REMOTE REPAIR DEMONSTRATION OF SOLAR MAXIMUM MAIN ELECTRONICS BOX

The remote repair demonstration of the Solar Maximum satellite Main Electronics Box using the SAMSIN® master/slave servo manipulator is described. The MEB was not designed to be repaired on-orbit and represented the most difficult repair task performed by NASA astronauts. The viewing system, tooling, support equipment and the manipulator system used to perform the replacement sequence is presented. A description of the technology demonstrations is provided along with guidelines for performing remote servicing. The SAMSIN® remote servicing system demonstrated how tasks can be effectively performed on space vehicles in an unstructured or unfamiliar remote environment.

Keywords: Remote Servicing Demonstration, Master/Slave Servo Manipulator, Bilateral Force Reflection, Manipulator Tooling, Viewing System.

1. INTRODUCTION

The successful completion of the Solar Maximum Repair Mission (SMRM) in April of 1984 introduced mew era in the Shuttle Program. The Shuttle demonstrated its capability for use as a repair vehicle to retrieve and deploy satellites and the astronauts, through Extra Vehicular Activity (EVA), were able to replace faulty modules designed for servicing, improve the performance of a scientific instrument by snapping a protective cover over its vent to shield it from interference and to replace an electronics box that was not designed to be repaired in space. These activities required a seven-hour EVA.

The Satellite Servicing Project at Goddard Space Flight Center (GSFC) issued an informal challenge to a number of U.S. industrial firms to demonstrate the capability of current state-of-the-art commercial remote servicing systems. It was suggested that companies could attempt to demonstrate the most difficult task performed by NASA, the Solar Maximum Mission (SMM) Main Electronics Box (MEB) repair sequence. This repair sequence was documented in an EVA Annex for the SMRM. Central Research Laboratories (CRL) accepted NASA’s challenge to demonstrate the remote servicing capabilities of its Servo-Actuated Manipulator System with Intellegence Networks (SAMSIN®) for performing the SMRM MEB repair. This effort was performed as a result of NASA’s interest in minimizing hazardous EVA exposure of astronauts and extending the productive Space Transportation System (Shuttle) time on-orbit through the use of a remote servicing aid system.

This paper discusses the on-orbit repair procedure and contrasts it with the remote demonstration procedure. A description of the servo manipulator system is provided and the lessons learned from this demonstration are also discussed.

2. SOLAR MAXIMUM MEB ON-ORBIT REPAIR PROCEDURE

The SMM was launched on February 14, 1980. For the first nine months of the planned two-year mission, the satellite collected data on solar flare activity. However, in late 1980 a failure in the Attitude Control Subsystem (ACS) module resulted in the inability to precisely point at observation areas on the sun, and it began to lose altitude. NASA estimated that a successful repair would restore the $77 million satellite at one-fourth of its replacement cost and would extend the operational life of the observatory several years.

Due to recovery problems, the two planned preflight EVA periods were reduced to one. The mission’s primary objective was to replace the ACS nodule, while the secondary objective was the more complicated repair procedure for the MEB. Another mission objective was to install a baffle over a vent on the x-ray polychrometer. A 31-page EVA sequence was developed to ensure successful replacement of the MED by three astronauts. Two astronauts were participating in EVA activity while the third astronaut operated the EMS from the aft flight deck of the Shuttle.

The tools required to perform this repair procedure were the MEB hinge assembly, panel support bracket, power tool with socket head cap screw and hex drive extensions, connector removal tool, connector installation tool, Essex ratchet, modified EVA scissors, and pin straightener.

The crew participated in 1-G and neutral bouyancy training at various NASA facilities for about one year in preparing for this mission. The time to complete the MEB repair mission in neutral bouyancy simulation was about 3-1/2 hours. The on-orbit repair time was about 2 hours.

3. EQUIPMENT AND FACILITIES

The Solar Maximum technology demonstrations were performed at the GSFC space station robotics laboratory using a full-scale mockup of the Solar Maximum satellite. The SAMSIN® remote servicing system is shown next to the Solar Max satellite mockup in Figure 1. The master system and operator viewing station is shown in the foreground. The slave system is shown positioned opposite the satellite’s MEB panel. The tool caddy and remote viewing system are also shown.

3.1 SAMSIN® Servo Manipulator System

SAMSIN® is a seven degree-of-freedom, bilateral, force reflecting, master/slave servo manipulator system. This system is anthropomorphic in function. All operator hand and arm movements at the master arm are duplicated at the slave arm resulting in manlike tasks being performed in the remote environment. All motions are force reflecting. The operator feels the forces he applies to the remote environment along with those being reflected back by the remote environment.

