SENSOR SELECTION
Accelerometers

Accelerometers are by far the sensor of choice for shock and vibration measurement. Accelerometers mount directly to (or in) the vibrating structure and proportionally converts mechanical energy to electrical when experiencing acceleration. Acceleration is generally represented with the gravitational constant ‘g’ which equals 9.81 m/s2. There are three main types of accelerometers:

  1. Piezoelectric Accelerometer

    Piezoelectric accelerometers are the most popular and widely used for industrial applications. They typically use lead zirconate titanate (PZT) sensing elements that product electric charge or output under acceleration. Piezoelectric accelerometers have very low noise and offer superior performance over capacitive MEMS or piezoresistive accelerometers in all vibration and most shock applications. Piezoelectric accelerometers come in many different variants: triaxial or single axis, high sensitivity for seismic applications down to low sensitivity for shock testing, and even have some types that can handle extreme environments including nuclear. The major downside of piezoelectric accelerometers is that they are AC coupled so they can’t measure the gravity vector or sustained accelerations. This also prevents the engineer from integrating the data for velocity or displacement information because of their intrinsic decay properties (although it can be integrated for higher frequency vibration). But again, piezoelectric accelerometers are generally the preferred choice for industrial testing applications for their performance benefits.

    Because piezoelectric accelerometers are so popular there are many different companies that sell these including: Measurement Specialties, Meggitt’s Endevco Corporation, PCB Piezotronics, Bruel & Kjaer, and Dytran. Generally the cost of a piezoelectric accelerometer will be in excess of $1,000 and they typically have long lead times of over 4 weeks.

  2. Capacitive 

    MEMS (micro-electro-mechanical systems) accelerometers more than likely refer to capacitive accelerometers; MEMS is just the fabrication technology. This fabrication technology has brought capacitive accelerometers into the mainstream though! They are by far the cheapest and smallest accelerometer options (as the name implies!); and capacitive MEMS accelerometers are the type found in your smart phone. These accelerometers can be mounted directly to printed circuit boards which has made capacitive MEMS accelerometers the preferred choice for electrical engineers. Their low cost (typically less than $10) and small size has made them popular but capacitive MEMS accelerometers have much poorer data quality, especially on the higher frequency and amplitude end. They should generally be avoided for industrial applications; but they are a DC coupled and a great option for human-based applications. Their low cost and power consumption does also make them a good choice for health monitoring.

    Capacitive MEMS accelerometers are very easy to purchase; and have short lead times. Using one will require some electrical design on your part though. The leading manufacturers of capacitive MEMS accelerometers include Analog Devices, Bosch Sensortec, and InvenSense.

  3. Piezoresistive Accelerometer

    Piezoresistive accelerometers are the premier type for shock testing. Piezoresistive accelerometers are strain gauge based so they require amplifiers and temperature compensation; but they have a very wide bandwidth (0 hertz to several thousand hertz) and low noise characteristics. Piezoresistive accelerometers can be gas or fluid damped which protects the accelerometer and prevents it from reaching its internal resonant frequency. Because they are DC coupled their output can be integrated to calculate velocity and displacement during shock events. Again, they are the premier type for shock testing; but piezoelectric accelerometers are preferred for vibration testing.

    The same companies that sell piezoelectric accelerometers also offer piezoresistive options. Piezoresistive accelerometers also tend to be in excess of $1,000 each and have longer lead times of over 4 weeks.

    Figure 1 provides a reference table that recommends an accelerometer type for different applications. If you want to dive a bit deeper into accelerometer selection check out Midé’s blog post. Endevco also has a nice white paper on selecting the right accelerometer. 

    Figure 1: This table provides a quick checklist on which accelerometer types work best for different testing applications. 
Vibration Meters

Vibration meters offer real time vibration analysis in a handheld unit so that maintenance decisions can be made quickly in the field. They either wire to a traditional accelerometer or some even, like the one shown in Figure 2, incorporate the accelerometer into the unit cutting down on wiring requirements and complexity. Vibration meters typically don’t allow the user to log long duration events (they may give you access to the last couple thousand points for some analysis); but they give RMS and peak-to-peak levels in real time. They also will typically have an algorithm to rate the overall vibration of your bearing or machine. Vibration meters can be a bit pricey at around $1,000 which sometimes won’t include the cost of the accelerometer (the Fluke 805 is over $2K that has the embedded accelerometer). If you are looking to do some more in depth vibration analysis or any shock testing, a vibration meter is probably not your best option. But for that quick go/no-go vibration testing of a piece of machinery, a vibration meter is unbeatable. Fluke is the leader in hardware and software for vibration meters; here is their vibration testing homepage.

