The design of a high precision powered hinge is complicated by the unavoidable presence of parasitic drag torque resulting mainly from friction and transfer of power, signals, and fluids across the hinge. Regardless of the type of drive system selected, it is impossible to completely eliminate all parasitic drag. However, the mechanism described here comes very close to providing a drag free system. All sources of parasitic drag torque are collected on a shaft which is powered by an electric motor independent of the main hinge drive. Under control of a sensor, the electric motor applies a compensating torque equal to that of the parasitic drag torque, allowing the main hinge drive to operate in a practically drag free environment with very high positioning precision

Jacquemin, G. G.

Rusk, S. J.

English

NASA Technical Reports Server

DRAG-COMPENSATED, PRECISION-POWERED HINGE SYSTEM
By G. G. daoquemin m and S. O. Rusk m
SL_MARY
The design of a high-precision powered hinge is complicated by the
unavoidable presence of parasitic drag torque resulting mainly from friction
and transfer of power, signals, and fluids across the hinge. Regardless of
the type of drive system selected, it is impossible to completely eliminate
all parasitic drag. However, the mechanism described here comes very close
to pro%id_ng a drag-free system. All sources of parasitic drag torque are
collected on a shaft which is powered by an electric motor independent of the
main hinge drive. Under control of a sensor, the electric motor applies a
compensating torque equal to that of the parasitic drag torque, allowing the
main hinge drive to operate in a practically drag-free environment with very
high positioning precision.
INTRODUCT ION
In the design of robotic arm_, precision pointing systems and other
mechanisms which require very accurate angular positioning, it is necessary
to find methods for minimizing or eliminating the parasitic drag torque. The
presence of parasitic drag torque introduces step functions into the torque-
vs.-displacement curve, the nonllnearity of which is further aggravated by
the effect of static friction. The parasitic drag is difficult to predict.
It is known to vary with angular position_ velocity, load, temperature, and
other variables, especially if wire bundles and flex hoses are routed around
the hinge. If the hinge drive mechanism design includes a gearboy gear
backlash introduces deadbands in _..ich positioning cannot be controlled.
The powered hinge mechanism presented in this paper eliminates gearing and
its associated problems by using dlrect-drive motors, and reduces parasitic
drag torque to that of one lightly loaded ball bearing.
POWERED HINGES - GENERALITIES
Powered hinges can be classed into two broad categories:
i. Deployables (nonretractable)
: 2. Remotely controlled c_inuously adjustable
The deployable systems of category 1 are usually spring-driven with
deployment rates control]ed by adjustable damp_s. Precision positioning in
the deployed position is provided by the lockup mechanism. This type of
powered hinge is of no interest in the following discussion.
The remotely controlled systems of category 2 cover an array of devices
adapted to various degrees of positioning precision, ranging from simple
powered door actuators to robotic arm hinges and high-precision pointing
systems. If high-precision pointing is not a requirement, relatively simple
*Lockheed Missiles & Space Company, _nnyvale, California
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devices can be designed using gear-train reduction drives. Such mechanisms
can sometimes be tolerant of significant parasitic drag torque so that
transfer of power and electric signals can be performed externally by means
of wire bundles or sllp rings and fluids via flexhoses. However, the design
of a hinge system with the high pointing precision of a fraction of an arc
sec. presents difficult problems which cannot be resolved by simple
conventional mechanisms. To meet such requirements, it is necessary to use
more complex electromechanisms.
GENERAL REQUIREMENTS
The hlgh-preclsion powered hinge system discussed in this paper was designed
to meet the following specifications:
1. Maneuver an 8000-1b payload at the end of a 75-in. arm in a vacuum/
zero-g environment.
2. Achieve fine pointing with a 0.50 arc sec. resolution and within
+--2arc sec. of the specified value.
3. Ensure that the Jitter does not exceed:
Freq., Hz. Max. Jitter, arc sec.
0 to 5 0.60
5 to 20 0.50
20 0.20
4. Provide fcr electrical power transfer across the hinge
(two lines).
5. Provide for signal transfer across the hinge (40 channels).
6. Provide for fluid llne transfer across the hinge (four lines)
or Into the hinge systems, if cooling is required.
The pointing precision requirements are not too meaningful when expressed in
terms of arc sac. In order to give a better appreciation of their severity,
they are expressed below in inches at 1 mile from the hinge point.
pointing: +_2 arc sac. = _ 0.62 in. at 1 mile
resolution: 0.50 arc sec. = 0.16 in. at 1 mile
Jitter: 0.60 arc sec. = 0.19 in. at 1 mile
0.50 arc sec. = 0.18 in. at 1 mile
0.20 arc sec. : 0.06 in. at 1 mile
BASIC CONCEPT
To provide a mechanism which will perform with the high precision cor,sistent
with the above specifications, it is necessary to neutralize the parasitic
