Analog Input Channels
Figure 1. Strain gages measure small changes in electrical resistance proportional to compression and tension.
A strain gage is a device whose electrical resistance varies in proportion to the compression and tension forces it is experiencing. Strain measurements typically involve very small variations in resistance, quantities on the order of millistrain. To measure such small changes in resistance, strain gages are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge. When all four resistive arms match with identical values, VO in Figure 9 will measure 0V. VO will be nonzero and vary when any of the resistive arms are unbalanced. Quarter-, half-, or full-bridge configurations are employed with varying levels of accuracy and ease of setup.
Figure 2. The general Wheatstone bridge consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.
For more information, read the Strain How-To Guide
Figure 3. The most common method for measuring load, pressure, and torque is to employ a full-bridge strain gage-based transducer.
The most common method for measuring load, pressure, and torque is to employ a full-bridge strain gage-based transducer. A load cell, used to measure load and force, consists of an array of strain gages, which measure the deformation of a structural member and converts it into an electrical signal. Pressure transducers operate under the same principle as load cells. Strain gages, mounted on a diaphragm where pressure is applied, measure the deformation of the diaphragm that is proportional to the pressure. Torque sensors are composed of strain gages that are affixed to a torsion bar. As the bar turns, the gages respond to the bar's shear stress, which is proportional to the torque.
Load, pressure, and torque sensors can output low- or high-level voltages, depending on its excitation requirements. Low-level sensors are typically powered by a measurement device and output mV signals. High-level sensors (or amplified sensors) require higher external power sources to operate, and output ±5 V, ±10 V, or 4-20 mA.
For more information, read the Pressure and Load How-To Guide
FBG optical sensors are becoming a popular alternative to conventional resistive foil strain gages due to the many benefits it provides, including longer deployed life span and simpler installation.
Figure 4. FBG strain gages are available in a variety of form factors including the above two, that you can weld and glue onto structures.
Optical sensors and fibers are nonconductive, electrically passive and immune to EMI-induced noise. Interrogation with a high-power light source enables measurements over extreme long distances with no loss in measurement accuracy. And unlike electrical sensors, one can daisy chain dozens of optical sensors on a single fiber connected to a single optical channel, greatly reducing the size, weight and complexity of the measurement system.
For more information, read a white paper on FBG optical strain gages.
The following section describes the necessary signal conditioning to make an effective strain, load, pressure, or torque sensor measurement. The basic requirements to make bridge-based measurements are bridge completion, excitation, amplification, bridge balancing, and shunt calibration.
Strain gage configurations are based on the concept of a Wheatstone bridge. Small variations in resistance that would unbalance the bridge shown in Figure 9 will cause a nonzero potential difference at VO, while an ideally balance resistive bridge will yield 0V at VO. The three types of configurations include quarter-, half-, and full-bridge depending on the number of active elements in the Wheatstone bridge. Each of these configurations is also subdivided into multiple configuration types determined by orientation of the active gage elements and the kind of strain measured.
Number of Active Elements
Table 1. The types of bridge configurations include quarter-, half-, and full-bridge depending on the number of active elements in the Wheatstone bridge.
Unless you are using a full-bridge strain gage sensor with four active gages, you need to complete the bridge with reference resistors. Therefore, strain gage signal conditioners typically provide half-bridge completion networks consisting of high-precision reference resistors.
The output of strain gages and bridges is relatively small. In practice, most strain and bridge-based sensors output less than 10 mV/V, which means 10 mV per volt of excitation. Therefore, strain gage signal conditioners usually include amplifiers that boost the signal level to increase resolution and improve signal-to-noise ratios.
Strain gages are often located in electrically noisy environments. It is therefore essential to be able to eliminate noise that can couple to strain gages. Lowpass filters, when used with strain gages, can remove the high-frequency noise prevalent in most environmental settings.
Strain gage-based signal conditioners typically provide a constant voltage source to power the bridge. 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 important that the excitation voltage be very accurate and stable. Alternatively, you can use a less accurate or stable voltage, and measure the excitation voltage to calculate the correct strain.
When a bridge is installed, it is very unlikely that the bridge will output exactly zero volts when no strain is applied. Slight variations in resistance among the bridge arms, pre-strained installation conditions and lead resistance will generate some nonzero initial offset voltage. Offset nulling can be performed by either hardware or software. In software compensation, you take an initial measurement before strain input is applied, and use this offset to compensate subsequent measurements. The hardware balancing method uses an adjustable resistance, a potentiometer, to physically adjust the output of the bridge to zero.
Shunt calibration is the procedure used to verify the output of a strain gage measurement system relative to some predetermined mechanical input or strain. Shunt calibration involves simulating the input of strain by changing the resistance of an arm in the bridge by some known amount. This is accomplished by shunting, or connecting, a large resistor of known value (Rs) across one arm of the bridge, creating a known ΔR. The output of the bridge can then be measured and compared to the expected voltage value. The results are used to correct span errors in the entire measurement path, or to simply verify general operation to gain confidence in the setup.
Figure 5. Shunt calibration involves connecting a large resistor of known value (Rs) across one arm of the bridge, creating a known ΔR.
Dynamic strain measurements require higher performing hardware than static or slow-changing strain. At the core is a faster ADC that has to both preserve its high accuracy and ability to measure small variations in voltages, while operating at high speeds. Other key elements of bridge-based signal conditioning such as filtering and amplification also need to operate properly at the specified measurement speeds and bandwidth.
National Instruments (NI) provides a variety of solutions for your bridge-based measurement system depending on sampling rate, isolation, and other needs of your application.
Figure 6. NI CompactDAQ is recommended for bridge-based measurements requiring simultaneous sampling and channel-to-channel isolation.
NI CompactDAQ is recommended for bridge-based measurements requiring simultaneous sampling, channel-to-channel isolation, and low to medium channel counts. Using multiple ADCs in a single module increases the overall sample rate on each channel and eliminates phase offset in between channels when sampling at higher speeds. You can choose modules with programmable bridge completion and excitation and up to 250 Vrms of channel-to-channel isolation and sampling rates of 50 kS/s/ch.
Figure 7. PXI bridge-based measurements are best suited for applications requiring high sampling rates and accuracy.
PXI bridge-based measurements are best suited for applications requiring maximum accuracy and bandwidth, as well as tight synchronization. The PXI Express bridge input module offers very low typical accuracy at 0.02 percent of reading with 24-bit resolution. The same module features simultaneous sampling and 25 kS/s per channel sample rate for dynamic measurements. The excitation voltage, bridge completion, and shunt calibration are all programmable by channel.
Figure 8. NI recommends the SCXI platform for a cost-effective solution to accurate bridge-based measurements.
NI recommends the SCXI platform for measuring bridge-based sensors with front-end signal conditioning. In addition, the SCXI quarter-bridge strain module is a cost-effective setup for measuring high-channel-counts of quarter-bridge strain gages. Lastly, the SCXI platform is recommended for bridge-based measurements that require channel-to-channel isolation.