Critical Air System Pressure Controller Senior Design Project, 2010 Morgan Hespe, Department of Electrical and Computer Engineering Mentor: Dr. Stanislaw Legowski, Professor of Electrical Engineering

Project Overview Design a printed circuit board controller Controller will operate 2 air compressors to maintain a desired pressure set by the user

Specific Goals/Benefits of Controller Alternates compressor starts to ensure equal run time. On low-pressure start-up, both compressors come on, but starts are staggered to reduce utility demand factor charges. This also reduces facility current inrush and local voltage sags.

Specific Goals/Benefits of Controller Monitors compressor performance. Initiates warning or alarm to notify user of faulty compressor. Alarm is interrupt reset. Controller has user adjustable settings through binary dip-switches mounted on board. Settings prominently displayed. Bang-bang controller reacts to large drop in pressure by bringing on 2 nd compressor.

Hardware Overview ATmega 16: 40-pin PDIP µController 16 K Bytes In-System Programmable Flash Memory I used 1 A/D channel I used 25 of the available 32 input/output pins System clock was set at 1 MHz via internal RC oscillator A/D system clock was set at 1 MHz/8 = 125 kHz (at 13 cycles per conversion: ~ 10k conversions/ sec) Timer system output compare was used to generate alarm/ warning codes

Hardware Overview Pressure Transducer: Typical response time of 100 µ seconds matches A/D setting of the µController Adjustable zero and span from 0 to 150 psi * Analog output voltage 0-10 VDC linearly proportional to pressure 35 mA minimum supply current

Hardware Overview (4) 3-digit 7-segment LED Display : ~8mA forward current Bright red display for industrial environments 12 total digits

Hardware Overview 4-to-16 line decoder/demultiplexer : Takes 4 bit address from µ Controller to de-assert channel corresponding to 7-segment digit of data to be loaded Maximum propagation delay from address input to output de-assertion is ~30 n seconds at VCC=4.5V 74HC154

Hardware Overview User-input settings : (3) 8-bit binary switches “load” user settings into D-type flip-flop latches. Latches are sequentially enabled by the ATmega 16 and read on bused port of ATmega 16 Maximum propagation delay from latch enable to output is ~11 n seconds at VCC=5V and load capacitance of 50 p Farads

Hardware Overview LED drivers : (12) BCD to 7-segment drivers take BCD data from ATmega 16 and “load” data at enable from demultiplexer Minimum latch enable pulse width is ~260 n seconds

Planning

Designing

Prototyping, First Tries

Designing

Results Transducer range was not as expected Propagation timing on bused lines was crucial Heat sink design on voltage regulator a must I learned a lot about IC’s, C-code, µControllers, prototyping, PCB design software, etc.

What I would do for next version: Surface mount components LCD display for settings SD card interface to adjust parameters in code