CAD for Biomedical Engineering Technology

Abstract

Purpose: give the reader a concise summary of the entire report such that the reader can get an idea of the report without actually having to read it in detail. It is a complete overview of the entire project.

Questions it answers: what was done, why was it done, how was it done, what were the results, what are the recommendations.

Details:

• 3 to 5 paragraphs
• One half to three-quarter pages in length.

• No illustrations

Write this section last, when the rest of the report is finished.

1. Introduction

The introduction to a report answers three questions: What? So what? Now what?

1.1 The Power Supply Project

Explain what this project set out to achieve, what is the context (course, program of studies), starting point, what the project achieved in the end.

1.2 Purpose of a Power Supply

Explain what a power supply is and does, and why it is important in electronic systems.

Some questions this subsection could answer: What does it do? What would it be used for? What could you do with it? What would you not be able to do if you didn’t have one?

1.3 Electrical Safety

Explain the electrical safety aspects of the power supply: earth ground, isolation, leakage currents, fuses, switches. Not too much detail for now. The details are to be explained in section 2.6.

1.4 Objectives of the Project

Clear statements of what this work is all about. Each objective/goal should be brief and clear, but it should begin with an overall statement.

A statement that embodies the project is required, to be followed by specific goals, listed below.

The main objectives of this project are:

1. ,
2. , and
3. .

(One set of design objectives should refer to the voltage and max. current output capability of the power supply, e.g. +15V @1A )

1.5 Layout of Report

The report is divided into six sections. Section 1 (this section) contains introductory material. Section 2 covers the design… Section 3 is... Finally, Section 7 concludes the work and suggests areas for future investigations.

List all the major sections briefly.

2 Design and Theory of Operation

A brief introduction of the contents of this section is needed here.

This section is where the reader would find out how the power supply works. The level of technical writing should be aimed at someone with your level of knowledge of electronics (they are assumed to have taken all the courses that you have), but are not familiar with the operation of this specific power supply design.

2.1 Overall Design

Describe briefly the initial schematic that was provided at the start of the course and explain that the power supply consists of two main sections: power and supervisory. Describe the option that you added, either as a separate subsystem (measuring current, temperature), or as in the power section (high current, variable output). Describe the functionality of each section briefly – what each section does but without details on how exactly it works (that’s for the detailed sections). Explain that the schematic was first tested a circuit simulation program to describe the theory of operation, and that it will become the starting point for the circuit board layout. Schematics should be easy to read, captioned, referenced in the text and relevant to the discussion (Figure 1, Figure 2).

Suggested illustration: block diagram of the complete power supply system, power and supervisory.

Refer to the appendix for the complete schematic. The complete schematic should be printed on B-size paper (17x11 landscape) and engineering folded (Figure 3) such that the folded page fits within the A-size report (8 ½ x 11), with the title block clearly visible. Place the schematic in the appendix.

2.2 Schematic Capture

Steps to enter the schematic: choose components, place them, connect them with wires, name some of the nets? Verify the connections? What components on the schematic are going on the PCB vs. those that are in the case or on the rear panel? Explain about the checkboxes to exclude components from simulation, PCB or parts list.

Suggest illustrations/screenshots: choosing a component from the library, wiring a component.

2.3 Components and Libraries

This is where the libraries of components are described. What goes into a component: schematic symbol, simulation model and PCB package (footprint)? How are those components organized? Suggested illustrations/screenshots: Compare the schematic symbol with the PCB package for a typical component. What work-around do you need if you want to use an animated model but also need that component to have a PCB package?

2.4 System Simulation Software

Discuss briefly the software that will be used to perform the simulations (Proteus VSM). Describe what it does and what changes you must make to the circuit schematic to enable the simulation. Describe briefly that it is able to simulate the microcontroller firmware on the Arduino.

Suggested illustration: screenshot of Proteus VSM in use.

2.5 Primary Circuit and Transformer

Describe the primary circuit: line cord, fuse, switch, transformer. Describe the power entry module. Explain how a certified power entry module is required in IEC 60601 and what it means for patient safety. Show calculations and explain what they mean.

Questions this section should answer: what is the power input (voltage, frequency)? What is the purpose of the fuses? Why is there a switch and a fuse in each leg (Line, Neutral) of the power input? What is the theoretical maximum power delivered to the secondary side (limited by fuse rating)? What type of transformer is it? What is the turns ratio? What is the VA rating of the transformer? What is the secondary voltage?

