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A robotic cutting tool for contaminated structure maintenance and decommissioning
Automation in Construction 58 (2015) 109–117
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A robotic cutting tool for contaminated structure maintenance and decommissioning Paul Matteucci, Francesco Cepolina ⁎ PMAR Laboratory, Dept. of Mechanics and Machine Design, University of Genova, via all'Opera Pia 15A, 16145 Genova, Italy
a r t i c l e
i n f o
Article history: Received 24 November 2014 Received in revised form 17 April 2015 Accepted 11 July 2015 Available online xxxx Keywords: Cutting tool Diamond wire Contaminated structures Safety robotics Decommissioning Nuclear power plants
a b s t r a c t With the global dependency on fossil fuels being undermined by growing prices and environmental concerns, nuclear power is returning as a solution for energy demands, however the increased cost of maintaining and updating ageing plants could render them nuclear alternative counter-economic. The development of tools specifically targeted at the maintenance field is vital to ensure the longevity and safety of existing power plants. This paper proposes a robotic tool capable of remotely cutting composite material structures. With design and engineering focused on the safety of the operator and automation, the proposed machine presents sufficient flexibility to be utilised in both maintenance and decommissioning of structures with low to medium levels of radioactive contamination. © 2015 Published by Elsevier B.V.
1. Introduction As of August 2014, 31 countries host over 430 commercial nuclear power reactors and 70 plants are in 16 countries under construction. Asia, Northern America and Western Europe have each about 120 operational nuclear plants (http://www.iaea.org/pris/). The original lifespan of a power plant was considered to be 30 years (United States. Congress. Office of Technology Assessment, 1993), however, in recent years, improvements in technology have allowed to extend the lifespan of the structures [1] to 40 years, with the possibility of further extension if the accurate maintenance is guaranteed [2]. It is however undeniable that many of the structures currently in existences are reaching a critical age, hereby they need to undergo significant maintenance and upgrades, or be shut down and decommissioned [3, 4]. This extension could be rendered counter-economic by the increasing cost of operations and a series of age-related issue such as radiation embrittlement of the nuclear power plant reactor vessel and the corrosion-induced failure of steam generators [5]. Whether or not nuclear power returns as a fundamental source of energy in the coming two decades most existing nuclear power plant sites will require some form of maintenance or decommissioning [6]. With most reactors being designed without factoring decommissioning or upgrades, these operations require extensive use of cutting machines [7] to cost-effectively remove large reactor components, making it vital to develop robotic systems which can perform such cutting operations whilst being safe, scalable and ⁎ Corresponding author. Tel.: +39 331 1379 527. E-mail address: [email protected] (F. Cepolina).
http://dx.doi.org/10.1016/j.autcon.2015.07.006 0926-5805/© 2015 Published by Elsevier B.V.
affordable. Existing cutting systems are not designed specifically for contaminated environments and employ virtually no autonomy either in the robotic platform or in the arm and tool [8, 9] with nearly all systems employing simple remote [10] control, tele-operation [11] or master/slave manipulation [12]. By encompassing modern powering, automation, sensors and enhanced remote control, this paper proposes a cutting tool designed to be used as a standalone robot, but which can also act as a mobile robot end-effector [13], which was engineered specifically to operate in both active and shutdown power plants. This was achieved by reducing the design elements which would render operation in a contaminated environment difficult, by implementing features to reduce the formation of dangerous dusts and polluting by-products, and by maximising the safety of the operator. Diamond wire was chosen as an abrasive cutting mean due to its increase in reliability over the past two decades, the industrial success acquired in cutting composite materials in off-shore structures and the successful use of diamond wire in decommissioning the Princeton test plasma fusion reactor [14]. Reactor cores are immersed in pools of filtered water which acts as both a coolant and a radiation containment; these present the ideal conditions for diamond wire cutting tool as water cools the diamonds and prevents burning and premature degeneration of the cutting wire. When compared to alternative cutting techniques, the diamond wire system offers the advantages of a cold cutting system: low thermal impact on the structure being cut and reduced risk of sparking (unlike gas mix cutting); however, unlike abrasive cutting, diamond wire cutting creates a low volume of potentially contaminated dust by only shearing a section equal to the diameter of wire being used
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and not requiring additional abrasive material which needs to be removed in the case of a maintenance operation and which aggravates the contamination in the case of decommissioning [15]. 2. Focus on research and innovations The main priority of engineering a system which operates in a contaminated environment is the safety of the people involved in the operation, followed by the safety of the working environment itself. Depending on the type and level of contamination of the environment, it is likely that the robotic system will remain on site once the operation is complete and will be disposed of together with the contaminated waste or suitably decontaminated. Based on the maintenance and decommissioning requirements the robotic system proposed is made by three main modules: a mobile platform, a manipulator, a cutting tool as working end-effector. Taking into account the heavy work made by the tool that apply very high forces to the structure to cut [16], the mobile platform has to be robust, able to overcome obstacles and able to keep stable during the cutting operations, may be after robust clamping on the structure. Platforms from the market with four separate tracks that can be tilted and raised up or down are considered for this purpose [17]. For heavy decommissioning tasks in radioactive environment, the manipulator on board the platform has to be robust, remotely controlled, waterproof in order to be washed by highpressure water jet for decontamination; few degree of freedom parallel kinematic hydraulically powered arm is considered with a wrist that can be equipped with instrumental end effector tool [7]. The paper focuses on the design and prototyping of the most innovative component that is the cutting diamond tool purposely developed for maintenance and decommissioning tasks. Current diamond wire tools are comprised of a simple frame upon which are mounted two or more pulleys; one which acts as a drive, pulling the diamond wire through the target structure, and at least one of the other pulleys acting as a tensioner to maintain tension on the drive pulley. Although applications in both the offshore [16] and in the infrastructures [18] have demonstrated that diamond wire is indeed ideal for decommissioning structures, these are not suitable for contaminated environments in their current form. Due to the low-technology fields in which these machines originate, they are almost exclusively hydraulically powered and deprived of sensors and automation. Industrially available diamond wires are designed to cut either stone or carbon steel and concrete materials; the use of hydraulics as a sole mean of powering the system has various drawbacks: high flow of high pressure oil is at constant risk of leaking, requires a large and inefficient external power unit and has range limitations. It was therefore necessary to custom design a wire which would be more suitable for cutting the softer 304 stainless steel common in many core reactor components, and to engineer a solution which limited the hydraulic system to the bare essential whilst implementing remote control and extending the range of operation to that which would be safe in a contaminated environment. Furthermore, due to safety and environmental concerns [19], the design was also required to be cost effective [20] as the machine would likely be decommissioned upon completion of a campaign. The proposed system utilises the concept of constant tension wire first patented by TS [21], where a spinning wire, held at a relatively constant tension by a pulley mounted on an actuator, is forced against a target to induce shearing. The wire is looped around two or more pulleys, one or more of which are powered. The recent improvement in permanent magnets and consequently in electric motors means that electric motors are now sufficiently compact to be used to drive the main pulley. The use of a permanent magnet, brushless motor to drive the cutting wire, leaves only feeding and clamping functions to be powered hydraulically. The main modules of the proposed cutting tool are visible in Fig. 1: the cutting module and diamond wire (1), it is powered by an electric motor assembled under the drive pulley; the cutting module support
