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Engineering Design

Engineering Design. How to build stuff that works, and make it work better. Dr. Tom Clarke, Second Year Electronics Laboratory Coordinator. Why learn design?. EEE courses teach the theory needed to do real engineering.

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Engineering Design

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  1. Engineering Design How to build stuff that works, and make it work better Dr. Tom Clarke, Second Year Electronics Laboratory Coordinator

  2. Why learn design? • EEE courses teach the theory needed to do real engineering. • Applying that theory happens later in projects, and some coursework. It helps you: • Motivate theoretical work • Become better at solving real problems • Perform better in your year 3 & 4 project work • Have fun on your EEE or ISE course Engineering design in Second Year Electronics Laboratory

  3. What is design? • 1. Specify the problem • 2. Explore top-level decomposition • 3. Discover relevant information from diverse sources • 4. Perform system analysis (where possible) • 5. Make assumptions, prioritise problems • 6. Perform detailed prototype design • 7. Evaluate prototype • test assumptions • identify problems & areas for further work • 8. Improve design Engineering design in Second Year Electronics Laboratory

  4. Design is not linear These activities are inter-dependent and concurrent Top-down Top-level Specify Assumptions Analysis Information Prototype Bottom-up Evaluate Improve Engineering design in Second Year Electronics Laboratory

  5. Putting it all together • Not all elements of design are “open”. • Some problems are closed, with specific constraints that allow only one solution • You will often find these problems in EEE coursework or exam problems • Use analysis to solve closed problems, to simplify the design space • No-one tells you which bits of analysis to do! • Design requires both analysis and creative exploration Engineering design in Second Year Electronics Laboratory

  6. Specify Example • Need to set bias voltage (Vb) input to alphanumeric LCD display module. • From LCD datasheet: • LCD module supply is 5v • Vb >0.5v, Vb < 3v • Circuit must adjust Vb to an unknown correct value in this range • This sets LCD contrast • Ib < 10uA • ILCD = 2mA (typical) Engineering design in Second Year Electronics Laboratory

  7. Top-down Bias Adjust Circuit +5V Bias Adjust Circuit LCD Module Vb GND Engineering design in Second Year Electronics Laboratory

  8. Multiple viewpoints Design ideas • Use voltage regulator IC? • Need to read datasheets to see how to make adjustable over required range • Use Zener diode (last year’s circuit) • Does not help since not variable • Use potential divider (P.D.) with variable resistor • Simplest solution if feasible • Could combine P.D. and Zener for slightly better stability • (probably not worth it) Engineering design in Second Year Electronics Laboratory

  9. Information Variable resistors • Preset resistors have three terminals, with a fixed resistance between the two ends and a slider which can move anywhere between the two ends. PR1 Engineering design in Second Year Electronics Laboratory

  10. This circuit will allow Vb to be adjusted between 0.5v & 2v What values R1, R3, PR2? Detailed design Detailed circuit design using variable resistor +5v R1 Vx 2v PR2 Vb 0.5v Vy R3 GND Engineering design in Second Year Electronics Laboratory

  11. Information Know your resistors • 22k resistor is not 22,000 ohms! • 22k resistor has specified tolerance (1%,2%,5%) • 2% tolerance: 0.98*22,000 < R < 1.02*22,000 • Variable resistors typically have tolerance 10% • Resistors have preferred values: • Fixed resistors available in E24 series and multiples 1,1.1,1.2,1.3,1.5,1.6,1.8,2.0,2.2,2.4,2.7,3.0,3.3,3.6,3.9,4.3,4.7,5.1,5.6,6.2,6.8,7.5,8.2,9.1 • Variable resistors only available: 1, 2, 5 and multiples! • Check catalogues and datasheets • Design for available precision & values Engineering design in Second Year Electronics Laboratory

  12. Analysis Analysis (ohms law) • R3 = 0.5/(2-0.5)PR2 • R1 = (5-2.5)/(2-0.5)PR2 • Choose PR2 first, calculate R3,R1 • How accurate do these ratios need to be? • If Vx>2V, Vy<0.5V the adjustment range includes the required range of 0.5-2V • Precision not required • R3,R1 can be smaller then calculated Engineering design in Second Year Electronics Laboratory

  13. Assumptions Assumptions • PR2 too low => more current used in circuit. • Assume want current as small as possible • PR2 too high => Vb will vary too much with LCD bias current change. • Datasheet does not say how much bias current changes so assume 10uA is possible (worst case, since we know it is < 10uA) • Datasheet does not say how accurate Vb must be: assume 10%. Engineering design in Second Year Electronics Laboratory

  14. More analysis + approximation Analysis • Approximate analysis • Assume Thevenin equivalent resistance at Vy = R3 (actually slightly smaller) • Assume OK at all other voltages if OK at Vy • 50mV > R3.Ib = R3.10uA => R3 < 5k • => total divider current = 1mA • Not too bad, but significant compared with ILCD • R1=50k, PR2=15k, R3=5k Engineering design in Second Year Electronics Laboratory

  15. Detailed design Are values realistic? • Variable resistors are available 10k,20k,50k • 15k not possible • Could scale by 2/3 • R1=33k, PR2=10k, R3=3k3 • These resistor values are all available • This is not good idea. Designing precisely to limits is dangerous. • Reduce R1, R3 by 20% to ensure coverage of entire range even if resistor values vary • R1=27k, PR2=10k, R3=2k7 (use E24 values) • Note that precise values don’t matter • R1=22k, PR2=10k,R3=2k2 would also be fine Engineering design in Second Year Electronics Laboratory

  16. Optimise circuit Optimise • Why bother with R1, R3? • Not really needed, but allows better adjustment • Resolution = minimum change in resistance value that a variable resistor can be adjusted to. • Typically 1%. 1.5V across PR2 =>15mV res • R3 missing => 2V across PR2 => 20mV res • R1 & R3 missing => 5V across PR2 => 50mV res • R3 probably not needed (1.5V -> 2V) • R1 maybe also not needed (1.5V -> 5V) Engineering design in Second Year Electronics Laboratory

  17. Guess which one is recommended in the LCD datasheets? Possible circuits +5v +5v +5v 30k R1 R1 27k 2v 10k Vb 50k Vb Vb PR2 PR2 PR2 20k 3k0 0.5v R3 GND GND GND Engineering design in Second Year Electronics Laboratory

  18. Design in EE2 & ISE2 Laboratory • Design activities during 1st half of each Term • Work through examples of design • Learn skills useful in project work • PCB design • Embedded system design • Prototyping • Measurement • Conducted in laboratory pairs • Assessed individually by demo, interview, & logbook Engineering design in Second Year Electronics Laboratory

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