Today, most scientists and engineers use personal computers for data acquisition in laboratory research, test and measurement, and industrial automation. To perform the tests outlined in this chapter as well as whole-system tests, a computer data acquisition system (DAS) is required.
Many applications use plug-in boards to acquire data and transfer them directly to computer memory. Others use DAS hardware remote from the PC that is coupled via a parallel, serial, or USB port. Obtaining proper results from a PC-based DAS depends on each of the following system elements:
The personal computer is integrated into every aspect of data recording, including sophisticated graphics, acquisition, control, and analysis. Modems connected to the Internet or an internal network allow easy access to remote personal computer-based data recording systems from virtually any place. This is very suitable when performing an actual solar system monitoring.
Almost every type of transducer and sensor is available with the necessary interface to make it computer compatible. The transducer itself begins to lose its identity when integrated into a system that incorporates such features as linearization, offset correction, and self-calibration. This has eliminated the concern regarding the details of signal conditioning and amplification of basic transducer outputs.
Many industrial areas commonly employ signal transmitters for control or computer data handling systems to convert the signal output of the primary sensor into a compatible common signal span. The system required for performing the various tests described in this chapter, however, needs to be setup by taking the standard requirements about accuracy of the instruments employed.
The vast selection of available DAS hardware makes the task of configuring a DAS difficult. Memory size, recording speed, and signal processing capability are major considerations in determining the correct recording system. Thermal, mechanical, electromagnetic interference, portability, and meteorological factors also influence the selection.
A digital DAS must contain an interface, which is a system involving one or several analog-to-digital converters and, in the case of multichannel inputs, a multiplexer. In modern systems, the interface also provides excitation for transducers, calibration, and conversion of units. Many DASs are designed to acquire data rapidly and store large records of data for later recording and analysis. Once the input signals have been digitized, the digital data are essentially immune to noise and can be transmitted over great distances.
One of the most frequently used temperature transducers is the thermocouple. These are commonly used to monitor temperature with PC-based DAS. Thermocouples are very rugged and inexpensive and can operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is non-linear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear:
(4.53)
The Seebeck coefficient (S) varies with changes in temperature, causing the output voltages of thermocouples to be non-linear over their operating ranges. Several types of thermocouples are available; these thermocouples are designated by capital letters that indicate their composition. For example, a J-type thermocouple has one iron conductor and one constantan (a copper–nickel alloy) conductor.
Information from transducers is transferred to a computer recorder from the interface as a pulse train. Digital data are transferred in either serial or parallel mode. Serial transmission means that the data are sent as a series of pulses, 1 bit at a time. Although slower than parallel systems, serial interfaces require only two wires, which lowers their cabling cost. The speed of serial transmissions is rated according to the baud rate. In parallel transmission, the entire data word is transmitted at one time. To do this, each bit of a data word has to have its own transmission line; other lines are needed for clocking and control. Parallel mode is used for short distances or when high data transmission rates are required. Serial mode must be used for long-distance communications where wiring costs are prohibitive.
The two most popular interface bus standards currently used for data transmission are the IEEE 488 and the RS232 serial interface. Because of the way the IEEE 488 bus system feeds data, its bus is limited to a cable length of 20 m and requires an interface connection on every meter for proper termination. The RS232 system feeds data serially down two wires, 1 bit at a time, so an RS232 line may be over 300 m long. For longer distances, it may feed a modem to send data over standard telephone lines. A local area network may also be available for transmitting information; with appropriate interfacing, transducer data are available to any computer connected to the local network.
4.11.1 Portable data loggers
Portable data loggers generally store electrical signals (analog or digital) to internal memory storage. The signal from connected sensors is typically stored to memory at timed intervals, which range from MHz to hourly sampling. Many portable data loggers can perform linearization, scaling, or other signal conditioning and permit logged readings to be either instantaneous or averaged values. Most modern portable data loggers have built-in clocks that record the time and date, together with transducer signal information. Portable data loggers range from single-channel input to 256 or more channels. Some general-purpose devices accept a multitude of analog or digital inputs or both; others are more specialized to a specific measurement (e.g., a portable pyranometer with built-in data logging capability) or for a specific application (e.g., temperature, relative humidity, wind speed, and solar radiation measurement with data logging for solar system testing applications). Stored data are generally downloaded from portable data loggers using a serial or USB interface with a temporary direct connection to a personal computer. Remote data loggers may also download the data via modem through telephone lines.
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