Crystal plays starring role in testing
04 July 2016
During the product development process, environmental testing is often used to validate the design criteria and to ensure that product reliability goals are met in the intended operational environment.
To do this, the testing technology itself must be reliable. Accelerometers based on PiezoStar, a piezoelectric material developed by the Kistler Group, are playing a key role in achieving that reliability.
Environmental testing is performed in industries such as automotive, aerospace and consumer products. It is a mainstay of best practice, ensuring that products achieve their intended life cycle in their operating environment.
Environmental testing involves testing different aspects of a product or component during the development process. Typically, test units are exposed to combinations of vibration stress (periodic, random and shock) and thermal stress (temperature dwells and cycling). These tests further divide into HALT (Highly Accelerated Life Testing) and HASS (Highly Accelerated Stress Screening). HALT is typically performed to discover design flaws before a product is manufactured. The pre-production product is exposed to extreme vibration and thermal stress cycles to the failure point to determine the true operating limit.
HASS is used on production assemblies using thermal and vibration stresses. HASS stresses products beyond the operational limits and to the destruction limits as determined in HALT testing. The intent is to identify weak components and manufacturing defects that will cause early failure. A combination of HALT and HASS testing improves product reliability, lowers warranty expenses and ensures a satisfied customer.
Environmental testing may also be used as a screening process in production. ESS (Environmental Stress Screening) is a process typically used on production electronic components for military and aerospace applications to force latent defects to manifest themselves through failure during the screening process. Products that survive the screening process will then have a higher reliability than a similar unscreened batch. ESS is typically performed as part of the manufacturing process or in acceptance testing when a new product is introduced.
For many reasons, it is crucial to manufacturers that such testing should accurately measure and characterise a product's life cycle. For instance, this knowledge helps maximise the financial and strategic advantages of bringing a product rapidly from design to market. It helps ensure that the product complies with safety and other regulations, with the warranty period offered, and the stated lifetime. It also impinges on long-term costs such as reworking or repair, and on the overall effect the product's perceived reliability has on its market position.
A further important consideration, for any company working in testing and calibration, is operating to the standards of competency needed for ISO/IEC 17025 accreditation. One of its requirements is that testing laboratories should be able to evaluate uncertainty in the measurement process. For a system such as an environmental test-rig, this typically involves calculating a Root Mean-Squared Sum of the separate uncertainties of each element comprising the system.
Until recently, accelerometers were a major source of uncertainty in measurements from environmental testing. For the popular IEPE (Integrated Electronics Piezo Electric) accelerometer, the core component is a piezoelectric crystal: one that develops a charge when mechanically stressed. A problem with commonly used materials, notably quartz and piezoceramics such as PZT, is that the amount of charge developed - the device sensitivity - varies with temperature. If the environmental test regime involves temperature cycling, acceleration measurements will be distorted. This produces uncertainties with both the input control (the shaker is monitored by one accelerometer) and the output data (the structural response of the tested product is monitored by another). Logging the test temperature and applying corrections to the accelerometer outputs can solve this problem but this is time consuming and prone to errors.
The need to compensate for temperature, however, is virtually eliminated by the use of Kistler’s PiezoStar IEPE accelerometers. These models are based on PiezoStar, a proprietary artificial mineral developed by Kistler in the mid-1990s. Combined with a high-gain hybrid microelectronic impedance converter, these materials produce sensors with very low change in device sensitivity with temperature variations.
As these materials share a crystallographic class with quartz, they can be used in the same cuts already developed for IEPE sensors, but have double the sensitivity of quartz, and a wider temperature tolerance. PiezoStar IEPE accelerometers are typically rated for -54°C to 165°C and have been successfully used in cryogenics down to -196°C. PiezoStar models such as the 8703A and 8705A are rated with a -0.004%/°C temperature sensitivity coefficient - a factor of ten better than those based on quartz and a factor of 40 better than those based on piezoceramics.
These properties make PiezoStar based IEPE sensors ideal for precision vibration measurement across a wide range of operating frequency and temperature. For these sensors, bias voltage does not drift radically with temperature (an electronic operating characteristic that ensures that the output signal will not be 'clipped', whatever the temperature). PiezoStar does not suffer from the long-term drift in sensitivity shown by piezoceramics. It does exhibit a small pyroelectric effect - that is, an electrical output with temperature change - but this is only significant with very rapid transient changes, not at the cycling rates (at most ±1.5°C/sec) used in environmental testing.
PiezoStar IEPE accelerometers come in several variants including ±50g and ±250g, ground isolated, and single axis linear and triaxial linear. As an “out-of-the-box” solution for precision vibration measurement in dynamic temperature applications, they complement ISO/IEC 17025 in situations where the highest standards of environmental testing procedure are crucial.
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