CHAPTER IV Computerized Data-acquisition Systems

Transcription

1 CHAPTER IV Computerized Data-acquisition Systems The signal outputting a sensor is usually an analog signal. Post-processing methods involve, however, complex mathematical formulations. The data recorded by the sensor have, therefore, to be digitalized before post-processing. This is the role of the computerized data-acquisition (DAQ) system. 1. Characteristics of DAQ systems: - The number of bits used to represent the signal: The higher is the number of bits, the higher is the accuracy of the digital representation of the analog input. - Input range: How many (max, min) volts the DAQ will accept (it can be unipolar: 0 to 5 V or bipolar: ± 5V). - Conversion speed: the speed it takes to convert the analog input into a digital output. - Unipolar single-slope integrating convert Figure 4.1. Ramp A/D converter process. Figure 4.2. Single-slope integrating A/D converter circuit. Instrumentation and Measurements \ LK\

2 A 8-bit single-slope integrating analog-to-digital converter has an input rage of 0 to 5 V. Determine the digital output for an analog input of 1 V. If an input is out of range. The analog-to-digital converter is said to be saturated. It is the responsibility of the experimentalist to ensure that the value of the input is lower than the limits. A 8-bit single-slope integrating analog-to-digital converter has an input rage of 0 to 5 V. Determine the digital output for an analog input of 1 V, using the above formulas. A 12-bit A/D converter with 2 s-complement output has an input range of -10 to +10 V. Find the output codes when the input is -11 V; -5 V; 0 V; V and 12 V. Instrumentation and Measurements \ LK\

3 - A/D converters errors: The typical errors associated with A/D converters are: - Linearity. - Zero error. - Sensitivity. Furthermore, since conversion from analog to digital signal requires converting a continuous function to a discrete one. A resolution error, called quantizing error has to be considered: V ru V input resolution error = ± 0.5 rl N 2 A 8-bit A/D converter has an input range of -5 to +5 V. Find the input resolution error. Note: if the inlet signal input has the order of magnitude of the input resolution error, it would be better to amplify the input signal before conversion. If data change rapidly in time, the A/D converter can cause an error, since it will not have enough time to convert the analog input into digital output. To solve this problem a sample-and-hold device is usually inserted before the A/D converter. The conversion can be performed every 1.5 μs or less (normal value: 10 to 25 μs). - Practical Analog-to-Digital converters The most common used A/D converter used in DAQ systems is a successive approximation converter. It is based on interval halving technique Figure 4.3. Successive approximations method (for 4-bit A/D converter) A 8-bit unipolar successive A/D converter has an input range of 0 to +5 V. An analog voltage of 3.15 V is applied to the input. Find the analog output. Instrumentation and Measurements \ LK\

4 If high sampling rates are required, faster A/D converters are used: parallel or flash A/D converters. In this type of converters, all the conversion process is performed in one step using several comparators and the conversion can be performed as fast as 10 ns. These A/D converters are adequate for 8-bit resolution. Half-flash A/D converter are also commonly used (2 flash converters and need two steps for a complete conversion). They perform a complete conversion in about 100 ns. Another type of very accurate, but slow, A/D converter is the dual-slope integrating converter. Figure 4.4. Dual slope integrating A/D. (a) block diagram; (b) integration process. V i t = 1 t0 V ; V i: input unknown signal and V r : reference signal. r The time for conversion is around 4 to 8 ms. Instrumentation and Measurements \ LK\

5 - Digital-to-Analog converters Computers (through DAQ systems) can be used to control instruments (run a pump, close a valve ). In this case a conversion from digital to analog signal is required. Figure 4.5. Digital to analog converter. It uses a set of switches to convert the digital signal to an analog one. Usually the signal produced is quite low and need therefore amplification. - Simultaneous Sample-and-hold subsystems: These subsystems are really critical went one has to ensure that there is a simultaneous reading of the data. Instrumentation and Measurements \ LK\

6 Types of Data Acquistion Systems Wireless Data Aquisition Systems Wireless data acquisition systems can eliminate costly and time consuming field wiring of process sensors. These systems consist of one or more wireless transmitters sending data back to a wireless receiver connected to a remote computer. Wireless transmitters are available for ambient temperature and relative humidity, thermocouples, RTDs, pulse output sensors, 4 to 20 ma transmitters and voltage output transducers. Receivers can be connected to the USB or Ethernet port on the PC. Serial Communication Data Acquistion Systems Serial communcation data acquistion systems are a good choice when the measurement needs to be made at a location which is distant from the computer. There are several different communication standards, RS232 is the most common but only supports tranmission distances up to 50 feet. RS485 is superior to RS485 and supports transmission distances to 5,000 feet. USB Data Acquistion Systems The Universal Serial Bus (USB) is a new standard for connecting PCs to peripheral devices such as printers, monitors, modems and data acquistion devices. USB offers several advantages over conventional serial and parallel connections, including higher bandwidth (up to 12 Mbits/s) and the ability to provide power to the peripheral device. USB is ideal for data acquisition applications. Since USB connections supply power, only one cable is required to link the data acquisition device to the PC, which most likely has at least one USB port. Data Acquisition Plug-in Boards Computer data acquisition boards plug directly into the computer bus. Advantages of using boards are speed (because they are connected directly to the bus) and cost (because the overhead of packaging and power is provided by the computer). Boards offered are primarily for IBM PC and compatible computers. Features provided by the cards can vary due to number and type of inputs (voltage, thermocouple, on/off), outputs, speed and other functions provided. Each board installed in the computer is addressed at a unique Input/Output map location. The I/O map in the computer provides the address locations the processor uses to gain access to the specific device as required by its program. Instrumentation and Measurements \ LK\