Suggested illustrations:

• Your schematic (Proteus) of the primary circuit;
• Proteus oscilloscope traces of the 120VAC primary and the secondary waveforms.

2.6 Safety Considerations in the Primary Circuit and Transformer

Describe the safety considerations in the primary circuit: what are the hazards and how are they addressed. Mention also the equipotential lug.

2.7 Rectifiers and Filters

Describe and discuss the part of the circuit that performs the rectification function (both positive and negative) and the DC filtering. Show and explain any calculations.

Suggested illustrations:

• Your schematic (Proteus) of the rectifier and filter block (positive and negative)
• Simplified schematic of current flow in the rectifiers and load during each half-cycle (see the section on rectifiers in your CIRE textbook).
• Proteus oscilloscope traces of the full wave rectified waveforms (positive and negative) compared to the secondary AC waveform;
• Proteus oscilloscope traces of the remaining ripple after filtering (add a small load to the circuit to accentuate the ripple).

2.8 Fixed +5V Regulator

Describe and discuss the part of the circuit that regulates the fixed +5V output. Show and explain any calculations. See the linear regulator section in your CIRE textbook.

Questions: What integrated circuit type is being used? How does this IC work? What are the specifications (input voltage, output voltage +- tolerance, output current)? What is the power dissipation? What additional components are required (diodes, capacitors, etc)?

Suggested illustrations:

• Your schematic (Proteus) of the +5V section
• Comparing the input DC voltage and ripple to the regulated and filtered output (use same vertical scale, add a 10Ω load to accentuate the input ripple)

2.9 +15V Regulator

Describe and discuss the part of the circuit that regulates the +15V output. Use terminology and specifications from the datasheet, e.g. Texas Instruments Inc. Show and explain any calculations. Suggestion: Work through and illustrate a sample calculation of the power dissipation in the regulator at several different load currents, e.g. 20mA, 100mA, 500mA.

Questions: What integrated circuit is being used? How does this IC work? What are the specifications (input voltage, output voltage +- tolerance, output current)? What is the power dissipation (at different load currents and input voltages)? What additional components are required (diodes, capacitors, etc)? What are the built-in protection features of the IC (current limit, SOA)?

• If you chose the high current option, this is where you would describe the parallel power transistor etc.
• If you chose the variable voltage regulator option, describe the circuit as suggested, and explain how the voltage is adjusted.

Suggested illustrations:

• Your schematic (Proteus) of the +15V section
• Simplified schematic to accompany the sample calculation of the power dissipation.

2.10 –15V Regulator

Describe and discuss the part of the circuit that regulates the –15V output.

Questions: What integrated circuit is being used? How does the regulator work? What are the specifications (input voltage, output voltage +- tolerance, output current) of the regulator? What is the power dissipation (at different load currents and input voltages)? What additional components are required (diodes, capacitors, etc) for the regulator?

• If you chose the high current option, this is where you would describe the parallel power transistor etc.
• If you chose the variable voltage regulator option, describe the circuit as suggested, and explain how the voltage is adjusted.

Suggested illustrations:

• Your schematic (Proteus) of the –15V section.

2.11 Voltage Monitor

Describe and discuss the part of the circuit that conditions the +15V and –15V outputs to reduce them to a range of 0—5V to be measured by the ADC inputs of the microcontroller. Explain why the 15V levels cannot be used directly. What topology of op-amp amplifier is used for each? What are the A_V voltage gain (reduction) values? Use the op-amp worksheet that was handed out week 3.

2.12 Arduino Microcontroller

Describe and discuss the Arduino Pro Micro added to the power supply. What type of microcontroller does it use? What is the supply voltage? What is the clock rate of the CPU? What are the input pins (analog, digital)? What do you get from an analog input pin when you read it? What do you get from a digital I/O pin when you read it? What outputs are being used? What is the purpose of the LED on I/O pin 10? What is the purpose of the momentary contact button?

2.13 LCD Module

Describe and discuss the LCD display module: what it is (size, type of LCD), how it is connected to the main board, how it is powered, how the contrast is adjusted, how it is backlit. Do not discuss the programming in this section, only mention that the display contents and the programming will be discussed in section 4.

