Gyroscopic apparatus

A free rotor gyroscope has a gimbal supported for limited rotation about a rotatable shaft by a first flexure suspension and a rotor attached to the gimbal by a second flexure suspension. Pickoff means rotate with the gimbal and detect its movements about its axis of suspension.

This invention relates to gyroscopic apparatus, and in particular to 
gyroscopes of the type usually called "free rotor gyroscopes". 
Free rotor gyroscopes having a relatively large and heavy rotor connected 
to a drive shaft by a universal joint permitting relative tilting between 
the rotor and shaft are well known. When the rotor is rotated at high 
speed it tends to maintain a fixed orientation in space despite movement 
of the drive shaft about the two axes perpendicular to the spin axis of 
the drive shaft. The change in the relative positions of the rotor and 
drive shaft may be measured by suitable forms of pick-off, in the form of 
a steady state signal for a particular relative alignment. If this steady 
state signal is to indicate a particular deflection of the rotor from the 
condition when it is in a plane perpendicular to the spin axis then this 
null position must be arranged to give zero pick-off output. This makes it 
necessary to trim the pick-off, a procedure which is often costly in time 
and electrical components. 
It is an object of the invention to provide gyroscopic apparatus of the 
free-rotor type which has a simpler pick-off arrangement than has hitherto 
been possible. 
According to the present invention there is provided gyroscopic apparatus 
which includes a rotatable shaft, a gimbal attached to the shaft by a 
flexure suspension allowing limited rotation of the gimbal relative to the 
shaft about an axis perpendicular to the spin axis of the shaft, a rotor 
attached to the gimbal by a further flexure suspension allowing limited 
rotation of the rotor relative to the gimbal about an axis perpendicular 
to the spin axis of the shaft and to the axis of rotation of the gimbal, 
and pick-off means rotatable with and spaced from the gimbal and 
responsive to rotational movements of the gimbal about its axis of 
suspension to deliver output signals representing the sense and magnitude 
of such movements. 
Preferably the pick-off means comprise conductive areas located on each 
side of the gimbal to which a d.c. polarising voltage may be applied.

