Rate of Change in Altitude Apparatus

This invention relates to an altitude computer which is adapted to supply a rate of change in altitude indicator with information to enable a pilot to safely operate an aircraft.

Most present day aircraft use air data computers to calculate information utilized in piloting an aircraft. These air data computers receive information relating to pressure data, indicated air speed, rate of ascent or descent and temperature in order to calculate true air speed and density altitude. The rate of climb and descent information is especially useful during take-off and landings in order that a pilot can provide a smooth transition of the aircraft into the flight pattern around airports.

Information relating to rate of ascent and descent is usually derived from a rate sensing apparatus associated with the altitude sensing aneroid of the type disclosed in U.S. Pat. No. 3,456,506. Such rate sensing apparatus include a hollow diaphragm or aneroid located in a sealed container. The interior of the diaphragm is vented to the surrounding environment. However, a flow restricting orifice limits communication between the sealed container and the surrounding environment. Any change in atmospheric pressure due to a change in altitude of the aircraft is immediately communicated to the interior of the diaphragm; however, the flow restricting orifice delays the change in atmospheric pressure communicated into the sealed container and thereby creates a pressure differential across the diaphragm. This pressure differential causes the diaphragm to expand or contract depending upon whether the aircraft is ascending or descending. A mechanical linkage connects the diaphragm to an indicator dial to provide the pilot with rate of change in altitude information. It has recently been discovered that the relationship between atmospheric pressure and altitude is only linear up to about 28,000 feet. Therefore, the accuracy of differential pressure rate sensing apparatus is limited to altitudes below 28,000 feet.

It is well known that altimeters utilized in aircraft have correcting means for compensating for the non-linearity relationship between indicated atmospheric pressure altitude and actual altitude. In an effort to simplify aircraft instrumentation we have devised a computer mechanism for utilizing the information supplied to the altimeter pointer to compute the rate of change in altitude of an aircraft.

The computer mechanism includes a detector with a sensor located on one side of the driving gear of the altimeter pointer. A light source on the opposite side of the driving gear is aligned with the sensor. The gear teeth on the driving gear intersect a beam of light transmitted to the sensor thereby causing the sensor to generate a sinusoidal signal which is indicative of the rotational speed of the gear. The sinusoidal signal represents the rate of change in altitude of the aircraft. The positive node of the sinusoidal signal is modified by a signal generator to produce an operational signal having a constant pulse width. Each constant pulse width operational signal actuates a first switch which allows electrical current to flow from a source at a fixed charging rate into a capacitor. At the end of each constant pulse width operational signal, the flow of electrical current from the source is interrupted and the capacitor is allowed to discharge to ground through a resistor. The charging and discharging of the capacitor when averaged with respect to time produces a steady state voltage whose amplitude is proportional to the frequency of the sinusoidal signal. This steady state voltage is transmitted through either an inverting buffer or a non-inverting buffer and amplified to a level sufficient to operate an indicator. A second switch which receives an operational signal corresponding to the rotational direction of the driving gear of the altimeter selects a flow path for the amplified voltage signal through either the inverting buffer or the non-inverting buffer to inform the pilot of the rate of change in altitude associated with an ascending or descending operation of the aircraft.

It is the object of this invention to provide a rate of change indicator with switch means which allows a capacitor to be charged at a constant rate for a period of time and thereafter permits the capacitor to be discharged at a variable rate in response to a sinusoidal signal. The charging and the discharging of the capacitor averaged with respect to time creates an operational signal which drives a pointer to inform an operator of instantaneous changes in altitude.

It is the object of this invention to provide a means for computing a rate of change in altitude of an aircraft by sensing the rotation of a gear train driving an output gear associated with an altimeter.

It is another object of this invention to provide a means for generating a voltage signal corresponding to the frequency of a sinusoidal wave signal derived by moving a mask with respect to a source of illumination and a phototransistor.

It is a further object of this invention to provide a rate of change in altitude indicator with an operational signal which is directly proportional to the speed of rotation of a gear train driving a pointer of an altimeter.

These and other objects will become apparent from reading this specification and viewing the drawing.

FIG. 1 is a schematic illustration of a rate of change in altitude apparatus made according to this invention; and

FIG. 2 is a graph illustrating the operational signals generated by various components associated with the altitude apparatus of FIG. 1.

The rate of change in altitude apparatus 10 shown in FIG. 1 includes a detector means 12 which supplies a generator 14 with an input signal indicative of altitude changes communicated to a pointer 16 of an altimeter 18. The generator 14 modifies the input signal to produce a constant width pulse signal. The constant width pulse signal actuates a first switch 20 which connects voltage source 22 to voltage generator 24 for a time period equal to the duration of the constant width pulse. The voltage generator 24 creates a voltage signal which is directly proportional to the frequency of the input signal generated by the detector means 12. The voltage signal is transmitted to a buffer 26 and amplified to a level sufficient to operate pointer 86 of an indicator means 28. The pointer 86 provides an operator with instantaneous visual information relating to the rate of change in altitude of the aircraft.

