M. D. Fowley and J. R. Brault
Westinghouse Savannah River Company
Aiken, SC 29808

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The Plutonium Immobilization Program will dispose of excess weapons grade plutonium by encasement in a solid ceramic form called a puck. Due to dose levels, the pucks will be handled by automated equipment. Robots will be used whenever the handling operations become complicated. Two such handling operations require a six-axis robot. End-effector design for these operations required versatility for the different equipment interfaces and robustness for the harsh environment.

A two-jaw angular gripper was required for one of the handling operation. A two-jaw angular gripper and three-jaw angular gripper were required for the other. The fingers for the two-jaw and three jaw grippers were initially designed to conform to the round circumference of the pucks. This design gave optimal contact area of the fingers on the pucks. The finger design was later changed to a multi-angular design, which provided four-point contact for the two-jaw gripper and eight-point contact for the three-jaw gripper. The multi-angular design would also re-center an off-center puck better than the round fingers.

Prior to acquisition of a robot, a pick and place machine was constructed from linear and rotary actuators to test dynamic handling effects of the end-effector. The pick and place machine provided valuable information relating to grip force, multi-angular finger design, flow control valves, and puck centering capabilities. Later, a six-axis robot was acquired to perform the automated operations. A mockup, representing the glove box containing the robot, was assembled to further develop the automated equipment. The mockup operated with non-radioactive, surrogate materials for simplicity of development and operation.

An extensive study was undertaken to reduce or prevent chipping, cracking, and dusting of the ceramic material caused by automated handling. Chipping, cracking, and dusting will create unwanted contamination and expensive cleanup in the actual facility. Flow control devices, finger pad materials, and end-effector sensors were tested during this process.

The Savannah River Site (SRS) will immobilize excess plutonium in the proposed Plutonium Immobilization Plant (PIP) as part of the Department of Energy's two track approach for the disposition of weapons-usable plutonium. The Department of Energy is funding and guiding the development and testing effort for the PIP by Lawrence Livermore National Laboratory (LLNL), Westinghouse Savannah River Company (WSRC) Savannah River Technology Center (SRTC), Pacific Northwest National Laboratory (PNNL), and Argonne National Laboratory (ANL). The method of immobilization is to encapsulate the plutonium in ceramic pucks that are roughly the size of a hockey puck, using a sintering process. A Can-in-Canister storage method will be used to further immobilize the plutonium. The Can-in-Canister method uses the existing design of a Defense Waste Processing Facility (DWPF) canister to house the plutonium pucks. The process begins with several pucks being stacked in a stainless steel can. Several of the stainless steel cans are stacked in a cage-like magazine. Several of the magazines are then placed in a DWPF canister. The DWPF canister is then filled with molten glass containing high-level, radioactive waste from the DWPF vitrification process. The Can-in-Canister method makes reclamation of plutonium from the pucks technically difficult and highly undesirable.

The mechanical requirements of the Can-in-Canister process, in conjunction with the amount of time required to immobilize the vast quantities of weapons-grade plutonium, would expose personnel to unnecessarily high levels of radiation if the processes were completed manually, in glove boxes. Therefore, automated equipment will be designed into the process to reduce or eliminate personnel exposure. Versatile articulated arm robots are used whenever the automated handling operations become complicated. There are two such operations in the initial stages of the Can-in-Canister process, which required a six-axis, articulated arm robot. The first operation is a press unloading process. The second operation is a tray transfer process.

The first operation is prior to the sintering furnaces. Plutonium, in oxide powder form, will be mixed with uranium oxide powder, ceramic precursors and binders. The combined powder mixture will be milled and possibly granulated. This processed powder will then be dispensed to a dual-action, cold press where it will be formed into green (unsintered) compacts. The unsintered compacts are white in color but are called a "green" pucks (see figure 1). The green pucks will be ejected from the press die, picked up by a robot, and moved to an inspection area. Acceptable green pucks will be palletized onto furnace trays by the robot. The loaded furnace trays will be stacked/assembled and transported to a furnace where sintering operations will be performed. In the second operation, the sintered pucks are unloaded from the furnace trays by a similar robot and move to a similar inspection area. Acceptable pucks will be placed onto transfer trays, which will carry them downstream to subsequent can loading operations.

