Anode Anode Cathode Mirror Piezoelectric dithering motor Readout detector Corner prism Light beams F r i n g e p a t e r n Gas discharge region Figure 10-98.  The relative scale size of a MEMS gyro.  Figure 10-97.  The ring laser gyro is rugged, accurate, and free  of friction.  Common Gyroscopic Instruments Vacuum-Driven Attitude Gyros Microelectromechanical Based Attitude and  Directional Systems Other Attitude and Directional Systems 10-56 While a large mechanical gyroscope spins in a plane,  its rigidity in space is used to observe and measure the  movement of the aircraft. The basis of operation of many  MEMS gyroscopes is the same despite their tiny size. The  difference is that a vibrating or oscillating piezoelectric  device replaces the spinning, weighted ring of the mechanical  gyro. Still, once set in motion, any out-of-plane motion is  detectable by varying microvoltages or capacitances detected  through geometrically arranged pickups. Since piezoelectric  substances have a relationship between movement and  MEMS for gyroscopic applications are used in small, general  aviation aircraft, as well as larger commercial aircraft.  Tiny vibration-based units with resistance and capacitance  measuring pick-offs are accurate and reliable and only a few  millimeters in length and width. They are normally integrated  into a complete micro-electronic solid-state chip designed  to yield an output after various conditioning processes are  performed. The chips, which are analogous to tiny circuit  boards, can be packaged for installation inside a dedicated  computer or module that is installed on the aircraft.  On aircraft, microelectromechanical systems (MEMS)  devices save space and weight. Through the use of solid-state  MEMS devices, reliability is increased primarily due to the  lack of moving parts. The development of MEMS technology  for use in aviation instrumentation integrates with the use of  ADCs. This newest improvement in technology is low cost  and promises to proliferate through all forms of aviation. The attitude indicator, or artificial horizon, is one of the most  essential flight instruments. It gives the pilot pitch and roll  information that is especially important when flying without  outside visual references. The attitude indicator operates  with a gyroscope rotating in the horizontal plane. Thus, it  mimics the actual horizon through its rigidity in space. As  the aircraft pitches and rolls in relation to the actual horizon,  the gyro gimbals allow the aircraft and instrument housing  to pitch and roll around the gyro rotor that remains parallel  to the ground. A horizontal representation of the airplane  in miniature is fixed to the instrument housing. A painted  semisphere simulating the horizon, the sky, and the ground  is attached to the gyro gimbals. The sky and ground meet  at what is called the horizon bar. The relationship between  the horizon bar and the miniature airplane are the same as  those of the aircraft and the actual horizon. Graduated scales  reference the degrees of pitch and roll. Often, an adjustment  knob allows pilots of varying heights to place the horizon  bar at an appropriate level.  [Figure 10-100] In modern aircraft, attitude heading and reference systems  (AHRS) have taken the place of the gyroscope and other  individual instruments. While MEMS devices provide part  of the attitude information for the system, GPS, solid state  magnetometers, solid state accelerometers, and digital air  data signals are all combined in an AHRS to compute and  output highly reliable information for display on a cockpit  panel.  [Figure 10-99] electricity, microelectrical stimulation can set a piezoelectric  gyro in motion and the tiny voltages produced via the  movement in the piezo can be extracted. They can be input as  the required variables needed to compute attitude or direction  information.  [Figure 10-98]