Signal Conditioning Requirements for Strain Gages
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.
Bridge Completion
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 signal conditioners 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.
Bridge Excitation
Strain gage 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 of around 3 V and 10 V are common.
Excitation Sensing
If the strain gage circuit is located away from the signal conditioner 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 signal conditioners 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 serve to regulate the excitation supply, to compensate for lead losses, and to 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 negligible effect on the measurement. You then can use the measured excitation voltage in the voltage-to-strain conversion to compensate for lead losses.
Signal Amplification
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 signal conditioners usually include amplifiers to boost the signal level, to increase measurement resolution, and to improve signal-to-noise ratios. For example, SCXI signal conditioning modules include configurable gain amplifiers with gains up to 2,000.
Offset Nulling (Bridge Balancing)
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.
Software Compensation
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 method is simple, fast, and 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.
Offset Nulling Circuit
The second bridge balancing method uses an adjustable resistor, or potentiometer, to electrically adjust the output of the bridge to 0 V.
Hardware Nulling Compensation
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 the device provides.
Shunt Calibration (Gain Adjustment)
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.
Linearization
While strain gages are close to linear, they do stray from linear at large strains. You need hardware or software to convert the voltage output of the strain gage into a strain measurement. The conversion formula you use can depend on the type of strain gage you use. Half-bridge and full-bridge strain gages offer more accurate conversion formulas.