach rendered both subsystems ineffective.

The CFM produced by the Shop-Vac would be sufficient for debris collection alone, as long as the path from the vacuum inlet to the debris collection site was streamlined. In order to achieve this, the maximum pipe size that would fit between the spindle collet and board was chosen. A smooth walled vacuum tube was also ordered to replace the original corrugated vacuum tube to decrease friction at the walls. Finally, a smooth 45 degree diameter reducer was designed and 3-D printed to minimize the losses due to the change in standard 1¼” diameter vacuum hose to the smooth walled ⅝” diameter hose. Testing was done using an anemometer attached at each joining area in the system to determine where the most losses were being incurred.

Figure 2. Shop-Vac AllAround Figure 3. Flow Reducer Design CAD Cross-Section

In order to handle the board security issue, the team redesigned a vacuum table that consisted of five layers with some carryover from the previous group: an aluminum basin attaching to the gantry, a gasket layer, an aluminum cover with an array of holes, a rubber sacrificial material with the same array of holes, and an aluminum clamping layer for the sacrificial material and board alignment. The original basin was modified to receive a ¼” hose barb which was attached to a vacuum pump. The team decided on a vacuum pump, as they do not output as much flow, but are capable of achieving a much higher pressure seal. The initial “MicroPump” vacuum pump selected met the target of providing 10 lb of holding force based on the specification sheet. However, upon initial testing, the pumps extremely low flow output proved to be insufficient for the imperfect sealing in the subsystem. The final pump selected is capable of higher flow to account for minor leaks and enough sealing pressure to both secure the board against the translational forces exerted by the cutting tool and maintain its flatness.

Fig 4. Parker “MicroPump” Fig 5. Medo Vacuum Pump Fig 6. Final Vacuum Table Design CAD Image

Overtravel Switches

Preventative measures are especially important on a machine designed with casual users in mind; as such, improved means of motor protection were devised. The original reed sensor system was improved upon with physically-triggered overtravel switches at the end of each axis.These sealed, dust-proof switches proved to be considerably more compact than their predecessors, while offering the reliability of a physical-contact switch generally preferred in industrial applications for reliability and repeatable performance. A pair of switches at opposite ends of each axis switch a 5V signal to a corresponding LED on the user control panel if/when depressed, while triggering an input into Mach3 to stop the machine and report an overtravel error to the user, thus preventing accidental damage to the stepper motor/lead screw assemblies.

Power Wiring

The machine is powered with 120 VAC. Incoming power is fed into a power switch that controls the power for the entire system. Following this switch, the power is distributed through a terminal block which feeds direct power to the computer through the power outlet and the emergency stop button (E-Stop). From the E-Stop, a distribution terminal block feeds power directly to the two AC/DC power supplies and indirectly to the spindle power supply and two control outlets through the safety relays.

The two AC/DC power supplies are mounted in the power panel located outside the enclosure. The first of these AC/DC supplies is a dual output channel supply with both +5V and +12V outputs. The UC300 controller board utilizes the +5V output on this supply. The I/O board controls, the control panel, and the LED lighting inside the cabinet utilize the +12V output. In addition, there is a distribution block for both of the channel outputs fed through the switches in the user panel, which control two of the four safety relays. The third safety relay is a solid state relay controlled by the UC300. This relay controls the coil of the fourth safety relay which gives the +12V power to the spindle and +24V power to the stepper motor drivers and enclosure fan.

I/O Board and Electrical Components

The PCB for the I/O Board is used to handle all the connections and logic from every subsystem to the UC300 microcontroller and Mach3. The design of the board was modified to include the necessary components for the new subsystems designed by the P15311 team, as well as to remove the unused subsystems by P14311.

The I/O board was designed in Eagle 7.4.0 Lite, utilizing the software’s excellent libraries. A limit on maximum board size was imposed due to using the free version of the software. The final design was a 4.3” x 3.4” double-sided board, with 36 surface mount resistors, 4 surface mount transistors, 37 through-hole components, 4 mounting holes, and a wire jumper that was needed to fit all the traces on the top side.

A second PCB board was designed to accommodate additional functionality and fix some of the mistakes made in the original I/O Board. This second board handles the logic and connections for the auto-homing photomicrosensors, as well as the overtravel limit switches. This board was designed as an isolation trace board and fabricated by the P15311 team on the actual PCB Isolation Router being discussed in this paper.

Figure 7: PCB Layout for I/O Board Figure 8. Secondary I/O Board Routed by Team

Monitor Mounts

In order for the stepper motors to control the gantry system the motors must receive commands from the computer. The previous team had the computer in the enclosure, but had the monitor, keyboard and mouse on a table next to the enclosure. To simplify the system the P15311 team attached two monitors (one for the CNC software window and one for the video feedback window), the keyboard, and the mouse to the enclosure. To do this, two monitor mounts were purchased, which were then mounted to the enclosure. However, with the weight of the monitors it caused the walls of the enclosure to flex, so the team also added a steel reinforcement bracket to reduce the deformation. Next the team created a piece that would connect the two monitors so they would move together. This same piece has a bend in it to provide a shelf for the keyboard and mouse. Ergonomic pads were then added to make the setup more comfortable for a user in a seated position.

Figure 8 (right): Monitor Mounts CAD Image

Modified subsystems

Video Feedback

To assist the operator, a camera was attached to the spindle plate, which allows the operator a close up view of what the bit is cutting. It is also beneficial for determining if a bit is broken. The video is displayed on the top monitor seen in Figure 8.