Sensor Measurement Fundamentals Series
Introduction to Data Acquisition Basics and Terminology Litkei Márton District Sales Manager National Instruments
What Is Data Acquisition (DAQ)? 3
Why Measure? Engineers need to determine system behavior to optimize performance Signals from sensors indicate how the system is operating Engineers can use that information to modify performance 4
In-Vehicle Data Logging In-vehicle data acquisition is a complex task that provides valuable information about actual on-road performance Diverse data needs to be collected from various sources up to 100 channels including transducers and buses such as CAN may be involved Mixed signals: vibration, RPM, strain, temperature, torque, load, pressure, velocity, CAN, and so on 5
What is Data Acquisition (DAQ)? 6
What is Data Acquisition (DAQ)? 7
What is Data Acquisition (DAQ)? 8
Components of DAQ System 9
Sensor Overview Converts physical phenomena into a measurable electrical signal Transducer or Sensor Physical Phenomena Signal 10
Sensor Examples Phenomena Transducers/Sensors Electrical Characteristics Temperature Thermocouples, resistive Low voltage output, nonlinear temperature devices (RTDs), thermistors Low resistance, nonlinear Light Vacuum tube, photo sensors Current loop output if current type (typical 4 to 20 ma) Sound Microphone Charge output, powered converter Force and pressure Strain gages, piezoelectric transducers Low resistance, low sensitivity, nonlinear Position and displacement Potentiometers, linear voltage differential transformer (LVDT), optical encoder LVDTs: Inductive, requires demodulation Vibration Accelerometer Charge output, powered converter ph ph electrodes 11
Signals Come in Two Forms: Digital and Analog Digital Analog 12
Digital Signals Digital signals have two states: high and low Digital lines on a DAQ device accept and generate transistor-transistor logic (TTL) compatible signals High State +5.0 V Indeterminate Low State +2.2 V +0.8 V 0 V State Rate 13
Signals Come in Two Forms: Digital and Analog Digital Analog 14
Analog Signals Analog signals are continuous signals that can be any value with respect to time. 15
Analog Terminology 4.71 V Level The instantaneous value of the signal at a given point in time. Shape The form that the analog signal takes, which often dictates further analysis that can be performed on the signal. Frequency The number of occurrences of a repeating event over time. 16
Components of DAQ System 17
Conditioning Signals for Quality Measurements Signal conditioning improves a signal that is difficult for your DAQ device to measure Signal conditioning is not always required Signal Conditioning Noisy, Low-Level Signal Filtered, Amplified Signal 18
Common Signal Conditioning Examples Transducer/Signals Thermocouples RTD (Resistance Temperature Detector) Strain Gage Signal Conditioning Amplification, Linearization, Cold-Junction Compensation Current Excitation, Linearization Voltage Excitation, Bridge Configuration, Linearization Common Mode or High Voltage Isolation Amplifier Loads Requiring AC Switching or Large Current Flow High-Frequency Noise Electromechanical Relays or Solid-State Relays Low-Pass Filters 19
Examining Common Signal Conditioning for Voltage Measurements Amplification Attenuation Filtering Isolation 20
Amplification Used on low-level signals Maximizes use of analog-to-digital converter (ADC) range and increases accuracy Increases signal-to-noise ratio (SNR) Amplifier 21
Example: Amplification and the Signal-to-Noise (SNR) Ratio 10 mv Signal 1 mv Noise 1000x Amplifier ADC 1 mv Noise» SNR = 10 10 mv Signal 1000x Amplifier ADC 22» SNR = 10,000
Attenuation Decreases the input signal amplitude to fit within the range of the DAQ device Necessary when input signal voltages are beyond the range of the DAQ device Attenuator 23
Filtering Filters remove unwanted noise from a measured signal and block unwanted frequencies Time Domain Lowpass Filter Time Domain Frequency Domain Frequency Domain 24
Isolation Isolation helps to pass a signal from its source to a measurement device without a direct physical connection Blocks high common-mode signals Breaks ground loops Protects your instrumentation Electromagnetic Isolation Capacitive Optical 25
Components of DAQ System 26
The Three R s of Data Acquisition: Resolution Resolution Range Rate 6-Bit Resolution Original Signal 3-Bit Resolution 27
The Three R s of Data Acquisition: Range Resolution Range Rate Range of -10V -2V to to 10V 2V Original Signal 3-Bit Resolution 28
The Three R s of Data Acquisition: Rate Resolution Range Rate Original Waveform (10 Hz) Sampling Rate = 100 25 11 Hz 29
Sampling Rate Considerations An analog input signal is continuous with respect to time. Sampled signal is series of discrete samples acquired at a specified sampling rate. Actual Signal The faster we sample, the more our sampled signal will look like our actual signal. If not sampled fast enough, a problem known as aliasing will occur. Sampled Signal 30
Amplitude Amplitude Aliasing Sample rate: how often an A/D conversion takes place Alias: misrepresentation of a signal Adequately Sampled Aliased Due to Undersampling 6 Frequency 2 Frequency 31
Following the Nyquist Theorem Prevents Aliasing Frequency To accurately represent the frequency of your original signal You must sample at greater than 2 times the maximum frequency component of your signal. Shape To accurately represent the shape of your original signal You must sample between 5 10 times greater than the maximum frequency component of your signal. 32
The Nyquist Theorem in Action Aliased Signal 100 Hz Sine Wave Sampled at 100 Hz Adequately Sampled for Frequency Only 100 Hz Sine Wave Sampled at 200 Hz 100 Hz Sine Wave Sampled at 1 khz Adequately Sampled for Both Frequency and Shape 33
Components of DAQ System 34
Why is Software Important? Total Measurement Application Cost = Hardware Price + Software Price+ Development Costs Visible Price Hidden Cost 35
Why is Software Important? 36%, Hardware and Software Price 64%, Development Costs Source: National Instruments Survey of Data Acquisition Customers and Prospects, 2005 36
Why is a Driver Important? Application Software LabVIEW, C,.NET Hardware Driver DAQmx Bus USB, PXI Express, Ethernet, PCI, Wireless DAQ Hardware Various Devices 37
Software Options Configuration Programming 38
Software Options Configuration LabVIEW Code Programming C Code DAQmxCfgSampClkTiming( taskhandle,, 10000.0,, DAQmx_Val_FiniteSamps, 1000 ); DAQmxClearTask( taskhandle ); 39
NI Data Acquisition Solution PXI Optimized for high channel counts and tight synchronization System NI CompactDAQ Customize with a variety of chassis and module types Desktop DAQ Install in a desktop PC slot for maximum data throughput Single Device 40 Portable DAQ Easily connect to any laptop or desktop with simple setup
Bridging the Hardware and Software Gap with NI-DAQmx NI-DAQmx is a single, free hardware driver that supports various development languages and hundreds of NI data acquisition hardware platforms. The mark LabWindows is used under a license from Microsoft Corporation. Windows is a registered trademark of Microsoft Corporation in the United States and other countries. 41
Choosing the Best Data Acquisition System for Your Application Your DAQ system requires the following: Appropriate signal conditioning Appropriate resolution Suitable sampling rate Right type of timing and triggering Correct bus Best internal calibration technique Most effective suite of software analysis/reporting 42
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