Achieve more innovative accelerometers with less (cost, size, weight and power) to achieve higher levels of inertial performance-GNSS internal-Global Navigation Satellite System Engineering, Policy and Design

2021-11-24 04:55:57 By : Ms. Loy Liu

Global Navigation Satellite System Engineering, Policy and Design

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Developed a breakthrough technology, and the hard work is far from over. Now you have to show potential customers that previously unheard of performance levels can be achieved with low-cost materials.

The challenge for Physical Logic’s chip designers is to convince manufacturers of inertial measurement units (IMU) and drones that the company’s revolutionary approach to microelectromechanical systems (MEMS) processing can be cost-effective, weight, and power (C-SWaP). ) The accelerometer provides support for several levels in the accuracy hierarchy. In short, MEMS can replace traditional mechanical or solid-state accelerometers in all applications to achieve tactical and even navigation-level inertial performance.

The key features of the innovative control design provide system-wide low noise, high linearity, almost zero vibration rectification error (VRE), strong stability and enhanced sensitivity.

Company CEO Aviram Feingold said: "Almost all of these [traditional mechanical accelerometers] can be easily replaced by physical logic closed loops." "If you want to compare these systems, you will see. These are mainly used for navigation. Among them, the achievement of 70g allows us to access about 90% of end-user applications. UAVs, helicopters, missiles, marine applications, now there are wireless applications, 30g is not enough, they need 50g or 70g."

Two variants of the innovative accelerometer design, open loop and closed loop, are based on a unique in-plane micromachining process. This is performed on a silicon-on-insulator (SOI) wafer assembled with an application-specific integrated circuit (ASIC) with an analog front end and temperature sensor in a custom, hermetically sealed, and compact LCC20 package.

The unique planar geometry of the sensor MEMS element allows it to use a highly symmetric comb structure based on a full-capacitance bridge topology. The in-plane concept can achieve excellent capacitance linearity in mass displacement. It also produces extremely low levels of thermomechanical noise, which is achieved by sealing under atmospheric conditions without the use of less reliable vacuum packaging.

Physical Logic's open-loop accelerometer series fills the gap between current MEMS accelerometers and traditional, heavier and more expensive mechanical accelerometers. The open-loop product line provides various input sensing ranges from 2g to 70g, which can be applied to tilt sensing, seismic sensing, resource exploration, vibration sensing, north finding and inertial navigation.

In-plane micromachining takes advantage of conventional MEMS, such as low-cost manufacturing and easy integration with microelectronics. However, it avoids the shortcomings of capacitive sensing with high impedance readout. The read node is extremely susceptible to parasitic leakage current and electromagnetic interference. In-plane micromachining uses a full-bridge capacitive sensing configuration to suppress parasitics and improve noise performance.

It can also improve linearity. Through the evenly distributed sensing capacitors on the MEMS chip area, the process further utilizes its symmetrical mechanical geometry to provide high stability performance. The simplicity of the design means that a variety of sensing ranges can be achieved with only minor changes to the architecture.

Physical Logic's open-loop MEMS accelerometer meets the requirements of tactical-level inertial performance (see Table 1, the key parameters of the open-loop 2000 series).

In order to overcome the inherent nonlinearity and further improve the bias stability performance, which is necessary to achieve navigation-level performance, further innovation is required. Therefore, Physical Logic has developed a closed-loop series, which is similar to in-plane micro-machining, but applies a rebalancing force on the mass to offset the inertial force caused by external acceleration. The closed-loop design compensates for acceleration and minimizes the displacement of the inspection mass

Equipped in this way, the closed-loop series goes one step further, replacing tactical and navigation-grade mechanical accelerometers. With a sensing range certification of up to 70g, it is ideal for high-end navigation applications such as drones, other self-driving cars, aviation and aerospace. It provides improved performance in the form of scale factor linearity, bias stability, and vibration correction.

For example, MAXL-CL-3030 has a dynamic range of 30g and a resolution of 20 bits. It is a fully integrated sensor packaged in a specially designed and manufactured LCC44 package. Please refer to Table 2, the key parameters of the closed-loop 3000 series.

Recently, Physical Logic announced that it has completed the certification and production preparations for the MAXL-CL-3050, which is a new type of 50g sensing range sensor based on the MAXL-CL-3000 series. Like the 15g and 30g configurations, the performance of the MAXL-CL-3050 enables it to compete with traditional mechanical accelerometers in the most demanding navigation-related applications. However, the new 70g sensor has reached a higher level of performance.

For open-loop and closed-loop product types, Physical Logic's powerful in-plane micromachining process can ensure high yield and high reliability, while maintaining the advantages of low C-SWaP MEMS while providing higher performance.

A few years ago, Physical Logic undertook a dual R&D project for capacitive MEMS accelerometers, aiming to achieve the inertial tactical-level performance of the open-loop accelerometer and the inertial navigation-level performance of the closed-loop accelerometer. The company's scientists have proposed a unique design that can provide the same MEMS manufacturing process flow for both product lines. In addition, the robust process ensures high yield and high reliability.

