Adjustment of zero spring rate suspensions

An adjustment system for a suspension system having the property of providing a zero spring rate response to limited vertical excursions from a reference level, the suspension system including a load suspension spring supported from a load spring anchor. The adjustment system includes a rigid reference element below the suspension spring, bi-directional drive means between the load spring and the load spring anchor, and a probe insertable into an aperture in the reference element. Circuit means are responsive to the location of the probe relative to the reference element, to adjust the elevation of the load spring relative to the reference level and thereby to tend to restore the reference element to the reference level.

FIELD OF THE INVENTION 
This invention relates to spring suspension systems which can exhibit zero 
spring rate response, and in particular to a device to adjust the system 
to a configuration which will provide this property. 
BACKGROUND OF THE INVENTION 
The dynamic testing of bodies intended for use in a non-gravitational 
environment is greatly complicated by the fact that the testing must be 
accomplished on earth where there is a gravitational field. Unfortunate 
experiences have shown that dynamic responses in space can differ greatly 
from those which are encountered on earth. Still, space is not the place 
to conduct interim dynamic testing of basic structures. It is far too 
expensive and is likely to provide only a single test which might not 
provide the necessary information. Instead what is required is a support 
structure which within sensible limitations of dynamic and dimensional 
ranges, and cost, can provide support for a body under test which will be 
the equivalent of the body's response to forces exerted outside of a 
gravitational field. 
Evidently the body under test must be physically supported, and it will be, 
by means of a spring suspension. However, this suspension is so 
constructed and arranged that, while still supporting the body, it 
exhibits a zero spring rate response to limited movements of the body. 
This characteristic provides behavior of the total system such that an 
absence of gravitational forces is simulated. 
The zero rate response is well-known in the fields in which it is of 
interest, and there are at least several basic schemes for attaining this 
result. The inventor herein does not claim to be the inventor of any zero 
rate system, including the one described herein. 
The problem with all of these systems is that the range of movement within 
which the zero rate effect exists is quite short. The inventor herein has 
been able to extend this range to a total movement of about 2.0 inches. 
Accordingly, to take advantage of this property of the system, it is 
important that, under the intended test load, the suspension be adjusted 
so the system is in a central datum position, preferably within a few 
thousandths of an inch of its very center. 
These systems are expected to carry substantial loads. An installation with 
perhaps 30 of these suspensions, each supporting 100 to 750 pounds of 
weight, is well within the expected parameters of such installations. 
Because there is zero spring rate at the datum position, the support 
condition can gently and very slowly drift. For this reason, adjustment is 
a slow and painstaking process which requires hours, and sometimes even 
days. At least to applicant's knowledge there does not exist any automatic 
and reliable means to accomplish this painstaking process. 
It is an object of this invention to provide adjustment means for a zero 
spring rate system which is automatic in the sense of providing means to 
hunt for a center adjustment. Clearly the adjustment system itself cannot 
involve a springing response, or the system would have no tendency to 
stop. This invention intends to provide an adjustment means devoid of 
springing response. 
BRIEF DESCRIPTION OF THE INVENTION 
An adjustment system according to this invention is used in combination 
with a zero spring rate suspension system. The suspension system includes 
a load spring which suspends the body to be tested. The extension of the 
load spring is in accordance with Hooke's Law, and is generally 
proportional to the weight supported because of its inherent property of 
spring rate (spring constant). It is important to observe that this 
extension must and does occur. This system does not render the body 
weightless. Instead, within a given and very limited amplitude range, it 
will exhibit the property of zero spring rate response to vibrational and 
other forces exerted on the body under test. The zero rate system includes 
a response spring which in combination with the load spring produces the 
zero rate response. 
It is further evident that because of the extension of the supported load, 
any datum point in the spring system in which zero rate response is to be 
expected must relate not only to the suspended weight, but also to the 
relative location of the reference level of the centered position at which 
the appropriate response is to be exerted. 
Further, especially when the suspended weight is part of a very large body, 
such as a scale model of a space vehicle, the suspending system must be 
located with reference to the geometry of the test body. The suspension 
system and the zero rate provisions must themselves be spatially 
adaptable, and finally mutually adjustable to provide the appropriate zero 
rate response. 