The inspection areas contain three separate devices: a platform scale for weight analysis, an array of lasers for dimensional analysis, and a digital camera for topological analysis. The digital camera will provide a snapshot of the top and bottom surfaces of the puck. Either an automated or manual process will be used to determine surface integrity of the puck. The array of lasers and platform scale are combined as one station. The lasers will measure thickness and diameter while the puck is sitting on the scale.

Figure 1
Furnace Tray with Sintered and Green Pucks

To successfully accomplish the operational tasks described in the two operations above, the end-effector of the robot must be versatile, lightweight, and rugged. Versatile to interface with the various equipment in the glove box. Lightweight to retain maximum capacity of the robot. Rugged to endure the dusty and radioactive environment. As a result of these demands, an extensive development process was undertaken to design the optimum end-effector for these puck-handling operations.

As an overall requirement, it was desired to keep the design of the robot end-effector as simple as possible. Simplicity usually equates to usefulness and durability. There were pros and cons for either type of actuation method (pneumatic or electric). But, pneumatic actuation was chosen for its simplicity and durability in a radioactive environment. Sensor feedback was necessary for the delicate movements around the furnace tray and for detection of process interruptions. But for simplicity, the number of end-effector sensors would be kept to a minimum.

It was determined early in the design process that at least two different types of end-effectors would be required for each of the operations. Therefore, a tool changing devise was incorporated into the end-effector design. The tool changer would also provide for simple end-effector maintenance when used in the actual PIP process.

A two-jaw, angular gripper was chosen early in the developmental process to be the primary gripper for each operation (see figure 2). The two-jaw configuration was chosen for two reasons. One, the puck could only be approached from the side while it was in the press. Two, the top and bottom surfaces of the puck were required to be free of obstruction when the puck was presented to the digital camera by the robot. The two-jaw configuration met both of these requirements.

Angular jaw movement of the gripper was chosen for durability and a greater range of jaw travel. The environment around the press was expected to be dusty due to the ceramic precursors. Ceramic powder collecting on the exposed jaw surfaces could work itself into the jaw mechanism and cause damage. In most cases, an angular jaw had less of an exposed jaw surface than a parallel jaw of similar travel. Therefore, an angular jaw configuration was thought to have greater durability for the puck-handling operations.

To firmly grasp a puck, the fingers of the gripper should completely envelop the puck perimeter. An angular gripper, with a jaw travel of 15° or greater, could be fitted with fingers that (somewhat) completely envelop the puck and permit a sideward approach. Parallel jaw grippers could not be fitted with fingers that completely enveloped the puck and permitted a sideward approach. It was later determined, when more of the interface components were specified, that a large amount of travel was undesirable. The angular jaw movement was still retained due to the durability issues. However, the fingers, redesigned for the lesser jaw travel, would not completely envelop the puck perimeter.

A two-jaw gripper will be used exclusively for the press unloading operation. However, in addition to a two-jaw gripper, a three-jaw gripper will be required for the tray transfer operation (see figure 2). The two-jaw gripper is necessary for the inspection area. The three-jaw gripper is necessary to remove the sintered pucks from the furnace tray. The necessity of both for the tray transfer operation requires a tool changing devise for the end-effector.

During the sintering process the pucks will shrink (diameter and thickness). It is possible that the pucks will shift from their original position as they shrink. The robot must know the exact location (in X, Y, and Z coordinates, relative to the robot origin) of each puck to successfully acquire and transfer them. One method to determine the new location of the pucks is with a digital camera or other visual means. This method is used extensively in the automated industry. However, to keep with the dictum of simplicity, a three-jaw gripper with adequate travel will be used instead of a visual system. When a puck shrinks, it will shrink within the original footprint of the unsintered puck. A three-jaw gripper, with adequate travel, could envelop the footprint of the unsintered puck and successfully re-acquire the shifted, sintered puck when the jaws are closed.

