What You Need to Know About Vibration Sensors

Chaos sometimes begins with a low rumble or a small shake. If a sensor is involved, it might be measuring the first vibrational movements of an earthquake or a mechanical failure in an industrial setting. It’s typically noise that gives us the first clue that trouble looms—and if trouble is what you’re trying to avoid, there can be two aspects of vibration you should pay close attention to:

1. The monitoring of vibration
2. Maintenance based on sensor data to avoid future issues

As machines or the electronics and components within machines begin to move back and forth, the vibration is preventing a smooth flow of energy. The flow is interrupted, hence the noise and the shake. Typically it’s overload due to some sort of stress, or the components themselves may have reached their useful life—gears, teeth, bearings, or belts may be in the process of failure.

Why monitor vibration? Vibrations produced by industrial machinery are vital indicators of machinery health. Vibration analysis is used as a tool to determine a machine’s condition and the specific cause and location of problems, expediting repairs and minimizing costs. Machinery monitoring programs record a machine's vibration history. Monitoring vibration levels over time allows prediction of problems before serious damage can occur.

Critical to vibration monitoring and analysis are machine-mounted sensors. Three parameters representing motion detected by vibration monitors are displacement, velocity and acceleration. Mathematically related, the parameters can be taken from a variety of motion sensors. Selection of a sensor proportional to displacement, velocity, or acceleration depends on the frequencies of interest and signal levels that are involved.

Displacement sensors are used to measure shaft motion and internal clearances. Non-contact proximity sensors sense shaft vibration relative to bearings or other support structure. The sensors are used in low-frequency (1 to 100 Hz) measurement and measure low-amplitude displacement typically found in sleeve-bearing machine designs. Piezoelectric displacement transducers solve issues associated with mounting non-contact probes and are more suitable for rolling element-bearing machine designs. Piezoelectric sensors yield an output proportional to the absolute motion of a structure.

Velocity sensors are used for low to medium frequency measurements (1 to 1000 Hz) and are useful for vibration monitoring and balancing operations on rotating machinery. They have lower sensitivity to high frequency vibrations than accelerometers and are therefore less susceptible to amplifier overloads. Overloads compromise low amplitude, low frequency signals. Traditional velocity sensors use an electromagnetic coil and magnet system to generate the velocity signal. Today, piezoelectric velocity sensors are becoming popular due to improved capabilities and their rugged nature.

Accelerometers are the preferred motion sensors for most vibration monitoring. They measure low to very high frequencies and are available in a variety of general purpose and application-specific designs. The piezoelectric accelerometer is reliable, versatile, unmatched for frequency and amplitude range, and popular for machinery monitoring.

When selecting any of the three types, it’s important to ask the following:

• What is the vibration level and frequency range?
• What is the temperature range?
• Is the environment corrosive or atmosphere combustible?
• Are intense fields (electromagnetic or acoustic) involved?
• Is there substantial ESD present?
• Are there sensor size and weight considerations?

For vibration analysis and condition monitoring, look at sensors with an AC or charge output. For continuous monitoring and machine protection, sensors with DC output are a better choice.

Five main features must be considered when selecting vibration sensors: measuring range, frequency range, accuracy, transverse sensitivity and ambient conditions. Measuring range can be in Gs for acceleration, in/sec for linear velocity (or other distance over time), and inches or other distance for displacement and proximity.

Frequency is measured in Hz and accuracy is typically represented as a percentage of allowable error over the full measurement range of the device. Transverse sensitivity refers to the effect a force orthogonal to the one being measured can have on the reading. Again, this is represented as percentage of full scale of allowable interference.

For the ambient conditions, such things as temperature should be considered, as well as the maximum shock and vibration the vibration sensors will be able to handle. This is the rating of how much abuse the device can stand before it stops performing, much different from how much vibration or acceleration vibration sensors can measure.

Examples of note

Looking at examples of vibration sensors, the MiniSense 100 Vibration Sensor from Measurement Specialties, Inc. (Figure 1) is a low-cost cantilever-type vibration sensor that offers high sensitivity at low frequencies. The active sensor area is shielded for improved RFI/EMI rejection. Rugged flexible PVDF sensing element withstands high shock overloads.

