Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSApplying the fluid jet principle allows the measurement ofvery small changes in air velocity that a differential pressuresensor cannot detect. A jet of air is emitted from a small tubeperpendicular to the flow of the air stream to be measured. Theimpact of the jet on a collector tube a short distance away causesa positive pressure in the collector. An increase in velocity ofthe air stream perpendicular to the jet deflects the jet anddecreases pressure in the collector. The change in pressure islinearly proportional to the change in air stream velocity.Another form of air velocity sensor uses a microelectroniccircuit with a heated resistance element on a microchip as theprimary velocity sensing element. Comparing the resistance ofthis element to the resistance of an unheated element indicatesthe velocity of the air flowing across it.PROOF-OF-OPERATION SENSORSProof-of-operation sensors are often required for equipmentsafety interlocks, to verify command execution, or to monitorfan and pump operation status when a central monitoring andmanagement system is provided. Current-sensing relays,provided with current transformers around the power lines tothe fan or pump motor, are frequently used for proof-ofoperationinputs. The contact closure threshold should be sethigh enough for the relay to drop out if the load is lost (brokenbelt or coupling) but not so low that it drops out on a lowoperational load.Current-sensing relays are reliable, require less maintenance,and cost less to install than mechanical duct and pipe devices.TRANSDUCERSTransducers convert (change) sensor inputs and controlleroutputs from one analog form to another, more usable, analogform. A voltage-to-pneumatic transducer, for example, convertsa controller variable voltage input, such as 2 to 10 volts, to alinear variable pneumatic output, such as 20 to 100 kPa. Thepneumatic output can be used to position devices such as apneumatic valve or damper actuator. A pressure-to-voltagetransducer converts a pneumatic sensor value, such as 15 to100 kPa, to a voltage value, such as 2 to 10 volts, that isacceptable to an electronic or digital controller.CONTROLLERSControllers receive inputs from sensors. The controllercompares the input signal with the desired condition, or setpoint,and generates an output signal to operate a controlled device.A sensor may be integral to the controller (e.g., a thermostat)or some distance from the controller.Controllers may be electric/electronic, microprocessor, orpneumatic. An electric/electronic controller provides twoposition,floating, or modulating control and may use amechanical sensor input such as a bimetal or an electric inputsuch as a resistance element or thermocouple. A microprocessorcontroller uses digital logic to compare input signals with thedesired result and computes an output signal using equationsor algorithms programmed into the controller. Microprocessorcontroller inputs can be analog or on/off signals representingsensed variables. Output signals may be on/off, analog, orpulsed. A pneumatic controller receives input signals from apneumatic sensor and outputs a modulating pneumatic signal.ACTUATORSAn actuator is a device that converts electric or pneumaticenergy into a rotary or linear action. An actuator creates a changein the controlled variable by operating a variety of final controldevices such as valves and dampers.In general, pneumatic actuators provide proportioning ormodulating action, which means they can hold any position intheir stroke as a function of the pressure of the air delivered tothem. Two-position or on/off action requires relays to switchfrom zero air pressure to full air pressure to the actuator.Electric control actuators are two-position, floating, orproportional (refer to CONTROL MODES). Electronicactuators are proportional electric control actuators that requirean electronic input. Electric actuators are bidirectional, whichmeans they rotate one way to open the valve or damper, andthe other way to close the valve or damper. Some electricactuators require power for each direction of travel. Pneumaticand some electric actuators are powered in one direction andstore energy in a spring for return travel.Figure 54 shows a pneumatic actuator controlling a valve. Asair pressure in the actuator chamber increases, the downward force(F1) increases, overcoming the spring compression force (F2),and forcing the diaphragm downward. The downward movementof the diaphragm starts to close the valve. The valve thus reducesthe flow in some proportion to the air pressure applied by theactuator. The valve in Figure 54 is fully open with zero air pressureand the assembly is therefore normally open.ENGINEERING MANUAL OF AUTOMATIC CONTROL33