Homework 6 - UMC Patent 3,643,513 (Potter)
Purpose/Capabilities of Device
The proposed patent describes an accelerometer designed to measure loads on an aircraft due only to maneuvers while ignoring other "parasitic" sources of acceleration such as wind gusts, weapons deployment, and the like. It is intended to keep records of when the aircraft is subjected to loads of a predetermined magnitude and length of time in order to monitor the fatigue of the aircraft materials.
Prior Art
Two prior patents had been issued for "mechanical switch type" accelerometers. The downsides of these accelerometers were that they required a large deflection per unit of acceleration in order to obtain low natural frequencies, which necessitated the use of very large switches, and that the only methods for filtering out high-frequency disturbances were through viscous or magnetic damping, which were difficult to control and didn't provide a sharp rolloff.
Proposed Device
The accelerometer proposed in the patent application uses a seismic mass connected to a circuit in order to produce a voltage waveform proportional to acceleration due solely to maneuvering. It records accelerations only when the output signal exceeds some reference value for a predetermined length of time.
Objects of the Invention
- New and improved accelerometer with:
- New and improved means for producing electrical signals proportional to acceleration loads
- New and improved means for discriminating between parasitic acceleration and maneuvering loads
- New and improved means for ignoring high-frequency impulses
- New and improved sensing and recording arrangement
Physical Description
The accelerometer consists of a casing with a permanent magnet assembly and a configuration allowing it to be mounted to a stud along its sensitive axis. A recess in the magnetic assembly houses a force coil wound about a non-temperature sensitive form and supported by 3 flexural members placed tangential to the coil assembly to allow for true linear motion along the sensitive axis. The configuration allows virtually no transverse motion and provides low hysteresis and increased sensitivity.
A capacitive displacement sensor is mounted in the casing, consisting of 2 parallel flat plates. The lower plate is carried on the force coil while the upper plate is mounted to the magnetic assembly. This configuration then constitutes a seismic mass, with the force coil and lower plate remaining largely stationary while the upper plate moves with the permanently fixed magnetic assembly. The seismic mass is located in a strong magnetic field, and current flowing through the force coil produces an electromagnetic field that interacts with the magnetic field to produce a force that restores the seismic mass to its normal position and produces an output signal proportional to the displacement.
Above this assembly are defined packaging areas for the associated circuitry and electrical components, and external leads protrude from the top of the casing.
Circuitry/Logic
The circuitry of the accelerometer can be broken down into 3 segments: a capacitance bridge and feedback loop for measuring the displacement of the seismic mass, an operational amplifier and associated filters to condition the signal, and a voltage divider network to measure the aircraft acceleration and record information when appropriate.
Capacitive Displacement Sensor and Feedback Loop
The capacitive displacement sensor is part of a capacitance bridge that is also connected to an oscillator. When the seismic mass is displaced, the bridge becomes unbalanced and applies a high frequency signal across a diode bridge which, in turn, produces an output voltage proportional to the degree of unbalance in the capacitance bridge. This voltage is applied to a differential amplifier which then sends a feedback current through the force coil to produce the necessary restoring force. This current then flows to a terminal and external indicating source which is connected to an RLC filter - in series between a resistor and the input to an operational amplifier.
Op Amp and Filters
The input filter to the op amp consists of a capacitor, choke, and resistor and its output filter consists of a capacitor and resistor. A reference input is provided by a pair of zener diodes connected between resistors, and the reference level is determined by the ratio of the resistances. The filter has a damping ratio of about 0.7 and a corner frequency of 7 cycles per second. This produces a slightly rising, under damped signal which is compensated for by the output filter, yielding a well-defined corner frequency and high rolloff which filters out high-frequency signal components.
Voltage Divider Network
The acceleration is measured by a voltage divider network. This consists of a number of individual voltage dividers with resistance values that are selected to produce voltages with a predetermined ratios to each other - corresponding to certain load factors.
- For instance, the first voltage divider may have a voltage of 1.5 V corresponding to a load factor of 2, with the next divider having a voltage of 2 V corresponding to a load factor of 4 (each voltage divider increasing in voltage by 0.5 V and increasing in load factor by 2)
Each voltage divider is connected to a voltage sensor (differential amplifier) which is then connected to a latch circuit, then to a gage which serves as a gate to a counter. A timer ensures that the network only records information when accelerations last for a predetermined length of time.
When a sufficient voltage is picked off the voltage divider, the amplifier enables the timer and a reset switch. The latch circuits then store the indication that the amplifiers have been turned on and send that information to the gages. The gages are opened by the information release gate if the voltage is still above the threshold at the end of the timer cycle and the signal is transmitted to the counters, which advance by one. The timer then resets and the process begins again. If the acceleration continues, the counters will advance again at the end of the cycle. Otherwise, if it falls below the reference value, the amplifier turns off and no signal is transmitted at the end of the timer cycle.
The latch circuit consists of a relay that is energized when its corresponding amplifier is turned on. This will then close the contact that attaches it to the reset switch and store the fact that a predetermined acceleration level was reached at some initial time. It then closes the contact to complete the circuit enabling the information release gate at a later time and advancing a counter advance coil by one count. The signal is then free to pass through to the counter on the main circuit.
Claims
- An accelerometer that counts the number of times an aircraft undergoes maneuvering loads of a certain magnitude; additionally:
- A configuration for mounting the accelerometer to the aircraft
- A method to produce a signal proportional to the acceleration load experienced
- A plurality of sensing and storing means corresponding to different values of acceleration
- Timing a cycle and producing a signal at the end of the cycle
- Means to initiate the timing cycle when acceleration signals reach some reference level
- A method to transmit the signal from sensors to recording devices at the end of a timer cycle
- A system to filter high-frequency components from the output signal waveform
- A number of counters
- Arranged to each be advanced by one of the sensing and storing devices
- A number of gates to pass information from the sensing and storing devices to the counters
- A system to detect when the signal exceeds some reference value and to disable the sensing and storing devices when the signal falls below the threshold
