This document provides step-by-step instructions for wiring and configuring your DAQ device for strain gage measurements. Before you begin using your NI DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing LabVIEW and NI-DAQmx document for more information.
You can measure strain with a strain gage, which is a device with electrical resistance that varies in proportion to the amount of strain in the device, and with signal conditioning. When using a strain gage, you bond the strain gage to the device under test, apply force, and measure the strain by detecting changes in resistance (Ω). Strain gages return varying voltages in response to stress or vibrations in materials. Resistance changes in parts of the strain gage to indicate deformation of the material. Strain gages require excitation, generally voltage excitation, and linearization of the voltage measurements.
Strain measurements rarely involve quantities larger than a few microstrain (µε). Therefore, measuring strain requires accurate measurements of very small changes in resistance. For example, if a test specimen undergoes a substantial strain of 500 µε, a strain gage with a gage factor of 2 exhibits a change in electrical resistance of only 2 × (500 × 10 -6 ) = 0.1%. For 120 Ω, this is a change of only 0.12 Ω.
To measure such small changes in resistance and compensate for temperature sensitivity, strain gages often use a Wheatstone bridge configuration with a voltage or current excitation source. The general Wheatstone bridge, shown in the following figure, is a network of four resistive legs with an excitation voltage, VEX, that is applied across the bridge. One or more of these legs can be active sensing elements.
Figure 1. Wheatstone Bridge
The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R1 and R2 compose one voltage divider circuit, and R3 and R4 compose the second voltage divider circuit. You measure the output of a Wheatstone bridge between the middle nodes of the two voltage dividers.
A physical phenomenon, such as a temperature shift or a change in strain applied to a specimen, changes the resistance of the sensing elements in the Wheatstone bridge. You can use the Wheatstone bridge configuration to help measure the small variations in resistance that the sensing elements produce corresponding to a physical change in the specimen.
A fundamental parameter of the strain gage is its sensitivity to strain, expressed quantitatively as the gage factor. Gage factor is the ratio of the fractional change in electrical resistance to the fractional change in length, or strain. The gage factor must be the same for each gage in the bridge.
The gage factor for metallic strain gages is usually around 2. You can obtain the actual gage factor of a particular strain gage from the sensor vendor or sensor documentation.
Nominal gage resistance is the resistance of a strain gage in an unstrained position. You can obtain the nominal gage resistance of a particular gage from the sensor vendor or sensor documentation. The resistance across each arm of the bridge must be the same for the bridge to be unstrained. For example, if you have two strain gages and two reference resistors, the gages must have the same nominal gage resistance, and the resistance of the reference resistors must be the same as the nominal gage resistance for the strain gages.
Configuration | Number of Active Elements |
Quarter-bridge | 1 |
Half-bridge | 2 |
Full-bridge | 4 |
Table 1. Strain Gage Configurations
NI-DAQmx strain virtual channels use the following equation to scale voltage readings to strain units.
Vr = (VCH / VEX)STRAINED – (VCH / VEX)UNSTRAINED
where VEX is the excitation voltage, and VCH is the measured voltage.
The following figure shows how to position a strain gage resistor in axial and bending configurations for the quarter-bridge type I.
Figure 2. Quarter-Bridge Type 1 Configuration
Quarter-bridge type I strain gage configurations have the following characteristics:
Figure 3. Quarter-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram in Figure 3:
The following figure shows how to position a strain gage resistor in axial and bending configurations for the quarter-bridge type II.
Figure 4. Quarter-Bridge Type II Configuration
Quarter-bridge type II strain gage configurations have the following characteristics:
Figure 5. Quarter-Bridge Type II Circuit Diagram
The following symbols apply to the circuit diagram:
The following figure shows how to position strain gage resistors in axial and bending configurations for the half-bridge type I.
Figure 6. Half-Bridge Type I Configuration
Half-bridge type I strain gage configurations have the following characteristics:
Figure 7. Half-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram:
The half-bridge type II configuration measures only bending strain. The following figure shows how to position strain gage resistors in a bending configuration for the half-bridge type II.
Figure 8. Half-Bridge Type II Configuration
Half-bridge type II strain gage configurations have the following characteristics:
Figure 9. Half-Bridge Type II Circuit Diagram
The following symbols apply to the circuit diagram:
The full-bridge type I configuration measures only the bending strain. The following figure shows how to position strain gage resistors in a bending configuration for the full-bridge type I.
Figure 10. Full-Bridge Type I Configuration
Full-bridge type I strain gage configurations have the following characteristics:
Figure 11. Full-Bridge Type I Circuit Diagram
The following symbols apply to the circuit diagram:
The full-bridge type II configuration measures only bending strain. The following figure shows how to position strain gage elements in a bending configuration for the full-bridge type II.
Figure 12. Full-Bridge Type II Configuration
Full-bridge type II strain gage configurations have the following characteristics:
Figure 13. Full-Bridge Type II Circuit Diagram
The following symbols apply to the circuit diagram:
The following figure shows how to position strain gage resistors in an axial configuration for the full-bridge type III. The full-bridge type III configuration measures only the axial configuration.
