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How does a piezo-electric accelerometer work?

Piezo-electric crystals are man-made or naturally occurring crystals that produce a charge output when they are compressed, flexed or subjected to shear forces. The word piezo is a corruption of the Greek word for squeeze. In a piezo-electric accelerometer a mass is attached to a piezo-electric crystal which is in turn mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibration the mass mounted on the crystal wants to stay still in space due to inertia and so compresses and stretches the piezo electric crystal. This force causes a charge to be generated and due to Newton law F=ma this force is in turn proportional to acceleration. The charge output is either is converted to a low impedance voltage output by the use of integral electronics (example: in an IEPE accelerometer) or made available as a charge output (Pico-coulombs /g) in a charge output piezo-electric accelerometer.

What are the different types of accelerometer?

There are many different type of accelerometers and each has unique characteristics, advantages and disadvantages. The different types include:

What is a single ended compression accelerometer?

A single ended compression accelerometer is where the crystal is mounted to the base of the accelerometer and the mass is mounted to the crystal by a setscrew, bolt or fastener.

What is an isolated compression accelerometer?

Single ended compression accelerometers can be susceptible to base strain and so to alleviate this problem the crystal is isolated from the base by mounting on an isolation washer or by reducing the mounting area by which the crystal is mounted to the base.

What is a shear type accelerometer?

A shear type accelerometer is where the seismic mass is attached to the crystal so that it exerts a shear load on the crystal rather than a compressive load. Shear type accelerometers are designed for applications that are likely to encounter significant base distortion from thermal transients or where they are mounted onto flexible structures.

What is a piezo-resistive accelerometer?

A piezo-resistive accelerometer is an accelerometer that uses a piezo-resistive substrate in place of the piezo electric crystal and the force exerted by the seismic mass changes the resistance of the etched bridge network and a whetstone bridge network detects this. Piezo-resistive accelerometers have the advantage over piezo-electric accelerometers in that they can measure accelerations down to zero Hertz.

What is a strain gage based accelerometer?

A strain gauged based accelerometer is based on detecting the deflection of a seismic mass by using a silicon or foil strain gauged element. A whetstone bridge network detects the deflection. The deflection is directly proportional to the acceleration applied to the sensor. Like the piezo-resistive accelerometer it has a frequency response down to zero Hz.

What is the useable frequency range? For an accelerometer to be useful the output needs to be directly proportional to the acceleration that it is measuring. This fixed ratio of output to input is only true for a range of frequencies as described by the frequency response curve.     The usable frequency response is the flat area of the frequency response curve and extends to approximately 1/3 to 1/2 of the natural frequency. The definition of flat also needs to be qualified and is done so by quoting the roll off of the curve in either percentage terms (typically 5% or 10%) or in dB terms (typically +/- 3db).

What is an IEPE accelerometer?

IEPE stands for Integrated Electronics Piezo Electric and defines a class of accelerometer that has built in electronics. Specifically it defines a class of accelerometer that has low impedance output electronics that works on a two wire constant current supply with an voltage output on a DC voltage bias. IEPE two wire accelerometers are easy to install, have a wide frequency response, can run over long cable lengths and are relatively cheap to purchase. The IEPE technology has generally replaced most 3 wire accelerometers and are broadly used for most applications except for specialist applications such as zero Hz accelerometers, high temperature applications or 4-20mA accelerometers used in the process industries.

What is an ICP accelerometer?

ICP is the trademarked PCB name for IEPE accelerometers. It stands for ‘Integrated circuit-piezo electric'.

What is a charge output accelerometer?

All piezo-electric accelerometers work by measuring the charge generated by a crystal that is being compressed or shear loaded by a mass influenced by acceleration. In most applications this high impedance charge output is converted to a low impedance voltage output by the use of integral electronics. However in some applications integral electronics are not appropriate such as high temperature or high radiation applications. Charge output accelerometers are self-generating and would typically have amplifying electronics mounted several feet away from the local heat or local radiation source.

What is dynamic range?