According to this invention, the response spring system has a null position 
where it is ineffective on the support of the load. When a datum point on 
the load spring system at equilibrium is coincident with the null 
position, the zero rate system will also be in full equilibrium. When the 
load is varied, such as by vibrational forces exerted on the suspended 
body, the load spring system will be expected to respond to what it 
perceives to be an increase or reduction in the suspended weight, and its 
response will involve the spring rate of the load spring. The response 
spring system will then intervene, for the purpose of eliminating the 
influence of spring rate, and this is effective within a short range of 
amplitudes. This invention is not involved in this response. After the 
system is initially adjusted, the adjustment system is passive. In fact, 
it is withdrawn from the active system. Its sole function is, before 
dynamic operation, to adjust the relative positions of the datum point on 
the load spring system to the null position of the response spring system, 
so the zero rate system where the test starts is within its zero rate 
range. 
According to a preferred but optional feature of the invention, a reference 
element is included in the suspension system depending from the load 
spring. The spring itself is above it, and the load is below it, so that 
its elevation relative to an upper anchored end of the load spring is a 
function of the weight of the suspended load. 
A response spring system has a reference level at which it does not exert a 
prevailing force along the upright Y axis of the suspension system. The 
response system has an anchor supporting an anchored end at this reference 
level. 
Motor means is provided to move one or the other of the anchored ends 
vertically so as to bring the reference level and the reference element 
into coincidence. A probe is movable along the reference axis into and out 
of the path of the reference element. The probe is withdrawn during system 
operation. It is inserted as part of the adjustment procedures. 
In the illustrated preferred embodiment the probe has an upper and a lower 
arm with rigid contacts which make contact with the reference element at 
the extremes of upper and lower zero effect movement of the reference 
element, which of course are the limits of this effect for the system. 
When contact is made between the contact and the reference element, the 
motive means is caused to operate, to cause one of said anchored ends to 
move relative to the other so as to tend to restore the coincidence of the 
center of the reference element and the reference level. After a few 
cycles of this type, the system will generally be in sufficient 
equilibrium that the probe can be withdrawn and the testing cycle can 
begin. 
According to a preferred but optional feature of this invention, the probe 
includes a load bar which remains in the path of the reference element, 
but does not impede its normal operation. It will, however, restrain the 
system against excessive amplitude movements which can occur when the 
amplitude of the load spring goes beyond the zero range. It is also of 
assistance while making an initial setting of the system.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows an example of a zero rate response support system 10 useful 
with the invention. Its objective is to support a load 11 from supporting 
structure 12 (sometimes called a "load spring anchor"). This may be such 
as a stanchion or a framework (an "anchor") supported from ceiling 
structure. Also, it could be a part of a frame which itself is supported 
by a stanchion or by other structure. 
In any event the load is connected to a cable 13 or other tension-type 
element, which in turn is connected to the lower end of a load spring 14. 
The upper end (the "anchored end") of the load spring is connected to 
adjustment means 15, preferably comprising a rotary electric motor 16 
which bi-directionally drives a lead screw 17. The motor is fixed to the 
load spring anchor (structure 12). 
Rotation of the motor in one direction will raise the upper, anchored, end 
20 of the load spring. Rotation in the other direction will lower the 
upper end of the load spring. Thus it is apparent that when a load is 
suspended by the load spring, operation of the motor will raise or lower 
the load without changing the extended length of the load spring. It is to 
be kept in mind that the load is freely suspended. 
A response system 25 has a response system anchor 26 which is most 
conveniently mounted to the same structure as the load spring anchor. One 
of the anchors, usually the load spring anchor is vertically movable 
independently of the other anchor along the vertical (y) axis 27. A 
response spring 28 in the response system is an adjustable-tension spring, 
appropriately pre-loaded in tension, connected at one of its ends 29 (its 
"anchored end") to anchor 26. 
A rigid response lever 30 is pivotally mounted to anchor 26 by a hinge 31. 
The hinge is spaced from end 29 of the response spring along the 
horizontal (x) axis. 
