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Overview

This page is a starting point for load, force, or torque measurement applications. It includes links to general concepts;signal conditioning required in order to measure load, force or torque; and the supported devices provided by NationalInstruments.

Table of Contents

General Concepts1.Signal Conditioning Required2.NI Supported Devices3.

General Concepts

A load cell is a member of a family of force transducers. This implies that a load cell creates an electrical signal orresponse when acted upon by a force or load. Most load cells work off of changes in shape of an object when subjected toforces, and from this change a measurement can be obtained. A typical load cell consists of an outer housing, a sensingelement, and a gauge or connections to signal conditioning equipment. Load cells are most practical for measuring eitherstatic loads or forces that vary at a rather slow rate.

Force measurements are not taken directly; rather, a force measurement is taken from measuring a physical property ofthe material used in the load cell as a result of the applied force. When the load cell experiences external forces it causesa characteristic of the load cell to change, this change is then what is used to determine the direction and magnitude of theforce.

Relevant LabVIEW Shipping Examples:Located in the Example Finder at Hardware Input and Output » DAQmx » Analog Measurements » Strain

Relevant ANSI C Shipping Examples:Located at \Program Files\National Instruments\NI-DAQ\Examples\DAQmx ANSI C\Analog In\Measure Strain

Relevant Visual Studio C# and VB .NET with Measurement Studio Shipping Examples:Located at \Program Files\National Instruments\MeasurementStudioVS2003\DotNET\Examples\DAQmx\AnalogIn\Measure Strain\AcqStrainSamples

Other example code posted on Developer Zone is listed in the related links section at the bottom of this document.

See Also:KnowledgeBase 2I7EBBJ3: How Can I Measure a Full-Bridge Transducer with the SCXI-1520? KnowledgeBase 2AJE1KJW: How Can I Read a Voltage Signal With the SCXI-1520?  KnowledgeBase 2APDE2JW: Can I Use the SCXI-1520 to Null the Offset of a Voltage Signal?  KnowledgeBase 2BIC1IRW: Why Do I Have an Offset in My Absolute Measurements with My SCXI-1520When Excitation Is Turned On? 

Signal Conditioning Required

Load cell measurement involves sensing extremely small changes in resistance. Therefore, proper selection and use ofthe bridge, signal conditioning, wiring, and data acquisition components are required for reliable measurements. Thefollowing signal conditioning parameters are applicable to any transducer that uses a Wheatstone bridge configuration.

Bridge ExcitationLoad cell signal conditioners typically provide a constant voltage source to power the bridge. While there is no standardvoltage level that is recognized industry wide, excitation voltage levels around 3 V to 10 V are common. While a higherexcitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors due toself-heating. It is very important that the excitation voltage be very accurate and stable. Alternatively, you can use a lessaccurate or stable voltage, and accurately measure, or sense, the excitation voltage so you can calculate the correctstrain.

Signal Amplification The output of load cells and bridges is relatively small. In practice, most load cells and load-based transducers output lessthan 10 mV/V (10 mV of output per volt of excitation voltage). With a 10 V excitation voltage, the output signal is 100 mV.

Therefore, load cell signal conditioners usually include amplifiers to boost the signal level to increase measurementresolution and improve signal-to-noise ratios. SCXI signal conditioning modules, for example, include configurable gainamplifiers with gains up to 2000.

Bridge Balancing, Offset Nulling 

Load, Force, and Torque Measurements

Document Type: Tutorial

NI Supported: YesPublish Date: Sep 6, 2006

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When a bridge is installed, it is very unlikely that the bridge outputs exactly 0 V when no strain is applied. Rather, slightvariations in resistance among the bridge arms and lead resistance generate some nonzero initial offset voltage. Thereare a few different ways that a system can handle this initial offset voltage.

Software Compensation–The first method compensates for the initial voltage in software. With this method, youtake an initial measurement before strain input is applied. This is also referred to as auto-zero. This method issimple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is thatthe offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to theoutput voltage, therefore limiting the dynamic range of the measurement.

1.

Offset-Nulling Circuit–The second balancing method uses an adjustable resistance, or potentiometer, to physicallyadjust the output of the bridge to 0 V. By varying the position of the potentiometer, you can control the level of thebridge output set the output to 0 V initially.

2.

Buffered Offset Nulling–The third method, like the software method, does not affect the bridge directly. Withbuffered nulling, a nulling circuit adds an adjustable DC voltage to the output of the instrumentation amplifier.

3.

 Shunt Calibration The normal procedure to verify the output of a load cell measurement system relative to some predetermined mechanicalinput or strain is called shunt calibration. Shunt calibration involves simulating the input of strain by changing theresistance of an arm in the bridge by some known amount. Shunting, or connecting, a large resistor of known valueaccomplishes this across one arm of the bridge, creating a known delta R. You can then measure and compare the outputof the bridge to the expected voltage value. You can use the results to correct span errors in the entire measurement path,or to simply verify general operation to gain confidence in the setup.

See Also:KnowledgeBase 3JFGH0XL: How Can I Access Strain Gauge Calibration Information Set in MAX in aLabVIEW VI? KnowledgeBase 2ZMFM2MW: How to Use Offset Nulling with External Excitation with the PXI-4220 or

SCXI-1520 KnowledgeBase 2FIF86UJ: Why Is The Residual Offset So Large After I Perform A Null Correction On AChannel Of My SCXI-1520? 

NI Supported Devices

Table 1.

NI Supported Device Terminal Block

SCXI-1520 SCXI-1314

PXI-4220 N/A

SCXI-1121 SCXI-1320

SCXI-1122 SCXI-1322

SCC-SG24 N/A

Related Links: NI-DAQmx Example: Ready-to-Run Strain Measurement NI-DAQmx Example: Digital Filtering with the SCXI-1520 NI-DAQmx Example: Nulling all Channels in a Custom Voltage with Excitation Task  Traditional NI-DAQ (Legacy) Example: Getting Started with Load Cells Using the SCXI-1520 Traditional NI-DAQ (Legacy) C Example: SCXI 1520 Continuous Acquisition Using Track and Hold Traditional NI-DAQ (Legacy) Example: Set SCXI-1520 Coarse or Fine Null Potentiometer Traditional NI-DAQ (Legacy) Example: Set SCXI-1520 Nulling Potentiometers with Settings Read from File Traditional NI-DAQ (Legacy) Example: Shunt Calibration for SCXI-1122 Module Traditional NI-DAQ (Legacy) Example: SCXI-1520 Manual Null Traditional NI-DAQ (Legacy) Example: Manual Strain-Null Calibration with File Output  

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