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Measuring Vibration

This guide helps you with basic vibration concepts, helps you understand how accelerometers work, and how different sensor specifications impact accelerometer performance in your application. After you decide on your sensors, you can view the required hardware packages and software to properly condition, acquire, and visualize vibration measurements. 

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What is Vibration? 

Vibration is the movement and/or mechanical oscillation about an equilibrium position of a body or machine. It can be periodic and repeated, such as the motion of a pendulum, or random, like the movement of a tire on loose rocks. Vibration can be expressed using metric units (m/s2) or it's gravitational constant g, where 1 g = 9.81 m/s^2. An object has the capability to vibrate in two ways: free vibration and forced vibration.

 

Free vibration is when an object or structure is displaced or impacted and then allowed to oscillate naturally. An example when you strike a guitar string it rings and then the sound eventually diminishes. Natural frequency usually refers to the frequency at which a structure “prefers” to oscillate at after a displacement. With that, resonance is the tendency for a system to oscillate more at certain frequencies. Forced vibration at  an object’s natural frequency causes energy inside the structure to build. 

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Figure 1. Structures may fail if their natural frequencies match environmental vibration.

Over time, vibration can increase even when an input forced vibration is small. If a structure has natural frequencies that match normal environmental vibration, the structure will vibrate more violently and prematurely fail. A forced vibration occurs when a structure has an altering force being applied. Alternating motion can cause an object to vibrate at unnatural frequencies which when occurring as an is imbalance in a washing machine is an obvious example. The machine shakes at a frequency equal to the rotation of the turnstile. Condition monitoring is the use of vibration measurements to indicate the health of rotating machinery such as turbines or pumps. By trending the vibration signatures and patterns off different machine parts over time, you can predict when a machine will fail and properly schedule maintenance for improved safety and reduced cost.

Measuring Vibration 

Vibration is traditionally measured using a ceramic piezoelectric sensor or accelerometer. Accelerometers rely on the piezoelectric effect, which occurs when a voltage is generated across crystals as they are stressed. A structures acceleration is transmitted to a seismic mass inside the accelerometer that generates a coordinate force on the piezoelectric crystal. This stress on the crystal generates a high-impedance, electrical charge proportional to the applied force and acceleration. 

The two common types of accelerometers are: Piezoelectric (charge mode) accelerometers and Integrated Electronic Piezoelectric (IEPE) accelerometers.

 

Accelerometers are contact sensors mounted directly on elements, such as rolling-element bearings, gearboxes, or spinning blades. The benefits of an accelerometer include linearity over a wide frequency range and a large dynamic range.

 

Charge mode accelerometers depend on an external amplifier or inline charge converter to amplify their generated charge, and lower their output impedance with measurement devices.

 

IEPE accelerometers have a charge-sensitive amplifier built inside them. This amplifier accepts constant current and varies its impedance according to the piezoelectric crystal. Measurement hardware made for these accelerometers have built in current excitation for the amplifier. It is then possible to measure this change in impedance as a change in voltage across the accelerometer inputs.

 

Lastly, proximity probes are non-contacting transducers that measure distance to a target. These sensors are exclusively used in rotating machinery to measure shaft vibration. A specific example is in turbo machinery where the flexible fluid film bearings and heavy housing does not allow vibrations to transmit well to the outer casing. In this case you use proximity probes instead of accelerometers to directly measure shaft motion.

Digital Telemetry Method:

The digital telemetry method has no contact points. The system uses a receiver-transmitter module, coupling module, and signal processing module. The transmitter module amplifies, digitizes, and modulates the sensor signal since it is integrated into the torque sensor. Then a radio frequency carrier wave is picked up by the caliper coupling module (receiver). A signal processing module then recovers the digital measurement data.

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Figure 2.  IEPE accelerometers output voltage signals proportional to the force of the vibration on the piezoelectric crystal.

Choosing the Accelerometer

Accelerometer versatility allows one to choose from a variety of designs, sizes, and ranges. Understanding the characteristics of the signal you expect to measure helps you decide on the different electrical and physical specifications for accelerometers.

Vibration Amplitude

The maximum amplitude of the vibration you are measuring determines the sensor range you will require. Attempting to measure the vibration outside of the sensor range distorts  the response. Ideally, you would use an accelerometer a lower mass and sensitivity to monitor high vibration levels.

 

Sensitivity

Sensitivity is an accelerometer's most important parameter. It describes the conversion between vibration voltage at a reference frequency, such as 100 Hz. Sensitivity units are mV per G. The exact sensitivity is determined from calibration and listed in the sensor's instruction document. Sensitivity is also frequency dependent and requires a full calibration across the usable frequency range to determine how sensitivity varies with frequency. Figure 4 shows the typical frequency response characteristics of an accelerometer. In general, use a low-sensitivity accelerometer to measure high amplitude signals and a high-sensitivity accelerometer to measure low amplitude signals.

 

Number of Axes

The most common accelerometer measures acceleration along only a single axis. This type is often used to measure mechanical vibration levels. The second type is a triaxial accelerometer which creates a 3D vector of acceleration in the form of orthogonal components. Use this type when you need to determine the type of vibration, such as lateral, transverse, or rotational.

Weight

 Adding mass to the structure can alter its vibrational characteristics and potentially lead to inaccurate data and analysis. The weight of the accelerometer should generally be no greater than 10% of the weight of the test structure.

Mounting Options

Another consideration is how to mount the accelerometer to the target surface for your vibration measurement. Four typical mounting methods:

  • Handheld or probe tips

  • Magnetic

  • Adhesive

  • Stud mount

 

Stud mounting is the recommended mounting technique, and since it requires you to drill into the target material it is generally reserved for permanent sensor installation. Other methods are for more temporary attachment. The various attachment methods all affect the measurable frequency of the accelerometer. Generally speaking, the looser the connection, the lower the measurable frequency limit. The addition of any mass to the accelerometer, such as an adhesive or magnetic mounting base, lowers the resonant frequency, which may affect the accuracy and limits of the accelerometer’s usable frequency range. Consult accelerometer specifications to determine how different mounting methods affect the frequency measurement limits. Table 1 shows typical frequency limits for a 100 mV/G accelerometer.

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What is Vibration
Measuring Vibration
Choosing the Right Accelerometer

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