Figure 2
Three-jaw and Two-jaw Angular Gripper
with Tool Changer and Adapter Plate

Prior to acquisition of a six-axis robot, a very simple pick and place machine was constructed to test the dynamic handling requirements of the end-effector. The Pick and Place machine was built from spare linear and rotary actuators and pneumatic valves. It could be fitted with a two-jaw gripper or a three-jaw gripper. A PC, using control software to sequence the pneumatic valves, controlled the pick and place machine movements. The pick and place machine provided valuable information relating to grip force, finger design, flow control valves, and much more.

Two criteria were initially established for the design of the gripper fingers. The first was that the puck had to be firmly held within the fingers. The relative position of the puck top and bottom surfaces must be well known throughout the press unloading and tray transfer operations (for gripping, inspection and palletization). The second was that puck handling required careful and controlled movements to prevent chipping, cracking, and other unnecessary damage to the puck.

Firmness of grip means grip force. Grip force is controlled by the amount of air pressure supplied to the pneumatic gripper. The greater the air pressure, the greater the grip force and the firmer the grip on the puck. The gripper fingers are made from aluminum for manufacturing ease and low weight. The aluminum fingers, unfortunately, produced a harsh impact on the sides of the puck as they snapped closed

Flow control valves were first used to offset the impact and retain the firmness of grip. The flow control valves restricted the flow of air so that the pressure built up gradually. With flow control valves, the fingers would softly impact the puck, then firmly grip the puck as the pressure built. The flow control valves lessened the impact on the pucks but required extra time for the pressure to build. The extra time only amounted to seconds but, as the overall immobilization process developed, a shorter cycle time was required to meet the production schedule. The shorter cycle time necessitated the elimination of the flow control valves in favor of a quicker method of lessening the impact on the pucks

Next, finger pads were used to reduce the impact on the puck. Various hard and soft elastomers were tried as finger pad material. The softer material proved best for lessening the impact. As a bonus, the elastomer pad provided a firmer grip due to the increase in friction between the pad and the puck. The firmer grip allowed a reduction in air pressure, which further reduced the impact of the fingers on the puck. Finger pads proved to be the best solution for the harsh impact problem.

Fingers, such as the ones shown in figure 2, were initially designed and tested with surrogate green and sintered pucks. These circular fingers conformed to the puck perimeter and distributed the grip force over a large area, which in theory would provide a very firm grip. However, the first surrogate pucks (made from plastic pipe filled with resin) were poor representations of the actual ceramic puck. The diameter and surface texture were different from the actual ceramic puck. The different diameter caused multiple point contact with the gripper fingers, which would concentrate the gripping force and increased the likelihood of chipping or cracking. In some cases, there was only two-point contact. The two-point contact caused an uneven load and created instability when the puck was moved. These initial results, although caused by poor surrogate pucks, were unfavorable and provoked a change in finger design. Ceramic pucks, made from surrogate materials, were later provided to more effectively develop the end-effector.

Parallel studies at LLNL used a two-jaw, multi-angular, finger design similar as the fingers shown in figure 3. One of the benefits of the multi-angular design was the ability to re-center a puck that was not in the center of the gripper target area. The flat sides that made first contact would push the puck to center. This "re-centering" would occur with the circular finger, but to a much lesser extent.

By design, the multi-angular fingers create the undesirable effect of multiple point contact (four-point contact with the two-jaw and six-point contact with the three-jaw). However, with the use of soft finger pads, the point contact would be spread over a larger area as the pad conformed to the sides of the puck. The multi-angular finger design, with pads, proved favorable during many of the initial gripper tests.

