Document 44j5KOEx58yKJ9JDz8dQGpaxQ

172 CHAPTB? 13 1965 Guide And Data Book TYPES OF CONTROL ACTION supply to a coil, to regulate the air temperature leaving the Control action may be divided into five types: coil. This control would be similar to that shown in Fig. l Thratting range is the total amount of change in the 1. Two-position action, Fig. 2, is the type in which the , 1 controlled variable required for the controller to move the coq. controlled device can be positioned only to a rntrimnin or trolled device through its complete stroke, from one extreme minimum position, or can be either on or off. A thermostat to the other. It is often adjustable to meet job requirements. which opens and closes a valve or startst ana stops aburaer is an example. <'-' :\ } } Controller differential, applying to two-position control action, is the difference between a setting at which the controller operatesto onejposition, and a eetting.at which it operates to the... .- Set point1 is the value -at which the controller is set and represents the desired value of the controlled variable. Control point is the actual value of the controlled variable at which the instrument is controlling. It will vary within the otter position. Tb^differential usually referred to in a'therinpstat rating a that differential in degreed which will become apparent - throttling range of the-controller according ,to' the demand o& , the system and other variabbi;. by raising and lowering the dial setting; This differential is usually'') - Offset is the difference between the set point and the actual referred to as the manual differential of the thermostat: When the';' . control point under stable conditions. This is sometimes abo same thermostat is applied to an operating system,, the.'total called drift, deviation, or droop.1 change in temperature that occurs between a call for more heat iwi ft call for less heat is usually greater than the manual dif ferential due-to thermostat lag. The differential encountered.on.. the job under control is referred to as operating differential. For example, a thermostat having a 2 deg differential when raising TTW0TTLM6 and lowering the dial setting may actually-produce.temperature variations ot 3 deg between system on ana system off. .- --V\--Vx--t / \ / \ / \otrnntttnsL ,Rg. 2....Two-Position Control ' 2. Timed two-position action is a common variation of straight two-position action often employed in room thermostats to re-' duce operating differential. In heating thermostats, a'beater ele ment is energized during the on periods, prematurely shortening the on time as the heater falsely warms the thermostat (heat anticipation). The same anticipating action can be obtained in cooling thermostats by energizing a better during thermostat off periods. In either case, the percentage of on time is varied in proportion to the system load, whilethe total cycle time-remains relatively constant. i . 3. Floating action. Fig. 3, is. the type,. as is..two-position action, in which the controller can perform only two operations, moving the controlled device, usually at a constant rate, either toward its open or closed position. Generally,', there is a neutral sons between the two positions which allows the. controlled device to stop at any position whenever the controlled-variable b within the differential of. the controller. When the controlled variable gets outside the differential of the controller) the con troller moves the controlled device in the proper direction. An example of Boating'oontrol b a static pressure controller position ing a damper or inlet vanes to maintain static pressure in the e duct. -. fig. 4....Proportional Control Showing Variations m , Controlled Variable as Load Changes 5. Proportional plus automatic reset action,. Fig. 5, com bines floating action with proportional action to achieve both the stability of proportional control, with a relatively wide throttling range, ana the invariable control point of fluting control-The system functions the-same as proportional action, the *ittrtmtie. reset serving to shift the control point back to the set point whenever any offset occurs. Reset rate b the number of times per minute the propor tional action b duplicated by the reset* action, and usually is expressed as repeats per minute. The reset rate in most con-' trailers b adjustable and must be carefully matched to the system characteristics to avoid unstable operation. Because automatic reset b necessarily slow-acting, it should be used only when load changes are;of reasonably long duration and when the maximum offset resulting from proportional oontrol alone b outside of acceptable limits. - :' Figs. 4 5 show the results of rather abrupt demand changes, which usually result - in two or three cyules before stability' b restored. More gradual demand changes will re sult