2.14 Option Subsystem

Describe and discuss the option that you added (the high current and variable output voltage options are described in the positive and negative regulator sections 2.9 and 2.10). Describe the IC(s) used and how they work: how the information provided by the option is delivered to the microcontroller; what special considerations (layout, placement, etc.) the option required; etc.

3 Printed Circuit Board

A short introduction to this chapter should identify the major subsections and what they contain. This is where the PCB design and fabrication process is described.

3.1 PCB Design Software (Proteus ARES)

Describe the features of the Proteus ARES design suite, how it is used, etc. How does it differ from a mechanical CAD package like AutoCAD? Is it freeware/payware? How does the Proteus ISIS schematic capture relate to the PCB design? What tool is there to help with placement of parts (explain the function of the “ratsnest” wires and force vectors)? What tool is there to check the board for overlaps and clearance violations (explain the DRC – design rule checking)? What tool is there to output artwork for PCB fabrication (explain the Generate Gerber/Excellon function)?

Suggested illustrations/screenshots: typical schematic capture window, typical board layout window.

3.2 Printed Circuit Board Design

Steps to design the PCB: placement of parts, routing of wires, etc. What were the mechanical constraints (board size, heatsink placement)? What did you have to do special for the regulators (and why)? What were the design rules? How are the two copper layers used to route tracks? What is a ground plane, what does it do and how is it achieved?

Suggested illustrations/screenshots: components, ratsnest wires, routed tracks, vias, regulators on the heatsink.

3.3 CAD/CAM Output

Brief section on Computer Aided Manufacturing (CAM) output and generating Gerber & Excellon files. What are the layers? Why only the copper layers (i.e. why not solder masks and silkscreen)? What is a Gerber file and what does it describe? What is an Excellon file and what does it describe? Why not just send the Proteus project file as-is to the board manufacturer?

Suggested illustrations/screenshots: dialog box, Gerber viewer.

3.4 Printed Circuit Board Production

Brief description of how the project PCBs were panelized (graphicode.com GC-PrevuePlus) to be processed together. Brief description of each stage of the PCB fabrication from plotting the phototool to drilling, plating and V-scoring the panel.

Suggested illustration: Photos of some of the steps from the Web. YOU MUST REFERENCE YOUR ILLUSTRATIONS e.g. (Eurocircuits, 2015).

4 System Firmware

A short introduction to the design, coding and debugging of the system firmware loaded into the EPROM of the Arduino microcontroller.

Do NOT copy the entire firmware source code file into the document. Source code formatting gets botched by Word’s document oriented formatting. Only quote short snippets of code, as needed to explain the firmware operation. Use a fixed font (e.g. Courier, Typewriter), remove spacing between lines, and preserve indenting. For the listing below (Listing 1), I used a text box (placed in line with text), inserted the snippet and added a caption at the top. There is a way of showing line numbers but it requires the use of tables instead of a simple text box.

Listing 1: Scaling the analog input and displaying the calculated voltage

sp15 = analogRead(pinp15);
lcd.write("V+:  ");
lcd.print((float)sp15*P15_SCALE);

When discussing a particular function like analogRead() or a variable like sp15, use the same font as your code snippets in-line with the rest of your sentence. The empty brackets on a function name are the usual way to show that the name refers to a function instead of a variable.

4.1 Firmware Functionality

Describe the functions of the firmware. What it measures, what is displayed (each item of information). This is where the ADC particulars can be explained in detail, i.e. you read a value from 0—1023 corresponding to 0—5V, then what? How do you work backward to the original 15V output (or the temperature, current, etc.)?

4.2 Firmware Development Environment

Describe the Proteus source code development environment, the Arduino IDE, the Windows drivers that need to be installed to program the Pro Micro from Windows or Mac OS X. What is the programming language used? How does the Arduino IDE differ from a full featured programming IDE?

4.3 Firmware Detailed Design

Describe each function, e.g. setup() in your code. Libraries and library functions only need to be discussed briefly, and only in terms of what they do for your program.

5 Power Supply Enclosure

A short introduction to this chapter should identify the major subsections and what they contain. This describes the enclosure design and fabrication.

1.1 Enclosure Design

Describe the enclosure. It consists of an off-the-shelf metal case that is modified for the project. Describe the package of drawings and templates received.

Refer to the appendix for the design drawing.

