Design And Implementation Of An Environmental Chamber Digital Control System

Abstract

This paper presents the design and implementat i o n (using a T I 9900 microcomputer system) of digi t a l control systems f o r an environmental chamber u t i l i z e d i n the study of plant growth. Two control systems were implemented -one t o control dry-bulb temperature and the other t o control t h e carbon dioxide content i n the chamber atmosphere. I n add i t i o n , real-time data a c q u i s i t i o n , automatic r es t a r t following power outages, and an i n t e r a c t i v e operating system have been achieved.The CO system i s a closed-loop modified proportional control system. The closed-loop tempera t u r e control system uti1 ized a proportional-plusintegral-plus-derivative d i g i t a l c o n t r o l l e r . The environmental chamber i s presently used i n research by t h e USDA a t A u b u r n University. 2 Thi s paper presents the design and imp1 ementat i o n of a d i g i t a l control system f o r an environmental chamber designed f o r the study of plant growth [ l ] . Two control systems were implementedone t o control dry-bulb temperature and the other t o control t h e carbon dioxide (CO2) content in t h e chamber atmosphere.The chamber i s constructed of plexiglass with measurerrients of approximately 0.7 x 1 . 3 x 2 meters.The chamber system i s hermetically sealed such t h a t both moisture content and carbon dioxide content can be accurately monitored.The hardware i n t h e temperature control system will be described f i r s t . The chamber i s cooled ( a i r conditioned) t o t h e extent t h a t heaters a r e required t o maintain the desired temperature. T h u s t h e heaters a r e the c o n t r o l l i n g elements in t h e closed-loop system. A temperature sensor t h a t has an output in t h e m i l l i v o l t range i s used. The temperature signal i s amplified and applied, t h r o u g h an A/D converter, t o a Texas Instruments TMS 9900 microcomputer system. The measured temperature i s then subtracted from the desired temperature, which i s stored i n the microcomputer. Next t h i s e r r o r signal i s processed by the TMS 9900 ( t h e system compensation), r e s u l t i n g in an output signal t o t h e heater i n t e r f a c e . The heater i n t e r f a c e i s a complex logic c i r c u i t [ l ] which converts the computer outp u t signal i n t o t r i a c s [ Z ] control pulses. The t r i a c s control the e l e c t r i c a l energy i n t o the heate r s by i n e f f e c t c o n t r o l l i n g the rms voltage applied to the heaters. The hardware description of t h e CO2 control system i s given next. The CO2 content of the chamber atmosphere i s measured by the gas analyzer i n p a r t per million (PPM). This signal i s compared t o t h e desired s e t p o i n t , and i f the e r r o r i s negative (a CO2 d e f i c i t ) , the computer opens a solenoid valve from a CO2 supply f o r a length of time depend e n t on che-error magnitude. The control system has no c a p a b i l i t y t o remove excess CO2. Because of the time lag required f o r the gas a n a l y s i s , ...