Why are accelerometers used as vibration and movement sensors?

The most economic and versatile sensor for vibration measurements is the accelerometer. On rotating machinery these measure the radial forces on shafts and bearings. As the machine rotates a sensor fixed to the machine experiences a force on each revolution pushing or pulling it depending upon the orientation of the machine shaft. As the machine runs, a periodic vibration is observed at the sensor. The output of the sensor can be used to analyse the intensity and frequency of these vibrations which are a measure of the condition of the machine.

On moving objects, such as vehicles, or large structures such as tall buildings, towers or bridges, acceleration will not be periodic or, if so, it will be at very low frequency. In this case an accelerometer will still yield valuable data on the magnitude and time over which the forces are experienced or exerted.

Artificially created ground and building vibrations experienced during building, blasting, drilling or piling activities can also be sensed by accelerometers but as the vibrations are usually random the data must be acquired and analysed in quite different ways to allow sensitive vibration logging.

Natural ground vibrations, earthquakes and tremors are also detected by accelerometers built into measuring systems known as seismographs. Natural ground vibrations have long periods and generally require much higher sensitivities than machine vibration measurements.

Shocks, such as experienced in crash, gunnery and explosive tests can also be measured by accelerometers though in this case there is usually a sudden deceleration at vehicle impact or acceleration at projectile release. The signal is transient over small fractions of a second but will be of very high magnitude. Sensors with very low signal filtering and optimized electronics are needed for this.

How to choose an Accelerometer?

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. 

How do shear mode accelerometers work ?

Shear mode accelerometer (vibration sensor) designs feature sensing crystals attached between a center post and a seismic mass. A compression ring or stud applies a pre-load force to the element assembly to insure a rigid structure and linear behavior. 

Under acceleration, the mass causes a shear stress to be applied to the sensing crystals. This stress results in a proportional electrical output by the piezoelectric material. The output is collected by electrodes and transmitted by lightweight lead wires to either the built-in signal conditioning circuitry of ICP sensors, or directly to the electrical connector for charge mode types.

By having the sensing crystals isolated from the base and housing. shear mode accelerometers excel in rejecting thermal transient and base-bending effects. Also, the shear geometry lends itself to small size, which promotes high frequency response while minimizing mass loading effects on the test structure.

With this combination of ideal characteristics, shear mode accelerometers offer optimum performance.

Is a shear mode sensor superior to compression mode?

What about flexure mode sensors? In recent years, shear mode sensors have gained popularity, while compression mode are often considered to be "old technology." Meanwhile, flexural mode sensors, once considered too fragile for industrial applications, are now making a comeback by incorporating special design techniques. Each construction method has inherent advantages and disadvantages. The construction method of a sensor is less important than its performance.

For each model, characteristics such as base strain and shock limits are quantified on the specification sheet and can be compared. For example, a well-designed compression mode sensor may have a lower base strain rating than a shear mode sensor. While this may be contrary to many peoples' intuition, it can be verified by comparing the values of the 101 (compression) versus the 101.01-6-1 (shear). In today's advanced designs, the right sensor for an application is determined by the performance yielded by different design techniques.

Why do some accelerometers measure acceleration and others velocity?

As a machine rotates and vibrates the bearings, for example, experience periodic accelerations, for that is what vibration is. However, we could also measure the velocity or displacement that accompany these vibrations. At normal machine speeds displacement is too small to yield useful data though velocity measurements have some interesting properties.

Firstly, the alternating velocity signal at low frequencies often has higher sensitivity and better signal to noise than the acceleration signal for vibration analysis and so we can use sensors designed to output this alternating velocity signal. These are called piezoelectric velometers.

Secondly, if the AC signal is conditioned to give its RMS (average intensity) value then a simple DC signal is generated which is ideal for monitoring and control processes. Usually, this is presented as a 4-20mA signal proportional to the range of the sensor in mm/sec and accessible directly by a PLC or other industrial controller. No signal analysis or special power supplies are necessary and extended cable runs are possible, easily up to a kilometre with good quality cable. These are often called velocity transducers.

What are the critical factors affecting the choice of cables for connecting accelerometers?

Standard industrial Monitran accelerometers are fitted with integral stainless steel over braided PTFE twin-core cable. This cable has a twisted pair of conductors, (power/signal & common) plus a shield which provides a high level of immunity to electrical interference when installed as recommended. This makes the cable ideal for most industrial applications.

Coaxial cables are also used though they have lower immunity to interference while they may be completely adequate in some situations such as the laboratory where high power machines are not in the vicinity. These cables work well with BNC and Microdot connectors and are used in a flexible coiled assembly for connecting portable analysers to sensors switchboxes. In miniature accelerometers coaxial cables are necessary as twin core cables, though superior, are too large and heavy.

To withstand the rigours of internal and external factory and works environments with elevated temperatures, humidity, oil and other chemicals plus rain and cleaning down with high pressure hoses requires a robust cable construction. Monitran’s standard cables use ETFE insulation for high temperature resistance and chemical inertness plus overbraiding with stainless steel mesh to protect against abrasion and wear.

The above cables are used up to 140ºC while perfluoroalkoxyethylene (PFA) insulation allows operation to 260ºC.

For long periods of immersion and operation down to 100m depth an external polyurethane coating over ETFE insulated cores with optional stainless steel overbraiding exhibits excellent water resistance and long life.

For lower temperature operation and less demanding environments like testing laboratories PVC insulated cables are acceptable and economical.

In the above examples the sensors are usually permanently mounted to machinery such that flexibility is not the most important criterion. However, for measurements with portable vibration analysers coiled, stretchable cable with PVC insulation offer convenience and economy.

The performance and reliability of results from vibration sensors depends critically on their electrical connection to the measuring system. Connectors and cables must be selected carefully to ensure optimum data collection, minimum signal interference and suitability for the working environment. Cables may be directly connected to the sensor or via a plug-in connector.

Factory environments demand cabling with high levels of noise rejection, protection from external electrical fields and radiofrequency interference. Where possible, cables will be twin-core shielded. The twin cores will be twisted together and carry the power/signal and return signals. However, where cable dimensions are too large to allow connection to small and miniature sensors then single core, screened (coaxial) cables are supplied.

Cables to constant-current and 4-20mA output sensors can be extended over long distances with no significant effect on signal quality. When connecting large numbers of sensors to a remote control room it is convenient and economic to combine the cables at a junction box and make the onward connection with multi-core cables. Each sensor needs its own pair while a common shield is acceptable.

Charge output sensors, however, have very low level signals and 10m should be regarded as a normal maximum length. Following charge amplification, though, extension may proceed as above.

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