Environmental testing: the transducers
Environmental testing: the transducers
When conducting an environmental test, signals are received from the device under test that describe the condition of the device in relation to a particular test parameter. Information is extracted from the signals and is used to determine whether the test is being performed as desired; and to monitor the operability or integrity of the device under test and so determine whether the device has passed or failed the test.
Each signal that is so used results from a measurement. The measurement is obtained from a sensor that is either connected to or oriented toward the device under test, and describes in some way the behavior of the device. (Our own nervous system is a good example, with our sensory organs measuring various characteristics of our environment, and transmitting signals to the brain for analysis and action.)
The variety of sensors used to make engineering measurements indicates the wide range of phenomena that can yield signals with environmental information, for test purposes:
Sensor type | Environmental parameter measured |
|---|---|
| Microphone | Acoustic noise in air; speech; music |
| Hydrophone | Acoustic noise in water |
| Geophone | Acoustic noise in earth; seismic disturbances |
| Thermocouple | Temperature |
| Flowmeter | Fluid flow |
| Pressure meter | Hydrostatic or dynamic fluid pressure |
| Accelerometer | Mechanical acceleration, velocity, displacement (in motion) |
| Strain gauge | Mechanical strain or tension (static) |
| EEG, EKG, pulse | Biomedical data |
Each of these sensors is a type of transducer that reacts to some physical phenomenon in the environment and produces a time-varying voltage that represents the changing amplitude of the quantity measured. Some situations or tests are adequately described by a single signal from the device under test. Other tests require signals from multiple measurement points to obtain a full description of the physical state of the device. For a flight or ground vibration test of an aircraft or missile, sensors may be needed at a very large number of points on the structure.
In vibration testing, the accelerometer is the sensor used to measure the amount of acceleration present, at the point where the accelerometer is mounted to the device under test. From the acceleration information, numerical integration is used to compute the velocity and displacement at that point on the device. The simplest accelerometer measures the acceleration in a single direction, or axis; however, two- and three-axis accelerometers are available that supply two or three independent signals, each of which measures the acceleration at that point in a particular direction.
Accelerometers are also available in a range of sizes, weights, and costs. It is desirable to use as small and low-weight an accelerometer as possible. This is especially true for devices that are themselves small in size (with limited areas on which to mount an accelerometer) and low in weight. An accelerometer that weighs a significant percent of the device weight would bias the mechanical response of the device and thereby invalidate the vibration test.
It is the job of the test engineer to select the number and location of accelerometer test (mounting) points on a device under test, and to choose appropriate accelerometers for each mounting point. The goal is to select the minimum number of points/accelerometers that will give sufficient vibration information to satisfy the test requirements; and to select points, and the accelerometers for them, such that when mounted the accelerometers will not significantly alter the mechanical characteristics of the device.
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