Steps for Reducing Noise in Your Measurements: Part 2, During the Test and Strain Measurements
Here is Part 2 in your step-by-step guide to reducing or eliminating noise from your strain measurements. In this article, we will assume that the Steps for Reducing Noise in your Measurements: Part 1, Planning are complete and that the strain gage, cable, and instrument selections are optimized and completed. The strain gage and instrument installation is complete and the test is ready to begin:
During the test:
1. Is noise apparent in your data really noise or is it signal?Turn off the excitation voltage and leave the strain gage connected to the instrument.If the “noise” is no longer apparent in your data, then it was not noise at all but was some form of active signal from the Wheatstone bridge.With the excitation voltage off, there can be no signal from the strain gage, and any signal remaining in your data can be considered unwanted noise.
2. Look at the frequency of the noise in your data, once you have determined in Step 1 that it is noise.Does the frequency match any known sources in your test environment?You may find some nearby equipment running at 50Hz or 60Hz which could be contributing to the noise, RF sources, or motor drives.With the excitation still off, turn off whatever equipment is not needed and check for changes (hopefully improvement) in your noise level.
3. Turn the excitation voltage on.Maximize your excitation voltage to give the best possible signal-to-noise ratio.Take advantage of the fact that output of a Wheatstone bridge is proportional to excitation voltage, and any increase in output means a proportional decrease in instrument gain.Reduced instrument gain means reduced noise.At this point, with the gages selected and installed, you can find the maximum voltage for the installation very easily:
a. Start with a low excitation voltage, zero your instrumentation, and observe the data.
b. Is the zero holding without drift?If so, increase the excitation voltage by an increment, about 1 V.
c. Repeat these steps until you reach the point at which you begin to see drift due to self-heating of the strain gage.For a static strain measurement, reduce the excitation by about 1V and you have maximized the best possible excitation voltage setting for the given strain gage and heat sink conditions.
d. Remember that for dynamic testing you can tolerate some slow drift in the output of the strain gage, and you may be able to further increase the excitation voltage and reduce noise.
4. If using a shielded cable, the best general advice is to connect the shield only to instrument ground.The shield at the strain gage end of the cable should be left unconnected.Connecting the shield to both the test article and at the instrument can create current flow on the shield, and actually increase the noise in the strain measurement.That being said, there are some circumstances where multiple grounding points on the shield may improve severe noise situations.Consult Micro-Measurements for more information.
a. If you are measuring static strain, you can take full advantage of the filtering offered by your signal conditioner or data system.Select a low-pass filter to eliminate unwanted noise frequencies.
b. For dynamic testing, you will need to select a low-pass filter frequency cutoff that will still pass the measurement frequency of your strain measurements.
6. Computers and interfaces can inject noise into the instrumentation.One way to determine if this is part of the noise in your data is to disconnect the Wheatstone bridge, and at the instrument itself, connect a 120Ω to 1000Ω resistor directly between the signal inputs (signal +, signal -); the excitation pins are left disconnected.Now record or observe the data.If noise is apparent, then the noise may be attributed to the instrumentation.While you are performing this check, also place your hand on the instrument chassis and observe the data.If you see an increase in noise, this can be an indicator of a poor earth ground connection.
Micro-Measurements has published Tech Note TN-501 Noise Control in Strain Gage Measurements with greater detail on this topic.
For more information about reducing noise in strain measurements, contact a Micro-Measurements Applications Engineer, Technical Sales Manager, or independent Sales Representative at email@example.com.