The Hubble Space Telescope (HST) Pistol Grip Tool (PGT) is a self-contained, microprocessor controlled, battery-powered, 3/8-inch-drive hand-held tool. The PGT is also a non-powered ratchet wrench. This tool will be used by astronauts during Extravehicular Activity (EVA) to apply torque to the HST and HST Servicing Support Equipment mechanical interfaces and fasteners. Numerous torque, speed, and turn or angle limits are programmed into the PGT for use during various missions. Batteries are replaceable during ground operations, Intravehicular Activities, and EVA's

Brown, Lee

Konkel, Carl

Richards, Paul W.

Smith, Chris

Wagner, Ken

English

NASA Technical Reports Server

6-27 
f 
. Richard;, Carl Konkel**, Chris Smith**, 
Town' and Ken c 
St r 
The Hubble Space Telescope (HST) Pistol Grip Tool (PGT) is a self-contained, micro- 
processor controlled, battery-powered, 3/8-inch-drive hand-held tool (Figure 1 ). The 
PGT is also a non-powered ratchet wrench. This tool will be used by astronauts 
during Extravehicular Activity (EVA) to apply torque to the HST and HST Servicing 
Support Equipment mechanical interfaces and fasteners. Numerous torque, speed, 
and turn or angle limits are programmed into the PGT for use during various missions. 
Batteries are replaceable during ground operations, lntravehicular Activities, and 
EVAs. 
Introduction 
Before each HST Servicing Mission the PGT is programmed via an RS-422 or RS-232 
interface. Fourteen torque settings and three speed settings can be programmed for 
each mission. A specific number of turns or angle can also be programmed with each 
torque setting. The programmed torque range is 2.7 - 33.9 Nem (2.0 - 25.0 ft4bf); the 
programmed speed range is 0.5 - 6.3 rad/s (5.0 - 60.0 rpm), and the available turn 
settings are 0.01 - 999.99 rotations or 1 - 1000 deg. Other programmable variables 
include stall time, sleep time, auto off time, torque thresholds, and angle thresholds. 
Integral to the PGT is a Multisetting Torque Limiter (MTL), which is a mechanical 
device for protecting hardware from the application of excessive torque. The MTL has 
6 settings that are ground re-configurable before each mission. The MTL torque range 
is 1.4 - 67.8 Nom (1 .O - 50.0 ftelbf). 
Overview 
The PGT is grouped into Functional Subsystems and physical Assembly Modules. 
The five Functional Subsystems are: Mechanical, Electrical, Battery, Software, and 
Operator Interfaces. There are five Assembly Modules: MTL, Power Train, Main 
Housing, Electronics Module, and Battery Module. A cutaway view of the PGT can be 
seen in Figure 2. 
A modular design simplifies assembly and maintenance and can be utilized during 
troubleshooting by exchanging modules with other PGT units. Each module can be 
installed and removed as a separate subassembly. All modules are fastened to the 
NASA, Goddard Space Flight Center, Greenbelt, MD 
Orbital Sciences Corporation, Greenbelt, MD 
Swales and Associates, Inc., Beltsville, MD 
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https://ntrs.nasa.gov/search.jsp?R=19960025621 2020-06-18T00:00:10+00:00Z
ly or through another module. 
sing battery shroud using a loc 
ule is secured 
The Mechanical Subsystem provides the necessary mechanisms and structure 
needed to apply output torque. The Assembly Modules with a majority of components 
from the Mechanical Subsystem include the MTL, Power Train, and Main Housing. 
Electrical Subsvstem 
The Electrical Subsystem provides the analog and digital functions as well as the 
platform required for software and computer control. This fault-tolerant, re- 
programmable electronic subsystem consists of the Electronics Module as well as the 
ON/OFF circuit breaker, the A/B mode switch, and the DCIDC converter. The 
challenge was to package this motor control system, along with a 25-measurement 
signal conditioning system, an 8-channel/l O-bit data recording system, and a reliable 
power supply into a volume of 245 cm3 (1 5 cubic inches). This challenge was met 
using proven state-of-the-art electronic design and assembly techniques, including an 
all-digital control scheme, multi-function microcontroller, off-the-shelf hybrid 
microcircuits, programmable logic, and two-sided surface mount technology. 
Batterv Subsvste m 
The Battery Subsystem which provides the power for the PGT consists of the self- 
contained Battery Module. The requirements for the Battery Subsystem were to 
maintain at least 28V with a 1A continuous current for the electronics and 8.25A pulse 
current discharge for the motor. As a goal, sufficient capacity for an entire 6 hour EVA 
was desired. These requirements also have to be met at the worst-case thermal 
conditions for the HST Servicing mission. 
Selecting the proper battery technology and cell size to meet these requirements yet 
keeping the battery small enough for an astronaut to translate with the PGT was a 
difficult process. Four technologies; Nickel-Cadmium (NiCd), Nickel-Metal-Hydride 
(NiMH), Lithium-Ion (Lilon), and Silver-Zinc (AgZn), were identified as potential PGT 
Battery technologies. Of the four candidates, Lilon cells possess the highest 
combination of cell voltage and volumetric energy density. The PGT Battery Module 
will be the first space qualified Lilon battery. 
The Software of the PGT computer implements three major functions; Servo-System, 
Fault Detection, Data Collection. The Servo-System is a closed loop feedback system 
which limits the torque, speed and the angle output of the PGT. The control is based 
upon Proportional and Integral control algorithm using pulse width modulation and 
the knowledge of the native motor speedhorque curve. The software will allow the 
motor to function at the lesser of three conditions; programmed torque, programmed 
speed, or the maximum output based upon the native speed/torque curve. The 
computer system also collects and stores fault data, diagnostic data, and performance 
