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Sensor Measurement Fundamentals Series

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Introduction to Data Acquisition Basics and Terminology

Jackie Byrne

Product Marketing Engineer

National Instruments

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What Is Data Acquisition (DAQ)?

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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

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Why Measure?

• 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

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What Is Data Acquisition (DAQ)?

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Components of a DAQ System

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Components of a DAQ System

Sensor/Signal DAQ Hardware Computer

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Sensor Overview


Converts physical phenomena into a measurable electrical signal

Transducer or

Sensor

Signal Physical Phenomena

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Sensor Examples Phenomena Transducers/Sensors Electrical Characteristics

Temperature Thermocouples, resistive temperature

devices (RTDs), thermistors

Low voltage output, nonlinear

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


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Components of DAQ System


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Signal Conditioning

• Improves a signal that is difficult for your DAQ device to

measure

• Not always required

Noisy, Low-Level Signal Filtered, Amplified Signal

Signal Conditioning


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Transducers/Signals Signal Conditioning

Thermocouple Amplification, linearization, cold-junction

compensation

RTD (resistance temperature

detector)

Current excitation, linearization

Strain gage Voltage excitation, bridge configuration,

linearization

Common mode or high voltage Isolation amplifier

Loads requiring AC switching

or large current flow

Electromechanical relays or solid-state

relays

High frequency noise Lowpass filters

Signal Conditioning Examples


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Analog

Rate State Shape

Level Frequency

Digital

Signal Classification

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The Three Rs of Data Acquisition


Resolution Range Rate

3-Bit Resolution

Original Waveform

6-Bit Resolution

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3-Bit Resolution

Original Waveform Range

of

10 V to -10 V

Range

of

2 V to -2 V

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Original Waveform (10 Hz)

Sampling Rate = 11 Hz



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Diversity of NI DAQ


Low-Cost Bus-Powered High-

Performance

NI CompactDAQ OEM

PCI/PXI PCI Express/

PXI Express

Ethernet Wireless USB

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Components of DAQ System


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Why Is Software Important?


Total Measurement Application Cost =

Hardware Price + Software Price + Development Costs

Hidden Cost Visible Price

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Why Is Software Important?


Source: National Instruments Survey of Data Acquisition Customers and Prospects, 2005

36%,

Hardware

and Software

Price 64%, Development

Costs

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Why Is a Driver Important?

At its core, NI-DAQmx is a powerful hardware driver for

controlling data acquisition hardware.

Hundreds of DAQ Devices

DAQ Hardware

USB, PXI Express, PCI Express

Bus

NI-DAQmx

Hardware Driver

LabVIEW, C, .NET

Application Software


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Configuration Versus Programming


Configuration Programming

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Configure

Channel

Configure

Timing

Configure

Triggering

Start

Acquisition

Read

Data

Clear

Task

NI-DAQmx LabVIEW API


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NI-DAQmx LabVIEW API

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DAQmxCreateAIVoltageChan( taskHandle, “Dev1/ai0”, “”, DAQmx_Val_Cfg_Default,

-10.0, 10.0, DAQmx_Val_Volts, NULL );

DAQmxCfgSampClkTiming( taskHandle, “”, 10000.0, DAQmx_Val_Rising,

DAQmx_Val_FiniteSamps, 1000 );

DAQmxCfgDigEdgeStartTrig( taskHandle, “/Dev1/PFI0”, DAQmx_Val_Rising );

DAQmxStartTask( taskHandle );

DAQmxReadAnalogF64( taskHandle, -1, 10.0, 0, data, 1000, &read, NULL );

printf( “Acquired %d samples. %d”, read );

DAQmxClearTask( taskHandle );

Read Data

Clear Task

Configure Channel

Configure Timing

Start Acquisition

Configure Triggering


NI-DAQmx C API

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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

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Technologies Behind NI Data Acquisition

Signal Streaming

Leverage the same NI-DAQmx API for communicating across

PCI, PCI Express,

PXI, PXI Express, USB, Ethernet, and WiFi

Timing and Synchronization Technology

System timing controller that provides high-speed digital I/O,

triggering, and synchronization between several modules and

chassis

Isolation

• Varying levels of electrical isolation to improve measurement

accuracy and device performance

• Isolated on a per channel basis, as groups of channels, or as a

complete system

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