Industrial automation.ppt

Editor's Notes

• #2: 1
• #3: . Engineers want to design and operate processes that remain in safe conditions, produce the desired amounts of high quality products and are profitable. Therefore, engineers must provide measuring devices for key variables and valves (or other devices, such as variable speed electric motors) to influence of “steer” the process
• #4: Process control requires sensors for measuring variables and valves for implementing decisions and itself work as decision makers. Successful process control requires appropriate instrumentation, so engineers should have the understanding of the principles of common instruments.
• #5: Perhaps the best advice for engineering students is that “instruments are always incorrect”. This surprising statement is not intended to undermine reasonable confidence in applications of sensors and valves. However, new engineers sometimes tend to accept instruments as exactly correct without evaluating the likely errors associated with their use. Depending on the instrument, the process operating conditions and the application, the instrument errors can be small enough to be insignificant or can be large enough to seriously degrade control performance. The engineer must evaluate each application during the process design and select an appropriate instrument. 
• #7: ----- Sensors to measure process conditions and valves to influence process operations are essential for all aspects of engineering practice. Engineers want to design and operate processes that remain in safe conditions, produce the desired amounts of high quality products and are profitable. Therefore, engineers must provide measuring devices for key variables and valves (or other devices, such as variable speed electric motors) to influence of “steer” the process. --- While sensors and valves are important in all aspects of engineering, they assume greatest importance in the study of automatic control, which is termed process control when applied in the process industries. Process control deals with the regulation of processes by applying the feedback principle using various computing devices, principally digital computation. Process control requires sensors for measuring variables and valves for implementing decisions ---- However, new engineers sometimes tend to accept instruments as exactly correct without evaluating the likely errors associated with their use. Depending on the instrument, the process operating conditions and the application, the instrument errors can be small enough to be insignificant or can be large enough to seriously degrade control performance. The engineer must evaluate each application during the process design and select an appropriate instrument. 
• #8: An instrument contain various parts that perform prescribed functions in converting a variable quantity or condition into corresponding indication. Primary sensing element: of an instrument receives energy from the measured medium and produces an output depending in someway on the value of the measured quantity. Variable Conversion element: merely converts the output signal of the primary sensing element (voltage or displacement) into a more suitable variable or condition useful to the function of the instrument Variable manipulating element: manipulates the signal represented by some physical variables to perform the intended task of an instrument. Data Transmission Element: transmits the data from one element to another, as simple as shaft and bearing assembly or as complicated as telemetry system Data Presentation Element: performs the translation function such as simple indication of a pointer moving over a scale.
• #9: To aid in the economics of industrial operations by improving the quality of the product and efficiency of production.
• #11: ---- Sensors are used for process monitoring and for process control. These are essential elements of safe and profitable plant operation that can be achieved only if the proper sensors are selected and installed in the correct locations. While sensors differ greatly in their physical principles, their selection can be guided by the analysis of a small set of issues, which are presented in this section. Each issue is introduced here with process examples, and details on the issues are provided in the remainder of this site for the most common sensors in the process industries. ---- The performance characteristics of an instrument are very necessary for choosing the most suitable instrument for specific measuring task. Static Characteristics: considered for instruments used to measure an unvarrying process conditions Dynamic Characteristics: for measuring quantities that fluctuates with time. ---- The relative importance of each issue depends upon the specific application; for example, one application might require excellent accuracy, while another might require only moderate accuracy, but high reliability. Generally, we find that the greater the requirements for good performance, the higher the cost for purchase and maintenance. Therefore, we must find the proper balance of performance and cost, rather than always specify the best performing sensor
• #12: The relative importance of each issue depends upon the specific application; for example, one application might require excellent accuracy, while another might require only moderate accuracy, but high reliability. Generally, we find that the greater the requirements for good performance, the higher the cost for purchase and maintenance. Therefore, we must find the proper balance of performance and cost, rather than always specify the best performing sensor
• #13: ---- Accuracy is needed for some variables, such as product quality, but it is not required for others such as level in a large storage tank.
