In the frame work of the project Shuttle Plume Impingement Flight Experiment (SPIFEX), a Load Measurement System was developed and fabricated to measure the impingement force of Shuttle Reaction Control System (RCS) jets. The Load Measurement System is a force sensing system that measures any combination of normal and shear forces up to 40 N (9 lbf) in the normal direction and 22 N (5 lbf) in the shear direction with an accuracy of +/- 0.04 N (+/- 0.01 lbf) Since high resolution is required for the force measurement, the Load Measurement System is built with highly sensitive load cells. To protect these fragile load cells in the non-operational mode from being damaged due to flight loads such as launch and landing loads of the Shuttle vehicle, a motor driven device known as the Load Cell Lock-Out Mechanism was built. This Lock-Out Mechanism isolates the load cells from flight loads and re-engages the load cells for the force measurement experiment once in space. With this highly effective protection system, the SPIFEX load measurement experiment was successfully conducted on STS-44 in September 1994 with all load cells operating properly and reading impingement forces as expected

Carroll, Monty

Le, Thang

Liu, Jonathan

English

NASA Technical Reports Server

N95-27291
Load Measurement System
With Load Cell Lock-Out Mechanism
Thang Le*
Monty Carroll"
Jonathan Liu*
Abstract
In the frame work of the project Shuttle Plume Impingement Flight Experiment
(SPIFEX), a Load Measurement System was developed and fabricated to measure the
impingement force of Shuttle Reaction Control System (RCS) jets. The Load
Measurement System is a force sensing system that measures any combination of
normal and shear forces up to 40 N (9 Ibf) in the normal direction and 22 N (5 Ibf) in the
shear direction with an accuracy of :L-0.04 N (i-0.01 Ibf). Since high resolution is
required for the force measurement, the Load Measurement System is built with highly
sensitive load cells. To protect these fragile load cells in the non-operational mode
from being damaged due to flight loads such as launch and landing loads of the Shuttle
vehicle, a motor driven device known as the Load Cell Lock-Out Mechanism was built.
This Lock-Out Mechanism isolates the load cells from flight loads and re-engages the
load cells for the force measurement experiment once in space. With this highly
effective protection system, the SPIFEX load measurement experiment was
successfully conducted on STS-64 in September 1994 with all load cells operating
properly and reading impingement forces as expected.
Introduction
The Space Shuttle is maneuvered in space by RCS jets. These jets can produce
significant plume impingement forces on surfaces that may be in the path of the jet
plume. Among these surfaces are the solar arrays on the planned International Space
Station Alpha or the Russian Mir Station. Due to the uncertainties of the analytical
plume force predictions of the Orbiter RCS jets in the vacuum of high Earth orbit and
the lack of direct empirical measurements, a project called SPIFEX was created.
The SPIFEX project consists of a 0.343-meter (13.5 in) diameter by 10-meter (32 ft)
long boom with an avionics package and plume sensor array attached to one end. In
operation, the Orbiter Remote Manipulator System (RMS), robot arm, grapples the
other end of the boom providing power and communication and positions the plume
sensor array over an RCS jet for a data point firing. The plume sensor data is recorded
in the SPIFEX avionics for later downloading.
*Mechanical Systems Laboratory, LockheedEngineering& Sciences Company,Houston, TX
381
https://ntrs.nasa.gov/search.jsp?R=19950020871 2020-06-16T07:17:41+00:00Z
A primary design factor for this project was minimizing cost. This factor led to no
redundancy in the SPIFEX operational avionics or mechanisms. Three motor-driven
mechanisms were used on SPIFEX: (1) a Two-Axis Drive to finely position the sensor
array at the end of the boom, (2) a Lock-Down device to secure the Two-Axis Drive and
sensor array for Space Shuttle launch and landing, and (3) a Load Cell Lock-Out
Mechanism to isolate the Load Measurement System from Space Shuttle launch and
landing loads. The Load Cell Lock-Out Mechanism is the topic of this paper.
Lock-Out Control System
