U.S. patent number 3,577,646 [Application Number 04/687,049] was granted by the patent office on 1971-05-04 for method of damping devices having oscillatory motion.
This patent grant is currently assigned to Lear Siegler, Inc.. Invention is credited to Harry Nils Eklund.
United States Patent |
3,577,646 |
Eklund |
May 4, 1971 |
METHOD OF DAMPING DEVICES HAVING OSCILLATORY MOTION
Abstract
A method for damping an oscillating mechanical system to bring
it to rest at or near its equilibrium position employing
iteratively a fixed damping cycle consisting of a first interval of
undamped motion followed by a second interval of supercritically
damped motion, the process being iterated a sufficient number of
cycles to position the system as close as desired to the
equilibrium position. A preferred application of this method of
damping is in connection with a meridian-seeking gyroscope, where
the first interval of undamped motion consists of free precession
of the gyroscope pin axle toward the meridional plane, followed by
a second interval of supercritical damping that positions the spin
axle closer to the meridional plane than at the release of the
motion. By repeating the appropriately timed intervals of the cycle
a number of times, the spin axle of the meridian-seeking gyroscope
can be brought as close to the meridional plane as desired.
Inventors: |
Eklund; Harry Nils (Pacific
Palisades, CA) |
Assignee: |
Lear Siegler, Inc. (Santa
Monica, CA)
|
Family
ID: |
24758812 |
Appl.
No.: |
04/687,049 |
Filed: |
November 30, 1967 |
Current U.S.
Class: |
33/301; 33/318;
33/324; 74/5.5 |
Current CPC
Class: |
F16F
15/02 (20130101); Y10T 74/1257 (20150115) |
Current International
Class: |
F16F
15/02 (20060101); G01c 019/38 () |
Field of
Search: |
;33/226 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hull; Robert B.
Claims
I claim:
1. A method of damping the motion of a mechanical system element
which is oscillating about an equilibrium position, the motion of
the element having a substantially undamped natural period of
oscillation, the method comprising the step of applying
supercritical damping force to the element for a selected interval
of time commencing from when the ratio of the instantaneous
velocity of the element toward its equilibrium position to the
instantaneous displacement of the element from its equilibrium
position is approximately equal to the ratio of .pi. to the product
of the undamped natural period of oscillation and the damping
ratio.
2. A method in accordance with claim 1 including the additional
steps of repeating the step a selected number of times so as to
further damp the motion of the element.
3. The method in accordance with claim 1 wherein the selected
interval of time, t, is given by the expression
undamped period, .psi..sub.o is the initial displacement from the
equilibrium position, .psi. is the displacement from the
equilibrium position at the end of said selected interval of time,
and .delta. is the supercritical damping ratio.
4. The method in accordance with claim 1 wherein the supercritical
damping force is proportional to the velocity of the moving
element.
5. The method in accordance with claim 1 wherein the supercritical
damping force is proportional to the square of the velocity of the
moving element.
6. The method in accordance with claim 1 wherein the supercritical
damping force is a function of the velocity of the moving
element.
7. A method in accordance with claim 1 wherein the supercritical
damping ratio is approximately 10.
8. A method in accordance with claim 1 wherein the supercritical
damping ratio is between 5 and 20.
9. The method of damping in accordance with claim 1 including in
addition a first initial step of applying supercritical damping
force to the element for a selected interval of time and a second
initial step of removing the supercritical damping force thereby
allowing the element to move undamped until supercritical damping
force is reapplied in accordance with claim 1.
10. The method in accordance with claim 9 wherein the time at which
the supercritical damping force is removed precedes the time at
which supercritical damping force is reapplied by an amount which
is approximately equal to the quantity t, where t is given by the
expression
where .delta. is the supercritical damping ratio and T is the
undamped natural period.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and means of controlling a
device having oscillatory motion, and more particularly to damping
the device to bring the moving element to rest in, or near, its
equilibrium position in a relatively short period of time, and in a
more precise and dependable manner than heretofore, which invention
has particular applicability to meridian-seeking gyroscopes.
Many devices, such as ordinary gravity pendulums, bifilar and
trifilar gravity pendulums, torsion pendulums, spring-centered
shafts, and spring-suspended masses, have elements that oscillate
or vibrate about an equilibrium position. Oftentimes the
oscillation is undesirable, such as for example when the device is
a meridian-seeking gyroscope for direct indication of a meridional
plane. A pendulous meridian-seeking gyroscope upon the earths's
surface having its spin axis horizontal tends to precess under
earths's rotation so as to align its spin axis with astronomical
north, provided the gyroscope is given angular freedom about its
vertical or azimuth axis.
Meridian-seeking gyroscopes are useful as instruments for surveying
and, in general, accurate direction finding. However, their
usefulness is limited because of the inability to easily and
quickly bring the spin axis in alignment with the meridian or
astronomical north. In the absence of added damping, it oftentimes
takes as much as two days or longer for the spin axis to come to
rest at its equilibrium position in the meridian plane after
oscillating with respect to this plane.
2. Description of the Prior Art
In the past oscillatory devices have been brought to rest at or
near their equilibrium position by applying some type of damping,
such as for example, viscous damping or eddy-current damping. In
some of the meridian-seeking pendulous gyroscopes a continuous
velocity-proportional damping has been applied where the damping
torque is less than the critical damping torque of the system. The
application of this damping torque will bring the oscillatory
element to its equilibrium position after a certain period of time.
However, it has been found that in many cases this period of time
is excessive and limits the usefulness of the device.
Presently used damping arrangements for meridian-seeking gyroscopes
provide a damping torque in azimuth proportional to either the
instantaneous angle of elevation of the spin axle, or to the
instantaneous angular velocity of the spin axle in the azimuth
plane. In either case, the magnitude of the damping torque is
chosen, so as to provide somewhat less than critical damping
torque, the actual magnitude being a compromise between that
required for adequate stability of the gyroscope, after coming to
rest in the meridian plane, and that required to bring the spin
axle within the allowable error angle, with regard to the meridian
plane, in a given time interval after uncaging of the
gyroscope.
Generally, such damping arrangements employ a damping ratio less
than unity (usually between 0.7 and 0.9 ), so that the spin axle
executes a damped oscillatory motion about the meridian or
equilibrium position before settling down within the allowable
azimuth error angle. The settling time for a meridian-seeking
gyroscope with such a damping arrangement may be of the order of 2
to 2 1/2 times the length of the undamped period of the gyroscope,
depending on the actual damping ratio and the magnitude of the
initial azimuth angle. However, due to variations in the value of
the damping ratio with temperature and other factors, this method
introduces a certain element of uncertainty, as to when the
amplitude of the oscillations of the spin axle about the meridian
have decayed below the magnitude of the allowable azimuth error
angle.
SUMMARY OF THE INVENTION
Thus, in accordance with this invention, devices that have
oscillatory motion about an equilibrium position are brought to
rest in a relatively short interval of time employing iteratively a
fixed damping cycle consisting of a first interval of undamped
motion followed by a second interval of supercritically dampened
motion, the process being iterated a sufficient number of cycles to
position the system as close as desired to the equilibrium
position.
