Design and Selection Considerations for High-Temperature Aerospace Accelerometers
- Thursday, 16 November 2006
Considerations include material selection, construction, and radiation tolerance.
Accelerometers used for shock and vibration measurement in extreme environments require special consideration in the design and manufacturing process. Certain unique applications can require the same accelerometer to function from -54 to 649 °C (-65 to 1,200 ° F). This might include such applications as vibration measurement on gas turbine engines, in flight, or in test cells; rocket motor vibration measurements; and thruster vibration. These accelerometers also may need to function in nuclear radiation environments, and possibly in a combination of temperature extremes and radiation. This might include nuclear power generation or space vehicle applications. Materials and construction must then be selected not only to enhance high temperature performance, but also to allow operation in the presence of gamma and neutron radiation
Sensor design and performance varies depending on choice of piezoelectric material. Piezoelectric materials can be either a single natural crystal, where the material is inherently piezoelectric, or ferroelectric ceramic, where a process known as poling applies a high voltage to the material to align polar regions within the ceramic element, resulting in a net piezoelectric effect (see Figure 1). Each material has unique features and advantages, which characterize its performance in various applications.
The highest temperatures and lowest pyroelectric output can be obtained with certain natural crystals, while ferroelectric ceramics offer extended frequency range and smaller size for equivalent output. Designs using ferroelectric ceramic in compression have greater pyroelectric output than a shear, or natural crystal, design. Pyroelectric output is caused by uniform temperature change in the crystals when compressed, and occurs on surfaces perpendicular to the axis of polarization. This output can be avoided by using high-pass filtering in the measurement electronics.
There are design trade-offs with high temperature sensors that also must be considered, affecting maximum temperature, bandwidth, and sensitivity. Ferroelectric ceramics typically exhibit twice the output as some of the natural crystals and nearly ten times that of Tourmaline, one of the more common natural crystals. Bismuth titanate-based ferroelectric can be used to 482 °C (900 °F). Various compounds can be added to alter characteristics, but higher temperature comes at the expense of output.
Single crystals are not so "natural" these days. Many crystals are grown in laboratories, rather than mined, resulting in higher, more consistent quality and new variations with higher output. The single crystal allows processing in both shear and compression modes. Typically, shear mode output is almost twice that of the compression mode. This allows versatility of design and performance optimization (see Figure 2).
Maximum accelerometer temperature is determined by the properties of the crystal material, known as the Curie temperature, as well as the minimum source resistance required for signal conditioning equipment. Source resistance, also known as the insulation resistance, will decrease as temperature increases. This is particularly important when considering the charge conversion electronics to be used with the accelerometer. Charge converters incapable of accepting this lower source resistance will not work properly.
As a sensor is exposed to temperature changes, several key parameters, such as sensitivity, source resistance, and sensor capacitance, will change. Changes should be predictable and repeatable. Changes beyond acceptable levels can indicate improper operation due to materials, assembly, or malfunction of the sensor.
The connection system, consisting of the connector and cabling, is also an important consideration for good measurements. A loose connector can result in high-level, low-frequency response. Connector reliability generally degrades above 492 °C (900 °F). To remedy this, accelerometers designed for temperatures of up to 649 °C (1,200 °F) have an integral cable attached to the sensor. By design, these integral cables are mechanically isolated from the seismic system to avoid base and cable strain. Materials selection of the cable, as well as a protective over-braid, promote ease of handling and bend forming during installation, as well as ruggedness.
For accelerometers used for shock and vibration measurements in nuclear radiation environments, an examination of transient and steady state radiation effects is necessary. This is to determine tolerable radiation doses, which will not affect accelerometer performance. An adverse reaction to radiation would be reduced output from the crystal, and deterioration of various materials that are radiation-intolerant. Generally, higher temperature crystals are more resistant to radiation effects.
This article was written by Margie Mattingly, senior program engineer at PCB Piezotronics, Depew, NY. For more information, visit http://info.ims.ca/5786-122 .