The Lock-Out mechanism of the Load Measurement System is driven with a low-power
DC gearmotor and with the end-to-end travel controlled by limit switch feedback to the
SPIFEX avionics. Due to limited 28-VDC power allocation for SPIFEX, a low-power DC
gearmotor with a high 4126:1 gear ratio and 13.5 N-m (120 in-lbf) peak torque
permitted the use of a small 28-VDC relay drive with less than 1-Amp capacity. This
combination provided a reasonable operation time of 15 seconds for the Load Cell
Lock-Out to open or close. A DC motor was chosen to drive this mechanism because
of the simplicity of the low-level control system and the simple open/close state of the
mechanism. A high-level means of controlling the Lock-Out mechanism was provided
from the SPIFEX avionics through a serial link to a Space Shuttle crew cabin laptop
computer.
Load Measurement System
The purpose of the Load Measurement System is to measure the magnitude and to
determine the direction of the plume force once the Load Measurement System is
placed in the plume field. The Load Measurement System consists of two major
components: a flat panel to catch the jet plume and an array of load cells to sense the
plume force. The panel is called the Load Plate, and the load cell array is known as
the Load Ring (Figure 1).
The Load Plate is a rectangular surface that provides a flat area to capture the
impingement of the RCS jet in the plume field. The Load Plate has a honeycomb
structure of aluminum. Woven graphite sheets are used for the plate close-outs for all
sides. This method of construction provides the necessary stiffness for the Load Plate
while minimizing its mass. The Load Plate minimal mass is required so that the Load
Measurement System can be responsive to the brief plume forces. A natural frequency
of 100 Hz or higher is required for the plate. The light-weight plate also prevents the
load cells from being subjected to strenuous forces during the non-experiment phases.
The Load Ring has six uniaxial strain gage load cells arranged in a circular pattern.
The load cells are installed with a universal joint at each end to form six struts between
the Load Plate and the base structure of the Load Measurement System. This
arrangement permits the Load Plate to have 6 degrees of freedom (DOF) movement
382
,,,,-
with respect to the base structure when lengths of the strut vary. In other words, once
a force is applied on the Load Plate, the infinitesimal movement of the Load Plate can
be detected by the reaction forces in the six struts. The reaction forces are the actual
compression or tension forces sensed by the individual load cells within the struts.
I
LOAD PLATE
_LOAD RING
Figure 1. Load Measurement System
The universal joints at the load cell ends prevent a bending moment from being
transferred and allow nothing but strictly axial load through the load cells. The six
struts with universal joints form a statically determinant structure from which the total
force exerted on the Load Plate can be derived from the individual axial forces in the
load cells. Thus, the input force can be computed from the force readings in the six
383
load cells to yield three force components and three moment components of the
applied force.
The Load Ring and the Load Plate are held together by several structural members as
illustrated in Figure 1. A majority of these members are made from aluminum because
of its light weight. A few members are made of stainless steel where large deflection is
not desirable.
Force Calculation
To calculate the external force, __, ]L__,El,, and moments, MX, MY, ME,, acting on the
Load Plate, the six load cell force readings are decomposed into component x-y-z
forces using the x-y connection position of the universal joints and orientation of the
load cells. The component forces are then summed into composite X-Y-Z forces acting
on the Load Plate. The composite X-Y-Z moments acting on the Load Plate are
calculated by summing the component forces times the distance from the axis of
interest. The axis of interest is defined by the offsets, Xeff, Yoff, Zoff, from the Load
Measurement System origin. The origin (0,0,0) is defined at the x-y center of the circle
of load cell struts and the x-y plane that passes through the load cell upper universal
joint pivot points. The resulting matrix equation is as follows:
FX FXoff
FYI FYoff
FZ FZoff
•"T.F -
MX MXoff
MY MYoff
MZ MZoff
where F is the vector of six force readings of the load cells, and T is the constant matrix
describing the Load Ring geometry. The matrix equation also shows where a static
force or moment offset can be removed from the result. Refer to the notes at the end of
this paper for the complete matrix derivation.
Load Cell Lock-Out System
The purpose of the Load Cell Lock-Out System is to provide an alternate load path for
the system so that the force passing through the load cells from the Load Plate to the
base structure can be minimized during the harsh vibration phase of launch and
landing of the Shuttle vehicle. During this vibration period all load cells are isolated, or
locked, for protection. The load cells then can be returned to the unlocked
configuration for operation during the load measurement experiment.
384
The Load Cell Lock-Out system is an over-center mechanism driven by a DC gear
motor (Figure 2). The upper end of each load cell is connected to the Load Plate via a
universal joint. The lower end of a load cell is connected to a linkage, also, through a
universal joint. In the operational, unlocked position, the linkage is spring loaded
against a hard stop (Figure 3). The Load Measurement System is then subjected to
plume forces for force measurement in this configuration, and all six load cells act as
the primary load path from the Load Plate to the base structure.
)3 REO' D
REO'D
'D
'D
(_3 REO'D
®
3 REO'D
3 REO' O
2
2 REO'D
2 REO'D
6 REO'D
REO'D
REO'D
R£O'O
®
)3 REO' D
REO'D
3 REO'D
Figure 2. Load Cell Lock-Out Mechanism
Once in the non-operational locked mode, a set of three hooks reach up and clamp
down the Load Plate to rigidize the plate to the base structure. In this configuration, the
linkage at the load cell lower end compresses the spring to separate the load cell lower
385
end from the hard stop to minimize the load transferred through the load cells. The 
clamping hooks act as the primary connection for load transferred from the Load Plate 
to the base structure. When the Load Measurement System is returned back to the 
operation mode, these clamping hooks are completely separated from the Load Plate 
so that only the load cells are subjected to the energy transfer. 
LOAD  
PLATE I d  
HOOK 
0 
BASE 
STRUCTURE 
LOAD CELL 
STRUT 
MOTOR DRIVEN f $' 5 ° F  T 
UNLOCKED CONFIGURATION 
r 
LOCKED CONFIGURATION 
Figure 3. Load Cell Lock-Out System Schematic 
Furthermore, the linkage springs are preloaded in such a way that when the load cells 
are compressed in the locked position, the spring compression force is equal to the 
load cell full-scale capacity. Therefore, the load cells measuring capability is 
maximized. The linkage spring system also provides a means of overload protection 
during the operation mode should the magnitude of the applied force exceed the spring 
preload. In this case, the spring is compressed, and the force magnitude passing 
through the load cell is confined. 
386 
All linkages and pivot pins in the drive system are made from stainless steel for higher 
strength. The DC gear motor has a high-ratio gearhead and is deactivated by limit 
switches once the mechanism is in a complete locked or unlocked state. Additional 
features include adjusting screws and a pip pin. Six spring adjusting screws allow the 
fine tuning of the spring preload at the linkages. Another six position adjusting screws 
provide the system with a means of alignment for the orientation of the Load Plate with 
respect to the base structure. The pip pin is used for quick disengagement of the motor 
from the shaft. This disconnection permits the mechanism to be manually actuated as 
desired using a standard 7/16 hex wrench. 
System Performance on STS-64 
The SPIFEX mission on STS-64 collected data for as many as 105 test points. Force 
measurement data were recorded in terms of voltage readings by the load cells and by 
their interpreted axial force in the individual load cells. All load cells were locked prior 
to payload installation and were not unlocked for the experiment, in orbit, until about 
eight weeks later. During the experiment, all six load cells were operational and 
reading forces properly. With the force magnitudes in all six load cells recorded, the 