The other end 32 of the response spring is connected to lever 30, spaced 
from the hinge. This arrangement provides a toggle-type lever system which 
is stable only when end 32 of the lever is on a line which also extends 
through end 29 of the spring and through hinge 31. The line through 29 and 
31 is defined as the "reference level 35" and extends along the (X) axis. 
When the lever is on either side of this level, the response spring exerts 
a force to move it up or down by exertion of a component of force along 
the (y) axis, biased to do so by the response spring. 
A reference element 40 is carried by cable 13, and is pivotally connected 
to the response lever at a location between the hinge and the point of 
attachment of the response spring to the response lever. Thus, up and down 
movement of the reference element (as the consequence of a varying load on 
the cable, for example), will cause the lever to move up and down. 
In FIG. 1, the response system has been shown off of its centered, 
reference level condition for purposes of illustration. When the system is 
optimally adjusted for a given load, the end 32 of the response lever will 
be coincident with the reference level, because that will be the center of 
the zero spring rate response range. If the lever is a straight member it 
will be horizontal. The operation of the device as described will be 
recognized by persons skilled in the art and will not be further described 
here, because an understanding of this function is not necessary to an 
understanding of the invention. 
It is obvious that when the system is loaded with loads of various weight, 
an initial gross adjustment of the vertical spacing between the upper end 
of the load spring and the reference level is useful but not necessary. 
Such devices as screws and the like could be used for this purpose. 
However, in most cases these will not be provided. Instead the system will 
gradually be brought into adjustment by this system even though it may 
take considerable time to do so. Ultimately, the purpose of this invention 
is to provide a very fine adjustment to place the suspension system in a 
carefully defined condition within the zero spring rate response range, 
and this can be accomplished for large bodies by the simultaneous 
operation of an adjustment system for each suspension. 
This invention accomplishes this objective by controlling the operation of 
motor 16 so as to raise or lower the reference element with the load 
already applied. In the illustrated embodiments this is accomplished by 
raising or lowering the anchored end of the load spring. It should be kept 
in mind that this system is totally passive once the system is adjusted. 
It has no part in the actual test operations. 
As best shown in FIGS. 2-4, the reference element 40 has an aperture 50 
with a vertical dimension 51, an upper edge 52 and a lower edge 53. The 
length of the vertical dimension will be discussed below. It is shown 
formed as a part of the continuing support of the load, although it could 
if desired be formed separately from it and be attached to it. 
The response system anchor or frame supports four guide wheels 55 which 
support rails 56,57 on opposite sides of probe 58. The rails form part of 
the response system anchor 26. The probe is centered on the reference 
level, and is movable laterally along horizontal X axis 59 (sometimes 
called a reference axis). A probe motor 60 is drivingly connected to the 
probe to move it bi-directionally along the horizontal axis. 
The probe is a rigid body 65 having an upper edge 66 and a lower edge 67. 
These edges are preferably parallel. They reduce to a leading end 68 from 
which a load bar 68a projects. 
The edges 66 and 67 carry contacts 69,70, which are electrically separated 
from the body by insulators 71,72. These are rigid, non-springing 
electrical contacts adapted to make surface-to-surface contact with 
respective edges of the reference element. The reference element provides 
an electrical ground. The contacts form respective parts of a circuit to 
control motor 16 shown in FIG. 5. 
FIG. 2 shows the system in its ideally adjusted position. The vertical 
spacing between the edges of 52 and 53 (dimension 51) of the reference 
element is generally between about 0.004 and 0.006 inches greater than the 
spacing between contacts 69 and 70, and is evenly divided on both sides 
when the system is optimally adjusted. However massive this system is, 
this adjustment device is intended to make this class of very small 
adjustment in order to make maximum use of the very relatively small 
amplitude available in which zero spring rate response is provided. As can 
be seen, this is a very close adjustment of a suspension system which can 
support a significantly heavy load. 
FIG. 5 illustrates the upper and lower edges 52 and 53 of the reference 
element 40. The reference element is conductive at the edges, and as will 
be seen, these edges constitute a rigid conductive surface to be 
non-springingly contacted by respective probe contacts. When the 
suspension system is too low, lower contact 70 will contact lower edge 53. 