Each slave arm has a 12 Kg (25 lb) capacity at the tong tip, while the master arm has a 7 Kg (15 lb) capacity at the master control handle. The seven degrees-of-freedom are driven by brushless DC servo motors mounted to a gear package. Two of the translational motions are driven by gears and linkage; the remaining five motions are driven by a combination of gears and tendon/pulley arrangements. This type of design results in a system that has low backlash, low friction and low effective inertia at the tong and control handle.

SAMSIN® incorporates a distributed microcontroller based digital control system. The master and slave joint processors communicate through a high speed digital serial link. Master and slave control is real time; the rate at which the slave arm will follow the master arm is 100 cm (40 in.) per second. The minimum slave arm loading which can be detected by an operator at the master control handle is approximately 0.5 Kg (1 lb) or 4 percent of peak capacity at a 2:1 force ratio.

3.2 Operator Control Handle

The control handle on the master arm is an opposed grip type that provides for slave tong control. Switches on the handles allow the operator to perform functions such as slave arm lock, slave tong lock and master-to-slave indexing without releasing the handle. The indexing feature is available for all motions except the grip and may be operated while the slave tong is locked. In addition, a 'deadman' function switch is also mounted on the handle. When the operator releases the handle, the slave brakes are set and power to the master and slave arms is removed.

3.3 Viewing System

All viewing of the remote work site was provided through closed-circuit TV, as shown in Figure 2. Three color and two black-and-white cameras were mounted on stands on or near the slave arm support stand. The cameras were positioned from the MEB in order to provide the operators with a close-up viewing area of 30 cm by 30 cm (12 in. by 12 in.). Initially, only three color cameras were used by the operators for remote viewing. These cameras provided good close-up views but were not sufficient for providing overall views. As training progressed, two black-and-white cameras were added to provide wide angle views. The wide angle views were necessary for the operator to monitor end effector position when not in the close-up views.

Remote controls for the color cameras included zoom, focus and pan/tilt. These camera lenses also had an auto iris feature. The black-and-white cameras were fixed focus with no remote control features.

A third black-and-white camera was added for providing close-up details when the right slave arm was removing #4-40 screws, removing electrical connectors. cutting tie wraps, and reinserting the electrical connectors. This camera was held by the left slave arm.

The cameras were positioned to show a top, front, and side view to the manipulator operators, analogous to an engineering drawing. The video monitors were arranged accordingly in the cabinet. The final camera arrangement enabled the operators to perform the remote repair of the MEB.

3.4 Orientation of Remote Manipulator and Video System

As shown in Figure 3, the remote manipulator system was positioned opposite the MEB panel in the closed position. The manipulator end effector was in a perpendicular orientation with respect to the MEB panel when the operators were working with thermal blankets or attaching the MEB hinge assembly. When the operator moved the master control handle forward, the slave end effector would respond in a likewise manner, moving toward the MEB panel as observed through the top and side view cameras. These cameras provided the operators with views of the end effector orientation and range. The front view camera was used by the operators to locate the end effector at the closed MEB panel surface.

When the MEB panel door was opened and the operator was removing #4-40 screws, removing connectors, cutting tie wraps, and reinstalling connectors, the manipulator end effector was oriented perpendicular to the MEB panel door, Figure 3. The end effector was now oriented at 90° with respect to the reference location for the viewing system. When the operator moved the master control handle to the left, the slave end effector would respond in a likewise manner, moving toward the MEB as shown by the top and front view cameras. These cameras now provided the operators with views of the end effector orientation and range. The side view camera was now used by the operators to locate the end effector at the MEB.

3.5 Remote Manipulator Tooling

CRL converted EVA tooling concepts used by the Shuttle crew to the SAMSIN® end effectors. These tools were remotely interchangeable and locked on the end effector by a ball-socket detent system. Because the tools were locked into place, a tethering system was not required. The tools used for the demonstration are shown mounted in the tool caddy in Figure 4. These tools included:

• Three battery-powered screw drivers:
  • one with an alien drive
  • one with an allen drive with screw capture
  • one with a slotted screwdriver blade
• Diagonal cutters
• Knife
• Arm-held camera

The battery-powered screw drive with the three accessory bits is shown in Figure 5. The standard allen drive was used where cap screws had retaining devices. The shrouded or captive allen drive retained the cap screw in the drive until the screw was removed by the other remote arm. The slotted screw driver was designed to drive out the screw only while in the threaded hole. It could not drive out the screw from the screw-retainer on the electrical connector. The operator was not required to count screw blade rotations.

3.6 Support Equipment

Support equipment unique to the Solar Max demonstration was the MEB hinge assembly and the panel support bracket. CRL took the liberty to slightly modify or design new support equipment to accommodate the remote repair procedure. CRL modified support equipment is shown in Figure 6.