composite force applied on the Load Plate can be calculated. A typical composite force 
of normal direction to the Load Plate is illustrated in Figure 4. The rectangular Load 
Plate has a dimension of 0.33 m by 0.51 m (1 3 in. by 20 in.). The force exerted per unit 
area can be easily obtained. 
0.04 0.01 
newton Ibf 
0.02 
( D o  
I 4.02 
0 
E 4.04 0 z 
8 4.06 
C 
0 a 4.08 
(D 
7 
B 
4 .1  
0 g 4.12 
. 0.005 
' 0  
4.005 
4.01 
4.015 
4.02 
4.025 
4.14 ' 4.03 
Tim e 1 second 
Figure 4. Typical Normal Composite Force Response 
381 
Conclusion 
The design work of the Load Measurement System was begun in June 1992. The 
system design was then presented at the SPIFEX Preliminary Design Review in 
October 1992 and at the SPIFEX Critical Design Review in July 1993. Fabrication of 
two complete units (one qualification unit and one flight unit) was completed in March 
1994. Photographs of the unit prior to installation on the SPIFEX boom are shown in 
Figure 5. The system was flown on the OV-103 (Discovery), STS-64 mission, in 
September 1994. 
The Load Measurement System has successfully collected plume force data. All load 
cells endured the harsh environment of launch and landing conditions and were 
completely functional during flight and postflight. Preflight and postflight calibrations of 
the system indicate no degradation of the force measurement capability and its 
sensitivity. This fact illustrates that the load cells were efficiently protected by the Load 
Cell Lock-Out System and its Motor Controller. 
The design of the 6-DOF force measurement platform utilizes only uniaxial load cells to 
measure all three force and three moment components as opposed to using complex 
multiaxis load cells. This concept can be adopted for various force torque 
measurement applications. The Lock-Out Mechanism idea can also be used to protect 
fragile instruments under severe conditions to ensure their functionality during the 
operating mode. 
Acknowledgments 
The authors wish to thank the SPIFEX team, which provided valuable support 
throughout the project. 
388 
Figure 5. The Load Measurement System with Lock-Out Mechanism 
389 
Notes - Load Plate Force Derivation
The general equations to calculate the forces acting on the Load Plate can be
determined given the following Load Ring geometric properties:
L
P
(z
r
PU, PL
i
= length of load cell strut between pivot points.
= angle of load cell relative to x-y plane.
= angle of load cell relative to x-axis.
= radius of load cell upper pivot points.
= upper and lower universal joint x-y pivot points.
= number of struts (6).
First, the angle of each load cell strut projected into the x-y plane is calculated relative
to the x-axis.
_ti := angle (PUx- PLx,PUy i - PLYi) (1)
Once the x-axis orientation of each strut is known including the Load Ring geometric
properties, the force, F, from each load cell is broken down into component x-y-z forces
and then summed to get the composite forces. The moments acting about the axis of
interest on the Load Plate are calculated and then summed to get the composite
moments. Any initial force or moment offset is also removed from the result. The
equations are given below.
Fx=F..cos(p).cos(_z_),,
WYi--Fi.ex)s ( _ )'sill ((Zi)
Fx=Ff sin(p )
Mxi=Fz" (PUy i - Yofl) + Fyi.Zoff
MYi--(-Fz)f(PUxt- Xoff)- Fx.Zoff
Mzi=FY f (PUx t - Xoff)- Fxt. (PUy i - Yoff) (2)
FX= Z Fxi - FXoff
(i= l.. 6)
l"l
FY-- L FYi- FYoff
0=1-6)
'¢"-!
FZ-- 2.a Fz- FZoff
(i==l.. 6)
MX= Z Mx,-MXoff
(i=l.. 6)
%"'!
MY= 2.a MYi- MYof
(i=l.. 6)
MZ= 2.a Mzi- MZoff
(i=l.. 6) (3)
390
To determine the forces, ]B', on each of the load cells, the equations (2) and (3) are
combined to form six simultaneous equations (4).
FX- Z Fi'c°s(_)c°s (ai)- FXoff
(_]..6)
FY- Z Fi'c°s (_) sin (ai) -FY°ff
(iml.. 6)
FZ= Z F..sin(l_)t - FZoff
(i_l.. 6)
(i-- 1.. 6)
(4)
The six equations in (4) are then placed in the following matrix format.
FX
FY
FZ
MX
MY
MZ
=T.F-
I FXoff
I FYoff
FZoff
MXoff
MYoff
MZoff (5)
391
Where the constant matrix T in (5) is defined as
TI, i := °°s(p)'c'°s (ai)
T2, i : = c'°s (I})" sin (_zi)
"]'3,i :-- sin(_)
T,,i_--E-_m_(P'x-×o_/-_,_P)_(_i)_o_]
T_,_:=E_P_._m(_).(_,-×o_)-_p_._ (_i).(P.y,- _o_)J (6)
392


Load measurement system with load cell lock-out mechanism

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