Figure 3
Multi-angular and Circular Gripper Finger Design

The re-centering capabilities of the multi-angular finger design give the puck handling process a certain degree of versatility. That is, the X-Y position of the puck, relative to the robot, can vary (slightly) without interrupting the process flow. This versatility is paramount for the gripping of the sintered puck from the furnace tray. As explained previously, a sintered puck can shift as it shrinks during the sintering process. The re-centering capabilities of the multi-angular finger design can successfully capture a shifted puck.

A study was performed with a sintered puck to determine the limits of the re-centering capabilities of the multi-angular finger design. In the study, a sintered puck was randomly placed off-center in the footprint of an unsintered puck. A three-jaw gripper was used for the study. Initially, the puck was placed on a wooden workbench, the fingers did not have pads, and a stainless steel mockup of a sintered puck was used. The stainless steel puck was easily re-centered at all extremes. Next, a surrogate sintered puck was used. The sintered puck also re-centered, but with slightly more difficulty due to the increased friction between the puck and the table. More difficulty was encountered when gripper pads were placed on the fingers. Once again the increased friction between the fingers and the puck hindered the re-centering capabilities. In every test, increasing the air pressure to the gripper helped the re-centering capabilities.

From these tests it was apparent that the finger pads had opposing effects. They increased the firmness of the grip, but hampered the re-centering capabilities. To overcome these opposing effects a composite finger pad was developed and tested (see figure 4). The composite finger pad was a foam rubber pad covered with a strip of stainless steel shim stock. The foam rubber provided the cushion effect and the shim reduced the friction to enhance the re-centering capabilities. Any firmness of grip lost by the shim stock could be accounted for with an increase in air pressure. The composite finger pad worked well in every finger configuration tested.

Figure 4
Composite Finger Pad

A six-axis Mitsubishi robot was purchased and is being tested for the press unloading and puck transfer operations (see figure 5). The robot is a 3 Kg model with an extended upper arm that provides an expanded work envelop. The test robot is the standard version but is available in a clean room version, which will be used for radioactive operation.

The end-effector design that was developed with the pick-and-place machine was incorporated onto the six-axis robot. From this point on development of the robot and end-effector was mainly focused on press unloading since the development paths of the press unloading operation and the tray transfer operation were parallel.

A mockup of the press unloading glove box was built around the robot. The mockup contained representations of the press, the inspection stations, and a furnace tray stacker/unstacker (SUS). The robot mockup provided a platform to develop the robot, develop the end-effector, coordinate the automated equipment interfaces, complete cycle-time studies, and much more. The dimensional and spatial relationship between the robot, press, inspection stations, and a SUS was based on existing prints or actual hardware. The sequential operation of the automated equipment in the mockup is controlled with a PC. Communication between each devise and the PC is via the RS-232 serial port.

Figure 5
Six-axis Robot in Press Unloading Glove Box Mockup

The press representation was constructed using the design of the actual puck press used at LLNL. Representations of the press platen and associated hardware were included in the mockup to simulate spatial restrictions for the end-effector (see figures 5 and 6). The press is located at the side of the glove box to keep the axis of the glove box free for linear transportation of material below the robot.

The SUS will present an empty furnace tray to the robot. An empty tray must be presented in the same position each time for the robot to successfully palletize the pucks. At first, the SUS was represented as a platform, which was the approximate size of the furnace tray. As the design of the furnace tray progressed, the platform was replaced by a plastic mockup of the tray (see figure 7). The actual SUS was placed into the mockup after it was designed, assembled, and tested (see figure 8).

The robot must remove the puck from the lower press ram. This method is different from similar press designs used in other industries. In the other designs the press product is pushed out onto the platen, away from the ram, as the press shoe delivers more powder. This action, if used for the press unloading operation, would present an easier target for the robot. However, this option was not used to reduce waste material. Early tests with the ceramic precursors indicated that minor bonding occurred between the puck and the lower ram. It was deemed that removing the puck with a twisting motion would produce the least amount of residue on the ram, when compared to a pushing motion by the shoe. The robot will provide the twisting motion for removing the puck from the press.

