Vibration Measurement at High Temperatures

“What do I have to do to measure vibration at high temperature?” is a common question for the extreme environments found in the automotive, aerospace and industrial vibration measurement fields.  Various sensor considerations come into play including sensing element material, casing/connector construction, signal conditioning and cabling.  Since there is no single right answer to this question, as you might imagine, the answer depends on just how high the temperature is.

For well over 80% of all vibration measurement applications, environmental conditions of less than 250 deg F (121 deg C) allow for standard ICP^® operation with built in microelectronics to simplify the power and signal conditioning needs.  These ICP accelerometer designs incorporate a microelectronic MOSFET or JFET amplifier to handle the high impedance charge conversion at the sensing crystal and provide a suitable low impedance voltage at the accelerometer’s signal connector to couple directly to a digital signal analyzer, scope or recorder.  Power (excitation voltage) and signal are delivered and returned over the same wire using a simple constant current signal conditioner.  This is the world’s single most common means of accomplishing measurement grade vibration sensing, whether in most laboratory measurements or field tests where “mother nature” is the only source of additional heat.

However, as temperature rises due to the friction of mechanical operation or the firing of a combustion engine, higher temperatures can begin to become challenging for the operation of the piezoelectric accelerometer's internal microelectronics.  Without proper design and selection of components, the elevated temperatures begin to effectively short the internal microelectronics as leakage increases back through the amplifier’s gate and produces a symptom observed as a falling bias voltage.

Fortunately, many piezoelectric ICP accelerometer designs can be specified to include high temperature electronics allowing operation as high as 325 deg F (163 deg C) For high temperature operation it is desirable to ensure that your sensor vendor of choice conducts additional screening of the finished sensors to ensure reliable operation at these elevated temperatures.  For example, a 100 hour soak at the 325 deg F temperature ensures proper operation by 100% screening.  This elevated temperature range is common in many automobile under hood measurement points as well as most engine block points away from the manifold.  An example of industrial machinery monitoring application includes the dryer section of a paper mill.   The high temp ICP designs provide all the convenience of tradition ICP operation including;

• low impedance operation
  • no costly special cables
  • able to drive long cable runs
  • no bulky charge amplifier box
  • no extra attention to cleanliness/insulation resistance
  • high resistance to RF/EMI/noise for excellent signal to noise
• simplified power requirements
  • simple constant current signal conditioning design low cost 
  • constant current supply often built into analyzer, recorder or data logger

As the temperature crosses the 325 deg F (163 deg C) mark and heads towards 500 deg F (260 deg C) internally conditioned ICP operation is no longer feasible.  While the standard PZT piezoelectric crystal in the sensing element continues to operate satisfactorily at these temperatures, the electronics and impedance conversion must be conducted remote from the sensors and high temperature environment.  This can be accomplished by using specially designed low noise cabling to connect the high temperature charge mode accelerometer to a remotely located selectable range laboratory charge amplifier or alternatively, cabled to a simple fixed range in-line charge amplifier, saving significant cost and complexity.  For further discussion on the similarities between charge and ICP mode (low impedance)  see the article "Charge vs ICP Accelerometers.". 

The construction of high temperature charge mode operation piezoelectric accelerometers start with a number of design considerations.  While high temperature flexible cabling can still be used at these temperatures, a great deal of attention is paid to the connectors.  To ensure long term reliability in thermally cycled environments, a glass sealed hermetic connector design is a must.  Solder joints internal to the sensor are replaced by spot welds and a long duration extreme temperature soak is required to stabilize and pre-age the high temperature crystal element.  Note that as temperature environments surpass the recommended operating range for a standard PZT high temperature charge mode sensor, the heat begins to depolarize the piezoelectric crystal and permanent loss of charge output can occur.  Applications for the up to 500 deg F (260 deg C) range include automotive brake caliper measurements, and exhaust points well downstream from the manifold and catalytic converter.  Aerospace applications include a host of gas turbine front section measurements including accessories/turbine, gear boxes and various mount points. Industrial applications include the  turbines for power generation as well as some steam lines.

The next plateau in high temperature measurements occurs at approximately 950 deg F (510 deg C).  At this temperature PZT based materials will depole, so bismuth titanate ferroelectric material in the compression mode can be used.  Additionally, single/natural crystal materials can be employed in either shear or compression mode.  With material selection comes tradeoffs in sensitivity, bandwidth, pyroelectric output, thermal coefficient and mounting.  Shear designs have performance advantages such as base strain and thermal transients.  Other internal construction considerations include spot welding of all internal signal leads and internal strain isolation features for integral cables.  In the best case, rigid high temperature hardline cabling is welded directly at the sensor connector end and to the flight qualified or industrially hardened charge amplifier on the other.  This ensures a complete chain of hermetic seal providing long term protection of the high impedance signal chain for faithful and reliable representation of the vibration signal.  With respect to packaging, some sensor housings are constructed from stainless steel (750 deg F max) though the material of choice at these temperatures is  inconel for its ability to withstand high temperature while maintaining its structural strength.  In automotive applications these specialized high temperature packages are typically needed when  mounting directly on the exhaust manifold or turbocharger and near the catalytic converter.  Industrial and aerospace applications include the hot section of gas turbines for  power generation and  aircraft. For laboratory/test applications, single ended operation is common, while  flight programs require differential operation. 

A final temperature plateau occurs at 1200 deg F (650 deg C) operation. This includes the hot section of gas turbine engines and locations directly on an automotive catalytic converter. Some manufacturers opt for natural crystal tourmaline as the sensing element although a new more efficient, low temp coefficient (LTC) material is now commercially available in both compression and shear designs.  The LTC design again affords better thermal response with the advantages of versatilities of crystal cuts and operation modes as described above.

As you can see, dynamic measurements are seldom “routine.”  It helps to leverage the proven methods from a trusted source.  I can also recommend an article from Quality Magazine written by my colleague, Margie Mattingly of PCB Piezotronics, for further reading on the detail design of high temperature piezoelectric accelerometers. 

To discuss measurements in these or other extreme environments contact your local field application engineer, the Aero, Auto or Industrial divisions at PCB Piezotronics or drop us a line here at The Modal Shop.