In part one of this series, we reviewed the internal structure of a 3-axis high precision MEMS accelerometer. In part two, we reviewed how to acquire a good starting dataset to establish baseline performance and validate what sort of noise levels to expect in subsequent data analyses. In this final installment of our series, we explore other factors affecting stability and then offer mechanical system design recommendations to improve the overall performance of a 3-axis high precision MEMS accelerometer.
Once thermal stresses in the design are well understood, another important aspect of inertial sensors is their long-term stability, or repeatability. Repeatability is defined as the accuracy of successive measurement under the same condition over a long period of time. For instance, taking two measurements of a gravity field in the same orientation with respect to gravity at the same temperature over an extended period and seeing how well they match. Repeatability for offset and sensitivity are of paramount importance when assessing the long-term stability of a sensor in applications that are unable to accommodate regular maintenance calibration. Many sensor manufacturers do not characterize or specify long-term stability in their data sheets. In ADI’s ADXL355 data sheet, for example, repeatability is predicted for a 10 year life and includes measured shifts due to the high temperature operating life test (HTOL) (TA = 150°C, VSUPPLY = 3.6 V, and 1000 hours), measured temperature cycling (−55°C to +125°C and 1000 cycles), velocity random walk, broadband noise, and temperature hysteresis. Repeatability as shown in the data sheet is ±2 mg and ±3 mg for X/Y and Z sensors, respectively. These measurements are important for evaluating long-term performance.
Repeatability under stable mechanical, environmental, and inertial conditions follows the square root law as it relates to time measured. For example, to obtain offset repeatability of the x-axis for 2.5 years (possibly a shorter mission profile for an end product), use the following equation: ±2 mg × √(2.5 years/10 years) = ±1 mg. Figure 1 shows an example HTOL test result of 0 g offset drift of 32 devices over 23 days. The square root law is clearly observable in this figure. It should also be highlighted that each part behaves differently—some perform better than others— due to process variation in fabrication of the MEMS sensors.
Figure 1. 500 hour long-term stability of the ADXL355. (Source: Analog Devices)
Mechanical System Design Recommendations
Armed with the knowledge from the previous discussion, it is clear that mechanical mounting interfaces and enclosure design will contribute to the overall performance of a 3-axis high precision MEMS accelerometer sensor as they will affect the physical stresses propagated to the sensor. In general, the mechanical mounting, enclosure, and the sensor form a second-order (or higher) system; therefore, its response varies between resonance or overdamped.
Mechanical support systems have modes that represent these second-order systems (defined by resonant frequency and quality factor). In most cases, the objective is to understand these factors and minimize their impacts on the sensing system. Thus, geometry of any enclosure that the sensor will be packaged in, and all interfaces and materials, should be chosen to avoid mechanical attenuation (due to overdamping) or amplification (due to resonance) within the bandwidth of the accelerometer application. The details of such design considerations are out of the scope of this article; however, some practical items are briefly listed:
PCB, Mounting, and Enclosure
- Securely attach the PCB to a rigid body substrate. The use of multiple mounting screws in combination with adhesive on the backside of the PCB offers the best support.
- Place the sensor close to a mounting screw or a fastener. If the PCB geometry is large (a few inches), use multiple mounting screws in the middle of the board to avoid low frequency vibration of the PCB, which will couple to the accelerometer and be measured.
- If PCBs are only supported mechanically by a groove/tongue structure, use a thicker PCB (greater than 2 mm thick is recommended). In the case of PCBs with larger geometry, increase the thickness to maintain stiffness of the system. Use finite element analysis, like ANSYS or similar, for the optimum PCB geometry and thickness for a specific design.
- For applications such as structural health monitoring where sensors are measured for a long period of time, the long-term stability of the sensors is critical. Packaging, PCB, and adhesive materials should be chosen to minimize degradation or change in mechanical properties over time, which could contribute to additional stresses on the sensor, and, hence, offsets.
