Data Acquisition and Control Computer
Digital i Signals
lect. A supervisory level computer may also be included to perform higher level monitoring, diagnosis, optimization and advanced control tasks (Figure 3.1). In the data-acquisition system, the input to the system is a physical variable such as temperature, pressure or flow rate. Such a physical variable is first converted to an electrical signal (a voltage or current signal) by a suitable transducer. Then it is transmitted to A/D converter. The digitized signal is sent to the computer. The computer records the data, monitors process status and generates control commands and messages to plant personnel. The control commands are converted to analog signals by the D/A converter and sent to the final control elements.
The Analog-to-Digital (A/D) converter, also called as encoder, is a device that converts an analog signal into a digital signal, usually a numerically coded signal. A/D converter is needed as an interface between an analog component such as a sensor or transducer and a digital component such as a computer.
The Digital-to-Analog (D/A) converter, also called as decoder, is a device that converts a digital signal into an analog signal. This converter is needed as an interface between a digital component and an analog component (a physical device) to operate the physical device, such as a control valve or a pump.
The conversion of an analog signal into a digital signal (binary number) is an approximation since an analog signal can take on an infinite number of values, whereas the numbers that can be formed by a finite set of digits are limited. This approximation process is called quantization. In other words, "quantizing" means transforming a continuous or analog signal into a set of discrete states. Since the number of bits in the digital code is finite, A/D conversion results in a finite resolution, rounding off an analog number to the nearest digital level and producing a quantization error.
The functionality and ease of use of commercially available data collection and processing packages have significantly improved over the years. Most commercial data acquisition software in the market are capable of
• capturing and recording process data over time
• data reconciliation and outlier detection
• custom tailoring data treatment according to the user's needs
• transferring the data to other software
• sending out commands or data to control instruments and final control elements
• alarm generation and handling
• inputting time series data from any device into any application program
• creating charts and graphs that automatically uptake real-time data from serial devices
• performing real time analysis of data
• storing and compressing the data
Most software work with popular operating systems such as Windows 2000 and Unix. User-friendly graphical user interface (GUI) of software provides a convenient environment for the user. Simple, menu driven, step by step set-up is possible in most commercial software due to the interactive nature of the GUI. Hierarchical password protection personalizes the user. In most applications, controllers can be designed and set points can be changed as a function of any parameter using simple pictorial function blocks avoiding any programming.
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