1.Relationship Analysis of Accelerometer Instability and Internal Impact Factors
Quartz flexible accelerometer comprises a header and a servo circuit, functioning as a balanced and regulatory system for force feedback. Within its header are components including a pendulum, a torquer, a soft magnet, a differential capacitance sensor, and others, forming an integrated complex of various materials such as quartz, magnet, coil, epoxy glue, and electronic circuitry.
Prior to assembly, all internal components undergo stabilization treatment. Nonetheless, following the complete manufacture and assembly of the product, parameters continue to drift in storage conditions due to the effects of time and environment, primarily attributed to temperature stress and residual stress.
Fig.1 Internal structure of quartz flexible accelerometer
1.1 Temperature Stress:
Temperature stress arises from the constraint of material temperature deformation, predominantly occurring within the core, where temperature undergoes significant changes due to Joule heat generated by the torque coil during electrification, particularly prominent in wide-range accelerometers. This stress primarily stems from the disparate thermal expansion coefficients of pendulous components such as torque coil, epoxy glue, and quartz. Notably, high-polymer epoxy glue undergoes a glassy to rubber state transition at specific temperatures, leading to significant alterations in mechanical properties like elastic modulus, thereby exacerbating temperature stress during temperature fluctuations.
1.2 Residual Stress:
Residual stress refers to the internal stress that persists within bodies for self-equilibrium subsequent to the removal of external forces or application of nonuniform temperature fields. It is generated during product machining, assembly, and other processes. Changes in residual stress primarily manifest in associative components because, during component assembly, residual stresses within them undergo release or redistribution under varying boundary constraint conditions. In the case of the accelerometer, residual stress primarily accumulates in the preloading ring welded joint or the torque coil joint during epoxy glue solidification.
Fig.2 See torque coil, Cross-section view of Q-F Accelerometer
From the overall point of view, the key parts of internal stress release are the adhesive torque coil and the preloading ring welded joints. Thus, the analysis is necessary to find out what kind of stress is conducive to the product stability.
2.Validity Analysis of Internal Stress Release by Environmental Stress
2.1 Temperature cycling
Epoxy glue can gradually release the stress in the process of temperature cycling, and with accumulation of various conditions such as temperature and its cycling number, its mechanical properties would change and tend to stable equilibrium state, i.e., physical aging. This is a relaxation process in which glassy polymers make the condensed structure transit from the nonequilibrium to equilibrium state through the micro-Brownian movement of small regional chain segments. Furthermore, physical aging bears all the following characteristics of the relaxation process:
1) being imitable—the method of heat treatment can be adopted to eliminate the history of sample storage or get the sample into the required state—and
2) being a self-deceleration process. Physical aging decreases the free volume and thereby reduces the activity of chain segments, which would lead into aging rate reduction. Then, the negative feedback self-deceleration process is formed, i.e., the closer it gets into equilibrium, the lower the rate is. Therefore, the applied temperature cycling stress can make epoxy glue release the thermal stress and get into the aging and equilibrium state.
Fig.3 Ultra-thin-Quartz-Accelerometer
2.2 Random vibration
Stress release by random vibration is achieved while the dynamic stress superposition results in the local plastic deformation. By applying an alternating force to metal components during vibration, if the sum of the dynamic stress amplitude and the residual stress at some points on the treated metal components reaches the yield limit, these points will produce lattice slip and microscopic plastic deformation, and this deformation starts at the maximum point of residual stress, that correspondingly makes these points release under constraints and thereby reduces the residual stress.
Random vibration works to provide mechanical energy to metal components, to improve the kinetic energy of crystal inside the workpiece, and to accelerate the speed in which lattice distortion can recover the equilibrium position. Then, under the impact of stress superposition, the crystal dislocation slip occurs inside the material, so residual stress would be released, redistributed, and rebalanced; also, the material matrix would be enhanced to resist deformation.
In essence, both temperature cycling and random vibration are ways of increasing internal energy by energy transfer. Random vibration has higher energy transfer efficiency and can greatly shorten the time of residual stress release, but vibration cannot completely eliminate residual stress. Temperature cycling is more effective for internal stress relief of the glue but is prone to bring about new distortion and secondary stress.
3.Summary
By conducting the mechanism analysis outlined above, we can ascertain that the accelerated stability test profile of the accelerometer comprises an amalgamation of temperature cycling and random vibration. Within this profile, the boundary conditions pertaining to factors such as temperature, temperature variation rate, and vibration levels can be deduced through enhancement experiments.
It's worth noting that the zero bias repeatability of the quartz flexure accelerometer ER-QA-01A is 10μg, and the scale factor repeatability is 10ppm. It is applied in aircraft carrier microgravity measurement systems, inertial navigation systems, and static angle measurement systems.
More Technical Questions
1. Optimizing Compensation Loop for Quartz Accelerometer
2. 2 Ways to Improve Shock Resistance Performance of Q-Flex Accelerometer
3. Parameters to Evaluate Performance of Quartz Flexure Accelerometers
4. Factors Affecting the Stability of Q-Flex Accelerometers
5. Structure Design of High Precision Quartz Flexible Accelerometer
6. Methods to Maintain the Long-Term Performance of Quartz Flexure Accelerometers
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