Referring now to FIG. 1, a gyroscope according to the invention has an 
annular rotor 10 pivoted about a diameter on a gimbal member 11. The 
gimbal member is itself pivoted about a perpendicular axis relative to a 
base plate 12 which is secured to a rotatable shaft 13. The shaft 13 is 
supported in bearings 14 and carries the rotor 15 of the gyro spin motor. 
Mounted above and below the gimbal are upper and lower conductive plates 
16 and 17 respectively secured to the base plate 12 and hence rotatable 
with it and with the gimbal 11. A cover 18 is attached to the base plate 
12 to cover the rotor 10 and gimbal 11, and the enclosure is usually 
evacuated and sealed. 
Each of the upper conductive plates 16 is connected to the diametrically 
opposed lower conductive plate 17, and each pair of plates is connected to 
one of two separate metal rings 19 and 20 which rotate in grooves 21 and 
22 in the body of the gyroscope. The grooves are lined with electrically 
conductive material which are connected to the necessary circuitry, to be 
described later. 
The body of the gyroscope also carries the stator 23 of the spin gyro 
motor, and a pick-off 24 used to indicate the angular position of the 
shaft 13 and the gimbal 11. An electrical connection, used to apply a d.c. 
polarising voltage, is made through a brush 25 to the rotating assembly. 
FIGS. 2 and 3 show the construction of the main parts of the gyroscope in 
greater detail. The same reference numbers are used as in FIG. 1, where 
appropriate. 
As shown in FIGS. 2 and 3 the gimbal 11 is supported on the base plate 12 
by a crossed-spring suspension 26, which allows the gimbal to pivot about 
the axis 27 shown in FIG. 3. The suspension may conveniently be of the 
type described and claimed in our U.S. Pat. No. 3,620,088. A similar 
suspension is used to provide two supports 28 for the rotor 10, allowing 
it to pivot about the axis 29 relative to the gimbal 11. The upper 
conductive plates 16 and lower conductive plates 17 are provided by 
conductive metallic layers deposited on ceramic plates. Each of the lower 
plates 17 is connected through a separate blocking capacitor CB to a 
different one of the rings 19 and 20. These rings do not make contact with 
their respective grooves 21 and 22: but the electrical output is 
transmitted by capacitive coupling between the two. 
A polarising d.c. voltage is applied via a connection 30 and two feeder 
resistors R to the two pairs of interconnected conductive plates 16 and 
17. In addition the two upper plates are connected together through a 
bootstrap capacitor CF. 
FIG. 4 illustrates the electrical connections and components of the 
gyroscope. The rotor 10 and gimbal 11 are shown schematically in a 
displaced position relative to the ceramic plates. The gimbal 11 is 
connected to the body of the gyroscope (i.e. to gound) by way of the hinge 
26 and the bearings 14. The d.c. polarising voltage V.sub.p may be 
adjusted by a variable resistor VR and is applied through the feeder 
resistors R to the interconnected conductive plates 16 and 17. The 
bootstrap capacitor CF is connected between the upper areas as shown. Each 
pair of interconnected conductive plates is connected through the blocking 
capacitors CB to separate capacitive slip-rings, represented by capacitors 
CS in FIG. 4. The slip rings are connected to buffer amplifiers A, the 
outputs of which are connected to a differencing circuit D which gives an 
output Vo. 
Mechanically, the gyroscope operates in a well-known manner. The gyro motor 
rotates the shaft 13, and hence the gimbal, rotor and associated 
components at a suitable high speed. The rotor 10 tends to maintain a 
fixed orientation in space, and hence any displacement of the body of the 
gyroscope causes the gimbal 11 to oscillate relative to the base plate 12 
as it rotates. Each pair of the conductive plates 16 and 17 forms a 
capacitor with the earthed gimbal moving between the electrodes of each 
capacitor. These capacitors are charged by the polarising voltage V.sub.p 
through the resistors R, which are of high value to prevent the charge on 
the capacitors from being attenuated by the resistors. The blocking 
capacitors CB prevent the d.c. polarisation from being applied to the 
slip-rings. 
When the gimbal is static relative to the conductive areas 16 and 17, 
whatever its actual distance from the conductive plates, there is no 
electrical output from the gyroscope. However, when the rotor is off-set, 
the gimbal oscillates at a frequency equal to the spin frequency of the 
shaft 13. Hence the earthed gimbal moves sinusoidally between the plates 
of the capacitors and produces an a.c. modulation of the d.c. polarising 
voltage. The output voltage Vo is given by 
EQU Vo=.theta..sub.x cos wt+.theta..sub.y sin wt 
where .theta..sub.x and .theta..sub.y are the rotor angular displacements 
about perpendicular axes relative to the case of the gyro, and w is the 
spin rate. This signal can be resolved into X and Y components with the 
aid of a timing signal indicating the angular position of the shaft, 
obtained from pick-off 24. 
The pick-off system described above, operating as it does by detecting the 
oscillation of the gimbal, develops an a.c. signal which may be 
capacitively coupled to the exterior of the gyroscope. This is obviously a 
more satisfactory situation than the known arrangement which detect steady 
displacement of the rotor and require an a.c. excitation voltage to enable 
the output to be extracted from the rotating parts of the gyro. In 
addition, as already stated, the pick-off system described above is 
insensitive to initial errors in the location of the gimbal, and such 
errors do not have to be trimmed out. Similarly any gradual changes in 
location of the parts will not develop any error. 
The use of the d.c. polarising voltage has a further advantage, in that it 
may be used to provide electrostatic back-off of the stiffness of the 
suspension hinges 26 supporting the gimbal 11. It is known that a partial 
cancellation of the spring stiffness has the advantage that a relatively 
stiff spring may support a heavy rotor without the large restoring torques 
which such a spring would normally exert. It is possible to cancel the 
spring torque completely, by adjusting the value of the d.c. polarising 
voltage. 
The boostrap capacitor CF is necessary to ensure that the total electrical 
charge in the capacitor circuit remains constant whilst allowing charge to 
transfer from one side of the capacitor circuit to the other as the gimbal 
oscillates. This produces an a.c. modulation. The capacitor CF also 
ensures that the electrostatic negative spring stiffness is unaffected by 
small changes in the voltage across the pick-off capacitors formed by the 
conductive plates 16 and 17. 
As already stated, the output from the pick-off system is in the form 
EQU V.sub.o =.theta..sub.x cos wt+.theta..sub.y sin wt 
The output must be processed to give the rotor angular displacement 
.theta..sub.x and .theta..sub.y relative to the gyro case. The output 
signal may contain unwanted noise components at multiples of the spin 
frequency w and these should be suppressed if possible. The most 
convenient way of obtaining only the desired components is to use two 
three-level phase sensitive rectifiers, locked to the rotor spin frequency 
w and phased 90.degree. apart. 
FIG. 5 shows an alternative electrical circuit to that of FIG. 4 in which 
the gimbal 11 is insulated from the base plate 12 and has the polarising 
voltage V.sub.p applied to it. The blocking capacitors CB are now no 
longer necessary and are omitted, and the resistors R are connected to 
earth to provide leakage resistance to maintain the average potential on 
the conductive plates at the same level as the base plate.