Data Acquisition on Aero

Data Acquisition on Aero-Engine 1 Introduction Data acquisition is the sampling of the real world to generate data that can be manipulated by a computer. Sometimes abbreviated DAQ or DAS, data acquisition typically involves acquisition of signals and waveforms and processing the signals to obtain desired information. The components of data acquisition systems include appropriate sensors that convert any measurement parameter to an electrical signal, then conditioning the electrical signal which can then be acquired by data acquisition hardware. The physical property or phenomenon could be the temperature or temperature change of a room, the intensity or intensity change of a light source, the pressure inside a chamber, the force applied to an object, or many other things. An effective data acquisition system can measure all of these different properties or phenomena. A transducer is a device that converts a physical property or phenomenon into a corresponding measurable electrical signal, such as voltage, current, change in resistance or capacitor values, etc.

Signals Signals may be digital (also called logic signals sometimes) or analog depending on the transducer used. Various examples of signal conditioning might be bridge completion, providing current or voltage excitation to the sensor, isolation, linearization, etc.

DAQ hardware DAQ hardware is what usually interfaces between the signal and a PC. It could be in the form of modules that can be connected to the computer's ports (parallel, serial, USB, etc...) or cards connected to slots (PCI, ISA, PCI-E, etc...) in the mother board. DAQ-cards often contain multiple components (multiplexer, ADC, DAC, TTL-IO, high speed timers, RAM).

DAQ software DAQ software is needed in order to the DAQ Hardware to work with a PC or other operating system. Examples of this kind of software are MATLAB and LabVIEW, both providing a high level graphical programming language.

2 Backgrounds Data acquisition technology has taken giant leaps forward over the last 30 to 40 years. For example, 40 years ago, in a typical college lab, apparatus for tracking the temperature rise in a crucible of sodium tungsten- bronze consisted of a thermocouple, a bridge, a lookup table, a pad of paper and a pencil. Today numerous options are available for gathering data. The optimal choice depends on several factors, including the complexity of the task, the speed and accuracy you require, and the documentation you want. Data acquisition systems range from the simple to the complex, with a range of performance and functionality. In industry field, due to the continuous development of the semiconductor integrated circuit technology in recent years, two American companiesTI and AD developed a sampling speed A / D devices in 100MbPs which hugely enhanced the performance of the DAQ hardware.

3 Theories Assuming an analog signal x (t) sampled once every Δt time. The time interval Δt is the sampling interval or the sampling period. Its reciprocal 1/Δt is referred to as a sampling frequency, the unit is the number of samples / sec. t = 0, Δt, 2Δt, 3Δt ......, values of the x (t) is known as sampling values. All x (0), x (Δt), x (2Δt) are sampled values. According to the sampling theorem, the minimum sampling frequency must be twice the frequency of the signal. The result of the sampling rate being too low is the frequency of the signal which is restored looks different from the original signal. This signal distortion called alias. Assumed that the sampling frequency fs is 100HZ, signal contains 25, 70,160, and 510Hz ingredients.

Below the Nyquist frequency (fs / 2 = 50Hz), signals can be properly sampled. And distortion will occur in the signal component of the frequency higher than 50HZ, respectively results in 30, 40 and 10 Hz distortion frequency F2, F3 and F4. The picture below shows the results of a signal with a suitable sampling rate and the low sampling rate for sampling. In order to avoid such a situation occurs, usually make the signal through a low-pass filterprior to signal acquisition (A / D), filtering off the signal component thatis higher than the Nyquist frequency. This filter is called anti-aliasing filter. Theoretically setting the sampling frequency 2 times the highest frequency component of the signal which were collected is enough, in fact, the engineering choice is of 5 to 10 times, and sometimes, in order to restore the waveform, or even higher. Typically, appropriate signal processing must be done after the signal acquisition, such as FFT, etc. There is a requirement on the number of samples, generally not only provide a signal cycle data sample, but 5 to 10 cycles or more samples. And hope that the total number of samples is the whole number of cycles. Here again, a difficulty occurs, we does not know, or do not know exactly what the frequency of the signal to be collected, therefore, not only the sample rate is not necessarily integer multiples of the frequency of the signal, and also cannot be guaranteed to provide a sample of the whole number of cycles. All we have is only a time sequence of discrete function of x (n) and the sampling frequency. This is the sole basis of the measurement and analysis. Data acquisition card, data acquisition modules, data acquisition, instrumentation, etc., are data acquisition tools.