Figure 2: The Fluke 805 vibration meter incorporates an accelerometer directly into the main electronics handheld unit to cut down on complexity and wiring.

Data Loggers

An often overlooked option for shock and vibration measurement is to use a data logger that combines the accelerometer with the data acquisition system, power, and memory into one package. This is the preferred option for engineers who need ease-of-use and portability. Apps on your smart phone can be considered simple data loggers but they tend to have a maximum sample rate of 100 Hz and poor data quality. Higher end data loggers like Midé’s Slam Stick X effectively bridge the gap to the more expensive shock and vibration measurement systems by incorporating a high quality piezoelectric accelerometer as opposed to the cheaper capacitive MEMS accelerometers found in most vibration data loggers.

Figure 3: The Slam Stick X is shown that incorporates the accelerometer, power, and data acquisition electronics into one small handheld package.

 

Shock and vibration data loggers generally have much shorter lead time (a few days) and lower cost ($500 to $2,000) than building your own vibration measurement system. There are a lot of different companies that make shock and vibration data loggers and a lot of different options; here’s a post comparing 6 different products.

Non-Contact Displacement Sensors

Although accelerometers are the most popular choice in shock and vibration measurement, displacement sensors measure the displacement of a vibrating structure. Calculating between displacement, velocity, and acceleration is accomplished with integration/differentiation (here’s a calculator for simple harmonic motion applications). The downside to using these is that it’s measuring relative motion between two structures. These are near impossible to use in the field because a fixed mounting and distance is required between the sensor and equilibrium position of the vibrating structure. They can also be quite a bit more expensive and complex than accelerometer based systems. That being said, displacement sensors can be preferred in some applications that prevent the use of accelerometers such as rotating components (although a data logger could be used), or when the accelerometer’s mass would too greatly influence the motion of the system. Generally displacement sensors should be avoided for shock testing for fear of damaging the sensors.

Figure 4: Laser displacement sensors (and capacitive sensors) offer a sensor option that allows direct measurement of displacement without contacting or interfering with your test article.

 

Laser displacement sensors (KEYENCE is the leader) and capacitive displacement sensors would be the two main sensor types that would be useful for vibration testing. These systems will typically be upwards of $5,000 and lead times over 4 weeks.

Microphones

Sound is not often thought of as a way to measure vibration; but it should be! After all sound, by definition, is a vibration that travels through the air in the form of pressure waves. Microphones offer a cost effective means of measuring high frequency vibration and is especially useful to determine how a system’s vibration changes with time. Health monitoring applications can greatly benefit from using a microphone on cost and simplicity.

Microphones aren’t limited to applications where cost is a concern; some acoustics applications will use high end microphones for vibration testing and analysis. You’ll notice a lot of the accelerometer companies also offering high end microphones, like PCB Piezotronics. Microphones and acoustic analysis can be a great option for some applications; but if you need absolute shock and vibration data, not relative change, then microphones probably won’t work. They also won’t be able to analyze modal shapes and specific/discrete points along your structure. But again, they are very effective for overall frequency analysis.

Strain Gauges

Often times the end goal of vibration testing is determining the stress and strain in your structure. Strain gauges can be an effective sensor type to directly measure the strain of your test article. A change in capacitance, inductance or resistance is proportional to the strain experienced by a strain gauge so that mechanical energy can be converted to an electrical signal. Strain gauges do present some challenges though; they can be very sensitive to temperature change, material properties of your structure, and the adhesive used. Instrumenting a structure with strain gauges is very much an art and difficult to do. They also require strain gauge amplifiers which are also difficult to work with. That being said, strain gauges are cost effective (from a material point-of-view, not labor), and allow the engineer to directly measure the strain in his/her structure.

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