arm (2); the clamping module (3) [24]; the hybrid control unit (4) which encloses the communication, control and powering of system functions. Various sensors are arranged both on the manifold and on the machine itself, providing feedback regarding the status of the machine: cutting motor absorption, feeding module position, wire tensioner position, clamping pressure, pump velocity, oil temperature, feeding circuit pressure. After the mobile robot reaches the working position the main steps of the operative cutting cycle are: arm end effector positioning guided by vision and robust clamping to the target structure, cutting tool set-up, diamond wire powering, frame sliding and cutting. Once the main hydraulic system was eliminated, the use of a hydraulic circuit pressurised externally became redundant, and it was decided to move the motor/pump for the feeding and clamping circuits onto the machine itself. This required the hydraulic manifold, reservoir and consequently the controlling electronics to also be shifted onto the machine. It was therefore decided to centralise all of these elements into a single enclosure, together with the brushless motor electronic driver; for simplicity this was called the Hydraulic, Communication and Control Unit (HCCU). To enhance feedback and therefore both safety and efficiency, sensors were installed on the robotic tool to monitor all the variables significant for the right development of the operation. Linear sensors were used to determine the cutting module position on its support frame, whilst most sensors were either enclosed in the HCCU (such as oil pressure, temperature, flow etc.) and all information for the cutting motor were provided to the controller from the motor driver (motor speed, power absorption, operating frequency etc.). Particular and extensive research was also dedicated to the control software, for both the embedded controller and the operator's control panel. The embedded controller provides more than just communication and control. The control software was developed to allow the system to behave as a simple tool, being controlled completely by an operator; to assist the operator with automatic cutting adjustments, or to act as a complete robotic system, autonomously performing the entire operation even if deprived of communication with the external environment. In any case the control panel interface was designed to be intuitive and easy to use for an operator used to working with traditional full hydraulic system. 3. Subsystem design 3.1. Cutting module The cutting module is composed of four pulleys, one of which powered, which spin a diamond wire at high speed and cut the target by shearing. The wire is kept at a constant tension by one of the pulleys which is mounted on a pre-charged tensioner [16]. Tests run using traditional hydraulic actuation allowed to determine the torque required to power the drive. An experiment was setup using a 34 cm3 hydraulic motor with a pressure sensor installed between the motor and the hydraulic umbilical. This model was chosen because it presents the largest single-motor machine currently in use and therefore the maximum amount of force expressed by a single cutting motor. The pressure was then recorded during an 80 minute cut, during which flow was maintained at a constant value 50 l/min. Return pressure was recorded at approximately 2 MPa. The recorded values for p and Δp (pressure variation across the motor) can be found in Fig. 2 By analysing the recorded data, it was determined that minimum pressure (free spinning) required by the motor is 9 MPa, whilst the maximum operating pressure was recorded at 14 MPa, during maximum surface cutting. Torque was determined using τ = (V · Δp · μ)/2π where V is the volume displacement in cm3, Δpis the pressure drop across the motor in MPa and μ is the efficiency of the motor, 0.85 as per constructor specifications. The motor was run at optimal cutting speed; the speed was then decreased until torque was insufficient to spin the wire. The values
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Fig. 1. Modules of the robotic cutting tool (1–4) and assembly (5).
were recorded and values for torque were found to be 41 Nm at rotor lock and 65 Nm at cutting velocity. In order to rightly size the electric actuation substituting the hydraulic one, the operation of the machine itself must be taken into account. When cutting, the maximum pressure needs be applied on 0 rpm only if the structure being cut collapses and the wire gets stuck in the cut. Attempting to unlock the wire using only the torque of the motor is almost always unsuccessful. If, otherwise, the cut needs to be suspended and then continued, the standard operating procedure is to retrieve the machine, in order to alleviate some of the pressure from the wire, and then continue the cutting operation once the wire has achieved sufficient speed. From the experiment it was determined that, in order to successfully substitute the existing hydraulic motors with an electric equivalent, the full rotational torque will need to be that of the cutting motor at full operation whilst the zero torque will only need to be that of the minimum rotation. In order for the electric propulsion system to effectively replace the existing hydraulic one, the motor used must be within reasonable size and weight parameters. It is occasionally the case that a machine will be tailored to fit exactly between two structures in order to perform a cut, when this occurs, the pulleys are the part of the machine with the maximum width and therefore the determining factor in sizing and positioning the machine, so the motor should not exceed in diameter that of the pulley. As it concerns the weight because weight is not a real limitation due to the nature of the machine, it was decided that a limit
would be set to conform to common weight lifting regulations (GCAL, 1992 #70) and ensure that a technician can disassemble the motor from the frame and maintain it without being assisted. 3.2. The support arm A U shaped support arm is in charge of feeding the cutting module making it shift along the two legs in order to perform the cutting operation. One feeding motor through a gear spin and endless screw drives the progress of the cutting module through the target to cut. The feeding circuit is an open circuit driving a motor and requiring a specific flow to operate. As per the cutting motor there is no need to account for the power required to pump the oil through the length of the umbilical. Pressure must be taken into account as it is the consequence of the force the feeding motor is encountering when pushing the cutting module and determines the speed at which the cut can be made and at which the machine can be retracted. Since feeding in the system at normal feeding rates can take from 30 min to up to several hours, based on the machine type, a fast feeding system was developed to return the machine to its retracted state in a short time span. This operation however requires a lower flow pressure than cutting as retracting the machine encounters none of the resistance of the cutting operation. An experiment was setup to evaluate the hydraulic requirements, by coupling a 12.9 cm3 hydraulic motor with a 20:1 worm-gear and then mounting it onto an endless screw assembly which drove a cutting module. Considering speed characteristics of the diamond wire, it was determined that a satisfactory dynamic range would be 5–15 mm/min under normal cutting conditions and 80 mm/min when resetting the machine. Under these conditions, maximum feeding flow to the 12.9 cm3 hydraulic was calculated at 4.128 l/min, and maximum pressure drop across the motor was found to be 7.5 MPa. The power requirement for the pump was given by Pmotor = (Δp · Q · 1.635)/100μ where Pmotor is power in kW and Δp is pressure drop across the motor in MPa, Q is flow rate in litres per minute and μ is the efficiency of the motor. 3.3. The clamping module
Fig. 2. Motor pressure variation during cut.
