# Use Cases

AHRS MEMS Accelerometer


Problem We Solve

Strap down inertial navigation systems require an initialization process that establishes the relationship between the aircraft body frame and the local geographic reference. This process called alignment generally requires the device to remain stationary for some period of time in order to establish this initial state. To initialize, the inertial reference system goes through a self-alignment process to align the vertical axis of the local level coordinate frame with sensed acceleration (leveling) and to measure the horizontal earth rate to determine the initial azimuth (gyro-compassing). If the initial attitude of the vehicle could be known, and if the gyros provided perfect readings, then the attitude processor would be sufficient. However, the initial attitude is seldom known, and gyros typically provide corrupted data due to bias drift and turn-on instability.

The role of MEMS accelerometer in an AHRS application is to provide with initial attitude reference (leveling) using gravity and provide attitude corrections during the flight required to correct the gyro drift.

Both MEMS gyros and accelerometers suffer from bias drift terms, misalignment errors, acceleration errors (g-sensitive), nonlinear effects (second order term or VRE), and scale factor errors. Accelerometers required for inertial navigation systems should have stable and repeatable bias, small bias temperature coefficients and practically non-existing non-linearity.

Why it is Important

Inertial navigation is the process of calculating the position and velocity of a body (such as an aircraft) from self-contained accelerometers and gyroscopes. Inertial Navigation Systems of middle accuracies, Attitude and Heading Reference Systems (AHRS) and Flight Control Systems (FCS), require gyros and accelerometers to predict the position of a moving object in free space.

AHRS are multi-axis sensors that provide heading, attitude and yaw information for aircraft or any subject moving in free space. FCS control an aircraft’s direction in flight and change speed as well. AHRS consist of gyroscopes, accelerometers and magnetometers on all three axes. Some AHRS use GPS receivers to improve long-term stability of the gyroscopes. A Kalman filter is typically used to compute the solution from these multiple sources.

How We Solve it

The Tactical grade accuracy AHRS or FCS use generally Fiber Optic Gyro (FOG) or Hemispheric Resonant Gyros (HRG). They need to be very accurate since they are used in the automatic flight mode and have to be accurate enough to prevent the collision to the ground during take-off and landing especially under fog and extreme weather conditions. For this type of applications usually MEMS or Quartz accelerometers with bias repeatability better than 2 mg over all conditions including temperature range, linearity, second order effects and axis misalignment are required.

The High-End and Medium grade accuracy AHRS function is to assist to the pilots’ sight or serve as backup systems, which do not require such high performance. This type of AHRS is very often used in the small civilian airplanes, Helicopters and some UAVs. In these cases, MEMS accelerometer and MEMS gyros are mostly used. Choice of accelerometer range for AHRS depends on the application and vibrating environments. Global accuracy require is around 5 mg for a high-end system and from 10 to 20 mg for a medium grade systems.

Safran’s MS1000 MEMS accelerometer is designed for inertial application such as AHRS.

Why Choose Us

Designed to address advanced, tactical grade inertial applications, the MS1000 is a new class of high performance accelerometers, based on Safran’s 25-year MEMS development and production expertise.

Key features:

  • Long Term Bias Repeatability : 1.2 mg (+/10g, typ)
  • Residual Bias Modeling error : 0.7 mg (+/10g, typ)
  • In-run bias stability (Allan Variance @ 10 s): 15 μg (+/10g, typ)
  • Non Linearity (IEEE Norm, % of full scale) : 0.3 %

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