1.2 Enclosure Fabrication

Describe briefly the work that was performed to cut out the openings.

Illustrations: photos of some of the stages, photos of the completed fabrication before assembly of components onto the front and rear panels (phone cameras are adequate).

6 Final Assembly and Testing

A short introduction to this chapter should identify the major subsections and what they contain. This is where the troubleshooting and assembly are described, including problems found and how they were resolved.

6.1 Circuit Board Assembly and Testing

Overall, this section answers the question: does it work?

Describe the steps to assemble and test each section of the power supply. This is where the design is checked against the specifications and design objectives (e.g. 5V±4%? 1A current? No ripple? etc.) are verified with the proper instrumentation (voltmeter, oscilloscope). For each objective that was stated in the introduction and theory sections, there should be a verification.

Test the firmware functionality: does the LCD display the correct information (voltage, current, temperature, or fault condition)

You may create sub-sections for each section of the PSU to match the subsections in Section 2 (use styles “Heading 3” and “Heading 4” as needed).

You may also create sub-sections for significant problems (e.g. short between negative supply and GND at the heatsink) that caused major repair work.

Illustrations:

• Screenshots of the primary AC waveform, secondary AC waveform, full wave rectification (without filter capacitors), DC with ripple;
• Photos of the various stages of assembly (phone cameras are adequate).

6.2 Enclosure Assembly

Describe how the populated circuit board and the components (transformer, power entry module, equipotential lug, etc.) fit into the enclosure and onto the front and rear panels.

Illustrations:

• Photos of the various stages of assembly (phone cameras are adequate);
• Multiple views of the completed enclosure

6.3 Complete Assembly Testing

Describe how the completed power supply is tested (switch, outputs, LCD display). The results can be summarized into a table of expected vs. measured values, or a checklist of features to be tested and the test result for each item.

7 Results and Conclusions

7.1 Summary of Results

Summarize the results of the design, fabrication and testing of the power supply – what went well and what did not. Were all the objectives achieved? Which ones were missed? Which ones were surpassed?

This should be half to a full page. One paragraph per idea – avoid glomming everything into a monster paragraph.

7.2 Recommendations

Suggestions are made for improvements in the conduct of the project, the design of the power supply, the operation of the CAD tools, the PCB fabrication process. Recommendations should be practical and based on your actual experience – in other words, something that you did, or that happened to you while working on the power supply should be the basis of the recommendation. Write them in point form or a short paragraph for each.

References/Bibliography

Proper references for the citations in the text. APA style preferred but any widely used system accepted (Chicago, IEEE, etc.). You should reference at least the CIRE textbook, the IC datasheets and some of the course material. Other possible sources: the ITL slides, the Robertson PCB book, online resources for Proteus, etc. Citations should be found throughout the text wherever appropriate.

Floyd, T. L. (2010). Principles of Electric Circuits: Conventional Current Version (9th ed.). Upper Saddle River, New Jersey: Prentice Hall.

Mehta, V. K., & Mehta, R. (2008). Principles of Electronics (11th ed.). New Delhi: S. Chand Publishing.

Robertson, C. T. (2004). Printed Circuit Board: Designer’s Reference Basics. Upper Saddle River, NJ: Prentice Hall.

Texas Instruments Inc. (2013, February). LM317: 3-Terminal Adjustable Regulator. Texas Instruments Inc.

Appendices

Appendix A: Complete Schematic

Place the schematics here.

If they fit on A-size paper (8 ½ x 11), they should be in landscape format to fill the page. If they are larger, print them on B-size paper (17x11), engineering folded. There should be a 1 or 2 line caption to explain the illustration to

Appendix B: Printed Circuit Board Design

Place the following illustrations here. They should be in default size (1:1). Each image should have a short caption (1 or 2 lines) to explain the illustration.

• Screenshot of PCB design: hide the grid and set the background to white to improve visibility.
• Image of the PCB bottom copper layer (export image from Proteus).
• Image of the PCB top copper layer
• Image of the PCB silkscreen layer to show component placement
• AutoCAD drawing of the PCB & Heatsink assembly

Appendix C: Power Supply Enclosure Drawing

Place the AutoCAD drawings for the case here (available on DC Connect as PDFs). They should be in default size (1:1). Each image should have a short caption (1 or 2 lines) to explain the illustration. Fit the image in landscape format to fill the page.