Figure 6
Press Platen and Obstructions

The inspection area consists of a platform scale for weight analysis, an array of lasers for dimensional analysis, and a digital camera for topological analysis. During the inspection sequence, the robot will first present the bottom and top surfaces to the digital camera. The end-effector must not obscure either surface during the visual inspection. The visual inspection process, whether manual or automated, will assess the surface quality of the puck. The platform scale and array of lasers are combined in the next step in the inspection sequence. As the puck rests on the scale the lasers measure diameter and thickness.

The puck is palletized on the furnace tray if it passes all of the inspection steps. The furnace tray has a four-wall design to support other trays stacked on top. The walls of the tray present an obstacle for puck palletization (see figures 1 and 7). A neutral material will be sprayed or laid in the tray bottom to prevent undesirable chemical reactions between the puck and the tray.

To reduce the size of the tray (and therefore the size of the furnaces) the spacing between each puck and the spacing between the pucks and the walls is constrained to ¾". This spacing limits the travel of the gripper jaws. Given the physical dimensions of the puck, and the design of the end-effector, the total jaw travel from gripping the puck to full open was aproximately 7° (per jaw). Unfortunately, the gripper of choice had 15° of travel per jaw. Placing the point where the puck is gripped at mid-travel retained the gripper of choice. This method proved helpful in other aspects, as will be discussed later.

Figure 7
Robot Palletizing a Puck in the
Plastic Mockup of the Furnace Tray

Figure 8
SUS in Glove Box Mockup

Sensors were incorporated on the end-effector to provide robot operational information. The PC monitors the output of the sensors directly or indirectly depending on the sensor.

Hall effect switches are used on the gripper to detect the full open and full closed positions of the jaws. The robot will monitor the output of these sensors directly and passed the information to the control PC. The puck is firmly gripped at mid-travel of the jaws. When the robot is ready to grab a puck the jaws must be fully open. The robot will check the state of the gripper prior to gripping the puck. If the jaws are not fully open a message from the robot to the control PC will stop the process. Conversely, the robot will check the state of the sensor after it has grabbed a puck. If the puck was missed or is not there the gripper will be fully closed. A message to the control PC will stop the process if the gripper is fully closed when it is supposed to have a puck.

Two different types of sensors were tested to prevent unintended contact with the furnace tray. The first was a collision sensor, having rotational, angular, and axial compliance as well as a trip sensor to indicate excessive compliance. The second was a multi-axis force/torque (F/T) sensor, which provided a linear output corresponding to a force and torque applied to the sensor. The control PC monitored the output of these sensors directly.

The collision sensor provided substantial compliance. The control PC monitored the tripping switch. When tripped the control PC would stop the process. The force or torque that cause the sensor to "give" (but not trip) was adjustable with air pressure. A higher pressure was used when the puck was removed from the press and inspected, to add stiffness to the end-effector. A lower pressure was used when the puck was around the furnace tray during palletization, to detect minor impact. This method worked reasonably well. However, the cantilevered weight of the gripper and puck caused a bias force in the Z-direction. The tripping force had to be set at a reasonable level above this bias force. Therefore, the robot would impact and exert a relatively large force on the furnace tray before tripping. Reducing the robot speed could soften the impact, but the force to trip was unacceptably high.

The F/T sensor provided force and torque feedback to the control PC at a fast rate. The control PC would monitor the feedback and "react" when a force or torque limit was exceeded, indicating an impact. Different force and torque limits are used during different steps of the press unloading process. A higher limit is used when the puck is removed from the press and inspected. A lower limit is used when the puck is palletized. The lower limits (in the X, Y, and Z direction) are only limited by the noise of the signal. The speed of the robot is also reduced while operating around the furnace tray during palletization. This allows sufficient time for the control PC to react when a limit is exceeded. The reaction, in most cases, will be to immediately stop the robot and move it to a safe location.

