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Feb. 28, 1950 W. B. McLEAN EAL 2,498,997
PHOTOELECTRIC INCLINOMETER AND ACCELEROMETER
Filed Oct. 27, 1948

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Patented Feb. 28, 1950 2,498,997

UNITED STATES PATENT OFFICE

2,498,997
PHOTOELECTRIC INCLNOMETER AND
ACCE LEROMETER,
William B. McLean and Irvin H. Swift, Inyokern,
Calif., assignors to the United States of
America as represented by the Secretary of the
Air Force
o Application October 27, 1948, Serial No. 57,544
5 Claims. (C. 250-23)
2
The invention described herein may be manu that the acceleration in question is in a direction
factured and used by or for the Government for perpendicular to the meter hand. In this ap
governmental purposes without payment to us plication any acceleration will produce a change
Of any royalty thereon. in meter current which may be utilized to main
This invention relates to a device for giving an tain the Speed of a moving object constant. In
electrical indication of the angle between a given a similar manner the angular velocity of a re
line and the horizontal. Also when properly Wolving object may also be maintained constant
mounted and orientated the device is capable of by this device when mounted with the hand per
giving an electrical indication of any accelera pendicular to a line through the center of ro
tion to which the device is subjected in a given 10 tation.
direction. The device may also be used as an effective
The device employs an ordinary direct current voltage regulator in a manner that will be ex
milliammeter movement with the hand of the plained later.
milliammeter weighted and arranged to intercept The details of a specific embodiment of the in
a beam of light focused on a photoelectric cell vention are shown in the accompanying draw
through a slot in the face of the meter. The ingS in which:
photoelectric cell operates through a vacuum tube Fig. 1 shows the special meter used in the in
to control the current through the meter so that vention,
any tendency for the meter hand to move results Fig. 2 is a diagram of the electrical circuit
in a change in the amount of light falling On the 20 used with the meter,
cell which causes the current through the meter Fig. 3 is similar to Fig. 2 but shows in addition
to change in Such a way as to oppose the tend the use of diodes to alter the characteristic of
ency of the meter hand to move. The meter the system.
hand therefore stays in substantially the same Referring to Fig. 1 a direct current meter f is
position while the current through the meter 25 shown having a face 2 and a hand 3. A small
varies as a function of the torque applied to the weight 4 is attached to the end of hand 3 which,
hand, in addition to adding weight to the hand, serves
As an inclinometer the device is useful as a dive also to mask part of slot 5 in face 2. A photo
angle meter in aircraft. In this application the electric cell 6 is mounted behind slot 5 and face 2
meter is mounted SO that the hand points along 30 and receives the light from light source 7 that
the line of flight and is movable in a vertical misses weight 4 and passes through slot 5. Slight
plane. The current through the meter is ad movements of hand 3 vary the amount of light
justed to just balance the pull of gravity on the falling on cell 6 which in turn varies the current
hand when the plane is in a horizontal position. through the meter in such a way as to oppose
When the plane is in the diving position the com 35 the movement of the hand, as will be explained
ponent of gravity perpendicular to the meter in connection with Fig. 2.
hand varies with the angle of dive. This causes In Fig. 2 the photocell 6, which is built into
the torque on the meter and the current through meter as indicated by the dotted line, has its
the meter necessary to balance this torque to vary anode connected to the positive terminal of a
With the angle of dive. 40 Source of a direct current 8 and its cathode con
When used as a dive angle corrector in toss nected through a high resistance 9, of the order
bombing equipment the dive angle meter pro of 25 megohms, to ground which is also the nega
vides the correct dive angle function plus a cor tive side of source 8. The grid of vacuum tube
rection for one-half the range wind. This is 0 is connected to the cathode of the photocell 6
due to the fact that if a range wind is present 45 So that voltages developed across resistor 9 by
the plane flies a curved path which is convex up the photoelectric current are applied between the
ward for a head wind and convex downward for grid and ground. The meter is connected in
a tail wind. The curvature of the path produces Series with resistors if and 2 between the cath
an acceleration normal to the path of plane ode of tube do and ground. Since the anode of
which makes the dive angle meter read an angle 50 tube fo is connected directly to the positive ter
which is too steep if there is a tail wind and too minal of source 8, the meter and resistors if and
shallow if there is a head wind. The net result 2 in parallel with whatever impedance may be
is a correction for one-half the error produced connected between terminal 3 and ground con
by the wind. stitute the entire load in pedance of the tube.