The dynamic range of an accelerometer is the range between the smallest acceleration detectable by the accelerometer to the largest. A piezo-electric accelerometer produces a charge proportional the force applied to the crystal, which due to the seismic mass on the crystal is proportional to acceleration applied. The piezo electric effect can be detected for very small forces or accelerations all the way through to very large accelerations. In most cases the smallest acceleration is dictated by the amplifying electronics noise floor and for high g levels to the voltage rail used by the power supply. The design of the accelerometer will also play a part in what shock g levels an accelerometer can withstand before the crystal is irreparably damaged or the structure holding the crystal is distorted. Compression accelerometers are the most shock resistant design of accelerometer.

Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensitivity of 100mV/g with electronics that has a maximum output of 5V will obviously have a dynamic range of +/- 50g while an accelerometer of sensitivity of 10mV/g will have a dynamic range of +/- 500g.

What is amplitude linearity?

The amplitude linearity of an accelerometer is the degree of accuracy that an accelerometer reports the output in voltage terms as it moves from being excited at the smallest detectable acceleration levels to the highest. This accuracy is qualified by the linearity. Typically the amplitude linearity is 1%. The dynamic range describes the minimum to maximum accelerations that can be detected. The output of an IEPE accelerometer can typically go from 100 micro g to 500g. This dynamic range is dependent on the electronics used with the accelerometer either internal or external, as is the output linearity over the dynamic range.

When should I use a velocity output accelerometer?

Velocity output accelerometers are usually used in condition monitoring applications where velocity is a much better parameter for judging the health of a machine. Doubling of velocity vibration equates to a doubling of the deterioration of the health of the machine. Velocity can also be used in lower frequency applications where the acceleration amplitude of vibration is too small to measure and the velocity vibration maybe of a higher and more meaningful value. Velocity vibration accelerometers are only really effective if the frequency of vibration is higher than 2Hz but more ideally 5 Hz.

How do I choose the sensitivity of an accelerometer?

Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensitivity of 100mV/g with electronics that has a maximum output of 5V will obviously have a dynamic range of +/- 50g while an accelerometer of sensitivity of 10mV/g will have a dynamic range of +/- 500g If the maximum g levels likely to be experienced is known then dividing this number by 5 volts will give the maximum sensitivity that should be used to get this dynamic range

Example: Vibration expected to be seen is 300g. Sensitivity will be 5 divided by 200 which equals 16.6 mV/g. The nearest sensitivity would be a 10mV/g accelerometer.

What is condition monitoring?

Condition monitoring is where the health of a rotating machine is monitored using vibration levels. As the health of a machine (example becomes unbalanced, fan blades corrode, bearing surfaces degrade) deteriorates so the amplitude of the vibration the machine generates increases. By monitoring the vibration levels over a long period of time this gradual deterioration of the health of the machine can be assessed until the vibration levels get to a point where the machine needs to be taken out of service and overhauled. Analysis of the frequency content of the machine vibration signal will indicate not only that the health of the machine has deteriorated but also root causes can be attributed to the problem.

Example: An 8 bladed pump running at 6000 rpm (100Hz) will produce a vibration signal with 100 Hz frequency if it becomes unbalanced, 200 Hz if it becomes mis-aligned 800 Hz if the blades become corroded and 43-47 Hz if the bearings start to go into oil whirl.

What is the natural frequency of an accelerometer?

The natural frequency of an accelerometer is the frequency where the ratio of output is at it highest. The natural frequency of an accelerometer is defined by the equation:

From a frequency roughly 1/3 to 1/2 of the natural frequency the ratio of output to input becomes non-linear and therefore makes measurements from this region difficult to interpret. Therefore the higher the natural frequency of an accelerometer the higher frequencies where the output to input is linear and the higher the frequencies that can be measured.

It can be seen from the formula for natural frequency that to increase the natural frequency the mass needs to be as small as possible and the stiffness needs to be as high as possible. A small mass usually means a lower sensitivity and this is true of most high frequency accelerometers.

What is the mounted natural frequency?

An accelerometer has a different natural frequency when it is in free space to that when it is mounted. The only frequency that is of interest to the user is of course the mounted natural frequency and is often the one quoted in the specifications. The mounted natural frequency is of course dependent on the stiffness of the mounting structure to which it is attached and is therefore quoted as the natural frequency of the accelerometer as installed according to manufacturers instructions. Gluing, magnetically mounting or loose bolting down to a surface will significantly reduce the mounted natural frequency.

