Abstract:
Apparatus and method for counting frequency of a signal with improved resolution. Frequency counters (10, 60, and 100) accumulate clock cycles from a reference oscillator (20) during a sample interval. In the simplest form of the frequency counter, the reference clock signal is inverted and both the noninverted and inverted clock cycles are accumulated in separate counters (40 and 44). The accumulated counts are totaled in a summing circuit (48) and divided by two to determine their average, thereby doubling the resolution of the frequency counter. A more complex embodiment of the invention corrects a raw count of cycles of an input signal (12) that are accumulated during an extended sample interval defined by successive rising edges of a sample signal (114). The fractional portion of a cycle of the input waveform that occurred prior to the beginning of the extended sample interval is added to the raw count and the fractional portion of the input waveform that occurred after the end of the extended sample interval is subtracted. These fractional portions are defined as ratios of a partial count of clock cycles to a full count of clock cycled. The partial count is the number of clock cycles occurring between a rising edge of the sample signal and the next rising edge of the input signal, while the full count is the number of clock cycles occurring between successive rising edges of the input signal. One-half clock logic circuits (126 and 136) double the resolution of the counts accumulated by a partial counter (122) and a full counter (142), thereby doubling the resolution with which the ratios of the two counts are determined.
Abstract:
Warning systems that monitor the altitude above ground and descent rate of an aircraft and generate a warning if the descent rate of the aircraft is excessive for the altitude in which the aircraft is flying are well suited to aircraft whose flight and operational characteristics are readily predictable but not as suitable for aircraft, such as tactical aircraft whose operating conditions are not so predictable. To overcome this problem, the criteria for providing the warnings are altered (30, 32) as a function of flight configuration (16, 18) to optimize the warnings for different flight conditions.
Abstract:
A fiber optic transducer utilizes a single optical fiber having one end surface, disposed adjacent a sensor element, cut at an angle to result in frustrated total internal reflection. The end surface of the optical fiber is spaced away from the sensor element. As the sensor element is displaced with respect to the end surface of the optical fiber, the amount of light reflected from the sensor element varies. Light injected into the optical fiber is reflected back toward the light source until the element comes into relatively close proximity with the angled end of the optical fiber. When this occurs, a portion of the light is transmitted across the gap and absorbed by the sensor element. The ratio of the light reflected from the reflective surface to the total light input into the optical fiber produces a signal representative of the displacement of the sensor element. This signal can then be converted to a corresponding temperature or pressure signal and displayed at a remote location. The transducer is capable of being used with temperature or pressure sensitive elements to provide either an analog output signal or a digital output signal. Also disclosed is a self test circuit for the transducer.
Abstract:
A system that warns the pilot of an aircraft performing low level maneuvers of a dangerous flight profile monitors the altitude of the aircraft above ground, and provides a first specific warning to the pilot if the altitude of the aircraft drops below a predetermined minimum altitude above ground. The system further monitors the roll angle and descent rate of the aircraft to provide a second specific warning if the descent rate of the aircraft exceeds a predetermined rate determined by the roll angle of the aircraft if the aircraft is below a second predetermined altitude above ground.
Abstract:
A transducer having compensation for a deflection due to an applied stress. The transducer includes a support ring (32) having a proof mass (34) cantilevered on a pair of flexures (38) between the magnets (26, 28) of a stator in which the transducer is mounted. Deflection of the support ring due to an imbalanced applied force is compensated by either moving the pads (30) used to mount the support ring, moving the centroid of capacitance (42) of the proof mass, or by modifying the support ring to provide a pair of moment arms (152), each approach insuring that an axis of deflection (102, 130) of the support ring is coaligned with the centroid of capacitance, thereby minimizing a bias error in the transducer output.
Abstract:
A vibrating beam force transducer that can be realized in a silicon micromachined device such as a micromachined accelerometer. The transducer includes a beam having a longitudinal axis, and a drive circuit electrically coupled to the beam for causing the beam to oscillate at a resonant frequency that is a function of a force applied along the longitudinal beam axis. The drive circuit provides an electrical current to the beam, and the beam, or a conductive portion thereof, conducts the current along a path that includes an axial component parallel to the longitudinal axis. A magnetic field is created intersecting the axial component, such that the electric current interacts with the magnetic field to produce a force that causes the beam to oscillate at the resonant frequency. In a preferred embodiment, the transducer has a double ended tuning fork configuration, and the current path extends along one beam and back along the other beam.
