GB1020560A - Improvements in or relating to control apparatus - Google Patents

Abstract

1,020,560. Automatic control of environment. INSTRUMENTATION SPECIALITIES CO. July 29, 1964, No. 30108/64. Heading G3R. In a system for controlling a condition the levels of two variable factors are sensed to provide signals when the levels are above or below preselected levels, means being provided to convert the signals into discrete outputs voltages in correspondence with the directional deviation of the variable factors which are employed to actuate one or more control elements to effect a change in level of the interacting variable factors. The temperature and humidity of an environmental chamber are controlled by a system responsive to the outputs of a wet and a dry thermistor temperature sensing devices. The air in the chamber is circulated by fans over heating and cooling coils and across a water-bath, varying the humidity, containing heating and cooling coils. The control system, as shown, comprises bridges 30, 31 having arms adjustable 33, 35 to desired dry and wet temperatures and arms 23, 24 comprising wet and dry bulb thermistors respectively. The D.C. signals from the bridges are fed to amplifiers 37, 38 having alternating outputs of positive phase when the temperatures are high and of negative phase when the temperatures are low. The outputs are combined by a subtractive resistance arrangement 41 to provide a humidity dependent output of positive phase when the air is too dry and of negative phase when the air is too humid. The outputs of amplifier 37 and arrangement 41 are fed to interconnected triodes 103, 104 and 105, 106 respectively which conduct for the temperature or humidity criteria indicated to selectively actuate relays 44-47. The contacts of the relays determine the inputs to a diode matrix 48 determining the combination of control elements 20, 12,18, 16a, 16b to be energized. The low and high water heating rates are determined by energizing a heater intermittently or continuously through a motorised switched arrangement see Fig. 4, not shown.