in a gmmthw curve. fig. 5.... Proportional Phis Automatic Reset Control Showing How.Control Point is Returned .to Set Point after a Load Change fig. 3.... Rooting Control Showing Variations in Controlled Variable as Load Changes 4. Proportional-action, Fig. 4, is the type in which the con trolled device is; positioned proportionately in response to alight -changes' in .the ^controlled .variable. It does not run through its complete* stroke, aB isTthe case with two-position oontrol, nor does it continue to move until the change in .the controlled variable, resulting from that movement,"a felt by the controller, as is the case with floating' control. Instead, it im mediately requirement. a position in proportion to the system An example of proportional oontrol is a thermostat in a fan discharge duct, actuating an automatic valve in'the steam AUTOMATIC CONTROL SYSTEM COMPONB4TS , In addition to controllers and controlled devices, many control systems include auxiliary apparatus such as switches and relays of various kinds, docks or timers, thermometers, gages, {dlot lights,.and other indicators for observing the operation of the system. Only those more commonly used will be described here. Controllers Automatic controllers have both a transducer. dement and a Hie sensing dement measures changes in the controlled variable end produces ft proportional effect on the transducer. Automatic Control 173 This effect may be a change in force, position, or electrical resistance, depending on the type of sensing dement used. ' ' The transducer converts the effect produced by the sensing dement into an effect suitable for operation of the controlled device. In pneumatic systems, a mechanical transducer regulates the pressure of a compressed air energy source aoDiied to the controlled device; inae--le-c-t,ri.c1--or elect_ro_n_ic_e/ra- 7ype5 of Sensing Elements Temperature sensing dements usually consist of (1) a bimetal strip: (2) a rod and tube of dissimilar metals, (3) a bellows, with or without a remote bulb, or (4) an electrical 1 A* bimetal element b composed of- two thin stripe of disBmUlar "metals fused together. Because the two metals have different coefficients of thermal expansion, the dement bends ee the temperature varies, sod produces a change in' position. Depending on the space available and the movement re quired, it may be in the form of a straight strip, U-ehaped, or wound into a spiral. This element finds its most common application in room thermostats, but b also used in insertion andimmeraon thermostats. 2. A rod-and-tefc element consists of a high-expansion metal tube inside of which b a low-expansion rod with one end attached to the rear of the tube. The tube changes length with changes in temperature, causing the free end of the rod to move. The rod- 3 A sealed-bellows element b either vapor-filled, gas-filled, or liquid-filled, after being evacuated of air. Changes of tem perature cause changes in pressure or volume of the gas or liquid, molting in a change in force or a movement. This element b often employed in room thermostats. A remote-bulb element b a sealed bellows or diaphragm to which a bulb or capsule b attached by means of a capillary tube, the entire system being filled with vapor, gas, or liquid. Changes of temperature at the bulb result in changes of pressure or volume which are communicated to the bellows or diaphragm through the capillary tube. The remote-bulb element's usatil where the temperature measuring point b remote from the desired thermostat location. It usually is provided with fittings suitable for insertion into & duct or into a pipe or tank.. * 4. A resistance element b made of wire with electrical re^; gbtance that changes with temperature changes. A thermistor is a nw4l HnH of semi-conductor in which - electrical resistance changes with temperature changes. A thermocouple b a union of two dteamflAr mHjdo joined at the ends, having generated voltage that varies as a function of temperature changes. These various elements are available in forms suitable for measuring room tem perature or for insertion into a duct, pipe, or tank. Humidity sensing dements are made of (1) hygroscopic or ganic materials or (2) an electrical resistance. 1. An organic element b usually mdA of human hair, wood, paper, or nirmJ monbrane. Changes in relative humidity cause the element to expand or contract. 2. A resistance element, for use with electronic controllers, frequently consists of a thin <vw>tjng of-hygroscopic salt on an hwnlnting form. The resistance ot the salt varies with the relative humidity. Pressure sensing dements can be divided into two general dames, depending upon pressure range. 