Abstract:
Prior steady-state accelerometers are subject to errors caused by differential thermal expansion between the force transducers and other accelerometer components. This problem is overcome by the present accelerometer that comprises a housing (32), a proof mass (30), a support (34,36) for mounting the proof mass with respect to the housing, and first and second force sensing elements (38,40). The force sensing elements have DC frequency responses, and are connected between the proof mass and the housing such that differential thermal expansion or contraction between the force sensing elements and the proof mass, support and housing results in rotation of the proof mass about a compensation axis (CA) normal to the sensitive axis (SA). The force sensing elements may extend from their respective points of connection to the proof mass in opposite directions parallel to the sensitive axis to their respective points of connection to the housing, and the force sensing elements may be connected to the proof mass at spaced-apart positions on opposite sides of the compensation axis.
Abstract:
A wind shear detection system compares air-speed with a composite signal derived from signals representative of longitudinal acceleration, normal acceleration, angle of attack and flight path angle to generate a shear signal. An enhanced version of the system is also compensated for roll angle, radio altitude and flap position. In a modified system, the accelerations are calculated along the velocity vector of the aircraft rather than along the horizontal axis to compensate for inaccuracies that could occur under extreme flight conditions such as high bank angle turns and dynamic maneuvers.
Abstract:
Disclosed is a method and apparatus for processing signals supplied by accelerometer assemblies in which one or more accelerometers are cyclically displaced in a predetermined manner so that signals representing the specific force experienced by the accelerometers and the angular rate experienced by the accelerometers are produced. The signal processor separately estimates the signal components of the signal being processed and provides an error signal by subtracting the estimated signal components from the signal being processed. The error signal is fed back through circuitry that controls the magnitude of the estimated signal components so that the value of each estimated signal component rapidly converges to the value of the signal components of the signal being processed. In an arrangement for determining the angular rate of one or more pair of cyclically displaced accelerometers, the signal processor includes a signal component that is in-phase with the signal that oscillates the accelerometer pair, a signal component that is in phase quadrature with the signal that displaces the accelerometer pair and a signal component that corresponds to random unmodulated additive noise. In this arrangement, a signal summing unit subtracts estimates of each signal component from the signal being processed to supply an error signal equal to (a.sub.1 -a.sub.1) Cos .omega.t+(a.sub.2 -a.sub.2) Sin .omega.t+(a.sub.3 -a.sub.3), where a.sub.1, a.sub.2, and a.sub.3, respectively represent the values of the in-phase, quadrature and random noise components of the signal being processed and a.sub.1, a.sub.2, and a.sub.3 represent estimates of those signal component values. To obtain a.sub.3, the error signal is scaled and integrated. To obtain the a.sub.1 Cos .omega.t signal estimate, the error signal is multiplied by Cos .omega.t, and scaled to obtain a signal representative of the derivative with respect to time of a.sub.1. This signal is then integrated and multiplied by Cos .omega.t. The signal component a.sub.2 Sin .omega.t is obtained in a similar manner by multiplying the error signal by Sin .omega.t, scaling, integrating and multiplying the integrated signal by Sin .omega.t. In such an arrangement, angular rate is obtained by scaling the a.sub.1 signal estimate. In addition, the signal estimates can be used in an inertial navigation system that employs the invention to eliminate misalignment of the accelerometer pairs and to improve system operation by eliminating phase shift between the signal source that oscillates the accelerometer pairs and the signals provided by the accelerometer pairs.
Abstract:
A method and apparatus is disclosed for passively determining altitude above the ground of an aircraft by use of an active emitter carried on a second higher altitude aircraft. Specifically, the altitude of the aircraft is determined by measuring the time difference between direct path radiation and reflected path radiation, taking into account the distance between an upper antenna which receives the direct path radiation and a lower antenna which receives the reflected path radiation and the angles of travel relative to the vertical between the direct path radiation and the reflected path radiation.