This module is present on most current machines and can be designed with different architectures, with double parallel jaws or as a self adaptable clamping tool for multiple seizure [22, 23], but sometimes
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it is not required on a custom built. It can be replaced with a frame fixed into position on the cutting target. The clamping circuit is a closed circuit, so no flow is required. The only requirements are that a sufficient pressure can be achieved to lock the clamps in place (20 MPa), and that there is a sufficient reservoir in the system to accommodate for the difference in volume occupied when the piston is retracted and part of the internal volume is occupied by the piston's rod, and is given by V = n ⋅ πr2 ⋅ l where V is the volume, n is the number of pistons mounted on the machine, r is the radius of the rod's cross section and l is the rod (or piston) length. Because the hydraulic circuit is flooded and bleeding of air is performed before operation, this will result in the only variable volume of oil in the system; however testing should be performed to determine and account for the variation of volume due to temperature change of the oil during operation. 3.4. The hybrid unit The hybrid unit contains three separate sections, as visible in Fig. 3: • Motor pump assembly: a pump connected to an electric AC motor which powers the hydraulic systems on the robotic tool; it is flooded with the oil reservoir and completely sealed except for three hydraulic hose connectors (pressure, return and drain) in the section wall and electrical connections for both motor control and sensors. • Electro valve manifold: controls the flow of oil to the individual functions of the robotic tool. The second section was engineered to contain the hydraulic manifold machined out a single aluminium block and utilising cartridge valves. This was vital to ensure a compact manifold which would fit within the confined cylinder internal diameter. Three umbilical hoses connect the motor/pump section to the manifold, which attach to the actuators on the machine. • Electronic section: it encloses the printed circuit board which controls the brushless cutting motor, and the custom-designed embedded
controller responsible for communication with the surface and driver, control of the manifold and automation. The centralisation of these functions into a single enclosed unit protects them from possible water infiltrations, facilitates removal for maintenance and, if necessary, allows flooding the module with oil to provide increased radiation shielding for the enclosed electronics. 3.5. Hydraulic circuit analysis Because the cutting motor is no longer powered hydraulically, the pump contained in the hybrid module is required to only produce a flow and pressure sufficient for the feeding and clamping. Moreover, to power these functions, oil is no longer required to be pumped through a hydraulic umbilical over a long distance. The unit is therefore engineered with a small motor-pump as opposed to the large motors which are currently powering the hydraulic power units. This dramatically reduces size and weight of the equipment needed by eliminating the need for a hydraulic power unit and spooler, and only requiring an electrical umbilical which can provide power and control, as well as cost for consumables. A series of solenoid-valves would replicate the functionality of a hydraulic control panel and be controlled digitally. The manifold which replaces the hydraulic control panel would be located on the machine and therefore would need to be more compact whilst maintaining all functionality. Integrated into the manifold would be a series of sensors which will monitor aspects of the hydraulic system. The volume of potentially contaminating oil is dramatically reduced: the volume of hydraulic oil required in the system is reduced by about 97%, from approximately 300 l to 10 l and flow is reduced by 99.4%, from 110 l/min on larger machines to a maximum of 4 l/min for fast feeding return functions.
Fig. 3. Details of the hybrid unit.
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A dramatic reduction is also found in the surface of hydraulic umbilicals at risk of damage or failure with a consequent spill of hydraulic oil: the distance of is reduce from 1800 m (9 function hoses of 200 m each), to just the length of umbilicals needed from the localised hydraulic pressure unit to the individual function, hence 10 to 40 m in all depending on size and model of the machine. Finally, the risk of contamination for the operator is eliminated as the remotely contained hydraulic circuit, without flow back to a hydraulic control panel, ensures that contaminated oil remains contained and never gets in close proximity to the operators. 3.6. Control system All control is driven through a custom designed controller. The individual feeding and clamping functions are realised through the hydraulic control circuit, which schema is represented in Fig. 4, analysed in the following points: • Feeding control flow does not require two individual circuits, one for low flow and one for high. Because the system can be tailored to the machine's needs and all machines use the same capacity feeding motor, maximum value of flow can be set to 4 l/min. • Clamping control also has no need for bidirectional non-return valves. A single non-return valve need only be installed on the return circuit to prevent loss of pressure when the machine is clamped in place. • General circuit, a main pressure release valve needs to be installed and set to maximum pressure of 22 MPa. A main hydraulic bypass controlled by solenoid is required to place the system under standby rather than start-stopping the motor repeatedly. The solenoid valves are driven by binary ports through relays, whilst the speed of the feeding module is controlled by a proportional flow valve controlled by PWM. All sensors are connected to analogue input ports. Communication with the external environment and with the electric motor driver is achieved through serial ports via rs-232 allowing serial communication of up to 1200 m. Alternative longer-range optic fibre communication was also tested. In this case the adapter was coupled with the rs-232 and allowed high speed, long range serial communication with no noticeable difficulties. The range and cost of optic fibre however have to be evaluated for the current application. Two main control programmes were developed and embedded into the two panels: • A Tester Panel, to monitor and perform tests on the functions of the system. • A Control Panel, to allow the operator to control the machine on the field.
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They work as a single programme, as the communication system, sharing common functions as string handling, delegate, etc. The Tester Panel (TP) was developed to allow testing of the hybrid module and for troubleshooting on the field. The simplified nature of the Tester Panel consisted in storing all incoming values simply as variables rather than complex class structure. The values are mirrored directly in the user interface. Each function generates a string which is recognised by the processor interface control board, by adding a prefix which determines the type of port which is required to be read or to be set. The Control Panel (CP) programme uses all values received by the machine in a simulated virtual machine by means of a complex structure of classes and inheritance as shown in Fig. 5. The child functions depend on the parent status. Each sensor is responsible for its own updating and communication with the real machine. For safety reason, in order to ensure that a user would not attempt to control functions which were disabled, thus creating confusion as to the actual behaviour of the machine, a functions control hierarchy has been implemented as can be seen in Fig. 6. Other security features include a further control implemented so that any attempt to shut down the pump would result in a warning and the opening of the isolation valve unless a shutdown is instructed. This reduces the amount of times the motor powering the hydraulic system is powered up and down during an operation and should increase the lifespan of the mechanical components. Isolating the hydraulic system will also cause the resetting of all functions which depend on it, in such a way that once hydraulic standby is closed the machine always presents itself in the same, safe and stationary status. The control system can operate with three degrees of automatic control: Manual, Semi-Automatic, and Full Automatic. Each strategy of control varies the modality used to analyse data being fed back from the sensors, and to react by forwarding commands to the machine. Manual Mode is for direct control on behalf of the operators. All controls are fed back to the control panel by the cutting machine and all changes in state come as a direct consequence of operator interaction. A watchdog system is implemented whereby if communication is lost between the robot and the control panel, both attempt reconnection for 60 s. If connection has not been re-established, the robot will force a shut-down of all systems, reset all valves of the manifold to default status, perform a check that all operations have been completed successfully, reset itself and await incoming communication from the control panel. Operator Assisted Mode assists the operator by trimming the cutting tool during operation. As for the manual mode, all information is fed from the machine back to the control panel. The control panel then opens or closes the feeding proportional valve based on feedback from the tensioner sensor, to ensure an optimal cutting
Fig. 4. Hydraulic manifold diagram.