a.ig torque such that the drive motor effectively operates a drag-free
system. It is also necessary to use DC torque motors in direct drive to
eliminate difficulties inherent with reduction drives and/or stepper motors.
The neutralization of the parasitic torque drag is provided by means of a
drag torque eliminator system (DTES). The DTES senses the presence of drag
torque and applies additional power independently from the main drive motor
in such a manner that the drag torque is balanced out without disturbing the
main drive.
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1985025199-086
lhe _in _]rive is provided by a DC torque motor controlled by an electronic
feedback control system which ensures the appropriate pointing precision.
Sne transfer of electric power Is performed by means of rotary transforme
,]_ng _C current $imJ!arly, the transfer of all electrical slgrals is
_rovided via rotary capacitive couplers. Advantage is therefore taken or the
property of both rotary transJrrmers and capacitive couplers which perform
their functions through air (or vacuum) gaps without physical contact, i.e.,
wlthout parasitic drag torque.
_ASIC HINGE MECHANISM
Figure 1 shows the hinge mechanism'J major components as designed for a
developmer_t prototype. To clarify the basic components of the system, Fig. 2
presents a schematic of the mechanism. It s _uld be noted that since this
d_vice was p_imarily intended for space use, all electric motors are provided
with identical backups for use in case of primary motor failure.
The syste_ is supported by two large self-aligning spherical roller bearings
inserted _n two pillow blocks. These roller bearings are mounted on stub
shafts over conic sleeves which are forced under the inner races. This
e×Fands the inner races radially to eliminate all interna] clearance and
apply controlled radial preloads needed to meet launch reqdlrements. As a
_esult of the preloads, these bearings exhibit a slgniflc_nt drag torque
_h_eh, together with the fluid coupler O-ring friction, prcvide the major
contributions to the system parasitic drag torque. The twc stub shafts are
connected through a large c_.indrical shell as shown in Fig. 1. Supported by
flexure bearings, the stub shaft assembly can rotate through small angles
with respect to the main hinge shaft. These flexures allow a total
diff_rentlal displacement of only +2° with a stiffness of 100-400 in-lb/rad
depending on the thickness of the flexure blades.
]he differential angular displacement between the stub shaft assembly and the
hinge shaft is detected and measured by a sensor as shown in Fig. 2. This
sensor controls the operation of a twist motor which adds sufficient torque
to the stub shaft assembly to counteract all parasitic torque drag collected
by the _tub shafts. Thus, the primary hinge drive motor operates in a
torque-free environment, thereby ensuring the desired hinge positioning
accuracy.
: The misalignment coupling in Fig. 2 is intended to provide enough compliance
to accommodate manufacturing tolerances in shaft alignment at that point.
T_is coupling allows for bending and for a_lal and lateral mismatching.
However, itJ torsional compliance is not significant.
PARTIAL ANALYTICAL MODEL
In order to more clearly describe the operation of the powered hinge, it was
found convenient to develop a simplified analytical model in which the
masses, inertias, and details of the electronic loops were left out. This
model is shown in Fig. 3.
It is shown in thls system that a displacement provided by the drive motor
acts directly upon the output branch and indirectly upon a parallel branch
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1985025199-087
which, by means of a detector and a feedback control system, provides power
to cancel out all parasitic drag torque.
An examination of the mechanism in Fig. i shows that some parasitic torque
drag must necessarily exist in both branches of the system. However, by
careful design, only one small ball bea_ing is left in the output branch to
ensure proper alignment of the encoder wheel. In zero gravity, the torque
drag of this bearing is expected to be at or near the noise level of the
branch. All ether sources of parasitic drag torque are collected on the
other branch to be sensed by the drag torque eliminator system. These
sources of parasitic torque incl,_de that of the main roller bearings, the
smaller ball bearir_s, and any external torque drag (such a_ that of the
rotating fluid coupl_ngs).
In operation, the position sensor detects an angular displacement _6 and
sends a signal to the twist motor. This signal closes tne loop mechanically
by rotating the stub shafts to cancel the displacement, thereby eliminating
the drag torque.
The response characteristics of this control system are not addressed in this
paper; however, the system is designed to function much faster than the
primary drive system to ensure a very small lag angle _6 and the intended
drag-free operation of the powered hinge.
ACTIVE FLEXURE
The operation of the flexure is shown in Fig. 4, which {llustrates one cycle
of motion. The flexure consists of two concentric hollow shafts held
together by thin flexible blades mounted as shown in _ig. 4-1. The tw_
shafts have a small radial clearance such that large lift-off and landing