When the suspension system is too high (or rather the system condition as 
defined as the reference element's elevation relative to the reference 
level), upper contact 69 will contact upper edge 52. In either event, such 
contact will energize motor 16 to drive it and also drive lead screw 17 to 
move the anchored end of the load spring in the direction to move the 
center of the reference element toward the reference level. Such motor 
operation will continue for as long as the contact is made. It is not 
desired to give the system a heavy driving impulse, because it is better 
for the suspension system to drift into final adjustment, which is within 
a very narrow dimension range, only a few thousandths of an inch. For this 
reason the motor is heavily geared down, and the adjustment is very 
gradual. 
A power supply 79, preferably 24 volts D.C., has a ground lead 80 which 
connects to the reference element, or more properly to both edges of it 
where contact will be made. 
A hot lead 81 has a first branch 82 which incorporates a normally open 
start switch 83. Branch 82 further connects to a motor down relay 85 and 
to a motor up relay 86. Specifically it connects to solenoid coils 87 and 
88, and to normally open latch switches 89, 90 of these relays. 
Hot lead 81 has a second branch 91 which connects the opposite sides of the 
latch switches. 
Leads 95, 96 connect respective solenoids to respective probe contacts 69 
and 70. It will be seen that closure of the start switch will complete a 
circuit from ground lead 80 through any contact which makes contact with 
an edge, through the solenoid, to actuate the respective relay. Actuation 
of the relay, by closing the latch switch, will hold the system in 
operation until contact is broken at the respective reference edge, even 
after the start switch has re-opened. The start switch will always be 
spring-loaded open unless momentarily closed. 
Motor 16 has two power leads 100, 101. The direction the motor will be 
driven is determined by which of these leads is connected to the hot lead. 
The other is of course connected to the ground lead. 
Application of power and selection of direction of drive is accomplished by 
the relays. The relays have a pair of power switches 102, 103 and 104, 
105. It will be observed that one side of all of these switches is 
connected to second branch 91 of the hot lead. The other sides of these 
normally-open switches are suitably connected to power leads 100 and 101. 
The power switches and latch switch of each relay are ganged for 
simultaneous opening and closing under the control of the respective 
solenoid. 
The circuit operation is straightforward. With the probe in place as shown 
in FIG. 2, and in its centered position, closure of the starting switch 
has no effect on the system. There is no completed circuit through a probe 
contact. 
If, however, the system is sufficiently off center, then a contact will be 
made. Still nothing happens until the start switch is closed. When it is, 
a circuit will be completed through the solenoid of the respective power 
relay. This will close all of its switches. Then a circuit for the 
solenoid continues to be made through the latch switch even after the 
start switch is opened. 
Power is supplied to the motor, and the load spring anchor is moved until 
contact is broken at the probe. This will drop out the coil and will break 
the circuit at all switches. If the system moves to contact the other side 
of the probe, the action will be in the reverse direction under control of 
the other relay if the start switch is closed again. Soon the system 
should be in equilibrium. 
Then the probe will be withdrawn, and the suspension system can move 
without restraint by the probe. It is, however, convenient for the load 
bar to be left in place. It is thin enough not to reduce the useful 
amplitude, but strong enough to restrain the system from excessive 
movement or support the entire test load. It is optional. 
The probe is moved by the probe member either manually or by the probe 
motor under control of a separate power circuit (not shown) which can 
drive the probe motor bi-directionally. 
There are other suitable circuits for control of the system, so the 
illustrated system is given by way of example, and not of limitation. 
Similarly, there are other useful zero spring rate response systems, so 
the illustrated system is also given by way of example and not limitation. 
This invention provides a useful means to adjust a zero spring rate 
suspension system within a very narrow range. The adjustment system is 
simple and rugged, and enables the suspension system to be optimally 
adjusted. 
This invention is not to be limited by the embodiment shown in the drawings 
and described in the description which is given by way of example and not 
of limitation, but only in accordance with the scope of the appended 
claims.