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Fig. 5. Class hierarchy of the CP prototype.
speed and that the wire never remains trapped in the cut. Reaction to variance is calculated based on the latest value's variation being greater than standard deviation over the last 20 s (sampling every 0.2 s), and a serial command is issued to the controller to increase proportional valve opening by 1% (increase of 0.2 l/min). Operators can supervise the operation and upon interacting with the Control Panel, control mode is automatically returned to Manual. Likewise if any of the sensors reports values which are outside of operating parameters warnings are issued to the operator and control is set to Manual. The same watchdog implemented in the manual mode is also implemented. Automated Mode takes over the entire cutting operation from the operator. Unlike other modes, information is sent back to the control software for monitoring purposes only. The firmware in the control board will open or close the proportional valve based on feedback from the tensioner, thus regulating the speed of the cut. The watchdog system is disabled in this mode. If communication is lost, the robot will continue the operation and continue cutting until the tensioner detects that the cut is complete or the cutting module is fully extended. Likewise if a sensor issues a warning, based on the criticality of the sensor, the
controller will either slow the machine down for several minutes, attempt to compensate for the problem, or stop all together and reset, issuing warnings to the operator. 4. Prototype construction Two prototypes were constructed. The first to test the motor with different gear, wire and pulley configurations, the second to test the capabilities of the system whilst performing an actual cut on a mockup steam dryer assembled by a nuclear power plant maintenance company. The first prototype (Fig. 7) was constructed out of carbon steel frame able to be embedded with three to seven pulleys. This allowed testing of different motors with various wire configurations including a variable numbers of idle pulleys onto the frame immersed in water connected by variable lengths of wire and driven by a single powered pulley. During the tests the following data was recorded: • Motor speed • Motor absorption
Fig. 6. Function control hierarchy.
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from aluminium but then redesigned to use cheaper and commercially available 220 mm diameter stainless steel tubes. A 0.75 kW AC asynchronous motor was purchased and coupled with a 1.9 cm3 gear pump and filters (Fig. 8-A). The hydraulic manifold (Fig. 8-B) was machined out of a single aluminium block and incorporated suitable cartridge valves, thus allowing it to fit within the constraints of the cylinder. All hydraulic elements (pistons, 12.9 cm3 feeding motor, hoses etc.) were assembled. The printed circuit board (PCB) of the controller was developed inhouse utilising a commercial microchip. The chip was due to the presence of two serial ports for communication and sufficient pins to connect nine analogue in sensors (six active plus three spare), nine digital i/o pins to control the manifold (six active plus three spare) and two PWMs to control the proportional valve and thus the cutting speed (one active, one spare). On the PCB are also present voltage controller, LEDs, resistors and capacitors. For the sensing, absolute optical encoders were mounted on the feeding motor (Fig. 9D) and a magnetostrictive sensor was installed on the frame to track the position of the cutting module. Finally, different types of diamond wires were sourced from an undisclosed supplier.
Fig. 7. Cutting module prototype.
• • • •
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Supply frequency Motor consumption Temperature of motor Temperature of water
5. Prototype testing The cutting robotic tool prototype was tested in an enclosed pool. Motor absorption was monitored as well motor temperature, overall pool temperature and cutting rate. The objective of the test was to test various motor/gear combinations, the absorption of the motor under a load comparable to that of a cutting wire and the consequential change in temperature of the reactor pool. The load was simulated by using three additional idle pulleys, for a total of six idle and one driven. In order to reduce motor heating, the diameter of the pulley was increased from 280 mm to 320 mm, thus giving the same linear speed to the cable but with a lower motor speed. Two wire configurations were tested. Both wires used the same steel-cable core, same diameter beads and medium to large sized
Tests based comparative evaluation suggested to use a submersible brushless, permanent-magnet small servo motor which outputs 21 Nm of torque at 4000 rpm in conjunction with a right-angle 3:1 gear which resulted in a final torque on the pulley shaft of 66 Nm and an ideal output speed of 1300 rpm. Once a suitable motor was identified, fabrication of the second prototype began. The frame was constructed out of stainless steel with the main plates for the cutting modules made out of aluminium. The communication, hydraulic and control cylinder was initially machined
A
B
C
D Fig. 8. A. Hydraulic motor/pump assembly. B. Hydraulic solenoid-valve manifold. C. PIC embedded controller on test bench. D. Feeding motor with absolute optical encoder.
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A
B
Fig. 9. Wire performance cutting SS304 in water A. Forty beads per metre cut. B. Fifty beads per metre cut.
industrial diamonds. The first cable (A) was fitted with a low number of beads per metre (40), the second (B) with a high number of beads (50 +). A series of SS304 box sections were cut over a 7 hour period. The wire performance is indicated in Fig. 1. It is interesting to notice the difference in cutting behaviour of the two wires. Due to the greater distance between beads, wire A proved to be more flexible than wire B; because of the angle created between the wire outside the target material, and the wire actually shearing, the increased flexibility created a sharper angle of entrance into the material, thus forcing the beads into the target and producing higher shearing force. In the long run however the smaller number of diamonds on the cable causes the shearing capabilities to be reduced, and with the reduction in diamond size, there is an increase in diamonds rendered useless by malleable material pasting. Wire B on the other hand, although never reaching the cutting efficiency of Wire A, maintains its shearing ability for longer due to the larger number of diamond, and thus the higher number of shearing elements as well as the reducing pasting of the wire. From this data it was decided that a reduced bead configuration would be chosen for shorter cutting operations (less than 5 h) to favour speed, whilst a higher density of
beads would be chosen when dealing with long cutting operations (5 h or more) in those cases when the wire could not be changed. During the trial, the motor temperature reached a peak of 65 °C which approaches a potentially damaging temperature. This however was not considered to be a concern as the 4 × 4 × 2 m pool was tested to be significantly smaller than the pools found in reactor cores and without induced circulation, causing the water temperature to increase from 19.4 °C to 37 °C. The second prototype was tested on a mock-up steam dryer assembled to simulate as accurately as possibly the conditions in a nuclear reactor. Due to safety concerns, the fully automated system was not used in this setup; however the controller-assisted cutting was enabled for much of the operation. The cut was successful and completed in just less than 6 h. Recordings from the sensors are displayed in Fig. 10. The motor power was controlled manually, and the assisted cutting algorithm worked effectively, adjusting the feeding speed in relation to the ease of cut. It is visible from the graph that an increase in cutting power corresponds to the robot increasing feeding speed as the force acting on the tensioner decreases.
Fig. 10. Main variables trend during the cut of the steam dryer mock-up.