Figure 14. Full-Bridge Type III Configuration
Full-bridge type III strain gage configurations have the following characteristics:
Figure 15. Full-Bridge Type III Circuit Diagram
The following symbols apply to the circuit diagram:
Common signal conditioning requirements for strain gages are bridge completion, bridge excitation, excitation sensing, signal amplification, offset nulling, shunt calibration, and linearization. You should calibrate your strain gage periodically to account for changes in the physical characteristics of the strain gage and in the material the gage is mounted to, to account for variations in the leadwire resistance, and to compensate for imperfections in the measurement system. Calibrating strain gages usually involves two steps: offset nulling, or bridge balancing, and shunt calibration, or gain adjustment.
Unless you are using a full-bridge strain gage sensor with four active gages, you must complete the bridge with reference resistors. Therefore, strain gage DAQ devices typically provide half-bridge completion networks consisting of two high-precision reference resistors. The nominal resistance of the completion resistors is less important than how well the two resistors match. Ideally, the resistors match well and provide a stable reference voltage of VEX/2 to the negative input lead of the measurement channel. The high resistance of the completion resistors helps minimize the current draw from the excitation voltage.
Strain gage DAQ devices typically provide a constant voltage source to power the bridge. Though there is no standard voltage level that is recognized industry wide, excitation voltage levels of around 3 V and 10 V are common.
If the strain gage circuit is located away from the DAQ device and excitation source, a possible source of error is voltage drops caused by resistance in the wires that connect the excitation voltage to the bridge. Therefore, some DAQ devices include a feature called remote sensing to compensate for this error. There are two common methods of remote sensing. With feedback remote sensing, you connect extra sense wires to the point where the excitation voltage wires connect to the bridge circuit. The extra sense wires regulate the excitation supply, compensate for lead losses, and deliver the needed voltage at the bridge. An alternative remote sensing scheme uses a separate measurement channel to measure directly the excitation voltage delivered across the bridge. Because the measurement channel leads carry very little current, the lead resistance has a negligible effect on the measurement. You then can use the measured excitation voltage in the voltage-to-strain conversion to compensate for lead losses.
The output of strain gages and bridges is relatively small. In practice, most strain gage bridges and strain-based transducers output less than 10 mV/V, or 10 millivolts of output per volt of excitation voltage. Therefore, strain gage DAQ devices usually include amplifiers to boost the signal level, increase measurement resolution, and improve signal-to-noise ratios.
When you install a strain gage, the gage probably will not output exactly 0 V when no strain is applied. Slight variations in resistance among the bridge legs generate some nonzero initial offset voltage. A system can handle this initial offset voltage in a few different ways.
This method of bridge balancing compensates for the initial voltage in software. With this method, you take an initial measurement before the strain input is applied. You then can use this initial voltage in the strain equations. This simple and fast method requires no manual adjustments. The disadvantage of the software compensation method is that the method does not remove the offset of the bridge. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, thus limiting the dynamic range of the measurement.
The second bridge balancing method uses an adjustable resistor, or potentiometer, to electrically adjust the output of the bridge to 0 V.
The third method, like the software compensation method, does not affect the bridge directly. A nulling circuit adds an adjustable DC voltage, positive or negative, to the output of the instrumentation amplifier to compensate for initial bridge offset. Refer to the device documentation to determine the hardware nulling methods your DAQ device provides.
You can verify the output of a strain gage measurement system by comparing the measured strain with a calculated strain value if the physical strain on the strain gage is known. The difference (if any) between the calculated and the measured strain can then be used for each measurement as a gain adjustment factor. If not all parameters of a strain measurement are known, you can simulate a mechanical strain by connecting a large known resistor in parallel with the strain gage. This resistor, called a shunt resistor, offsets the zero voltage of the bridge. Because the value of the shunt resistor is known, you can calculate the mechanical strain corresponding to the voltage drop of the resistor. You can then compare this voltage to the voltage output of the strain gage undergoing the same mechanical strain. This gain adjustment factor (calibration factor) can then be applied to every measurement.
Before connecting any signals, locate your device pinout.
Figure 16. Device Terminals Help
You can use MAX to quickly verify the accuracy of your measurement system setup. Using an NI-DAQmx Global Virtual Channel, you can configure a strain measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.
Follow these steps to begin:Figure 17. Creating an NI-DAQmx Virtual Channel
Figure 18. Device Physical Channels
Figure 19. Setting Up a Strain Channel in MAX
The next step is to physically connect the strain gage to your DAQ device.
Figure 20. Strain Connection Diagram
The connection diagram above indicates which pins on your DAQ device should be wired according to the physical channel you selected. In this example, a full bridge type I configuration uses pins 2, 3, 6, and 7, corresponding to AI+, AI-, EX+, and EX- on an NI 9237 C Series module.
When you configure a strain measurement task, you can use the Strain Gage Calibration Wizard to calibrate your strain gage. Complete the following steps to calibrate the strain gage:
Figure 21. Strain Gage Calibration Wizard
Figure 22. Measure and Calibration Window
Use NI-DAQmx global virtual channels to preview your measurements.
Figure 23. Previewing a Strain Measurement in MAX
You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.