As mentioned previously, the three-jaw gripper will be used to re-center and pick up a sintered puck from the furnace tray. One of the conclusions that was alluded to in the re-centering study was that re-centering became more difficult (more force was required) as the friction between the puck and the tray surface increased. The tray surface will be a neutral sprayed-on of laid-on material. A few of the laid-on materials being considered are very hard and porous ceramics. The surface quality of this material will create high friction with the sintered puck. The force required to re-center a puck on this material may approach the robot motor overload limits. Furthermore, the act of re-centering the puck may chip particles off of the bottom of the puck, creating unwanted contamination. It may be necessary to "find" the shifted puck on the furnace tray to prevent these unwanted effects.

There are several ways for the robot to locate the puck on the tray. Two ways will be explored if the three-jaw method develops insurmountable problems. One is with a digital camera. This method has been employed in other glove box mockups and can be used in the tray transfer glove box. However, a concern exists that the furnace trays could discolor from the sintering process so that it would be difficult to distinguish the pucks from the tray (the sintered pucks are typically dark gray and the trays are typically white).

The second method will use the F/T sensor to "feel" the offset of the puck. The X and Y forces will be zero if the puck is exactly centered in the closed jaws of a three-jaw gripper. There will be a non-zero value for either the X or Y (or both) forces when the puck is off center as the jaws try to force the puck to center. The control PC can recognize the non-zero forces and, by vector analysis, signal the robot to move towards center.

The radiation levels in the actual glove boxes slowly damage certain materials and electrical components. The materials and electrical components of the end-effector are extremely prone to damage due to their proximity to the puck. If possible, radiation resistant parts will be used. If not, the ALARA concept will be used to increase the life expectancy and decrease the maintenance requirements of susceptible parts. ALARA is a method of reducing exposure to radiation workers by keeping exposure As Low As Reasonably Achievable. This is accomplished by the three principles of 1) reducing the exposure time, 2) working at a safe distance, and 3) using appropriate shielding. Keeping the radioactive materials away out of the glove boxes until they are ready for automated handling can reduce the exposure time. The movements of the automated equipment can be programmed to keep susceptible parts away from the radiation source. And shielding can be used as needed or when the first two principles cannot be employed.

Dusting has occurred with several ceramic powder recipes that have been tested for the sintering process. Dusting in the actual PIP press unloading glove box will contaminate other equipment in the glove box with radioactive material. The radioactive material is accountable and must be contained and reused.

The contaminated dust can be managed several ways. Vacuum lines can be placed around the press where dusting is more prevalent. These vacuum lines can remove the dust as it is produced, limiting the spread of contamination. In addition, the glove box can be designed so that the dust collects in areas that are easily policed. The robot, or other programmable, automated equipment, can clean these areas at predetermined intervals.

Disposal of replaceable equipment in the glove box will be difficult and expensive when the equipment becomes contaminated. In most cases, the equipment will be located as far away as possible from the dusty areas. Removable covers that are easily decontaminated will be employed when relocation is not an option. The robot will be a clean-room version. The clean room version has tighter seals and a method to pressurize the housing. Pressurizing the housing will prevent contaminated particles from entering.

The puck handling glove box mockup will eventually be installed in the Ceramic Prototype Test Facility (CPTF) at the Clemson Environmental Technologies Laboratory (CETL). This effort is part of the South Carolina Universities Research and Education Foundation (SCUREF) program that links WSRC with local universities for advanced learning opportunities. This facility will take the development process to the next phase by interfacing major automated components in the production line around the puck handling glove box. The facility will include mills and blenders to prepare the green puck ingredients, a press, and a sintering furnace. The entire facility will be automated and controlled through a central PC. This facility will use non-radioactive surrogate materials to produce the pucks. The installation is slated to begin in CY01.