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The motor power was controlled manually, and the assisted cutting algorithm worked effectively, adjusting the feeding speed in relation to the ease of cut. It is visible from the graph that an increase in cutting power corresponds to the robot increasing feeding speed as the force acting on the tensioner decreases. The initial slow feeding speed is due to the wire encountering large flat surfaces of the box tubular used to assemble the mock-up, a similar slow down was detected when approaching completion of the cut. 6. Conclusions and future developments A robotic system is proposed suitable for contaminated structure maintenance and decommissioning. The system uses diamond wire for cutting. The standalone teleoperated robot is purposely developed to be safe for users and for the working environment itself. A hydraulically powered robust clamping secures the robot to the structure to be cut, a hydraulically driven feeding mechanism moves the diamond wire toward the part to be cut, and a brushless motor drives the cutting wire. A custom designed hybrid control unit encloses the communication, the control and powering of system functions. Two cutting tools prototypes have been constructed and successfully tested. Overall the robot works effectively and efficiently. All proposed objectives are met: the operator uses the robotic system to cut at a safe distance (300 m), the integrated feeding control algorithm reduces the human error. Recently the robot has successfully completed a cut of a steam dryer for upgrading purposes in an undisclosed US reactor and is in list for a similar operation in Japan in the near future. Whether nuclear power plants become a technology of the past in the search for cleaner solutions, or are maintained and modified for increased life span, civil engineering in contaminated environments is a necessity. The proposed remotely controlled robot minimises the amount of dangerous waste and polluting substances generated during the cutting process. The need of materials, resources and energy involved in the decommissioning process is also minimised. The proposed robotic tool guarantees that any human operator, working with the system or in close vicinity to contaminated environments, is fully safeguarded, in conformity with the highest level of health and safety standards. Some improvements are necessary in order to allow the robot to operate in highly contaminated environments; the feeding adjustment algorithm needs to be further developed, the embedded controller should be enhanced and radiation shielded to protect the electronic devices from damage and interference. The use of a compact new generation brushless motor will allow to reduce the cutting actuation system in size and weight; maintenance will be simplified and cost will be reduced. Acknowledgment TS Tecnospamec srl, patent holder for the Diamond Wire Cutting System, CUT Offshore and CUT Nuclear are gratefully acknowledged. The authors also thank the editors and referees for their careful review.
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References [1] J. Rust, G. Rothwell, On the optimal lifetime of nuclear power plants, Industrial Organization 9512002. EconWPA, 2002. [2] Tweed Katerine, APS argues to extend lifespan of nuclear reactors to 80 years, IEEE Spectr. (December 12 2013). http://spectrum.ieee.org/energywise/energy/nuclear/ aps-argues-to-extend-lifespan-of-nuclear-reactors-to-80-years. [3] Tommy Hansson, Thomas Norberg, Andreas Knutsson, Patrik Fors, Camilla Sandebert, Ringhals Site Study 2013—An Assessment of the Decommissioning Cost for the Ringhals Site, Swedish Nuclear Fuel and Waste Management Co., Stockholm (Sweden), 2013. [4] S. Slater, Decommissioning and Remediation after a Nuclear Accident. 3 Case Studies, 2013. [5] S.Y. Choi, Y.H. Choi, Piping Failure Analysis in Domestic Nuclear Safety Piping System, Spring Conference of KSME, Pusan, 2003. [6] KwanSeong Jeong, ByungSeon Choi, JeiKwon Moon, Dongjun Hyun, JongHwan Lee, IkJune Kim, GeunHo Kim, et al., An evaluation on the cutting technologies for decommissioning of the large cylinder parts in the major components of NPPs, Ann. Nucl. Energy 71 (2014) 340–342. [7] Jeong Kwan Seong, Geun Ho Kim, Jei Kwon Moon, Byung Seon Choi, Cutting Technology for Decommissioning of the Reactor Pressure Vessels in Nuclear Power Plants, 2012. [8] R. Molfino, M. Zoppi, Heavy duty uuv for offshore plants decommissioning, CLAWAR 2006, 9th International Conference on Climbing and Walking Robots, Brussels, 2006. [9] Xiaojie Tian, Yonghong Liu, Rongju Lin, Pengfei Sun, Renjie Ji, An autonomous robot for casing cutting in oil platform decommission, Int. J. Control. Autom. 6 (4) (2013) 356–362. [10] K. Kravarik, S. Brecka, S. Kosnac, L. Blazekova, Advanced ca technologies and remotely controlled manipulators used for the decontamination of the a-1 npp jaslovske bohunice, International Conference on Safe Decommissioning for Nuclear Activities, Berlin, 2002. [11] L. Cragg, H. Hu, Application of mobile agents to robust teleoperation of internet robots in nuclear decommissioning, IEEE Int. Conf. Ind. Technol. 2 (December 2003). [12] D.W. Seward, M.J. Bakari, The use of robotics and automation in nuclear decommissioning, 22nd International Symposium on Automation and Robotics in Construction ISARC 2005, Ferrara, 2005. [13] P. Matteucci, R. Molfino, Development of a robotic cutting device for contaminated structure decommissioning, International Conference CLAWAR 2009, Istanbul 2009, pp. 811–818. [14] K. Rule, E. Perry, R. Parsells, Diamond wire cutting of the Tokamak fusion test reactor, state of the art technology for decontamination and dismantling of nuclear facilities, Tech. Rep. 395, Princeton Plasma Physics Laboratory, 1999. [15] W. Ian Hore-Lacy, Decommissioning nuclear facilities, Encyclopedia of Earth (National Council for Science and the Environment, Washington, USA), 2006. [16] Rezia M. Molfino, Matteo Zoppi, A robotic system for underwater eco-sustainable wire-cutting, Autom. Constr. 24 (2012) 213–223. [17] Seungho Kim, Seung Ho Jung, Sung Uk Lee, Chang Hoi Kim, Ho Chul Shin, Yong Chil Seo, Nam Ho Lee, Kyung Min Jung, Application of robotics for the nuclear power plants in Korea, Applied Robotics for the Power Industry (CARPI), 2010 1st International Conference, IEEE 2010, pp. 1–5. [18] Liwen Cao, Guohua Liu, Wenbin Jia, Peng Jing, Haibing Zhang, Study on the wear and cutting mechanism of oil pipeline cutting with diamond wire saw, Bridges 10 (2014) (9780784412619-122). [19] Christophe Martin, Aurélien Portelli, Franck Guarnier, Myths and representations in French nuclear history: the impact on decommissioning safety, Safety, Reliability and Risk Analysis: Beyond the Horizon 2013, p. 393. [20] Ian Seed, Paula James, John Mathieson, Laurie Judd, Rosa Elmetti-Ramirez, Ana Han, Investing in International Information Exchange Activities to Improve the Safety, Cost Effectiveness and Schedule of Cleanup-13281, 2013. [21] Tecnospamec T. Patents, GE91a000143Italy (1992), US 5361748 (1994), EU 0540834 (1992) [22] Shunichi Suzuki, Demolition and Removal of Structures Damaged or Contaminated as a Result of the Fukushima Accident, 2013. [23] Cepolina Francesco, Gripping device with flexible elements, patent WO 2005002797 A1, 13 January 2005 [24] Francesco Cepolina, Francesco Matteucci, Rinaldo C. Michelini, Self Adaptable Clamping Tools for Multiple-Seizure, IEEE International Conference on Intelligent Manipulation and Grasping IMG04, 1–2 July, Genova 2004, pp. 308–312.