- Avoid making assumptions about the natural frequencies of the enclosure. Calculation of natural vibration modes in the case of simple enclosures and finite element analysis in the case of more complex enclosure designs will be useful.
- Stress buildup from soldering the accelerometer to a board has been shown to cause offset shift of up to a few mg. To alleviate this effect, symmetry in PCB landing pattern, thermal pads, and conduction paths through copper trace on PCB are recommended. Closely follow the soldering guide provided in the accelerometer’s data sheet. It is also observed that, in some cases, solder annealing or thermal cycling prior to any calibration are helpful to relieve the stress buildup and to manage longer term stability issues.
Potting compounds are widely used to secure electronics inside an enclosure. If the sensor package is an overmold plastic, such as land grid array (LGA), use of potting compounds is highly discouraged due to their temperature coefficient (TC) mismatch with the enclosure material resulting in pressure being exerted directly on the sensor and then offset. A 3-axis high precision MEMS accelerometer that comes in a hermetically sealed ceramic package significantly protects the sensor from the TC effect. But potting compounds can still contribute to stress buildup on the PCB as a result of material degradation over time, potentially causing strain on the sensor through small warpages to the silicon die. It is generally recommended to avoid potting the sensors in applications in which high stability over time is required. Low stress conformal coatings such as parylene C could provide some form of moisture barrier as a substitute for potting.8
Air Flow, Heat Transfer, and Thermal Balance
To achieve the best sensor performance, it is important to design, locate, and utilize the sensing system in a setting where temperature stability is optimized. As this article shows, even small changes in temperature can show unexpected results due to differential thermal stresses on the sensor die. Here are some tips:
- The sensor should be positioned on the PCB so that thermal gradients across the sensor are minimal. For example, linear regulators can generate significant amounts of heat; therefore, their vicinity to the sensor can cause temperature gradients across the MEMS that may vary with current outputs over time in the regulator.
- If possible, the sensor module should be deployed in areas away from air flows (for example, HVAC) to avoid frequent temperature fluctuation. If not possible, thermal isolation outside or inside the package are helpful and can be achieved with thermal insulation. Note that both conduction and convection thermal paths need to be considered.
- It is recommended to choose the thermal mass of the enclosure such that it damps environmental thermal fluctuations in applications where environmental thermal changes are inevitable.
This article has shown how the performance of a high precision MEMS accelerometer can be degraded without adequate consideration to environmental and mechanical effects. Through holistic design practices and a focus at a system level, discerning engineers can achieve excellent performance for their sensor system. As many of us are experiencing unprecedented stresses in our lives, it is useful to realize that, similar to accelerometers, it is never the stress that kills us—it is our reaction to it!
|Paul Perrault is a senior staff field applications engineer based in Calgary, Canada. His experience over the last 17 years at Analog Devices varies from designing 100+ amp power supplies for CPUs to designing nA-level sensor nodes and all current levels in between. He holds a B.Sc. degree from the University of Saskatchewan and an M.Sc. degree from Portland State University, both in electrical engineering. In his spare time, he enjoys back-country skiing in hip-deep powder, rock climbing on Rockies’ limestone, scrambling and mountaineering in local hills, and spending time outdoors with his young family. He can be reached at firstname.lastname@example.org.
|Mahdi Sadeghi is a MEMS product application engineer in the AIN Technology Group at Analog Devices. He received his Ph.D. in electrical engineering from the University of Michigan, Ann Arbor, in 2014. His Ph.D. thesis and work as a research fellow at the Engineering Research Center for Wireless Integrated Microsystems (ERC WIMS) focused on the development of sensing microsystems for unmanned air vehicles and autonomous mobile platforms. His experience includes microhydraulic sensors and actuators, microfluidic systems, inertial sensing system design for wearables, and sensing solutions for condition-based monitoring applications. He can be reached at email@example.com.
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