Document 2jpZrbp7VR6np5eoaoBX2xw4L

966 CHAPTER 38 1958 Guide changes in outdoor temperature is an example. This system depends for its operation on a prearranged relationship between outdoor temperature and heat input to the building, and room temperature has no effect on the controller. As there is no feedback, the control corrects only for those disturbances of room temperature caused by changes in outdoor temperature. TYPES OF CONTROL SYSTEMS Control systems are divided into five main groups according to the pri mary source of energy: 1. A self-contained system combines the controller and controlled device in one unit and employs the power of the measuring system to effect the necessary corrective action. The measuring system derives its energy from the process under control without amplification by any auxiliary source of energy, and may be of the sealedbellows or remote-bulb type as described under Types of Measuring Elements CONTROLLER T _(THERMOSTAT) CONTROLLED VARIABLE (AIR TEMPERATURE) , MEASURING ELEMENT (REMOTE BULB) ,-- CONTROLLED"' | DEVICE (VALVE) CONTROL AGENT (STEAM) jj^FEED BACK Fig. 1. Essentials of a Control System Temperature changes at the bellows or the remote bulb result in pressure or volume changes of the enclosed media which are transmitted directly to the operating device of the valve or damper. 2. A pneumatic system utilizes compressed air, usually at a pressure of 15 to 25 psig, as a source of energy. This is supplied to the controller which in turn regulates the pressure supplied to the controlled device. 3. A hydraulic system utilizes a suitable liquid under pressure as the source of energy. The pressure often is considerably higher than in a pneumatic system, but in other respects the systems are similar. Hydraulic systems find their chief use in applications where large forces are required for operation of the controlled devices. 4. An electric system utilizes electric energy, either low or line voltage as the energy source. The electric energy supplied to the controlled device is regulated by the controller either directly or through relays. 5. An electronic system also utilizes electric energy, but employs an electronic amplifier to increase the minute voltage variations of the measuring element to values required for operation of standard electrically-controlled devices. Measuring ele ments usually are of the resistance type, but thermocouples also are employed. Combination electronic-pneumatic systems utilize compressed air for operation oi the controlled device by converting the output of the electronic amplifier into suitable air pressure changes by means of an electronic-pneumatic transducer. TYPES OF CONTROL ACTION Control action may be divided into five types: 1. Two-position action. Fig. 2, is the type in which the controlled device can be positioned only to a maximum or minimum position, or can be either on or off. A ther mostat which opens and closes a valve, or starts and stops a burner, is an example. 2. Timed two-position action is a common variation of two-position control action in which the time of on periods are prematurely shortened. This type of action usu ally is employed only in room thermostats, and is accomplished by a heater elemen Automatic Control 967' in the thermostat which is energized during the on periods. The percentage of: time on is varied in proportion to the system load. .; . Controller differential applying'to two-position control action, is the difference between the setting at which the controller operates to one position and the setting at which it operates to the other position. As an example, if a pressure controller starts a pump at 12 psi and stops it at 15 psi, the differential is 3 psi. It is sometimes o ss- ac < o> u Fig. 2. Two-Position Control Fig. 3. Floating Control Showing Variations in Controlled Variable as Load Changes *IG* 4- Proportional Control Showing Variations in Controlled Variable as Load Changes iToJ I COr,aye a CODtroIIer in which the differential can be changed by jusiment. This is designated as an adjustable differential. trnb'pf aftion< Kg. 3, is the type, as with two-position action, where the con- conatonf Perf?rTM only two operations: moving the controlled device usually at a be.tweennpnt.oi ra L e,lther towards its open or its closed position. Generally there is a anv the, two positions which allows the controlled device to stop at trollw wf whenever the controlled variable is within the differential of the con- the 11 the eootrolled variable gets outside the differential of the controller, floatim/TM fr ?)ore the controlled device in the proper direction. An example of a boiler C0Dtr01 1S & "re box draft controller positioning a damper in the breeching of ProtioHtnn07/1!0"?2 action, Fig. 4, is the type where the controlled device is positioned run throng * m repnse to slight changes in the controlled variable. It does not continue* Us comP'ete stroke, as is the case with two-position control, nor does it movement;,, un,til the change in the controlled variable resulting from that atelv lelt by. . controller, as is the case with floating control, but it immedi- J umes a position in proportion to the system requirement. mg an6ai?t^Pletf Prportional control is a thermostat in a fan discharge duct actuatIeavine the^n-i v^e in the steam supply to a coil to regulate the air temperature g tne coil. This control would be similar to that shown in Fig. 1. Throttling range is the total amount of change in the controlled variable required