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Automation in Construction journal homepage: www.elsevier.com/locate/autcon
A robotic cutting tool for contaminated structure maintenance and decommissioning Paul Matteucci, Francesco Cepolina ⁎ PMAR Laboratory, Dept. of Mechanics and Machine Design, University of Genova, via all'Opera Pia 15A, 16145 Genova, Italy
a r t i c l e
i n f o
Article history: Received 24 November 2014 Received in revised form 17 April 2015 Accepted 11 July 2015 Available online xxxx Keywords: Cutting tool Diamond wire Contaminated structures Safety robotics Decommissioning Nuclear power plants
a b s t r a c t With the global dependency on fossil fuels being undermined by growing prices and environmental concerns, nuclear power is returning as a solution for energy demands, however the increased cost of maintaining and updating ageing plants could render them nuclear alternative counter-economic. The development of tools specifically targeted at the maintenance field is vital to ensure the longevity and safety of existing power plants. This paper proposes a robotic tool capable of remotely cutting composite material structures. With design and engineering focused on the safety of the operator and automation, the proposed machine presents sufficient flexibility to be utilised in both maintenance and decommissioning of structures with low to medium levels of radioactive contamination. © 2015 Published by Elsevier B.V.
1. Introduction As of August 2014, 31 countries host over 430 commercial nuclear power reactors and 70 plants are in 16 countries under construction. Asia, Northern America and Western Europe have each about 120 operational nuclear plants (http://www.iaea.org/pris/). The original lifespan of a power plant was considered to be 30 years (United States. Congress. Office of Technology Assessment, 1993), however, in recent years, improvements in technology have allowed to extend the lifespan of the structures [1] to 40 years, with the possibility of further extension if the accurate maintenance is guaranteed [2]. It is however undeniable that many of the structures currently in existences are reaching a critical age, hereby they need to undergo significant maintenance and upgrades, or be shut down and decommissioned [3, 4]. This extension could be rendered counter-economic by the increasing cost of operations and a series of age-related issue such as radiation embrittlement of the nuclear power plant reactor vessel and the corrosion-induced failure of steam generators [5]. Whether or not nuclear power returns as a fundamental source of energy in the coming two decades most existing nuclear power plant sites will require some form of maintenance or decommissioning [6]. With most reactors being designed without factoring decommissioning or upgrades, these operations require extensive use of cutting machines [7] to cost-effectively remove large reactor components, making it vital to develop robotic systems which can perform such cutting operations whilst being safe, scalable and ⁎ Corresponding author. Tel.: +39 331 1379 527. E-mail address: [email protected] (F. Cepolina).
http://dx.doi.org/10.1016/j.autcon.2015.07.006 0926-5805/© 2015 Published by Elsevier B.V.
affordable. Existing cutting systems are not designed specifically for contaminated environments and employ virtually no autonomy either in the robotic platform or in the arm and tool [8, 9] with nearly all systems employing simple remote [10] control, tele-operation [11] or master/slave manipulation [12]. By encompassing modern powering, automation, sensors and enhanced remote control, this paper proposes a cutting tool designed to be used as a standalone robot, but which can also act as a mobile robot end-effector [13], which was engineered specifically to operate in both active and shutdown power plants. This was achieved by reducing the design elements which would render operation in a contaminated environment difficult, by implementing features to reduce the formation of dangerous dusts and polluting by-products, and by maximising the safety of the operator. Diamond wire was chosen as an abrasive cutting mean due to its increase in reliability over the past two decades, the industrial success acquired in cutting composite materials in off-shore structures and the successful use of diamond wire in decommissioning the Princeton test plasma fusion reactor [14]. Reactor cores are immersed in pools of filtered water which acts as both a coolant and a radiation containment; these present the ideal conditions for diamond wire cutting tool as water cools the diamonds and prevents burning and premature degeneration of the cutting wire. When compared to alternative cutting techniques, the diamond wire system offers the advantages of a cold cutting system: low thermal impact on the structure being cut and reduced risk of sparking (unlike gas mix cutting); however, unlike abrasive cutting, diamond wire cutting creates a low volume of potentially contaminated dust by only shearing a section equal to the diameter of wire being used
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and not requiring additional abrasive material which needs to be removed in the case of a maintenance operation and which aggravates the contamination in the case of decommissioning [15]. 2. Focus on research and innovations The main priority of engineering a system which operates in a contaminated environment is the safety of the people involved in the operation, followed by the safety of the working environment itself. Depending on the type and level of contamination of the environment, it is likely that the robotic system will remain on site once the operation is complete and will be disposed of together with the contaminated waste or suitably decontaminated. Based on the maintenance and decommissioning requirements the robotic system proposed is made by three main modules: a mobile platform, a manipulator, a cutting tool as working end-effector. Taking into account the heavy work made by the tool that apply very high forces to the structure to cut [16], the mobile platform has to be robust, able to overcome obstacles and able to keep stable during the cutting operations, may be after robust clamping on the structure. Platforms from the market with four separate tracks that can be tilted and raised up or down are considered for this purpose [17]. For heavy decommissioning tasks in radioactive environment, the manipulator on board the platform has to be robust, remotely controlled, waterproof in order to be washed by highpressure water jet for decontamination; few degree of freedom parallel kinematic hydraulically powered arm is considered with a wrist that can be equipped with instrumental end effector tool [7]. The paper focuses on the design and prototyping of the most innovative component that is the cutting diamond tool purposely developed for maintenance and decommissioning tasks. Current diamond wire tools are comprised of a simple frame upon which are mounted two or more pulleys; one which acts as a drive, pulling the diamond wire through the target structure, and at least one of the other pulleys acting as a tensioner to maintain tension on the drive pulley. Although applications in both the offshore [16] and in the infrastructures [18] have demonstrated that diamond wire is indeed ideal for decommissioning structures, these are not suitable for contaminated environments in their current form. Due to the low-technology fields in which these machines originate, they are almost exclusively hydraulically powered and deprived of sensors and automation. Industrially available diamond wires are designed to cut either stone or carbon steel and concrete materials; the use of hydraulics as a sole mean of powering the system has various drawbacks: high flow of high pressure oil is at constant risk of leaking, requires a large and inefficient external power unit and has range limitations. It was therefore necessary to custom design a wire which would be more suitable for cutting the softer 304 stainless steel common in many core reactor components, and to engineer a solution which limited the hydraulic system to the bare essential whilst implementing remote control and extending the range of operation to that which would be safe in a contaminated environment. Furthermore, due to safety and environmental concerns [19], the design was also required to be cost effective [20] as the machine would likely be decommissioned upon completion of a campaign. The proposed system utilises the concept of constant tension wire first patented by TS [21], where a spinning wire, held at a relatively constant tension by a pulley mounted on an actuator, is forced against a target to induce shearing. The wire is looped around two or more pulleys, one or more of which are powered. The recent improvement in permanent magnets and consequently in electric motors means that electric motors are now sufficiently compact to be used to drive the main pulley. The use of a permanent magnet, brushless motor to drive the cutting wire, leaves only feeding and clamping functions to be powered hydraulically. The main modules of the proposed cutting tool are visible in Fig. 1: the cutting module and diamond wire (1), it is powered by an electric motor assembled under the drive pulley; the cutting module support