A preferred application of this method of damping is in connection
with a meridian-seeking gyroscope, where the first interval of
undamped motion consists of free precession of the gyroscope spin
axle toward the meridional plane, followed by a second interval of
supercritical damping that positions the spin axle closer to the
meridional plane than at at the release of the motion. By repeating
the appropriately timed intervals of the cycle a number of times,
the spin axle of the meridian-seeking gyroscope can be brought as
closed to the meridional plane as desired.
The method of the present invention has general applicability to
any oscillatory mechanical system with plural degrees of freedom
and has specific applicability to systems which obey the
differential equation
Such systems, which include nonpendulous and pendulous
meridian-seeking gyroscopes can be represented graphically in a
phase-plane diagram by writing the equation for the system in the
form
where x in the displacement of the system from its equilibrium
position,
and is the angular natural frequency of the system, t is the time,
and plotting v versus x. Phase-plane diagrams are very useful in
analyzing oscillatory of vibratory systems. An example of
phase-plane diagram analysis is set out at pages 353--363 in the
book entitled "Mechanical vibrations," Fourth Edition, by J.P. Den
Hartog, published by McGraw-Hill Book Company.
When .delta.=0 the trajectories in the x, v -plane are circles, the
radii of which are the initial values, x.sub.0, of the
displacement. Then .delta. is different from zero, and much greater
than unity, the phase-plane trajectories become virtually straight
lines everywhere, except in the neighborhood of an asymptote whose
equation is given by x'+(.delta.- .delta..sup.2 -1) x=0, which all
other trajectories but one approach asymptotically and proceed
along to the stable, singular point at the origin of the phase
plane. A point P(x,v ), the representative point of the state of
the system, progresses in time along a trajectory defined by the
initial condition of the damped motion, and, upon approaching the
asymptote, will proceed along this to the origin, which it will
reach only after infinite time. One exception to this approach to
the origin is the trajectory whose equation is x'+(.delta.+
.delta..sup.2 -1) x=0, and which represents a straight line through
the origin. No trajectory can cross this straight line, but can
only approach it along the above asymptote and will be referred to
here in a special sense as a second asymptote of the system, along
which the motion rapidly proceeds to the proximity of the singular
point at the origin. These two asymptotes are trajectories of
slowest and fastest approach respectively, to the singular point of
the origin.
The method of damping the system, whose equilibrium position
coincides with the singular point in the phase plane, consists in
releasing the system with zero velocity from its displaced
position, allowing the representative point to proceed along the
circular trajectory, defined by the initial displacement, until the
representative point arrives at the intersection between the
undamped trajectory and the damped trajectory represented by the
"fast" asymptote. At this point the damping is switched on, and the
representative point, P(x,v), moves rapidly along the fast
asymptote into the proximity of the stable singular point at the
origin, which is the equilibrium position of the system.
The fast asymptote is, thus, the equivalent of a switching line,
except that the switching action is not initiated by the fast
asymptote, but rather by determining the interval of the undamped
motion so that at the time of switching the representative point
will be at the fast asymptote.
However, due to errors in the timer, it may happen that the
switching from the undamped to the damped state of the system
occurs either too early or too late so that, instead of proceeding
directly to the origin along the fast asymptote, the representative
point will proceed along one of the neighboring trajectories,
become "captured" by the "slow" asymptote and, thus, never reach
the origin, or the equilibrium position. To remedy this failing,
the original cycle, consisting of a first time interval of free,
undamped motion followed by second timed interval of damped motion,
is repeated by starting a new interval of undamped motion from the
final position of the representative point at the slow asymptote.
After completing the second cycle, the point will again virtually
come to rest on the slow asymptote. The second encounter of the
representative point with the slow asymptote represents a position
closer to the origin, than at the first encounter after the first
damping cycle. Thus, by iterating the process a number of cycles,
it is possible to get as close to the origin as desired, despite
errors in timing of the free, undamped interval of motion.
It is apparent that the slow and the fast asymptotes, which are
characteristic of the system, serve as reference or guidelines, and
that in particular the slow asymptote will establish a fixed
reference line, from which the undamped motion will start each
cycle, thus preventing a cumulative error in position of the system
state point, or representative point P, for a single, constant
error in the timing of the undamped interval of motion.
This type of damping process is of particular value in the damping
of a meridian-seeking gyroscope so that it will come to rest in or
near to the meridional plane rapidly and from any initial
displacement.
The use of the fast asymptote and the slow asymptote as "switching
lines" is a special case of the more general approach of employing
any two selected switching lines, one to determine the time for
applying the supercritical damping and the other to determine the
time for removing the damping and allowing undamped or proper
motion.
The determination of the interval of time for the representative
point P to intercept the selected switching line at the end of the
initial undamped motion is simplified if the moving element of the
system starts with no initial velocity. This is accomplished by
applying supercritical damping simultaneously with the release of
the system from its initial displacement. This brings the slow
asymptote into existence, and as a result the representative point,
which is now moving on a damped trajectory, cannot cross the slow
asymptote, but comes virtually to rest thereon, so that the
undamped motion will be initiated from the slow asymptote.
The apparatus for selectively applying the damping force includes a
means for sensing the speed at which the element moves toward its
equilibrium position. The apparatus further includes a means
responsive to the sensing means and thus to the instantaneous
velocity of the element for generating a damping force that is
proportional to the velocity of the moving element. This damping
force is applied to the moving element through a timer which sets
the interval of time during which the damping force is applied and
also sets the interval of time between the applications of the
damping force, when iterative damping is employed.
The time interval between the applications of the damping force, as
well as the magnitude of the damping force are adjustable to
compensate for changes in operating conditions, for example to
compensate for changes in latitude when the moving element is the
spin axle of a meridian-seeking gyroscope.
Alternatively, the generated damping force may be proportional to
the square of the instantaneous velocity on it may be primarily
proportional to the instantaneous velocity below a first selected
magnitude of velocity and primarily proportional to the square of
the instantaneous velocity above a second selected magnitude of
velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present
invention may be understood more fully and clearly upon
consideration of the following specification and the accompanying
drawings in which:
FIG. 1 is a phase-plane representation of the normalized motion of
an undamped oscillatory or vibratory element;
FIG. 2 is a phase-plane representation of the normalized motion of
an over-damped or supercritically damped moving element having
oscillatory motion;
FIG. 3 is a block diagram of the damping apparatus in accordance
with the present invention;
FIGS. 4 and 5 are graphs of the damping ratio versus time of the
iterative damping in accordance with the present invention;
FIG. 6 is a phase-plane representation of the general method of
iterative damping in accordance with the present invention;
FIGS. 7 and 8 are phase plots graphically depicting a preferred
method of iterative damping in accordance with the present
invention;
FIG. 9 is a phase-plane representation of the motion of an undamped
element and a supercritically damped element along the fast
asymptote;
FIG. 10 is a phase-plane representation graphically depicting the
iterative damping in accordance with the present invention for a
moving element having a negative initial angle; and
FIG. 11 is a phase-plane representation of large initial angle
undamped motion of a moving element which has a restoring force
that is proportional to the sine function for the displacement.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Devices that have elements that oscillate or vibrate about an
equilibrium position may be analyzed more easily by considering
mathematical equations for the motion of the element and graphical
plots of the solutions of the equations for the motions of the
device.
