MEMS have become widely available at low cost because of their use in the automotive industry. Many cars now have some type of skid control or active handling assist system, and MEMS gyros provide the information necessary to determine if the car is skidding or about to exceed its control limits. The car guys are only interested in very short term rates of change, so an automotive quality MEMS cannot, by itself, perform the necessary calculations to determine an aircraft's attitude over a long interval. The trick, which Avidyne is the first to perform, is to take the poor quality but low cost gyro output from a MEMS and transform it into an AHRS that tracks aircraft attitude with the precision of a spinning rotor gyro.
The key to the AHRS is the computer power that massages the raw output from the MEMS gyros, and that is what Avidyne tapped into. One way to provide long-term stability to an electronic gyro is to feed GPS track information into the solution. GPS calculates a position once each second, not fast enough to act as a gyro replacement, but good enough to remove the large errors that accumulate over time in a low quality gyro. However, if performance of the gyro is GPS dependent you would need to prove continuous availability in order to certify the gyro, and nobody can show 100 percent GPS availability.
Instead of using GPS for long-term stability, Avidyne uses the natural stability of an airplane combined with air data information. The Avidyne people realized that an airplane spends most of its time in steady 1G flight, and in that mode a pendulum type of device knows up from down and can realign the gyro. This alignment of the gyro with the earth's actual horizon is called erection, and all gyros must have a system that periodically erects the gyro referenced to the earth beneath it. Avidyne uses pendulums to erect the gyro when the airplane is in steady un-accelerated flight and then consults the air data sensors to know when to turn the erection logic off and let the MEMS gyros track changes in attitude during maneuvering.
An air data computer is the other key sensor element of the Entegra system. An electronic air data computer is simple and cheap to design, at least compared to the challenges of building a low-cost AHRS. The air data computer uses pressure transducers to measure pitot and static air pressure and a temperature probe to provide the necessary data to calculate altitude, airspeed, vertical speed and true airspeed. This information can also be fed into the AHRS logic, along with magnetic heading change, to let the system know that the airplane is maneuvering and the erection logic must be turned off until the airplane returns to steady state level flight.
The other inputs to the Entegra are the flux gate magnetic heading sensor that all slaved compass systems use, navigation information from GPS, VOR, ILS and other sensors that may be in the airplane. The entire Entegra is contained in a single box that extends only 9.4 inches behind the panel. The system weighs 12 pounds.
