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Unformatted text preview: 02-1 02-1 Process State, Sensors, and Interfacing Function of sensors: convert physical quantity to information. Information will (usually) be read by computer via an interface. Ultimately, the information, in the desired form, will be stored in a memory location. The following steps are typical: 1: A transducer converts process state to a raw electrical quantity. 2: A conditioning circuit converts the raw electrical quantity into a useful electrical quantity. 3: An analog-to-digital converter (ADC) converts the useful electrical quantity to information. 4: A buffer and interface store, format, and present the information to a computer. 5: An interface routine reads the information, converts it to the desired form, and stores it in the desired place. 02-1 EE 4770 Lecture Transparency. Formatted 13:25, 23 December 1997 from lsli02. 02-1 02-2 02-2 Process State and Process Variables The process state is the current condition of the process, down to infinitesimal detail. The process variable is a part or characterization of a process state, usually in terms of a common measure. For example, consider a coffee maker. Process state: amount of water in carafe, water temperature, chemical description of water in carafe, type of coffee beans, etc. Process variable: temperature of water. Process variable value: 70 C. Characteristics It is impossible to know the complete process state (because of infinite detail). It is impossible to know the exact value of a process variable. A process variable value, however, can be determined to a high degree of precision. 02-2 EE 4770 Lecture Transparency. Formatted 13:25, 23 December 1997 from lsli02. 02-2 02-3 02-3 Dimensions Basics A process variables value is usually expressed as the product of a number and a dimension. For example, let process variable T be the temperature of water in a coffee maker carafe. Then a value for T might be 60 C. An equivalent value might be T = 333 . 15 K. Notation Dimensions will be written in Roman (upright) type. For example, mA, V, and m. Symbols representing values (variables) will be written in italic type: T , x , and R . Thus, 3 V V means three vee volts. 02-3 EE 4770 Lecture Transparency. Formatted 13:25, 23 December 1997 from lsli02. 02-3 02-4 02-4 Algebraic Manipulation of Dimensions In expressions, dimensions are manipulated in the same way as num- bers and variables....
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- Fall '99