In one embodiment of this invention the device having an
oscillatory element having periodic motion about an equilibrium
position is a meridian-seeking gyroscope, although the invention is
in no way limited to this particular device for it is equally
applicable to any device having oscillatory motion, such as
ordinary gravity pendulums, bifilar and trifilar pendulums, torsion
pendulums, spring-centered shafts and spring-suspended masses, for
example. However, a meridian-seeking gyroscope will be employed in
describing the invention.
In general there are two types of meridian-seeking gyroscopes,
either of which could be used in describing the invention. These
are the pendulous gyroscopes and the nonpendulous gyroscopes. The
pendulous meridian-seeking gyroscope will be employed in this
description.
Such a gyroscope includes a spinning rotor, spin axle, suitably
mounted in a stator frame, with means for keeping it spinning. The
gyroscope proper is enclosed in frame, which is a pendulous
horizontal axis gyrocompass has 2.degree. of freedom, 1.degree. of
freedom about an elevation axis and another degree of freedom about
an azimuth axis. In using a pendulous gyroscope to describe the
invention, the spin axle movement will be taken to be synonymous to
movement of the frame enclosing the gyroscope proper of the
illustrative gyroscope.
For small angles of displacement the equations of motion of a
damped pendulous meridian-seeking gyroscope may be written in the
simplified form
where C is the damping moment coefficient about the azimuth axis;
.psi. is the azimuth angle, with the angle being positive to the
east; H is the angular momentum of the gyro rotor about the spin
axis; .OMEGA. is the local horizontal component of the earths's
spin velocity, .omega..sub.e ; .THETA. is the elevation angle,
which is positive upward; M is the pendulous moment about the
elevation axis; and t is time.
In these equations, 1 and 2, the damping about the elevation axis
and the moments of inertia of the gyro container and the gyro wheel
about the two axes perpendicular to the spin axis have been
neglected. These quantities are negligibly small compared to the
remaining quantities, and are generally ignored in analyzing the
performance of a pendulous meridian-seeking gyroscope. The absence
of these quantities in the following mathematical equations in no
way invalidates the results of the analysis, but makes the system
amenable to phase-plane representations. Additionally, it is
convenient to normalize equations 1 and 2 by introducing
nondimensional time defined by .UPSILON.=.omega..sub.o t, where
.omega..sub.o is the natural undamped frequency of the system given
by
and by setting
.delta. is commonly called the damping ratio and is defined as the
ratio of the actual damping moment coefficient, C, to the
hypothetical damping coefficient having the value 2H/.omega..sub.o
, which hypothetical damping coefficient produces critical damping.
When these substitutions are made in equations 1 and 2 the
following results:
As is well known, the end of a spin axle of an undamped gyroscope
traces an elliptical path in a vertical plane which is
perpendicular to the meridional plane with the major axis in the
azimuth plane and the minor axis in the meridional plane. With the
substitution
the elliptical path normally traced by the spin axle becomes a
circle, which greatly facilitates the representation of the motion.
The damped motion will, of course, be correspondingly modified
since the elevation and azimuth axes now have the same scale
factor.
Separating the variables in equations 3 and 4 results in the
equations
Equations 5, which represents the azimuthal motion, is a second
order differential equation having a single parameter .delta.,
which denotes the damping ratio of the system. Thus, .delta.<1
represents an under-damped system, the solution of which can be
expressed by a damped sinusoid. .delta.=1 results in "critical"
damping which gives rise to a damped linear algebraic function,
while .delta.>1 represents an over-damped or supercritically
damped system. Finally, in the case where .delta.=0, the solution
is an undamped sinusoid representing, for small angles, a simple
harmonic motion.
The solution of the equations, 5 and 6, for the undamped case and
for the supercritically damped case, where .delta.>>1, are
respectively given by
.psi.=.psi..sub.0 cos .tau.-.xi..sub.0 sin .tau. (5a )
.xi.=.psi..sub.0 sin .tau.+.xi..sub.0 cos .tau. (6a) for .delta.=0,
and the set of solutions
for .delta.<<1, where .psi..sub.o is the initial azimuth
angle, .xi..sub.o is the initial elevation angle, and .eta.=
.delta..sup.2 -1.
Equation 5 is a general equation for a damped linear oscillatory
system that has a single degree of freedom. A
single-degree-of-freedom spring system is an example of such a
system. A discussion of this system and the resultant differential
equation is set out in the Den Hartog book "Mechanical Vibrations"
at pages 24 to 26. Additionally, equation 5 is the same for both
pendulous and nonpendulous meridan-seeking gyroscopes for their
azimuth motion, where .psi. is the azimuth angle and .delta. is the
damping ratio about the azimuth axis. For a nonpendulous gyroscope,
however, the natural frequency of the undamped motion is given
by
, where I is the moment of inertia about the azimuth axis.
For purposes of better illustrating the present invention the
behavior of the gyroscope is depicted in a special type coordinate
plane known as the phase plane in which the nondimensional azimuth
velocity, d.psi./d.tau., of the spin axle is plotted against the
azimuth displacement, .psi., of the spin axle. Phase-plane diagrams
are useful in analyzing oscillatory systems and a specific example
is set out at pages 353--363 of the above-referred to book by Den
Hartog. By introducing the abbreviation
equation 5 can be rewritten as follows:
in which the dimensionless time .tau. has been eliminated to obtain
an equation in .psi. and .psi.'. The solution of equation 7 is the
locus for the spin axle in the .psi., .psi.' plane.
A point P may be employed on the phase-plane diagram to represent
the state of the system at any instant. Thus, point P represents
the displacement and velocity of the end of the spin axle at a
particular time and will be hereafter referred to as the
representative point P.
When the damping ratio .delta.=0, equation 7 reduces to
which has the well-known solution
.psi.'.sup.2 +.psi..sup.2 =C (9) where the constant on the right
side is determined by the initial conditions. Thus, if at .tau.=0,
.psi.'=0 and .psi.=.psi..sub.o, this constant, C, will be equal to
.psi..sub.o.sup.2 and equation 9 becomes
.psi.'.sup.2 +.psi..sup.2 =.psi..sub.o.sup.2. (9a) This is the
equation of a family of concentric circles of radius .psi..sub.o
and corresponds to the undamped precessional motion of the gyro due
to the action of the pendulous moment in conjunction with the
earths's rotation.
The phase-plane representation of equation 9a is depicted in FIG. 1
with curves a, b, and c representing some of the possible
solutions. In FIG. 1, the representative point P moves clockwise
with constant angular velocity along the circular trajectory of
radius .psi..sub.o. The instantaneous position of point P is
defined by the angle .tau., which, according to convention, is
negative. The instantaneous values of .psi. and 104' are thus given
by .psi.=.psi..sub.o cos .tau. and .psi.'=.psi..sub.o sin
.tau..
