Engineering Manual o.. - HVAC.Amickracing

ELECTRONIC CONTROL FUNDAMENTALSTYPICAL SYSTEMFigure 1 shows a simple electronic control system with acontroller that regulates supply water temperature by mixingreturn water with water from the boiler. The main temperaturesensor is located in the hot water supply from the valve. Toincrease efficiency and energy savings, the controller resetsthe supply water temperature setpoint as a function of theoutdoor air temperature. The controller analyzes the sensor dataand sends a signal to the valve actuator to regulate the mixtureof hot water to the unit heaters. These components are describedin COMPONENTS.INPUTSMAIN SENSOR(HOT WATER SUPPLY)OUTDOOR AIRSENSORREMOTESETPOINTADJUSTMENTELECTRONICCONTROLLERCONTROLLERRETURNFROMAC SYSTEMPOWERINPUTFINAL CONTROLDEVICEFROMBOILERHOT WATERSUPPLY TOHEATINGSYSTEMFig. 1. Basic Electronic Control System.C3096COMPONENTSAn electronic control system includes sensors, controllers,output devices such as actuators and relays, final controlelements such as valves and dampers, and indicating,interfacing, and accessory devices. Figure 2 provides a systemoverview for many electronic system components.SENSORSA sensing element provides a controller with informationconcerning changing conditions. Analog sensors are used tomonitor continuously changing conditions such as temperatureor pressure. The analog sensor provides the controller with avarying signal such as 0 to 10V. A digital (two-position) sensoris used if the conditions represent a fixed state such as a pumpthat is on or off. The digital sensor provides the controller witha discrete signal such as open or closed contacts.Some electronic sensors use an inherent attribute of theirmaterial (e.g., wire resistance) to provide a signal and can bedirectly connected to the electronic controller. Other sensorsrequire conversion of the sensor signal to a type or level thatcan be used by the electronic controller. For example, a sensorthat detects pressure requires a transducer or transmitter toconvert the pressure signal to a voltage or current signal usableby the electronic controller. Typical sensors used in electroniccontrol systems are included in Figure 2. A sensor-transducerassembly is called a transmitter.TEMPERATURE SENSORSFor electronic control, temperature sensors are classified asfollows:— Resistance Temperature Devices (RTDs) change resistancewith varying temperature. RTDs have a positive temperaturecoefficient (resistance increases with temperature).— Thermistors are solid-state resistance-temperature sensorswith a negative temperature coefficient.— Thermocouples directly generate a voltage as a function oftemperature.Resistance Temperature DevicesIn general, all RTDs have some common attributes andlimitations:— The resistance of RTD elements varies as a function of temperature.Some elements exhibit large resistance changes,linear changes, or both over wide temperature ranges.— The controller must provide some power to the sensor andmeasure the varying voltage across the element to determinethe resistance of the sensor. This action can cause the elementto heat slightly (called self-heating) and can create aninaccuracy in the temperature measurement. By reducingthe supply current or by using elements with higher nominalresistances the self-heating effect can be minimized.122ENGINEERING MANUAL OF AUTOMATIC CONTROL