radial and axial loads transmitted from the inner main shaft to the outer
stub shaft can be taken by direct contact of the two shafts. The radial
clearance between the two shafts is small enough that the flexure blades are
not unduly stressed during static load. This clearance is also selected to
ensure that no contact is made between shafts in the normal zero-g operation.
As can be seen in Fig. 4-1, the main shaft (output) is inside the stub shaft
(drag eliminator system). Tne main shaft is eonnecte_ to the main motor; the
stub shaft is connected to the twist motor and runs with the drag-producing
roller bearing.
When the main motor is activated, the main shaft rotates. Fig. 4-2 shows the
small rotation _ , which brings the flexures almost in contact with the edges
of the main shaft windows. Th_ sensor, upon detecting this misalignment,
energizes the twist motor, which rotates the stub shaft and restores a n_ll
position at the displacement angle _ , thereby driving the roller bearing and
any other drag-producing components which may be connected to the stub shaft.
In a practical application, the angl, _ is made as small as possible; in
this instance, 1 are sac The lag of the stub shaft over the main shaft is
not perceptible, i
1985025199-088
EFFECT OF PARASITIC DRAG TORQUE ON SYST_ RESPONSE
In order for this system to perform with the expected efficiency, the torque
required to deflect the flexures : _st be as low as possible. This
requirement implies a combination of low spring rate and small angular
displacement. Bottoming out of the flexure is highly undesirable. Figure 5
shows a representation of a typical torque-vs.-displacement curve for a
flexure where the bottoming angle is +2 °. In a well-designed system, the
sensor must be capable of detecting a displacement of i arc sec. so that the
flexures remain virtually undeflected with no parasitic torque being prodoced.
The residual torque along the output branch (see Fig. 3) must be very small,
slnce it will not be eliminated.
All other sources of internal and external drag torque are applied to the
"Eliminating" branch (Fig. 3) to avoid disturbing the output branch. The
disturbance introduced by parasitic drag torque is shown in Fig. 6, which
compares the displacement under the same torque of a one-degree-of-freedom
system with and without friction drag. This plot shows how troublesome the
effect of static friction is. In order to obtain a displacement, it is
necessary to apply a torque at least equal to that produced by the static
friction. However, as soon as the torque corresponding to the static
friction is reached, the motion starts and the friction coefficient drops to
the lower value corresponding to the dynamic condition. The system then
jumps to a position of equilibrium, such as 61 . By comparison, a
friction-free linear system with the same stiffness and under the same torque
would reach its equilibrium position at 02, going in a controlled manner
through all intermediat_ angles as shown by the straight line passing through
the origin.
Since friction is the major contributor to the paraz[tic torque drag, it is
clear that a high-precision hinge cannot be designed unless it can be
operated in a friction-free manner.
SOURCES OF PARASITIC DRAG TORQUE
In the general hinge system, sources of parasitic drag torque are to be found
in the following components:
i. Main bearings (rollers or ball bearings)
2. Auxiliary motors bearings (ball bearings)
3. Encoder bearing (ball bearing)
4. Power line transfer system
5. Sisals transfer system
6. Fluid lines transfer system
In the powered hinge discussed here, only Item 3, the encoder bearing,
remains effective as a minor source of parasitic drag torque. Item_ 4 and 5
are transmitted by induction and capacitive coupling through r gaps (or
vacdum gaps) while the remaining three, Items i, 2, and 6, elJminatea by
the twist motor feedback loop.
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1985025199-089
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' CONCLUDING RF_ARKS
A full-s_ze model of this powered hinge has been designed and built for test
purposes. This model (Fig. ?) meets the requirements specified in this
paper. It has been successfully subjected to qualitative running tests of
the main drive motors without any drag relief.
Precision pointing tests have not been carried out at this stage because of
delays in the design of the electronic feedback systems. However, analytical
simulation techniques have shown that excellent controlled performance could
be achieved at any selected angle. It should be noted that the hinge
rotation angle is not limited, it can be a fraction or any number of
revolutions.
In its _-esent configuration, this precision powered hinge is a large device
adapted to 2erform the functlon_ represented by its specifications. Its
large size wa_ convenient for prototype fabrication, development, and
testing. Application to smaller devices will require changes in its
configuration to accommodate more severe space restrictions.
In addition to obvious aorospace applications, the principles of this
mechanism should be adaptable in miniaturized form to robotics systems and
other devices.
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Drag-compensated, precision-powered hinge system

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