In more particular detail, the detector 12 includes a light source such as a light emitting diode 30 which is positioned adjacent a driving gear 32 in the gear train of the pointer 16 of the altimeter 18. A light responsive photocell 34 is located adjacent the driving gear 32 opposite the light emitting diode 30. The individual teeth 38 on the driving gear 32 periodically interrupts the light from diode 30 to the photocell 34 whenever gear 32 is rotated by servomotor 36. The periodic interruption of the light beam causes the photocell 34 to produce a sinusoidal change in altitude signal, as illustrated by line 40 in FIG. 2. This change in altitude signal is transmitted to an amplifier-limiter 42 of the generator means 14. The amplifier-limiter 42 modifies the altitude signal by converting the positive node of the sinusoidal signal into a square wave signal, illustrated by line 44 in FIG. 2. The square wave signal has the same frequency as the sinusoidal signal. The square wave signal is transmitted to a constant width pulse generator 46 where the leading edge of the square wave signal triggers the generation of a pulsating signal, illustrated by line 48 in FIG. 2.

Each pulse of the pulsating signal is transmitted to the base 50 of a PNP transistor 52 in the first switch means 20. The collector 54 of the transistor 52 is connected to the direct current source 22. The pulsating signal raises the electrical potential of the base 50 to allow a fixed current to flow from the emitter 58 into junction 60 of the voltage producing means 24 for a time period equal to the duration of each pulse of a pulsating signal. The fixed current communicated from the emitter 58 raises the voltage across capacitor 62 at a constant rate as illustrated by line 64 in FIG. 2. When the current flow terminates, the voltage in the capacitor 62 reaches a peak 66. Thereafter, the capacitor 62 discharges through resistor 68 at an exponential rate which follows a curve 70, as illustrated in FIG. 2. However, before capacitor 62 is completely discharged, another pulse from the generator 46 again turns the transistor 52 on and allows current to recharge the capacitor 62 from point 72 to peak 66. We have found that when the voltage represented by peak 66 and point 72 are averaged with respect to time, a steady state voltage as illustrated by line 74 in FIG. 2, is produced across the capacitor 62. The value of this voltage is directly proportional to the frequency of the sinusoidal signal of the input signal generated in the detector 12.

The steady state voltage is transmitted to an inverting buffer 74 and to a non-inverting buffer 76 in buffer member 26. The steady state voltage transmitted to buffers 74 and 76 is amplified to a level sufficient to operate the rate of change in altitude indicator 28. The buffer member 26 includes a flip-flop switch 80 which is connected by lead 82 to a control means 84 which operates the servomotor 36 of the altimeter 18. The control means 84 is adapted to produce a positive signal in response to an increase in altitude, and a negative signal in response to a decrease in altitude of the aircraft. A positive signal presented to flip-flop switch 80 allows a positive amplified voltage to flow from the non-inverter buffer 76 and drives pointer 86 in a clockwise direction with respect to a null position 88, whereas, a negative signal presented to flip-flop switch 80 allows a negative amplifier voltage to flow from the inverter buffer 74 and drive pointer 86 in a counterclockwise direction with respect to the null position 88.

The dial 90 of the rate of change in altitude indicator 28 is part of the instrument panel in an aircraft. When the aircraft is flying at a constant altitude, pointer 86 remains at the null position 88.

If the aircraft changes altitude, altimeter servomotor 36 responds to an input signal and rotates gear 32, clockwise for ascent and counterclockwise for descent, to provide pointer 16 with information relating to the present altitude of the aircraft.

Rotational movement of gear 32 causes the photocell 34 to generate a sinusoidal signal, whose frequency is proportional to the angular rotation speed of the gear 32, through the sequential interruption of the light beam from diode 30. Each positive node of the sinusoidal wave signal is amplified and converted into a square wave signal. The leading edge of each square wave signal triggers a pulse of predetermined width which activates the transistor switch 52 to allow electrical current to flow from source 22 to the summing junction 60 of the voltage generator 24 for a time period equal to the time period of the pulse. Electrical current flowing from the transistor 52 charges a capacitor 62 at a constant rate until fully charged at peak 66 during this time period. Thereafter, the pulse terminates, and capacitor 62 discharges to ground 92 through resistor 68. However, before the capacitor 62 is completely discharged, another pulse triggers the switch means 20 to recharge the capacitor 62 to peak 66' shown in FIG. 2. When the peak voltage 66, 66', 66", . . . 66.sup.n and the discharge intersection voltage 72, 72', 72", . . . 72.sup.n are averaged with respect to time, a steady state voltage is generated across capacitor 62. This steady state voltage is communicated to inverting and non-inverting buffers 74 and 76 where the steady state voltage is amplified and directed through the flip-flop switch 80 as either a positive or negative signal depending upon the rotational direction of gear 32. The steady state voltage drives the pointer 86 away from the null position 88 to provide the pilot with information relating to the rate of change in altitude of the aircraft.