An alternative way of considering the undamped precessional motion
of the spin axis of a meridian-seeking gyroscope would be to plot
displacement .xi.which is proportional to the displacement along
the elevation axis, against the displacement .psi. along the
azimuth axis, that is, to plot the behavior of the gyro using the
.psi., .xi. coordinate plane. Since .xi. equals negative .psi.'
(equation 6), this coordinate plane may be represented by taking
FIG. 1 and rotating the phase-plane representation through
180.degree. about the .psi. axis. This will produce the more
familiar .psi., .xi. plane, in which the motion proceeds
counterclockwise, and the angle .tau. is positive.
For supercritically damped systems, where .delta.>>1, it will
be seen that the solution for equation 7 results in a phase-plane
representation that is drastically different from the plot where
.delta.=0, which is depicted in FIG. 1. The solution curves of
equation 7 for .delta.>>1, form a family of trajectories as
depicted in FIG. 2. These trajectories are virtually straight
parallel lines, such as lines d-h in FIG. 2, except in the region
surrounding the straight line marked A1-A1. The equation for line
A1-A1 is .psi.' +(.delta.-.sqroot..delta..sup.2 -1 ) .psi.=0. This
line as an asymptote that cannot be crossed by any solution curve
and all of the latter will therefore approach this line
asymptotically and will then move along it toward the singular
point at the origin, 20.
For example, as shown in FIG. 2 a trajectory point P starting at
given initial conditions .psi.=.psi..sub.o, .psi.'=.psi..sub.o '
will travel on the trajectory f passing through the point
.psi..sub.o, .psi..sub.o ' approaching A1-A1 asymptotically while
moving toward the origin 20, which is the point of stable
equilibrium.
The motion along asymptote A1-A1 is the slowest approach to the
origin, which approach is representative of the "creeping" motion
generally encountered in overdamped systems.
In the present invention a supercritical damping ratio is employed.
For illustrative purposes it is assumed that this damping ratio is
10 times the critical damping. Such a large value of damping ratio
as .delta.=10, causes the motion along the line A1-A1 to be so slow
as to make the settling time of the gyro hundreds of times longer
than can be practically tolerated. However, as will be described
hereinafter, the otherwise undesirable characteristic of the slow
asymptote is employed in the method and apparatus of the present
invention to aid in bringing the oscillatory or vibratory element
to rest at or near the equilibrium point in a relatively short
interval of time.
The trajectory along the line A2-A2, which passes through the
origin 20, as shown in FIG. 2, constitutes a trajectory of fastest
approach to the origin. Thus, if a trajectory originates on this
asymptote, A2-A2, or is by some means made to enter thereupon, the
representative point P will move rapidly along the asymptote toward
the origin and reach the proximity thereof in the shortest possible
time. The equation for line A2-A2 is
.psi.'+(.delta.+.sqroot..delta..sup.2 -1 ) .psi.=0.
Since the origin is a stable singular point, the representative
point P travelling along the fast asymptote comes to rest in the
proximity of the meridional plane, which plane passes through the
origin and is perpendicular to the .psi. axis.
The slow and fast asymptotes A1-A1 and A2-A2, respectively, in FIG.
2 are displaced from the axes .psi., 104' by an angle .alpha.. This
angle .alpha. is directly dependent upon the selected damping ratio
.delta., and since by definition,
.delta. is directly dependent on the basic characteristics of the
system. As the damping ratio .delta. increases, the angle .alpha.
decreases and for an infinite damping ratio .delta., .alpha. would
be zero so that the fast asymptote would coincide with the .psi.'
axis and the slow asymptote would coincide with the .psi. axis of
the phase-plane representation in FIG. 2.
As depicted in the phase plane, where .psi. is assumed to be 10,
FIG. 2, the family of curves resulting from the solution of
equation 7 consists of trajectories that are virtually straight
parallel lines, except in the vicinity of the slow asymptote,
A1-A1. The trajectories for .delta.=10 are more nearly
perpendicular to the .psi. axis than shown in FIG. 2 and the
subsequent FIGS. of the drawing. This is because the angle .alpha.
is exaggerated to show more clearly the relationship of the terms
of the equations. For example, the angle .alpha. in FIG. 2 is
depicted as being between 6.degree. and 7.degree., while in actual
practice the angle is less than 3.degree. for a damping ratio
.delta.=10.
As the damping ratio .delta. is increased above the assumed value
of 10, the trajectories become straighter and more parallel to each
other up to the point where the damping ratio is infinite and the
trajectories become parallel to the .psi.' axis. Conversely, as the
damping ratio is decreased below 10, the family of trajectories
become less straight and more curved up to the point where
.delta.=0, with a resultant family of concentric circles.
Trajectories for .delta.=1.25 are shown in FIG. 8.23 on page 356 of
the above-identified book. The shortest possible time in which the
gyro spin axis can be brought to the meridian plane, as seen by
combining FIGS. 1 and 2, is to bring the representative point P
along a circular trajectory representing undampened motion onto the
fast asymptote A2-A2 and then allow point P to follow this
asymptote to the origin, which is located in the meridian
plane.
An apparatus for performing the method of the present invention is
depicted schematically in the block diagram of FIG. 3. For
illustrative purposes, the device to be controlled is assumed to be
a meridian-seeking gyroscope having precessional motion with
displacement about both azimuth and elevation axes.
The operation of the control system depicted in FIG. 3 is described
with reference to the phase plane as representatively depicted in
FIGS. 1 and 2. The azimuthal movement of the spin axle of the
gyroscope is set forth by equation 7 and conforms to the solutions
of this equation, when .delta.= 0 and .delta.=10 as depicted in
FIGS. 1 and 2, respectively. The inputs to the gyroscope, which are
shown schematically on FIG. 3, are .xi.' and -2 .delta..psi.' to
which the gyroscope responds with the outputs .psi. and .psi.'.
When .delta. equals zero the only input is .xi.' and the gyroscope
executes a simple harmonic motion, the precessional motion depicted
in FIG. 1. On the other hand, for .delta. equal to 10, the motion
of the gyroscope, which is described by the representative point P,
will be along one of the trajectories shown in FIG. 2.
The control system operates on the azimuth motion of the spin axle,
which is the oscillatory of vibratory element of the
meridian-seeking gyroscope 1 in FIG. 3. The control system includes
a pickoff 1 coupled to the azimuth motion of the spin axle of the
gyroscope 1 and has an output representative of the displacement
and velocity of the gyroscope spin axle in the azimuth plane. The
output of the pickoff 2 is amplified by amplifier 3 and applied to
motor 4. The output of the motor 4 is coupled back to the input of
the pickoff 2 through a speed-reduction mechanism, such as a gear
box 5. The displacement output of the gear box combines with the
displacement output from the gyroscope to form a summing junction
6. The coupling between the gear box 5 and the motor 4 and the gear
box 5 and the summing junction 6 is mechanical, which mechanical
coupling is shown by dotted lines in the drawing. Thus, the azimuth
motion of the gyroscope 1 is followed up by a servosystem
consisting of pickoff 2, amplifier 3, motor 4, and gear box 5.