Figure 1: The MiniSense 100 Vibration Sensor is a cantilever-beam accelerometer that detects continuous or impulsive vibration or impacts (Courtesy of Measurement Specialties).

These sensors can be used to detect continuous vibration or sudden impact in such applications as washing machine load imbalance, vehicle motion sensor, anti-theft devices, vital signs monitoring, and tamper detection. Features include:

• High Voltage Sensitivity (1 V/g)
• Over 5 V/g at Resonance
• Horizontal or Vertical Mounting
• Shielded Construction
• Solderable Pins, PCB Mounting
• Low Cost
• < 1% Linearity
• Up to 40 Hz (2,400 rpm) Operation Below Resonance

Another feature-packed example is the ADIS16223 Digital Tri-Axial Vibration Sensor from Analog Devices (Figure 2) a vibration sensor system that combines the company’s iMEMS sensing technology with signal processing, data capture, and a convenient serial peripheral interface (SPI). The SPI and data buffer structure yield convenient access to wide bandwidth sensor data. The 22 kHz sensor resonance and 72.9 kSPS sample rate provide a frequency response that is suitable for machine health applications.

Figure 2: ADIS16223 Digital Tri-Axial Vibration Sensor (Courtesy of Analog Devices).

In this case, an internal clock drives the data sampling system during a data capture event, which eliminates the need for an external clock source. The data capture function has four different modes that offer several capture trigger options to meet the needs of several applications. The ADIS16223 also offers a digital temperature sensor, digital power supply measurements and peak output capture. It is available in a 15 mm x 15 mm x 15 mm module with a threaded hold for stud mounting with a 10-32 UNF screw. Applications include vibration analysis, shock detection and event capture, condition monitoring, machine health, instrumentation diagnostics, safety shutoff sensing, security sensing including tamper detection.

Also well worth considering, particularly for energy harvesting applications, is the Midé Technology Volture Piezoelectric energy harvester (Figure 3), which converts otherwise wasted energy from mechanical vibrations into usable electrical energy. The Volture does this by using normally brittle piezoelectric materials.



Figure 3: Midé’s Volture Piezoelectric Energy Harvester converts wasted energy from vibration (Courtesy of Midé).

The piezoelectric materials are packaged in a protective skin with pre-attached electrical leads, producing a robust component with no soldered wires. The skin also provides electrical insulation and defense against humidity and harsh contaminants. Applications include:

• Industrial health monitoring network sensors
• Condition Based Maintenance Sensors
• Wireless HVAC Sensors
• Mobile Asset Tracking
• Tire Pressure Sensors
• Oil and Gas sensors
• All air, land and sea vehicle sensors
• Battery and hard-wired power replacement

The first step in applying a device such as the Volture is to fully understand the vibration environment in which the part will be operating. The most effective means to accomplish this is to measure the vibration using an accelerometer, capture the data, and perform an FFT (Fast Fourier Transform) on the data to extract the relevant frequency information.

Some applications will not require this step since their dominant frequencies are well known. An example of this would be a 120 Hz AC motor or a 60 Hz appliance. However, most applications will require some form of vibration characterization to be successful.

If you do not know what the vibration frequency content and amplitude levels are, Midé offers a vibration characterization product and service, the VR001. The “Slam Stick” VR001 is a small device that can be easily installed into many different vibration environments. Simply attach it to the vibrating structure using any means available (double sided tape is recommended), hit the one button and wait for the recording to finish. Next, plug the Slam Stick into a USB port and download the data just like it was a memory stick.

Vibration analysis is an essential part of machinery maintenance. By monitoring pumps, motors, fans and other types of rotating machinery, troubleshooting and fault diagnosis will be easier, and ultimately will result in cost saving. This article has outlined some of the critical parameters that must be considered when choosing a vibration sensor. Following these “to-do’s” and considering examples such as the parts presented will increase the effectiveness of your vibration monitoring program and improve the productivity of machinery as well as allow you to consider energy harvesting applications.