What is base strain sensitivity?

Base strain sensitivity is the erroneous signal that is generated by an accelerometer when the base is subjected to bending, torque or distortion either by mechanical movement or thermal stressing. The relative movement of the base of the accelerometer squeezes the crystal in an accelerometer and the seismic mass mounted on the crystal. Base strain is where the base distorts the mass while acceleration causes the seismic mass to distort the crystal. These two forces on the crystal are indistinguishable and so reduction of the base strain is vital for good signals only to be generated. The more indirectly that a crystal is mounted to the base under strain the less sensitive the accelerometer is to base strain. Single ended compression sensors are the most prone to base strain sensitivity and shear type accelerometers the least. Isolated compression accelerometers are a good compromise between have good base strain immunity and the disadvantages that shear type accelerometers bring in terms of sensitivity an robustness.

What is cross sensitivity or transverse sensitivity?

An accelerometer produces a charge output when the crystal is compressed. That same crystal also produces a charge, albeit a much smaller one, when a shear load is exerted on the crystal. The accelerometer therefore produces a charge when it is vibrated in the axis 90 degrees to the main axis of measurement, which is indistinguishable from acceleration in the main axis. Conversely shear type accelerometers produce an erroneous signal when they experience cross axis acceleration only this time it loads the crystal in compressive mode.

The sensitivity of the accelerometer to a transverse vibration is known as the transverse sensitivity and is typically less than 5% of the sensitivity to an "on axis" acceleration.

What are the differences between quartz crystal based and ceramic crystal based accelerometers?

How do I mount an accelerometer?

The mounting of an accelerometer effects its frequency response. The mounted natural frequency is dependent directly on the stiffness of the mounting. The higher the stiffness the more the mounted natural frequency approaches its maximum. The least stiff mounting of an accelerometer is magnetic mounting and the highest stiffness is using a high tensile setscrew tightened to the correct torque mounted on a hard flat surface. Other mounting methods come in between these two extremes.

It is important to ensure that the site chosen for the accelerometer is ground flat for at least an area larger than the base of the accelerometer. A slight smear of Silicone grease will ensure a stiff bond between accelerometer and structure.

When using a mounting stud it is important to ensure that the stud does not bottom out in either the base of the accelerometer or the drilled location hole. High tensile strength set screws that have a shoulder will prevent this eventuality from happening.

What is a quickfit mount?

A quickfit mount is used in installations where the accelerometer will be removed between monitoring the acceleration or velocity vibration yet will be repeatedly place back in the same location. Such installations include machine health monitoring using data collectors.

What signal conditioning do I need for my accelerometer?

• a) Overall voltage levels in either RMS or peak to peak
• b) Spectral content analysis
• c) Snap shot time domain analysis

What is an isolated stud?

An accelerometer isolated stud is used in application where the possibility for ground loops exists which can corrupt the output of the sensor. Isolated studs do reduce the frequency response of the accelerometer somewhat so caution should be taken if high frequency data needs to be measured.

What is the tribo-electric effect? Tribo-electric effect is when a spurious signal is generated by a charge output accelerometer by the movement of the co-axial cable. To prevent the tribo-electric effect the low noise cable needs to be clamped down as close to the accelerometer as possible. See How do I install a charge amplifier?

What is the output of an IEPE accelerometer? An IEPE accelerometer is a two-wire sensor that requires a constant current supply and outputs an AC voltage output on a DC voltage bias. The DC bias is often removed by the use of a decoupling capacitor.

What frequency response do I want from my accelerometer? The frequency response of the accelerometer needed for testing depends on what frequencies of vibration are required to be measured. An accelerometer should have a high enough natural frequency as to capture all the frequencies required to be measured. Problems start to arise however when the vibration content of the acceleration to be measured gets close to the natural frequency of the accelerometer. In these instances distortion of the acceleration by the high gains seen near the natural frequency can give a false picture of the reported acceleration amplitudes at high frequencies. To overcome this problem one of two things needs to happen: a) a higher frequency accelerometer needs to be used b) If the higher frequencies are not required to be measured then using a low pass filter should filter them out.

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