3.6.1 MEB Hinge Assembly. The MEB hinge assembly had no provisions for being remotely handled or maneuvered. CRL modified the hinge assembly by adding a grasping block near the tether ring. A C-section was provided for this block in order to accommodate the standard CRL tong jaws. In addition, the flanges on the C-section were coated with a black marker in order to provide the operator with visual assistance in remotely grasping the hinge assembly.

3.6.2 Panel Support Bracket. The panel support bracket used by the EVA astronauts was difficult to handle and operate remotely. The purpose of this bracket was to hold the MEB panel 'door' in an open position while the electrical connectors were being removed and reinstalled. The original bracket contained too many degrees-of-freedom to control remotely. In addition, it could not be maneuvered effectively in a 1-G environment since there were no provisions for grasping the bracket at its center of gravity with the standard CRL tong jaws. Therefore, CRL designed a new panel support bracket. The old bracket attached to the bottom of the satellite frame and MEB panel by tightening knobs. The CRL bracket attached to the top of the SMM structure by two cap screws held captive in the bracket. The panel door was held in place when the bracket arm was swung into position. A remote grasping block was provided similar to the grasping block in the MEB hinge assembly.

3.6.3 Tape Caddy. CRL also designed a tape caddy, positioned in front of the tool caddy, to store tape. Tape was used for two purposes. The primary purpose was to hold the thermal blankets in place during the repair procedure. A secondary purpose was to capture thermal grommets in the MEB panel thermal blanket.

3.7 Operators

CRL used two manipulator operators and one camera operator for performing the remote repair demonstration. The camera operator was also responsible for monitoring the repair procedure and providing cues to the manipulator operators for tool changes. Normally, one manipulator operator controls the two master arms of the manipulator system. In view of the complexity of this remote repair procedure, limited training time and the number of demonstrations to be performed, two manipulator operators were used, one for each master arm. This approach demonstrated how two operators can work in a cooperative fashion. Because the manipulator system is bilateral and force reflecting, obstacle avoidance was not an issue. The operators could feel when the arms encountered an unforeseen obstacle or when the slave arms accidentally interfered with each other.

3.8 Training

Some of the SMRM equipment used during neutral bouyancy training was made available to CRL for training and tool development. CRL constructed a mockup of the SMM structure to accommodate the SMRM training equipment (Figure 7). CRL trained on this mockup for approximately 30 hours in order to verify the CRL tooling and to gain experience using the remote viewing system.

4.0 REMOTE REPAIR DEMONSTRATION

A technology demonstration was done at the GSFC space station robotics laboratory between February 26 and March 4, 1987 using a full-scale mockup of the Solar Maximum satellite. A majority of individuals knowledgeable about satellite servicing felt that the remote repair of the Solar Max MEB was not possible using a remote servicing system. The purpose of this demonstration was to demonstrate the capabilities and the limitations of bilateral force reflecting servo manipulator technology.

Twelve demonstrations were conducted during this period which were subsets of the full 31-page EVA procedure. In addition, one complete replacement procedure was also performed. These demonstrations were attended by about 400 NASA engineers, scientists, astronauts and government officials at GSFC during the five-day period.

4.1 Subset MEB Demonstrations

A subset demonstration was developed that was appropriate for repeat demonstrations and still illustrated most of the tasks required to remove the 'ailing' MEB. This demonstration initially required 45 minutes to perform. The completion time for the last demonstration was 25 minutes, indicating that SAMSIN’s operators were still on a learning curve. This time improvement can be attributed to increased familiarity with the remote viewing system and the full-scale satellite mockup.

The subset demonstration began with the thermal blankets folded back and consisted of five tasks. These tasks were:

1. Installing the panel hinge assembly
2. Removing two of fourteen non-captive #10-32 screws from the MEB panel
3. Installing the panel support bracket to hold the panel open
4. Removing six of twenty-two #4-40 screws
5. Removing three of eleven electrical connectors

4.2 Complete MEB Demonstration

The full demonstration procedure included twelve dexterous manipulation tasks. These tasks were:

1. Cutting plastic Kapton tape holding the thermal blankets
2. Folding, unfolding and taping temporary and permanent thermal blankets
3. Installing the MEB panel hinge assembly
4. Removing fourteen #10-32 non-captive cap screws and reinstalling four #10-32 captive cap screws
5. Installing the panel support bracket
6. Removing twenty-two #4-40 slotted-head connector screws
7. Removing eleven subminiature connectors by their back shells
8. Cutting plastic tie wraps holding the wiring harness
9. Removing an 'ailing' MEB from the hinge assembly and reinstalling a ‘new’ box onto the hinge assembly
10. Reinserting the subminiature connectors by their back shells
11. Removing Velcro straps from thermal blankets
12. Using Velcro straps as cable ties

Tasks not performed were cutting the grounding strap and thermister wire and installing the thermal closeout. These tasks were a part of the normal MEB repair procedure.