• #15: Conditions: the same measurement procedure the same observer the same measuring instrument, used under the same conditions the same location repetition over a short period of time.
• #22: Typical examples of compensating calculations are the square root applied to the orifice flow sensor and the polynomial compensation for a thermocouple temperature sensor. The engineer should not assume that a compensation for non-linearity has been applied, especially when taking values from a history database, which does not contain details of the measurement technology.
• #23: Reliability is affected by maintenance and consistency with process environment. Also, some sensors are protected from contact with corrosive process environment by a cover or sheath (e.g., a thermowell for a thermocouple), and some sensors require a sample to be extracted from the process (e.g., a chromatograph).
• #24: The process for determination by measurement or comparison with a standard of correct value scale reading on a meter or other measuring instrument.
• #26: Speed of Response: is the rapidity with which an instrument responds to the change in the measured quantity. Fidelity: degree to which an instrument indicate the changes in the measured variables without dynamic error. Lag : is a retardation or delay in the response of an instrument to the changes in the measured quantity Drift: is a undesired change or gradual variation in output over a period of time that is unrelated to changes in output, operating conditions or load.
• #28: ---- Most sensors will function properly for specific process conditions. For example, many flow sensors function for a single phase, but not for multi-phase fluid flow, whether vapor-liquid or slurry. The engineer must observe the limitations for each sensor.   Some sensors can have direct contact with the process materials, while others must be protected. Three general categories are given in the following. ----- Sensors in direct contact must not be degraded by the process material.     The sheath usually slows the sensor response.   Samples must represent the fluid in the process.
• #29: ------ The measurement is displayed for observation by plant personnel. Typically, the display uses analog principles, which means that the display presents the measurement as a position in a graphical format, which could, for example, be the height of a slide bar or the position of a pointer. Often, the value is displayed as a line on a trend plot that provides the values for some time in the past. In addition, the measurement can be displayed as a digital number to provide more accuracy for calibration. Finally, measurements that are transmitted to a digital control system can be stored in a historical database for later recall and for use in calculating important parameters useful in monitoring process behavior, for example, reactor yields or heat transfer coefficients. ------ A sensor can display the measurement at the point where the sensor is located. This information can be used by the people when monitoring or working on the equipment. A measurement that has only local display involves the lowest cost, because the cost of transmission and interfacing to a digital system are not required. Note that no history of these measurements is available unless people record the values periodically. ---- Some equipment is operated from a local panel, where sensors associated with a unit are collected. This enables a person to startup, shutdown and maintain the unit locally. This must be provided for units that require manual actions at the process during normal operation (loading feed materials, cleaning filters, etc.) or during startup and shutdown. Usually, the values displayed at a local panel are also displayed at a centralized control room. ------ Many processes are operated from a centralized control room that can be located a significant distance (e.g., hundreds of meters) from the process. The measurement must be converted to a signal (usually electronic) for transmission and be converted to a digital number when interfaced with the control system. A centralized control system facilitates the analysis and control of the integrated plant. ----- In a few cases, processes can be operated without a human operator at the location. In these situations, the measurements are transmitted by radio frequency signals to a centralized location where a person can monitor the behavior of many plants. Typical examples are remote oil production sites and small, safe chemical plants, such as air separation units.
• #30: Currently, sensor technology is experiencing a dramatic change. While the basic physics and chemistry of sensors are not changing, sensors are being enhanced by the addition of microprocessors at the location of the sensor. This change makes the following features possible that were not available with older technologies. ------ The “signal” from the sensor is no longer simply a single value representing the measured value. The sensor can transmit additional information, including diagnostics and corrected estimates of a variable based on multiple sensors, e.g., orifice pressures and density. All values can be transmitted digitally, which allows many sensor values to be sent by the same cabling, which reduces the cost of an individual cable for each measurement, as required with analog transmission. ---- - The sensor can provide sophisticated diagnostics of its performance and warn when a measurement might be unreliable. ----- The sensor can identify unusual signal characteristics and eliminate noise or “spikes” according to methods defined by the engineer. ------ The range of a sensor can be changed quickly to accommodate changes in process operating conditions.