1. For. pressures or vacuums measured in pounds per square mefa or inches of mercury, the element b usually a bellows, diaphragm, or Bourdon tube. One ride of the element may be' open to the atmosphere, in which case the element responds to pressures above or -below atmospheric. A differential-pressure riement has connections to both odes so that it will respond to the difference between two pressures. 2. For low ranges of pressure or vacuum- which are usually measured in inches of water, such as the static pressure in an air duct, the measuring element may be an inverted bell im(rased in oil, a large slack'diaphragm, or; a large flexible Burial bellows. The element usually b of the differential type when employed in'conjunction' with orifices. Pitot tubes, and similar accessories, may be used to measure flow, velocity, or liquid level as well as static pressure. - Sensing elements for other purposes, such as flame detec tion, or for measuring smoke density, specific gravity, current, CO* CO, etc., are often necessary for the complete control of a heating, ventilating, or air-conditioning system. Types of Transducers Transducers differ widely; some regulate the application of pneumatic energy, while others regulate the application of electrical energy. They also differ according to the type of con-, tool action produced. Five types are in common use: 1. Electric transducers are normally combined with senring elements having a force or position type output to obtain various control modes. a. For two-position control, the transducer may be a ample eleo- trical contact which starts a burner; pump, etc., or which actu ates a spring-return valve or damper operator. SPDT switching circuits are used to control a 3-wire unidirectional motor operator. SPDT circuits also are used for heating-cooling applications. Either SPST or SPDT circuits may be modified to obtain timed two-position action. b. -For floating control, the transducer output is a SPDT switch ing circuit with a neutral zone where neither contact b made. This type of control b normally used with reversible motor opvatore. c. For proportional control, two types of transducer systems are in common use. (1.) The sensing element moves a wiper arm across apotentir ometer to vary the voltage applied to a balancing relay. The relay controls the operation of a reversible motor operator, and b re balanced by a variable voltage feedback gnal from ,a second potentiometer in the motor operator. (2.) The sensing element positions a floating type switching circuit with a neutral zone, which controls the operation of a re versible motor operator. The control b rebalanced by a small solenoid, energised by a variable voltage feedback signal from a potentiometer in the motor operator. ' 2. An electronic transducer uses an electronic amplifier to detect the small voltage changes generated by the elements. Two types of systems with different transducer outputs are in common use. a. In one system the amplifier output actuates an electric relay or relays. The control mode may be two-position, floating,-or pro portional, depending upon the amplifier and relay arrangement. For proportional control, relays control the operation of a-reveirible motor operator, and rebalance b obtained by a variable voltage feedback signal impressed on the amplifier from a po tentiometer in the motor operator. b. Another system uses the amplifier output to actuate an electrohydr&ulic relay which - in tom varies the position of a- hydraulic operator. The control mode b proportional, and re-' balance b provided by a spring in the operator. 3. 'Electronic-pneumatic transducers use the output of an elec tronic amplifier to actuate an electropneumatic relay, resulting in' a variable air pressure applied to the controlled device. The con trol mode may be two-position or proportional, depending upon the amplifier and relay arrangement. 4. Pneumatic transducers are normally combined with sensing elements having a force or position output to obtain a variable air pressure output. The control mode is usually proportional, but other modes may be used. They are generally M*--ifiAd as oonrelay type and relay type. a. The nonrelay type of pneumatic transducer b the simplest form. It uses a restrictor in the air supply and a bleed oosxle. " The sensing element positions a flapper which varies the nozzle opening, resulting in a variable air pressure output applied to the , controlled device, usually a small pneumatic operator. The non- relay type b limited to applications requiring small volumes of y air. ... 1 b. In a relay type pneumatic transducer, the variable force pn> duced by thesensingelement either directly, or indirectly through a restrictor, nozzle, and flapper arrangement, actuates a relay of substantial air handling capacity. The force produced by the out put pressure change b rebalanced against the force produced by til* Harming element. * Pneumatic transducers are further classified by construction as direct- or revcreo-actuig-' ' 5:,` is.