As an accelerometer the device is mounted SO 55 The voltage between the grid and cathode of

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 2,498,997
3 4
tube 0 is equal to the difference between the sary are then adjusted to balance the hand in
voltage developed across resistor 9 and the volt the position shown in Fig. 1. If the meter is
age between terminal 3 and ground. An increase mounted so that the hand moves in a horizontal
in the amount of light falling on cell 6 causes an plane the torque produced by the hairspring will
increase in voltage drop across resistor 9 and b3 the only restoring force on the hand, where
causes the space current of tube 0 and the volt as if the plane of movement is not horizontal
age W to be adjusted to a higher value. Similarly the effect of gravity on the hand must be taken
a decrease in the excitation of cell 6 reduces the into account. In the presence of an acceleration
voltage across resistor 9 and likewise the Space the meter hand tends to rotate in one direction
current of tube 0 and the voltage W are reduced. O or the other depending upon whether the ac
When the device is to be used as a dive angle celeration is positive or negative. This tendency
meter the meter is mounted as shown in Fig. 1 for the hand to rotate results in a change in
with the hand pointing in the direction of flight meter current for reasons already explained and
and movable in a vertical plane. In this applica this change in current, or in the voltage V, is
tion, with the airplane in level flight, there are 5 a function of the acceleration. The current
two counterclockwise torque producing forces through the meter under the given acceleration
acting on the hand, namely, the force of gravity is completely independent of circuit character
acting at right angles to the hand and the force istics and may be varied only by changing the
exerted by the hairspring of the meter, the latter magnetic flux or spring forces of the meter.
however is preferably adjusted to zero. By ad This is the chief advantage of this arrangement.
justment of resistor 2 the current through the A use for the above described control circuit
meter may be adjusted to produce a clockwise apart from the measurement of gravitational or
torque just equal to the above mentioned Counter acceleration forces is as a voltage regulator.
clockwise torque with the hand in its normal Referring again to Fig. 2 the circuit may be
position which, in this case, is the horizontal used to maintain a substantially constant voltage
position shown in Fig. 1 with the weight 4 par between the terminal 3 and ground in spite of
tially masking the slot 5. When the direction changes in the load connected to this terminal.
of the airplane is tilted downward as in a dive When used as a voltage regulator the meter is
the component of gravitational force perpendic preferably mounted so that the hand moves in
ular to the hand becomes less and the counter 30 a horizontal plane. The tension of the hair
clockwise torque due to this force is reduced. As Spring and the value of resistor 2 are then
a result of this the hand tends to move upward, adjusted to balance the meter as shown in Fig. 1
however in so doing the amount of light falling on and to give the desired voltage between terminal
the photocell is decreased. This operates to de 3 and ground. If the load is increased the
crease the current through the meter and thare 35 current through meter tends to drop and this
fore reduces the clockwise torque resulting from CauSeS the hand to begin rotation in a counter
this current. The two effects are opposed so that clockwise direction. However, this results in more
the hand remains in substantially the same posi light on the photocell which acts to increase the
tion and the effect of changing the angle of dive Space current of tube fo and to oppose any reduc
is to change the value of current through the 40 tion in the voltage W. The increase in space
meter and the voltage W. There is therefore a current also causes an increase in meter current
known relationship between this voltage and the which opposes the tendency of the hand to rotate
angle of dive. This relationship is given by the So that the hand stays in substantially the same
following equation: position. An increase in load impedance causes
45 the current through the meter to increase and
V=RIo COSA-B the hand to tend to rotate in a clockwise direc
Where: tion. However, this reduces the light on the
R=the total resistance between the Cathode of photocell which reduces the space current of tube
tube fo and ground. 0 and opposes a rise in output voltage. There
Io=value of current needed to balance the hand 50 fore the voltage V is substantially unaffected by
in a horizontal position if the meter hairspring load impedance variations.