arm (2); the clamping module (3) [24]; the hybrid control unit (4) which encloses the communication, control and powering of system functions. Various sensors are arranged both on the manifold and on the machine itself, providing feedback regarding the status of the machine: cutting motor absorption, feeding module position, wire tensioner position, clamping pressure, pump velocity, oil temperature, feeding circuit pressure. After the mobile robot reaches the working position the main steps of the operative cutting cycle are: arm end effector positioning guided by vision and robust clamping to the target structure, cutting tool set-up, diamond wire powering, frame sliding and cutting. Once the main hydraulic system was eliminated, the use of a hydraulic circuit pressurised externally became redundant, and it was decided to move the motor/pump for the feeding and clamping circuits onto the machine itself. This required the hydraulic manifold, reservoir and consequently the controlling electronics to also be shifted onto the machine. It was therefore decided to centralise all of these elements into a single enclosure, together with the brushless motor electronic driver; for simplicity this was called the Hydraulic, Communication and Control Unit (HCCU). To enhance feedback and therefore both safety and efficiency, sensors were installed on the robotic tool to monitor all the variables significant for the right development of the operation. Linear sensors were used to determine the cutting module position on its support frame, whilst most sensors were either enclosed in the HCCU (such as oil pressure, temperature, flow etc.) and all information for the cutting motor were provided to the controller from the motor driver (motor speed, power absorption, operating frequency etc.). Particular and extensive research was also dedicated to the control software, for both the embedded controller and the operator's control panel. The embedded controller provides more than just communication and control. The control software was developed to allow the system to behave as a simple tool, being controlled completely by an operator; to assist the operator with automatic cutting adjustments, or to act as a complete robotic system, autonomously performing the entire operation even if deprived of communication with the external environment. In any case the control panel interface was designed to be intuitive and easy to use for an operator used to working with traditional full hydraulic system. 3. Subsystem design 3.1. Cutting module The cutting module is composed of four pulleys, one of which powered, which spin a diamond wire at high speed and cut the target by shearing. The wire is kept at a constant tension by one of the pulleys which is mounted on a pre-charged tensioner [16]. Tests run using traditional hydraulic actuation allowed to determine the torque required to power the drive. An experiment was setup using a 34 cm3 hydraulic motor with a pressure sensor installed between the motor and the hydraulic umbilical. This model was chosen because it presents the largest single-motor machine currently in use and therefore the maximum amount of force expressed by a single cutting motor. The pressure was then recorded during an 80 minute cut, during which flow was maintained at a constant value 50 l/min. Return pressure was recorded at approximately 2 MPa. The recorded values for p and Δp (pressure variation across the motor) can be found in Fig. 2 By analysing the recorded data, it was determined that minimum pressure (free spinning) required by the motor is 9 MPa, whilst the maximum operating pressure was recorded at 14 MPa, during maximum surface cutting. Torque was determined using τ = (V · Δp · μ)/2π where V is the volume displacement in cm3, Δpis the pressure drop across the motor in MPa and μ is the efficiency of the motor, 0.85 as per constructor specifications. The motor was run at optimal cutting speed; the speed was then decreased until torque was insufficient to spin the wire. The values
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Fig. 1. Modules of the robotic cutting tool (1–4) and assembly (5).
were recorded and values for torque were found to be 41 Nm at rotor lock and 65 Nm at cutting velocity. In order to rightly size the electric actuation substituting the hydraulic one, the operation of the machine itself must be taken into account. When cutting, the maximum pressure needs be applied on 0 rpm only if the structure being cut collapses and the wire gets stuck in the cut. Attempting to unlock the wire using only the torque of the motor is almost always unsuccessful. If, otherwise, the cut needs to be suspended and then continued, the standard operating procedure is to retrieve the machine, in order to alleviate some of the pressure from the wire, and then continue the cutting operation once the wire has achieved sufficient speed. From the experiment it was determined that, in order to successfully substitute the existing hydraulic motors with an electric equivalent, the full rotational torque will need to be that of the cutting motor at full operation whilst the zero torque will only need to be that of the minimum rotation. In order for the electric propulsion system to effectively replace the existing hydraulic one, the motor used must be within reasonable size and weight parameters. It is occasionally the case that a machine will be tailored to fit exactly between two structures in order to perform a cut, when this occurs, the pulleys are the part of the machine with the maximum width and therefore the determining factor in sizing and positioning the machine, so the motor should not exceed in diameter that of the pulley. As it concerns the weight because weight is not a real limitation due to the nature of the machine, it was decided that a limit
would be set to conform to common weight lifting regulations (GCAL, 1992 #70) and ensure that a technician can disassemble the motor from the frame and maintain it without being assisted. 3.2. The support arm A U shaped support arm is in charge of feeding the cutting module making it shift along the two legs in order to perform the cutting operation. One feeding motor through a gear spin and endless screw drives the progress of the cutting module through the target to cut. The feeding circuit is an open circuit driving a motor and requiring a specific flow to operate. As per the cutting motor there is no need to account for the power required to pump the oil through the length of the umbilical. Pressure must be taken into account as it is the consequence of the force the feeding motor is encountering when pushing the cutting module and determines the speed at which the cut can be made and at which the machine can be retracted. Since feeding in the system at normal feeding rates can take from 30 min to up to several hours, based on the machine type, a fast feeding system was developed to return the machine to its retracted state in a short time span. This operation however requires a lower flow pressure than cutting as retracting the machine encounters none of the resistance of the cutting operation. An experiment was setup to evaluate the hydraulic requirements, by coupling a 12.9 cm3 hydraulic motor with a 20:1 worm-gear and then mounting it onto an endless screw assembly which drove a cutting module. Considering speed characteristics of the diamond wire, it was determined that a satisfactory dynamic range would be 5–15 mm/min under normal cutting conditions and 80 mm/min when resetting the machine. Under these conditions, maximum feeding flow to the 12.9 cm3 hydraulic was calculated at 4.128 l/min, and maximum pressure drop across the motor was found to be 7.5 MPa. The power requirement for the pump was given by Pmotor = (Δp · Q · 1.635)/100μ where Pmotor is power in kW and Δp is pressure drop across the motor in MPa, Q is flow rate in litres per minute and μ is the efficiency of the motor. 3.3. The clamping module
Fig. 2. Motor pressure variation during cut.