A typical pickoff and servoloop for a meridian-seeking gyroscope is
described in the copending U.S. Pat. application Ser. No. 529,325,
filed Feb. 23, 1966, by Leonard R. Ambrosini and assigned to the
same assignee as this application now U.S. Pat. No. 3,512,264. For
purposes of illustration, it will be assumed that the gyroscope 1,
pickoff 2, amplifier 3, motor 4, and gear box 5 of FIG. 3 are
similar to the corresponding elements in the referenced
application.
The output of the motor 4 is also applied to a tachometer-generator
7, the output voltage of which is applied to an amplifier and
demodulator 8. The output voltage of the tachometer-generator 7 is
representative of the angular velocity and is proportional to the
angular velocity of the spin axle in the azimuth plane and is
applied to a timer and damping controller 9 for generating a
damping torque about the azimuth axis, which torque is proportional
to the instantaneous velocity of the spin axle in the azimuth
plane. The damping torque is applied to the oscillatory element,
that is to the gyroscope 1 through a torquer 10.
In the servo described above, the gear box 5 allows the motor 4 to
run at a relatively high speed for more uniform motion. This
relatively high speed also increases the output voltage of the
tachometer-generator 7 mounted on the shaft of motor 4. The
relative displacement between the gyrodriven part of the pickoff 2
and the motor-driven part of the pickoff 2 generates an error
signal .epsilon.. This error signal is amplified by amplifier 3 and
applied to the motor 4 with such a polarity that it reduces the
pickoff error signal. Phase and amplitude compensation may also be
provided in the amplifier to improve the servoresponse.
Additionally, the stability of the servo may be improved by
applying the velocity feedback signal from the output of the
tachometer-generator 7 through a feedback network 12 to the input
of the amplifier 3 as shown in FIG. 3.
The output voltage of the tachometer-generator 7, which is
proportional to the azimuth velocity d.psi./dt, is amplified and
demodulated in the amplifier and demodulator 8 and then fed to the
timer and damping controller 9, which controls the inputs to the
gyroscope torquer 10.
The gyroscope torquer 10 is depicted to FIG. 3 by coils 13 and 14,
whose axes are at right angles to one another. Coil 13 is mounted
on the gyroscope housing and carries a constant current I. Coil 14
is mounted on the servo followup of the gyroscope and carries a
variable current I.sub.2 which is proportional to the
tachometer-generator 7 output voltage, which is proportional to the
output velocity d.psi./dt of the gyroscope. Coil 13 cooperates with
coil 14 so that when both coils are appropriately excited a torque
will be developed about the azimuth axis of the gyroscope. The
gyroscope torquer 10 thus furnishes the damping term
(2.delta..psi.') about the azimuth axis of the gyroscope. The
torque exerted by one coil on the other is given by T=C.sub.1
I.sub.1 I.sub.2, where C.sub.1 is a constant of proportionality.
Since I.sub.1 is constant and I.sub.2 is proportional to .psi.', it
is evident that T= C.sub.2 .psi.', where C.sub.2 is another
constant of proportionality. By giving I.sub.1 the proper value so
that C.sub.2 =C.sub.1 I.sub.1 =2 .delta., one thus achieves that T=
2 .delta..psi.', which is the desired instantaneous value of the
damping torque to be impressed upon the gyroscope. While the
damping torque is shown in FIG. 3 as being applied to the gyroscope
or moving element by way of a torquer, it could be applied in any
suitable manner. For example, it could be applied by use of some
type of coulomb damping or viscous damping.
After the gyroscope has reached its operating speed and has
generally been oriented in the direction of the meridian by some
other means, for example a magnetic compass, the meridian-seeking
operation may be initiated by uncaging the gyroscope. At the time
the gyroscope is uncaged, power is applied to the control system of
FIG. 3 from the power source 15. At this time the timing sequence
established by the timer and damping controller 9 begins.
Timer and damping controller 9 includes a constant current
generator 16 and a variable current generator 17, which is
responsive to the output voltage of the tachometer-generator 7
through the amplifier 8 and thereby responsive to the instantaneous
velocity d.psi./dt of the gyroscope about the azimuth axis. The
outputs of the generators 16 and 17 are applied to the torquer 10
through transmission gates 18 and 19, respectively. The conduction
states of the transmission gates 18 and 19 are controlled by the
output of a variable timer 21. When the variable timer has an
output signal of a particular polarity the gates will be placed in
their conduction state to pass the signal from the associated
current generator. In this manner, the duration of the interval of
each damping pulse may be controlled as well as the interval
between the damping pulses by programming the variable timer 21.
Additionally, the application of the first damping pulse may be
controlled and may be timed to take place at the moment of
uncaging, as representatively shown in FIG. 4, or at some later
selected time, as shown in FIG. 5. In the charts of FIGS. 4 and 5,
the application of the damping ratio .delta. is shown on a time
scale, the time intervals being predetermined by the variable timer
21. The curves 22 and 23 of FIGS. 4 and 5 occur at the same time as
the output signal from the variable timer 21 which opens the
transmission gates 18 and 19. The time scale and the intervals of
time on FIGS. 4 and 5 are shown for an illustrative
meridian-seeking gyroscope having a natural undamped period of 240
seconds at a particular latitude.
Ideally, during the initial period after uncaging, the gyroscope
precesses without damping for the length of time required for the
representative point P on the circular trajectories of FIG. 1 to
intersect the fast asymptote A2-A2 of FIG. 2. At this time, the
full damping torque is switched on by the timer, and as a result,
the representative point P travels along the fast asymptote A2-A2
into the meridian plane where it comes to rest.
The method includes in the preferred case the use of the
asymptotes, in the displacement-velocity representation (FIG. 2) of
a supercritically damped system, as reference lines or switching
lines, to determine when the damping should be applied or removed.
The asymptotes, which are defined in the displacement-velocity
plane of the motion by the magnitude of the supercritical damping
ratio of the system, pass through the origin of the
displacement-velocity plane and have the slopes of (-.delta.-
.delta..sup.2 -1 ) and (-.delta.+ .delta..sup.2 -(1 ),
respectively, for the "fast" and the "slow" asymptote. For high
supercritical damping, the slow asymptote forms a relatively small
angle with the displacement axis, while the fast asymptote forms an
equal angle with the velocity axis. Motion originating on or
entering upon the slow asymptote requires infinite time to reach
the equilibrium position, generally known as a "creeping motion,"
while motion along the fast asymptote reaches the proximity of the
equilibrium position in the shortest possible time.
In the preferred method, the fast asymptote represents the ideal
switching line for applying the damping torque. Instead of using
the fast asymptote as the switching line, or the fast and the slow
asymptotes as switching lines, any two arbitrarily selected
switching lines can be used, as shown in the phase-plane
representation of FIG. 6, which is a combination of the circular
trajectories of FIG. 1 and the family of trajectories for
.delta.=10 of FIG. 2. The lines A1-A1 and A2-A2, respectively,
represent the slow and fast asymptotes, for a given supercritical
damping ratio.