These twelve tasks were accomplished in three hours. A detailed breakdown of time versus task for the on-orbit repair was not available. Time versus task was given in the EVA annex for the neutral buoyancy simulations. For the purpose of comparing times for the EVA simulation with the remote demonstration, the twelve manipulative tasks described above were grouped into the eight general procedures given in the procedures. This comparison is given in Table 1. Times for the EVA neutral buoyancy simulation have been adjusted to reflect the task not done by remote operations. The operators took two 15-minute breaks during this demonstration. The break times are not reflected in Table 1.

The two procedures that required significantly more time to perform remotely were the removal and securing of the thermal blankets. These procedures took twice the time to perform remotely. Thermal blankets are difficult to handle in a 1-G environment. In addition, it is easy to accidentally cut into the thermal blanket while cutting the Kapton tape. Remotely, this is more difficult because of the viewing system utilized and the 0.5 Kg (1 lb) force sensitivity of the manipulator system.

The other six procedures identified in Table 1 were completed remotely in one hour and 48 minutes as compared with two hours and 45 minutes for EVA neutral buoyancy simulation. Removing connector screws and reinstalling the electrical connectors were done substantially quicker by remote operations (38 minutes versus 60 minutes, and 27 minutes versus 55 minutes, respectively). This can be attributed to the view provided to the operator by the mobile camera.

The MEB repair sequence could only be accomplished remotely using two arms. For some operations and procedures, both arms were used in parallel. Other operations were done using both arms, but in a serial fashion. Table 2 summarizes the remote tasks requiring parallel arm operation, whereas Table 3 summarizes the serial arm operations.

However, there were some tasks where more than two remote arms would have been desirable. Specifically during the cutting of Kapton tape and reinstalling of the electrical connectors, three arms would have been beneficial. The procedure for cutting the Kapton tape would be enhanced if one arm was used for positioning the mobile camera, while two other arms were working on the tape and thermal blankets.

The operators had to make a mental adjustment when the roles of the video monitors changed (see Section 3.4). Roles of the monitors changed when the repair procedure moved from working with the MEB panel closed to working on the MEB panel when opened. The front view monitor that was used for observing end effector location changed to end effector range and orientation. This mental adjustment made the MEB repair more difficult to accomplish remotely.

Ideally, the manipulator base would always remain opposite the remote work area and the remote viewing system would be fixed with respect to the manipulator base. Then, the roles of the video monitors would remain constant, thus simplifying viewing of the remote repair procedure by the operators.

Handling the wire bundle and manipulating the Velcro 'tie wraps' was extremely difficult. For this procedure, one arm could have been used to hold the wire bundle, while a second and third arm were grasping the Velcro straps and securing the wire bundle.

While reinstalling the electrical connectors, one arm was positioning the mobile camera while the other arm was being used to insert the connectors into the retaining clips. A third arm would have provided assistance in keeping the individual connectors from interfering with the operation.

5.0 MISCUES

Operator errors resulting in dropped support equipment only happened twice and occurred during the first demonstration. The MEB hinge assembly was accidentally dropped when the operator inadvertently released the grip lock. The second error occurred during a tool exchange sequence. The left tong jaw was not seated properly on its parking post when the jaw, was disengaged from the end effector.

Bending pins on connectors during reinstallation never occurred during the full remote demonstration and only once during rehearsals. Pins were bent due to incorrectly spaced aluminum guides on the 'new' MEB panel.

6.0 SUMMARY

The remote servicing system represented by CRL's teleoperated master/slave system is in keeping with NASA's efforts to minimize hazardous EVA exposure of astronauts and to extend productive on-orbit Shuttle time. The SMRM MEB repair demonstration illustrated how such a system can effectively perform tasks on satellites and other space vehicles in an unstructured or unfamiliar remote environment.

CRL offers the following six general comments regarding remote operations:

1. Flat grasping points simplify the remote operations for handling and positioning objects.
2. Viewing systems should enable the operators to precisely locate objects at the work site and also to orient tools or supporting equipment.
3. Visual indicators on support equipment for grasping and alignment improve the operator's ability to use this equipment in a remote environment.
4. Viewing systems should remain fixed with respect to the manipulator base.
5. The remote manipulator system should be positioned directly opposite work surfaces, if possible, to minimize role changes for video monitors.
6. Operator feedback, in addition to a viewing system, should also include a bilateral force reflecting control system and an audio system. Audio feedback for a space environment may be provided to the operator by a vibratory sensor at the remote work site that is capable of detecting mechanical vibrations.

7.0 REFERENCES

1. Essex Corporation Report Number H-85-05, 'Solar Max Repair Mission, Final Report'. Contract NAS5-27345, June 25, 1985.
2. JSC-14082, Annex 11, Revision A, 'Extravehicular Activity Annex, SMRM', March, 1984.