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Feb 2nd, 2011
 
Next Generation of MEMS gyroscopes and inertial combo sensors from SensorDynamics
 
SensorDynamics is a well known manufacturer of automotive grade MEMS gyroscopes and inertial combo sensors such as SD721 (single axis gyroscope), SD755 (gyroscope plus accelerometer) and SD78x family (gyroscope plus 3D accelerometer) mainly for Rollover and ESC.
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Introduction

SensorDynamics is a well known manufacturer of automotive grade MEMS gyroscopes and inertial combo sensors such as SD721 (single axis gyroscope), SD755 (gyroscope plus accelerometer) and SD78x family (gyroscope plus 3D accelerometer) mainly for Rollover and ESC. Driven by these applications considerable advances in reduction of manufacturing costs, package size and power consumption have been made.

With the new family of gyroscopes in QFN package, namely SD705 (in-plane single axis gyroscope), SD706 (out-of- plane single axis gyroscope), SD742 (dual axis gyroscope), SD740 (3D gyroscope) and SD746 (3D gyroscope plus 3D accelerometer) SensorDynamics now applies this know how to the non-automotive market where many new applications are arising. For instance, today, even small handheld devices have enough computing power to handle the signals of several inertial sensors for solving complex tasks like gesture recognition. Consequently, SensorDynamics today provides a fully integrated 6DoF-IMU  in a tiny 6x6x1.2 mm QFN40 package with a moderate power consumption of only 16 mW for the consumer market.
SensorDynamics is addressing the high-end consumer, industrial, automation, medical and health-care market and therefore uses the stable and proven QFN package, a package that allows sensors to be calibrated with low bias and sensitivity errors over a wide temperature range up to 125°C.


Technology
The SensorDynamics’ MEMS fabrication makes use of the reliable PSMX2 process. This silicon technology based process provides a 10 to 20µm thick polycrystalline silicon for the micro-mechanically active structure, thus making it best suited for single axis, dual axis, and triple axis gyroscope sensing elements with high resonant frequencies, which is highly desired to give the structures a high resistivity against shock and vibrations.

Figure 1 shows the mechanically active structure of a 3D gyroscope. Basically this structure consists of four coupled masses that are forced to resonant oscillations by means of electrostatic excitation. Whenever this oscillating structure is subjected to a rotation, the so called Coriolis force is acting perpendicularly to the rotation axis and to the direction of oscillation and thus forcing the structure to a secondary movement. The amplitude of this secondary oscillation is a measure of the rotational speed.

For a complete 6DoF-IMU a 3D accelerometer must be added to the 3D gyroscope. SensorDynamics has a unique dual cavity technology available that allows integrating the gyroscope together with the accelerometer on a single die. This technology provides adjacent, hermetically sealed cavities with different internal pressure, optimized for the different needs of two sensor types.

 
Figure 1: Active Structure of a 3D gyroscope

The inertial sensors discussed in this paper are all packaged in the proven QFN40 package. This package is extremely stable over temperature and can be employed up to +150°C. SensorDynamics only uses QFN packages with L-shaped leads that provide well visible soldering joints at the package rim. This makes the QFN also suitable for automotive applications.

Product properties
Inertial Sensors can be optimized for different performance parameters such as shock resistivity, vibration robustness, temperature drift, and bias stability. The QFN package supports all of these parameters, especially temperature drift and bias stability.

As an example figure 3 shows the result of a Root Allan Variance (RAV) measurement of a single axis gyroscope in QFN package, SD706. The RAV is used to determine the intrinsic bias stability of a sensor, i.e. the stability in the absence of external influences like temperature changes and vibration. For this purpose the sensor signal is sampled over several days at constant ambient conditions. Subsequently the measurement series is divided into certain time intervals and average values are calculated for these intervals. The standard deviation (root variance) of the average values is plotted for different averaging intervals.

In figure 3 the interval time is referred to as cluster time. The minimum of the RAV plot is considered as the intrinsic bias stability.

Figure 2:  SD746, 6DoF-IMU in QFN40 package

Table 1 provides an overview of the family of SensorDynamics gyroscopes and combo sensors in QFN40 package. The specified errors are maximum errors over the whole temperature range from -40°C to +85°C. The measurement range of the accelerometer of the SD746 is ±8g. All sensors feature a continuous self diagnosis function that immediately alarms the user if a MEMS element has a malfunction.

Figure 3: Root Allan Variance of a single axis gyroscope in QFN package (SD706)

Table 1: SensorDynamics gyroscopes in QFN package. For axis definition refer to figure 2. 
*) Measurement range up to ±4096°/s on customer demand


Applications
SensorDynamics is targeting three main areas of application for the new inertial sensors. First area is navigational systems where the position of a moving object has to be determined. In this application the sensor signals must be integrated over time in order to convert rotational speed into angle and acceleration into distance traveled. However, due to the unavoidable bias errors the position error will increase with integration time. Hence, sensors such as the SD706 with outstanding intrinsic bias stability are well-suited for such applications.  

A second area of application is platform stabilization where unwanted movements have to be compensated, such as the rocking movement of a crane or image blur in cameras.  MEMS inertial sensors are well-suited for such an application, since there is no need to integrate the sensor output signal. 2D or 3D gyroscopes such as SD742 and SD740 fit well into these applications, especially if the application is safety relevant. This is because the safety level can be improved by making use of the continuous self test of these devices.

The third main area is motion capturing for instance gesture recognition for handheld devices, but also movement analysis in medical research or for computer animated movies. The SD746 is an ideal fit for these applications because it is temperature compensated over the whole operating range making external bias compensation dispensable.


Outlook
SensorDynamics is addressing further optimization of the performance parameters of its inertial single- and combo-sensors, e.g. improvement of power consumption and noise figures. These parameters are strongly related to ASIC design and technology.
Another field of continuous improvement is shock resistivity and vibration hardness. Here the thickness of the MEMS active structure is a key parameter because thicker structures can be made stiffer thus allowing higher resonant frequencies.

In order to enhance the sensor system approach, SensorDynamics will add more functionality to its modules like interfaces for other sensors (e.g. AMR sensors) and have a focus on application software for fast and easy sensor employment.

SensorDynamics will also put efforts into making a 6DoF-IMU available for the automotive market. For this purpose the temperature operating range will be enlarged from +85°C to +125°C and the device will be qualified according to AEC-Q100.


 
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