S1 and S2 are switching lines passing through the origin of the
.psi., .psi.' plane and forming an angle .gamma. between them. The
angle .gamma. is proportional to the time interval of undamped
precessional motion along a circular trajectory from S1 to S2 in
the phase plane for small angles of .psi..sub.o, such that the
system is essentially linear. A timing sequence in accordance with
the present invention is provided whereby the gyroscope is uncaged
at an initial azimuth angle .psi..sub.o and then allowed to precess
undamped through an interval .tau..sub.o at the end which the
undamped precession circle 30 intersects switching line S2 at point
C.sub.0. At this point the supercritical damping is switched on and
the representative point P proceeds along the damped trajectory 31,
which is parallel to the fact asymptote, A2-A2, until at B.sub.1,
it is intercepted by the switching line S1. The damping is now
removed and the gyro allowed to precess freely with the
representative point following a circular trajectory 32 for an
interval given by the angle .gamma. between the switching lines S1
and S2, at the end of which interval it is intercepted by the
switching line S2 at point C.sub.1. With accurate timing the time
intervals corresponding to the arcs B.sub.0 C.sub.0 ; B.sub.1
C.sub.1 ; B.sub.2 C.sub.2, are all equal. Similarly, the damped
trajectory segments C.sub.0 B.sub.1 ; C.sub.1 B.sub.2 ; C.sub.2
B.sub.3, etc. all correspond to equal time intervals. As a
consequence, the representative point will follow the zigzag path
between the switching lines until after a sufficient number of
cycles it reaches the proximity of the meridian plane.
The ratio between the azimuth displacement amplitudes of any two
consecutive cycles is constant. Thus,
constant. One can then express the amplitude after n complete
cycles by .psi.n =(.psi..sub.o cos .phi.) .rho..sup.n where .rho.
is the angle between the switching line S1 and the .psi. axis.
One notes from FIG. 6 that
will decrease as .phi. gets larger and .gamma. gets smaller,
resulting in a greater reduction in amplitude after a given number
of cycles. However, unavoidable errors in timing make such an
improvement illusory since the absolute error in a timing interval
could under these conditions, even be equal to the length of the
interval itself. As a result, the "switching lines" will not remain
in their fixed, predetermined positions, but will shift relative to
each other, in such a way as to cause the magnitudes of the angles
.phi. and .gamma. to vary in a prohibitive and indeterminate
manner. To overcome the possibility of errors in timing, the slow
asymptote is alternatively used as one of the switching lines. In
this way, every undamped interval will be commenced from a fixed
reference line.
Although the use of the slow asymptote as a reference for the
initiation of the undamped intervals tends to ensure that the end
of the undamped interval and the beginning of the following damping
interval will occur at the point where the trajectory of the
undamped precession intersects the fast asymptote, other
circumstances may conspire to end the undamped precession interval
either before or after intersection with the fast asymptote. Thus,
the timer will be subject to errors, for example, the local
latitude is not always accurately known, the period of the
gyroscope increases as the initial amplitude gets larger, etc., all
of which adds up to the fact that seldom, if ever, will the damping
interval be initiated on the fast asymptote, when it is selected as
a switching line.
Thus, even when the fast asymptote is selected as the switching
line for the application of damping, the damping is made iterative,
i.e., the cycle, made up of the undamped interval and the damping
interval, is repeated a sufficient number of times, so as to reduce
the amplitude of the gyroscope to an acceptable value. The
iterative damping, with the slow and fast asymptotes used as
"switching lines," is depicted graphically in FIGS. 7, 8, and
10.
In FIG. 7, it is assumed that the error, .DELTA..tau.
(corresponding to an error of .DELTA..tau./.omega.o in real time),
in the undamped time interval
is negative, so that the damping interval is initiated prematurely,
at B, an angle .DELTA..tau. ahead of the fast asymptote, thus
making the switching line S3 pass through B. As a result, the
damped motion now takes place along the trajectory 40 which passes
through point B. The magnitude of the error angle .DELTA..tau. is
exaggerated in FIGS. 7, 8, and 10, similar to the exaggeration of
the angle .alpha. for ease of drawing. These angles are only
illustrative and are not limiting.
The trajectory 40 is virtually parallel to the fast asymptote
A2-A2, and is "captured" at point C by the slow trajectory A1-A1,
on which it comes to a virtual stop and then "creeps" toward the
origin at an extremely low rate. A few seconds after the
representative point has been "captured" by the slow asymptote, the
damping is switched off by the timer, and the gyroscope resumes its
free precession, with the representative point P now moving along
the circular trajectory 41 or arc CD. Since the angle COD= AOB=
.beta., it is clear that, at the end of the undamped time interval
the representative point P will be at D on the "switching line" OB,
or S3. This switching line S3 is defined by the equation
.psi.+.psi.' tan (.alpha.+.DELTA..tau. )=0, while the ideal
switching line, i.e., the fast asymptote has the equation
.psi.+.psi.' tan .alpha.a=0. At the end of the undamped time
interval, the timer 21 again switches on the damping and the
representative point now proceeds along the damped trajectory 42
through D, until it is captured at E by the slow asymptote. The
iteration may be continued in this manner, until the amplitude of
the gyroscope oscillation has been reduced below the magnitude of
the allowable error.
When the timing error is positive, and the undamped trajectory
extends an angle .DELTA..tau., beyond the fast asymptote, the
damping process will have the appearance shown in FIG. 8, which is
self-explanatory in view of FIG. 7, with the second switching line
now being defined by the equation: .psi.+.psi.' tan (.alpha.
-.DELTA..tau.)= O.
In order to determine the attenuation of the gyroscope amplitude
after a certain number of iterations of the damping process, one
may apply the following considerations: the angle .alpha. is less
than 3.degree. for .delta.=10, so that by reference to FIGS. 7 and
8, it is seen that .psi..sub.1 .apprxeq..psi..sub.o .DELTA..tau.
or
Since this ratio is constant for a given error, .DELTA..tau., one
obtains readily the residual amplitude of the gyroscope oscillation
after n iterations: .psi..sub.n =.psi..sub.o (.DELTA..tau.).sup.
n.
For an exemplary meridian-seeking gyroscope one can, for the
purpose of illustration, assume that one-quarter period of undamped
precession will require about 60 seconds. The transversal of the
arc between the slow and fast asymptote then requires,
approximately 56 seconds for a damping ratio of 10. Assuming the
very pessimistic value of .DELTA..tau.=0.05 rad., or .DELTA.t=2
seconds, the undamped interval will be about 56 seconds .+-. 2
seconds. From both analytical and graphical investigations, it has,
furthermore, been found that 12 seconds of time is adequate for the
damped interval, as well as for the initial damping interval. The
time interval sequence to be generated by the timer 21 will thus
have the appearance shown schematically in FIG. 4, where the
negative timing error, .DELTA..tau., of minus 0.05, or the
equivalent of minus 2 seconds has been assumed.
