Document 914GkyyJbjoDGX8VjjOEKKx4D

946 , CHAPTER 38 1957 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 source or ENERGY CONTROLLER (THERMOSTAT) I MEASURING CONTROLLED VARIABLE i ELEMENT 4(AIR TEMPERATURE) '(REMOTE BULB) c01 L CONTROLLED DEVICE fVALVE) CONTROL AGENT (STEAM) AIR FLOW Fig. 1. Essentials op 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 utlilizes 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 of 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 element Automatic Control 947 in the thermostat which is energized during the on periods. The percentage of time un 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 Fig. 2. Two-Position Control gs PjrFERENTlAL o 5> Ftg. 3. Floating Control Showing Variations in Controlled Variable as Load Changes Fig. 4. Proportional Control Showing Variations in Controlled Variable as Load Changes desirable to have a controller in which the differential can be changed by manual ad justment. This is designated as an adjustable differential. 3. Floating action, Fig. 3, is the type, as with two-position action, where the con troller can perform only two operations: moving the controlled device usually at a constant rate either towards its open or its closed position. Generally there is a neutral zone between the two positions which allows the controlled device to stop at any position whenever the controlled variable is within the differential of the con troller. When the controlled variable gets outside the differential of the controller, the controller moves the controlled device in the proper direction. An example of floating control is a fire box draft controller positioning a damper in the breeching of a boiler. 4. Proportional action, Fig. 4, is the type where the controlled device is positioned proportionately in response to slight changes in the controlled variable. It does not run through its complete stroke, as is the case with two-position control, nor does it continue to move until the change in the controlled variable resulting from that movement is felt by the controller, as is the case with floating control, but it immedi ately assumes a position in proportion to the system requirement. An example of porportional control is a thermostat in a fan discharge duct actuat ing an automatic valve in the steam supply to a coil to regulate the air temperature leaving the coil. This control would be similar to that shown in Fig. l. Throttling range is the total amount of change in the controlled variable required