is adjusted to Zero tension. The condensers 4 and 5 and the resistor 6 in
B=constant determined by the torque of the Fig.2 are for the purpose of suppressing parasitic
meter hairspring. Oscillations that might occur in the circuit. The
A=dive angle or angle between the axis of the 5. condenser 5 performs the added function of
plane and horizontal. causing a more rapid action of the System in op
If the airplane while diving is subjected to a posing displacements of the meter hand. This is
tail wind the path flown is convex upward. This due to the fact that the potential across condenser
curved path produces an acceleration normal to 5 cannot change instantly and therefore initially
the path of the plane and therefore causes a 50 the full effect of a voltage change across resistor
counterclockwise torque on the meter hand. This 9 due to a displacement of the meter hand is ap
causes the meter to indicate a smaller dive angle plied between the grid and cathode of tube fo
than in the absence of wind as required to cor thus causing a rapid initial change in the current
rect for the effect of the wind. Likewise the through meter f. The condensers 4 and 5 may
convex downward path flown in the presence 65 have capacities of about 0.0005 microfarad and
of a head wind produces an acceleration in the 0.5 microfarad respectively.
opposite direction to the above and causes a The shape of the curve relating the voltage W.
clockwise torque to be exerted on the meter hand. to the dive angle or the acceleration may be
This causes the meter to indicate a Steeper dive modified by the use of diodes as shown in Fig. 3.
angle as required to correct for the effect of 70 The diodes become operative at various values of
the head Wind. meter current to change the load resistance of
When used as an accelerometer the device is so the tube and therefore the slope of the above men
mounted that the acceleration to be measured tioned characteristic. The potentials of the cath
is normal to the meter hand. The resistor ode and anode of diodes 7 and 8 respectively are
2 and the tension of the hairspring if neces s determined by the potential divider 9-20. The
 2,498,997
5
anode of diode 7 and the cathode of diode 8 are electric cell and to the anode of said tube, and
Connected to points on the load resistance of means for taking an output voltage from across
tube 0. When the current through resistors said series connected meter movement and load
and 2 reaches a sufficiently high value that the resistance.
anode of diode 7 becomes positive with respect to 2. Apparatus as claimed in claim 1 in which a
its cathode the diode begins to conduct thus low condenser is connected in shunt to said load re
ering the load resistance of the tube. Similarly sistance to increase the Speed of response of said
the diode f8 becomes conductive when the cur circuit.
rent through resistors f and f2 has fallen to a 3. Apparatus as claimed in claim 2 in which a
sufficiently low value for the anode of diode 9 O Condenser is connected between said grid and
to become positive with respect to its cathode. said point of reference potential to by-pass high
More diodes may be used to change the slope of frequencies.
the characteristic at other points if desired. 4. Apparatus as claimed in claim 3 in which a
What we claim is: plurality of diodes are provided with each diode
1. Apparatus of the class described comprising s having one of its electrodes connected to a point
a direct current ammeter movement having a on said load resistance and the other of its elec
meter hand attached thereto and movable by said trodes connected to a point of positive potential
movement, said hand having a relatively wide relative to ground.
portion, a photoelectric cell and a source of light, 5. Apparatus as claimed in claim 1 in which
means providing an aperture between said source 20 said meter hand is weighted.
of light and said cell, said aperture being So posi WILLIAM B. McLEAN.
tioned that it is partially covered by said rela IRWIN H. SWIFT.
tively wide portion, a vacuum tube having an
anode, a cathode and a grid, means for applying REFERENCES CITED
the output voltage of said photoelectric cell be 25 The following references are of record in the
tween said grid and a point of reference poten file of this patent:
tial, a load resistance for said tube, means con UNITED STATES PATENTS
necting said meter movement and said load re
sistance in series between the cathode of said Number Name Date
tube and said point of reference potential, means S0 2,141,175 Dawson ------------- Dec. 27, 1938
for applying operating potentials to said photo