If the initial amplitude is, say 5.degree., the amplitude of the
gyroscope after four iterations of the damping cycle will be
.psi..sub.4 =5.degree. (0.05).sup.4 =3.125.times.10.sup..sup.-5
degrees .apprxeq.0.1 seconds of arc. In fact for this small initial
amplitude sufficient accuracy is obtained in most cases with only
three cycles of iteration, since .psi..sub.3 =5.degree.(0.05)
.sup.3 .apprxeq.2 seconds of arc. Furthermore, for many
applications even two iterations, for which .psi..sub.2 =40 seconds
of arc would be adequate.
The interval for damping along the fast asymptote for the
illustrative gyroscope having a natural undamped period of 240
seconds may be calculated by reference to the phase plot of FIG.
9.
The two asymptotes on the phase plot are derived in the following
manner. The diagram depicts the solution for the equation
Setting
and solving equation (7) for .psi.', one obtains
which, for any constant value of m, defines a straight line through
the origin of slope -1 /m+2.delta.. Thus, in the .psi., (.psi.'
plane (the phase-plane), every solution curve or trajectory must
cross the isocline, given by equation (12), with the slope m.
Isocline is the mathematical term for a curve, such that when
crossed by a family of trajectories, every trajectory crosses the
curve at the same slope with respect to the coordinate axes. By
assigning different values to m, a family of isoclines results, the
m -values of which define the slope of the trajectories that cross
them.
There are several specific values of m that are of special
interest. Thus, the isocline for m=.infin. is .psi.'=0, i.e., the
.psi.-axis, and all trajectories must cross this axis at right
angles to it. Of particular interest is the case in which the
directions of the trajectories are the same as the direction of the
isocline, or
Substituting this value of .psi.'/.psi. in equation (12) gives a
second degree algebraic equation in m, the roots of which are
Substituting these two values of m in equation (12) gives the two
isoclines
Since the slopes of the trajectories must be parallel to these two
isoclines at every point on them, the trajectories cannot cross,
but must approach these lines asymptotically. Equations (12a) and
(12b) thus define two asymptotes of the phase plane. The asymptote
defined by equation (12a) has a small negative slope, while the one
defined by equation (12b) has a large negative slope. Equation
(12a) defines the slow asymptote and equation (12b) defines the
fast asymptote. These equations can be rewritten in either of the
following forms.
Slow asymptote:
.psi.+ (.delta.+.eta.).psi.' =0 (12ai) or .psi.'+
(.delta.-.eta.).psi. =0 (12aii)
Fast asymptote:
.psi.+ (.delta.-.eta.).psi.' =0 (12bi) or .psi.'+
(.delta.+.eta.).psi. =0 (12bii)
The asymptotes are drawn on the phase-plot of FIG. 9, with the slow
asymptote being line 45 and the fast asymptote being line 46.
Assuming a representative point P starting at the intersection of
the circular trajectory for undamped precession with the fast
asymptote and the application of damping at this time, the time
interval for damping can be calculated.
The equation (12bii) for the fast asymptote, line 46 can be written
in the form
and solving this for .tau.
At the beginning of the damping interval, .tau.=0 and
.psi.=.psi.(0), which makes
The dimensionless time along the fast asymptote becomes then
Since .alpha. is small, less less than 3.degree. for .delta.=10,
one may set ##SPC1##
Again, because of the smallness of .alpha., one can write
Consequently, the dimensionless time .tau. along the fast asymptote
is given in terms of the distance .psi. from the origin:
corresponding to the real time.
Since 1n (0)= -.infin., it is seen that even along the fast
asymptote it theoretically requires infinite time to reach the
origin. This presents no problem, however, since it is not
necessary to reach the origin, but only to get sufficiently close
to it in a reasonable time. To obtain a numerical answer, as an
example, let it be assumed that the time to approach within one
second of arc of the origin, which is very little error or
displacement from the equilibrium position, is desired for a linear
system with an initial azimuth angle of
radian. The .psi.-axis position of one second of arc is then,
approximately, 5.times.10.sup..sup.-5 .psi.o.
Substituting this value of .psi. and the natural undamped period
for the frequency .omega.o in the expression for t, (equation 18 )
the following result is obtained:
For the illustrative gyroscope having a natural undamped period T
of 240 seconds and a damping ratio of 10, the time to reduce the
displacement from 0.1 radians to 1 second of arc is
t =13.3 seconds.
For the example of an initial displacement of .psi..sub.o =0.1
radians and a residual displacement of .psi.=1 second of arc the
ratio .sigma. between the damped interval of time and the undamped
interval of time may be written
The general expression for the ratio between the damped interval of
time and the undamped interval of time is
The above analysis also holds true for negative azimuth angles. A
phase representation of iterative damping for an initial negative
azimuth angle and a negative timing error is shown in FIG. 10.
All of the previous considerations and results have been based on
the assumption that the initial angle, .psi..sub.o, of the
gyroscope oscillation is small, say less than 5.degree., so that
the undamped precession angle, .tau..sub.o, is independent of the
magnitude of .psi..sub.o, that is, the system is assumed to be
linear. Actually, this is of course not generally true as the
magnetic compass used for rough alignment of the meridian-seeking
instrument with the meridian, may have a large unknown declination
at the point of setup of the instrument, and as a result the
initial angle .psi..sub.o may become too large to justify the
assumption made in setting up equation 1, namely that sin
.psi.=.psi.. Consequently, the more exact equation for the undamped
precessional motion of the gyroscope, in the phase-plane
coordinates, is
If this equation is integrated twice and solved for the
dimensionless time, .tau., one obtains the undamped precession
interval between the two asymptotes:
The above integral can be solved in terms of elliptic functions,
but this rather lengthy procedure can be evaded by resorting to an
approximation based on the following reasoning: Since the angle
.alpha. is small, of the order of 3.degree., one may, for the sake
of simplicity, instead of considering the arc
consider the full arc (.pi./2), i.e., the quarter period. If one
now compares the length of the quarter period, for the case that
.psi..sub.o approaches zero with that for say, .psi..sub.o
=45.degree., one obtains the difference
.DELTA..tau..sub.1 = [.tau..sub.o ] .sub. o 45 -[.tau.o] .sub. o
o
=1.633- 1.571
=.062 radians
or
Thus, at the end of the first undamped precession interval,
.tau..sub.o, the representative point P will be lagging by an angle
.DELTA..tau. of about 0.062 radians, in addition to the assumed
timing error .DELTA..tau..sub.o. The ratio of the amplitudes after
1 cycle of damping will then, be approximately
so that the initial 45.degree. angle is now reduced to
approximately 5.degree.. As a consequence, in the next and the
following damping cycles, the error, .DELTA..tau..sub.1, due to the
initial angle, vanishes, and only the assumed timing error of 0.05
radians is present. Consequently, after 4 cycles of damping, the
initial amplitude of 45.degree. is reduced to
.psi..sub.4 .apprxeq.0.112 .times.(0.05).sup.3
.times.45.degree.