The MEMS manufacturing process usually follows an out-of-plane design. The final product needs to be vacuum packed, because in the out-of-plane configuration, the damping of the large parallel plate electrode is very serious. There are always reliability issues with vacuum sealing. The in-plane process can utilize atmospheric pressure sealing to improve reliability and simplicity.

The out-of-plane sensor that uses the gap change principle sacrifices linear performance. Due to its capacitive area change architecture, in-plane processing eliminates this. And, as mentioned earlier, the full-bridge capacitive sensing architecture provides parasitic suppression and improved noise performance, both of which are difficult to achieve with out-of-plane devices.

Finally, in-plane processing improves performance during vibration, which is an extremely important performance indicator for UAVs, especially UAVs.

"The main advantage is almost zero VRE," said Aviram Feingold, CEO of Physical Logic. "The reality here is that the VRE amplitude in a closed-loop accelerometer is much better, 10 times better than an open-loop accelerometer.

"Take the application of a package delivery system as an example, where safety is the most important parameter. The vibration of the propeller caused by an accident will have a huge impact on the navigation of the drone. Therefore, this is one of our main end-user target applications. : Our contact with drone manufacturers that specialize in package delivery systems."

Figure 1 shows the in-plane micromachining scheme. Both open-loop and closed-loop structures include an array of capacitor electrode plates, mechanically coupled to a common inspection mass (the rotor shown in Figure A) and the same electrode plate array connected to a fixed silicon frame (stator). The careful design of the support spring limits the inspection The mass moves linearly in a plane parallel to the plane of the fixed electrode plate. The capacitance between the two plate arrays is variable and depends on the detection mass displacement.

Many navigation applications require input acceleration measurements up to 50g or 70g. This may be difficult to achieve without losing other key performance parameters. This requires a very close understanding and careful management of the overall error budget.

The sensing range of an open-loop accelerometer is usually proportional to the spring constant. Increasing the spring constant for a higher sensing range will also increase the bias charge sensitivity, thereby reducing performance. In non-navigation applications, this compromise is acceptable, but navigation-grade accelerometers must provide both high performance and high sensing range.

The closed-loop operation uses capacitive sensing and feedback voltage to determine the sensing range, eliminating the trade-off between performance and sensing range, optimizing these two results. Therefore, the theoretical error budget developed by the company leads to MEMS designs that achieve low bias sensitivity and superior performance over temperature and time.

“An interesting application is resource development,” Feingold said. “There you need underground navigation. You don’t have GPS time and it’s very difficult to make long-term measurements underground. These companies now know that we have quantified our performance. , We started to get feedback, in terms of accuracy, we provided better results than our competitors."

Figure 2 shows the open-loop and closed-loop accelerometers before capping.

"Fundamentally, both open and closed loops are based on the same planar technology, allowing a very linear transfer function from acceleration to voltage to the capacitance we see," added Lisa Koenigsberg, the company's chief technology officer. "Because we don’t just use the gap effect, we optimized the design to get the highest possible sensitivity. With a planar configuration, optimization is much easier. This is the real reason why VRE and scale factor have good performance. Because we have a lot of sensitivity Degree, so it is very linear in terms of core technology.

"When we close the loop, we basically have three capacitor parts that are all mechanically connected. Now the acceleration is the voltage we apply to these actuators. This is how we get a high sensing range because it only depends on the voltage. .

"The benefit of the closed loop is to benefit from everything and get all the good performance from the low range. The high range is not achieved by motion or displacement, but by voltage. So you can get a high range from, say, a 2g accelerometer And small deviations, but you can measure 50g or 70g."

Each production unit in open-loop and closed-loop configuration has passed acceptance and qualification tests, including temperature, vibration, and centrifuge measurements, to reach its designated g-level. The temperature cycle fits the baseline temperature model of deviation, scale factor, and misalignment behavior, followed by the temperature storage cycle temperature operation cycle. The dynamic test includes vibration and shock curves in multiple directions. Finally, the unit repeats the first temperature cycle to test the repeatability of the model.

All in-plane processing equipment has shown excellent performance in terms of environmental exposure, verifying the long-term accuracy of all parameter ranges. There are specific statistics in each product data sheet. In addition, the closed-loop series achieves excellent scale factor linearity, VRE and excellent stability.

"The other thing we are doing here," Feingold said, "To convince our customers, we are running life tests to accelerate C-Swap. We actually use accelerometers for many months, below 60 degrees. Each We will take them out every month, conduct some tests and bring them back. We test again and again to show very good results."

The advancement of MEMS accelerometers in the past 20 years has opened up new opportunities in many areas of motion sensing. With the explosive growth of unmanned aerial vehicles, the accuracy requirements for high dynamic and high vibration platforms have increased the downward pressure on cost, weight, size and power. The unique advantages of the in-plane MEMS architecture introduced by Physical Logic means that acceleration, vibration, shock, tilt, and rotation in high-gravity applications can be more accurate than previously achieved in a small and light form factor. Open-loop and closed-loop MEMS accelerometers can now replace high-cost instruments in most high-end applications.

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Can elliptical Galileo satellites be used for RTK?

Washington View: The Big Wheels Continue to Turn-Rolling to 5G on the River