Load, Force, and Torque Measurements

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 National Instruments.

General Concepts

A load cell is a member of a family of force transducers. This implies that a load cell creates an electrical signal or response when acted upon by a force or load. Most load cells work off of changes in shape of an object when subjected to forces, and from this change a measurement can be obtained. A typical load cell consists of an outer housing, a sensing element, and a gauge or connections to signal conditioning equipment. Load cells are most practical for measuring either static 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 of the material used in the load cell as a result of the applied force. When the load cell experiences external forces it causes a characteristic of the load cell to change, this change is then what is used to determine the direction and magnitude of the force.

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\Analog In\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-1520 When Excitation Is Turned On?

Signal Conditioning Required

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

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

Signal Amplification
The output of load cells and bridges is relatively small. In practice, most load cells and load-based transducers output less than 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 measurement resolution and improve signal-to-noise ratios. SCXI signal conditioning modules, for example, include configurable gain amplifiers with gains up to 2000.

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

1. Software Compensation–The first method compensates for the initial voltage in software. With this method, you take an initial measurement before strain input is applied. This is also referred to as auto-zero. This method is simple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is that the offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, therefore limiting the dynamic range of the measurement.
2. Offset-Nulling Circuit–The second balancing method uses an adjustable resistance, or potentiometer, to physically adjust the output of the bridge to 0 V. By varying the position of the potentiometer, you can control the level of the bridge output set the output to 0 V initially.
3. Buffered Offset Nulling–The third method, like the software method, does not affect the bridge directly. With buffered nulling, a nulling circuit adds an adjustable DC voltage to the output of the instrumentation amplifier.

Shunt Calibration
The normal procedure to verify the output of a load cell measurement system relative to some predetermined mechanical input or strain is called shunt calibration. Shunt calibration involves simulating the input of strain by changing the resistance of an arm in the bridge by some known amount. Shunting, or connecting, a large resistor of known value accomplishes this across one arm of the bridge, creating a known delta R. You can then measure and compare the output of 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 a LabVIEW 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 A Channel Of My SCXI-1520?

NI Supported Devices

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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