.apprxeq.6.3.times.10.sup..sup.-4 degrees
.apprxeq.2.27 seconds of arc.
It now becomes evident that even with much larger initial angles,
one can achieve a rapid attenuation of the gyroscope amplitude of
oscillation. For example, taking .psi..sub.0 =90.degree. and the
same timing error of 0.05 rad., one obtains in a similar manner
after 4 cycles of damping
.psi..sub.4 .apprxeq.0.233.times.0.0552.times.(0.05).sup.2
.times.90.degree..times.3600=14 sec. of arc. Obviously, by
increasing the number of damping cycles to 5 or 6, adequate
alignment with the meridian can be obtained with initial angles
approaching 180.degree. , and with a total time for alignment of
about 8 min. (n=7). This is in contradistinction to the long time
required for alignment of a continuously damped exemplary gyroscope
when the initial angle approaches 180.degree..
FIG. 11 is a phase-plane representation of the undamped
precessional motion of the gyro for initial angles between 0 and
+180.degree.. The portrait is shown only for one quadrant, since
the curves in the other 3 quadrants are the mirror images of the
adjacent quadrants. The curves begin to deviate appreciably from
the circular form, associated with small initial angles, as the
latter go beyond, approximately, 5.degree.. This is made evident by
the dashed curve 50, which represents .tau..sub.o =const., which
curve intersects all of the precession curves at points of equal
time, .tau..sub.o, from the time of uncaging. Thus, with the timing
of the undamped precession interval kept constant at the value
.tau..sub.o, the damping will be switched in at the point, where
the particular curve intersects the curve 50. This will cause the
representative point to proceed along a damped trajectory toward
the slow asymptote 51. Whether it arrives at the asymptote before
switching to the next undamped precession interval takes place,
depends on the magnitude of the initial angle and on the length of
time that has been allotted to the damping interval. In the diagram
of FIG. 11, there is illustrated the case of the gyroscope being
uncaged at an initial angle of 45.degree. , which case has been
considered in the discussion of the numerical evaluation of the
settling accuracy.
In FIG. 11, the representative point P proceeds along the undamped
precessional curve 52 for the interval of time allotted for
undamped precessional movement. The damping is then applied and the
representative point P now proceeds along a damped trajectory 53
and approaches the slow asymptote 51. At the end of this first
cycle the azimuth angle is reduced from 45.degree. to less than
5.degree.. The damping cycles are then repeated until the
oscillatory element is within the allowable error with respect to
the meridian plane. It should be noted that for large angles,
neither the asymptotes nor the damped trajectories are straight
lines. However, this is of no consequence for a qualitative
evaluation of the behavior at large initial angles.
For purposes of illustration, it has been assumed that the damping
ratio .delta.=10. However, this damping ratio may be varied over a
wide range.
The larger the damping ratio becomes, the shorter will be the time
interval required to deprive the gyroscope or other oscillatory
element of the velocity gained during the undamped precession
interval. Thus, in the limit, as .delta. approaches infinity, the
trajectories of FIG. 2 becomes parallel straight lines,
perpendicular to the .psi.-axis, while the slow and fast asymptotes
become coincident with the .psi.-axis and .psi.' -axis,
respectively.
This condition can, of course, not be realized. Furthermore, it
does not appreciably shorten the time of a single cycle. One notes
that the undamped precession interval is now
while the interval along the damped trajectory is zero. Thus, for
the previously considered gyro period of 240 seconds, the time of
the undamped precession interval,
will be 60 seconds, as compared with 54 seconds for .delta.=10,
assuming a negative timing error of 2 seconds. Thus, for four
iterations after an initial damping interval of 12 seconds, the
total saving in time would be 36 seconds.
Although this case is only academic, it indicates nevertheless that
very little time is saved by using even a very large value of
.delta.. This is, of course, due to the fact that most of the time
required for a complete cycle is used up in the more or less
constant undamped precession interval. Furthermore, the power
required for the damping torques increases at least as
.delta..sup.2. Thus, increasing the magnitude of the damping ratio
from .delta.=10 to, say, .delta.=50, would require at least 25
times more power, as well as larger coils, in order to effect a
time saving of approximately 13 percent.
Consider now the case that the damping ratio be reduced from
.delta.=10 to, say .delta.=2.5. This increases the time along the
damped trajectory by about 4 times that for .delta.=10, while the
length of the undamped precession interval reduces to about 44
seconds for .DELTA..tau.=0. The time for a complete cycle including
the initial damping interval is now (4) (12)+ 44=92 sec. This is an
increase of approximately 35 percent over the 68 seconds per cycle
for .delta.=10, which increase would be unacceptable in many
applications.
Thus, one can conclude from both practical and theoretical
considerations that, although no optimum value of the damping ratio
exists per se, an optimum range of the damping ratio may be found
to exist between, say .delta.=5 and .delta.=20, although .delta.
may be any value over 1. The assumed value of .delta.=10, used in
describing the iterative damping process, can, therefore, be
considered representative of this process.
When the meridian-seeking gyroscope is used at different latitudes,
it will be necessary to adjust the timing and damping control so as
to compensate for the change in latitude. However, such
compensation need not be exact; an error of .+-.1.degree. in
latitude can be easily tolerated. The need for the compensation of
the timing control is due to the fact that the period of the
gyroscope is a function of the latitude:
so that in order to avoid an error in the undamped precession
interval, it is necessary to vary the latter with latitude in the
same way as T varies, i.e., instead of making the undamped
precession time interval t.sub.o, constant, one makes it vary in
the same manner as T:
for this purpose, a latitude compensator 25 is provided in the
control system of FIG. 3 which will allow the same proportional
adjustment of each of the individual undamped precession time
intervals depicted in FIGS. 4 and 5. This can be achieved in
several different ways, depending on the type of timing device
employed. With mechanically or electromechanically driven timers,
the speed of the timer may be varied in inverse proportion to the
gyroscope period, i.e. in proportion to cos .lambda., so as to
expand or contract the timing sequence in a corresponding manner.
In the case of an electronic timing device, either a variation in
the electronic time constants, or a variation in the integrator
voltage of a timing device, proportional to the gyroscope period,
may be utilized for the adjustment of the timing control.
Although the damping control system and method have been described
on the basis of employing torque linearly proportional to the
velocity of the azimuth motion, damping torques that are not
linearly proportional to the azimuth velocity may also be used. One
may, for example use a damping torque proportional to the square of
the azimuth velocity
T =C .psi. .psi. (26).
Alternatively, one may use a damping torque which for small azimuth
velocities is predominantly proportional to the velocity, but which
for high azimuth velocities is predominantly proportional to the
square of the velocity. Such a torque is given by
T= C[1+k .psi. ].psi. (27
where the value of the constant k is appropriately chosen so as to
suit the requirements.
Various changes may be made in the details of construction of the
system and in the method without departing from the spirit and
scope of the invention as defined by the appended claims.
* * * * *