data. The various faults that can be detected are Software, Temperature, Current, 
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Voltage, and Torque. The fault history is stored in non-volatile memory to allow for 
trouble shooting during development, testing, and use. 
Ope rator lnte rfaces 
The Operator Interface Subsystem provides the means for all necessary operator input 
and provides visual feedback through markings, an alphanumeric display, and status 
and warning tight emitting diodes (LEDs). The operator selects the configuration of the 
PGT through various switches and collars. The selection coflars include the MTL 
Torque collar, Ratchet collar, Programmed Torque collar, and the Speed collar. The 
Speed collar also has a CAL setting which tests the LEDs, the Alphanumeric Display 
and calibrates the torque transducer. The switches include the ON/OFF circuit 
breaker, the A/B Mode switch, and the trigger. 
Configuration, performance, and fault information is shown to the operator via the 
LEDs and Alphanumeric Display. The LEDs consist of the POWER. (green), FAULT 
(red), and TORQUE (greenhed). The Alphanumeric Display shows the programmed 
torque, programmed speed, direction, actual peak torque, and actual cumulative 
rotation since the last configuration change. Fault messages are also presented to the 
operator in the Alphanumeric Display. 
Assembly Modules 
The MTL, located at the forward end of the PGT rotates with the output shaft. There are 
six torque settings which are ground re-configurable before each mission. The MTL 
torque range is 1.4 - 67.8 N*m (1 .O - 50.0 ftelbf). Each setting increases the torque of 
the previous setting by 1.4 - 20.3 Nom (1 .O - 15.0 ft-lbf). The highest value 
implemented will be 41.3 Nom (30.5 ft-lbf), which is the maximum design limit of the 
PGT in the manual mode. The torque settings are selected by the operator by 
unlocking the thumb switch and rotating the MTL torque selector collar. The thumb 
switch prevents inadvertent reconfiguration of the MTL. 
Power Train 
The PGT Power Train consists of the motor, gearing, and torque transducer, and 
ratchetklutch mechanism. The PGT motor is a 28V, four pole, brushless dc motor. 
The motor uses samarium cobalt magnets on the rotor and employs hall effect sensor 
commutation. The gearing is a 2-stage planetary gear set with a reduction ration of 
11O:l. 
The operator selects the motorized or manual mode of the output shaft by rotating the 
ratchet collar and aligning the visual indicator with the desired setting. There are three 
settings: Ratchet Clockwise (RCW), Motorized (MTR), and Ratchet Counterclockwise 
(RCCW). In the MTR setting a clutch is engaged and the motor output shaft is 
connected through the MTL which to the PGT output shaft. In the RCW and RCCW 
settings the clutch is disengaged and either the clockwise or counter-clockwise ratchet 
pawl is engaged. When in either the RCW or RCCW settings, the load path is through 
the ratchet pawl to the Main Housing. There is also a hall effect sensor electrical 
inhibit, which allows motorized operation in the MTR mode, but inhibits motorized 
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operation in the ode. This inhibit revents motor operation during 
inadvertent cont 
The Main Housing consists of the Collar/Switch Subassembly, and a two-piece 
housing, which consists of the Power Train housing, the handle and trigger and the 
battery shroud. The Collar/Switch Subassembly contains the Speed collar, Torque 
collar, ON/OFF circuit breaker switch, A/B mode switch, and the DC/DC Converter. 
Electronics Module 
The Electronics Module consists of an analog circuit board and a digital circuit board 
mounted in an aluminum housing. The major components of these boards are a 
87C196KD microcontroller, a Clock Oscillator, Memory Address Latches, EEPROM, a 
Erasable Programmable Logic Device (EPLD) EMP5130, 82C55 port expander, 
DAC-8412 quad digital to analog converter, Strain-gauge Torque Transducer signal 
conditioning circuit, temperature (Motor, Battery and Motor-drive Hybrid) signal 
conditioning circuits, Hardware Torque Warning Circuit, Latch-up current-limiting 
circuits, and a Motor Drive Hybrid circuit. 
Batterv Module 
Each PGT Battery Module contains ten Lilon cells connected in series to create a 41V 
open circuit and a 36V nominal closed circuit battery. The battery design was 
optimized to approximately 2.4 A-hr capacity using modified C sized cell. The cell 
pack is then packaged into a nickel-plated aluminum housing. A cover is fastened on 
top to form the battery module. On one end of the module is a rectangular sub- 
miniature connector which provides an interface to the tool for positive and negative 
power lines. A 10A fuse on the negative line provides over-current protection in the 
event of a short in the tool. The battery modules and the battery shroud are keyed with 
alignment grooves to prevent incorrect insertion. 
The battery module design includes a temperature sensor to sense batterykell 
temperature. If battery temperature increases to above nominal level, the 
Alphanumeric Display will indicate a high battery temperature warning to the user. If 
battery temperature increases and approaches an unsafe level, a software command 
is given to remove power using the circuit breaker. 
Conclusion 
The application of satellite mechanism technology and design methods was critical in 
the production of a compact, ergonomic power tool which meets the accuracy, 
reliability, and safety requirements for the HST Second Servicing Mission. 
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Figure 1 
Pistol Grip Tool 
External View 
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001 
Fig 
Pistol 
~utaway View 
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A computer controlled power tool for the servicing of the Hubble Space Telescope

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