|Tuesday, 19 September 2006|
A data acquisition system mostly includes transducers, transmitters, amplifiers and other means for provision of the signal representation by their measurement and/or monitoring. Transducers are used in a wide variety of engineering applications to convert various physical quantities to an electrical signal. An example of one type of such a transducer is the pressure transducer, which usually includes a pressure sensor and some associated circuitry for producing an electrical output. A pressure transducer typically provides an output voltage or current that is a function of pressure applied to the sensor. A linear variable differential transformer (LVDT) is an electro-mechanical transducer which allows for the measurement of very small motions in a structural mechanical device. LVDTs are used for measuring position, pressure, load weight, angular position, and acceleration. Thermocouples have been employed for many years to measure temperatures in a wide variety of applications. Thermocouples produce an output voltage proportional to the temperature difference between the measurement junction and a reference junction. Sensors are generally coupled in transducers through at least a rudimentary signal conditioning circuit to electronic instrumentation for use in an application. Modem sensing systems typically include a sensor, a microprocessor, and an A/D converter coupled there between. Sensors typically provide a low-level signal which needs to be amplified before being of use.
Data acquisition systems may be coupled to sensing and other input devices by signal lines, some providing analog signals, and digital signals. The external input is generally an analog signal, although digital signals, frequently on-off switching, pulse-width modulation, or serial data protocols, are also involved. The inputs, though, come in many forms with many different characteristics, be they pneumatic, hydraulic or electronic. The amplitudes and/or frequencies of the analog signals may be sampled at a predetermined rate to generate a digital representation at the times of sampling. Modern applications, control schemes and devices usually necessitate the use of electronic inputs in one form or another. The amplitudes of analog signals may represent temperature, pressure, velocity or even light intensity. Generally, a data acquisition system typically is composed of transducers for measuring and providing electrical signals, signal conditioning hardware performing amplification, isolation and/or filtering, and measurement or DAQ hardware for receiving digital and analog signals and providing them to a processing system, a computer based DAQ system may further include analysis hardware and software for analyzing and appropriately displaying the measured data. The DAQ hardware is typically plugged into one of the I/O slots of the computer system. The field signals are provided to the DAQ hardware.
Signal conditioner is a critical component in a DAQ system because it performs many functions such as signal amplification, filtering, electrical isolation, linearization, transducer excitation, switching and multiplexing. Most sensors and transducers generate signals that must be conditioned before a measurement or DAQ device can reliably and accurately acquire the signal. This front-end processing is referred to as signal conditioning. Predistortion, a signal conditioning technique which is used in connection with for example power amplifier and transmitter systems, is used to compensate for distortion caused by the power amplifier or similar system by predistorting the input signal to the power amplifier with the inverse of the distortion characteristics of the power amplifier. A signal conditioner may create excitation for certain transducers such as strain gauges and resistance temperature detectors, which require external excitation voltages or currents. Due to imperfections in sensor element manufacturing, the sensor signal conditioner must be able to compensate the transducer output signal for the span and offset of the sensor output over the operating temperature range. Signal conditioners also may linearize voltage levels from transducers so that a simple scaling function can convert the voltage to the measured phenomena. Signal conditioning systems may also perform a switching or multiplexing function to increase the functionality of the measurement and automation system.
There are various types or classes of signal conditioning systems. Adaptive signal conditioning systems can be found in all areas of electronics and communication, and are generally used for adaptively conditioning the input signal to a signal conversion system such as an amplifier chain or any other suitable system in order to continuously provide a desired output signal of the overall system. The fast fourier transform (FFT) is a known method of converting a signal into its spectral components for analysis and manipulation. The FFT has utility in a vast variety of applications including signal processing, image enhancement, etc. FFTs are particularly useful in the conditioning of an output signal of a sensor or other type of detection device. Different types of sensors are used for sensing parameters such as quantity, pressure, temperature, position, etc. By taking the FFT of the output signal, it is possible to condition the signal in order to obtain information such as phase and amplitude. Signal conditioning becomes critically important for wireless systems because electronic inputs can have different characteristics and ranges. Through signal conditioning, radio frequency (RF) wireless data acquisition systems can convert the input to a conditioned electronic signal which is used to modulate a carrier frequency which is then transmitted as a radio frequency signal to equipment in another location.