This module is present on most current machines and can be designed with different architectures, with double parallel jaws or as a self adaptable clamping tool for multiple seizure [22, 23], but sometimes
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it is not required on a custom built. It can be replaced with a frame fixed into position on the cutting target. The clamping circuit is a closed circuit, so no flow is required. The only requirements are that a sufficient pressure can be achieved to lock the clamps in place (20 MPa), and that there is a sufficient reservoir in the system to accommodate for the difference in volume occupied when the piston is retracted and part of the internal volume is occupied by the piston's rod, and is given by V = n ⋅ πr2 ⋅ l where V is the volume, n is the number of pistons mounted on the machine, r is the radius of the rod's cross section and l is the rod (or piston) length. Because the hydraulic circuit is flooded and bleeding of air is performed before operation, this will result in the only variable volume of oil in the system; however testing should be performed to determine and account for the variation of volume due to temperature change of the oil during operation. 3.4. The hybrid unit The hybrid unit contains three separate sections, as visible in Fig. 3: • Motor pump assembly: a pump connected to an electric AC motor which powers the hydraulic systems on the robotic tool; it is flooded with the oil reservoir and completely sealed except for three hydraulic hose connectors (pressure, return and drain) in the section wall and electrical connections for both motor control and sensors. • Electro valve manifold: controls the flow of oil to the individual functions of the robotic tool. The second section was engineered to contain the hydraulic manifold machined out a single aluminium block and utilising cartridge valves. This was vital to ensure a compact manifold which would fit within the confined cylinder internal diameter. Three umbilical hoses connect the motor/pump section to the manifold, which attach to the actuators on the machine. • Electronic section: it encloses the printed circuit board which controls the brushless cutting motor, and the custom-designed embedded
controller responsible for communication with the surface and driver, control of the manifold and automation. The centralisation of these functions into a single enclosed unit protects them from possible water infiltrations, facilitates removal for maintenance and, if necessary, allows flooding the module with oil to provide increased radiation shielding for the enclosed electronics. 3.5. Hydraulic circuit analysis Because the cutting motor is no longer powered hydraulically, the pump contained in the hybrid module is required to only produce a flow and pressure sufficient for the feeding and clamping. Moreover, to power these functions, oil is no longer required to be pumped through a hydraulic umbilical over a long distance. The unit is therefore engineered with a small motor-pump as opposed to the large motors which are currently powering the hydraulic power units. This dramatically reduces size and weight of the equipment needed by eliminating the need for a hydraulic power unit and spooler, and only requiring an electrical umbilical which can provide power and control, as well as cost for consumables. A series of solenoid-valves would replicate the functionality of a hydraulic control panel and be controlled digitally. The manifold which replaces the hydraulic control panel would be located on the machine and therefore would need to be more compact whilst maintaining all functionality. Integrated into the manifold would be a series of sensors which will monitor aspects of the hydraulic system. The volume of potentially contaminating oil is dramatically reduced: the volume of hydraulic oil required in the system is reduced by about 97%, from approximately 300 l to 10 l and flow is reduced by 99.4%, from 110 l/min on larger machines to a maximum of 4 l/min for fast feeding return functions.
Fig. 3. Details of the hybrid unit.
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A dramatic reduction is also found in the surface of hydraulic umbilicals at risk of damage or failure with a consequent spill of hydraulic oil: the distance of is reduce from 1800 m (9 function hoses of 200 m each), to just the length of umbilicals needed from the localised hydraulic pressure unit to the individual function, hence 10 to 40 m in all depending on size and model of the machine. Finally, the risk of contamination for the operator is eliminated as the remotely contained hydraulic circuit, without flow back to a hydraulic control panel, ensures that contaminated oil remains contained and never gets in close proximity to the operators. 3.6. Control system All control is driven through a custom designed controller. The individual feeding and clamping functions are realised through the hydraulic control circuit, which schema is represented in Fig. 4, analysed in the following points: • Feeding control flow does not require two individual circuits, one for low flow and one for high. Because the system can be tailored to the machine's needs and all machines use the same capacity feeding motor, maximum value of flow can be set to 4 l/min. • Clamping control also has no need for bidirectional non-return valves. A single non-return valve need only be installed on the return circuit to prevent loss of pressure when the machine is clamped in place. • General circuit, a main pressure release valve needs to be installed and set to maximum pressure of 22 MPa. A main hydraulic bypass controlled by solenoid is required to place the system under standby rather than start-stopping the motor repeatedly. The solenoid valves are driven by binary ports through relays, whilst the speed of the feeding module is controlled by a proportional flow valve controlled by PWM. All sensors are connected to analogue input ports. Communication with the external environment and with the electric motor driver is achieved through serial ports via rs-232 allowing serial communication of up to 1200 m. Alternative longer-range optic fibre communication was also tested. In this case the adapter was coupled with the rs-232 and allowed high speed, long range serial communication with no noticeable difficulties. The range and cost of optic fibre however have to be evaluated for the current application. Two main control programmes were developed and embedded into the two panels: • A Tester Panel, to monitor and perform tests on the functions of the system. • A Control Panel, to allow the operator to control the machine on the field.
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They work as a single programme, as the communication system, sharing common functions as string handling, delegate, etc. The Tester Panel (TP) was developed to allow testing of the hybrid module and for troubleshooting on the field. The simplified nature of the Tester Panel consisted in storing all incoming values simply as variables rather than complex class structure. The values are mirrored directly in the user interface. Each function generates a string which is recognised by the processor interface control board, by adding a prefix which determines the type of port which is required to be read or to be set. The Control Panel (CP) programme uses all values received by the machine in a simulated virtual machine by means of a complex structure of classes and inheritance as shown in Fig. 5. The child functions depend on the parent status. Each sensor is responsible for its own updating and communication with the real machine. For safety reason, in order to ensure that a user would not attempt to control functions which were disabled, thus creating confusion as to the actual behaviour of the machine, a functions control hierarchy has been implemented as can be seen in Fig. 6. Other security features include a further control implemented so that any attempt to shut down the pump would result in a warning and the opening of the isolation valve unless a shutdown is instructed. This reduces the amount of times the motor powering the hydraulic system is powered up and down during an operation and should increase the lifespan of the mechanical components. Isolating the hydraulic system will also cause the resetting of all functions which depend on it, in such a way that once hydraulic standby is closed the machine always presents itself in the same, safe and stationary status. The control system can operate with three degrees of automatic control: Manual, Semi-Automatic, and Full Automatic. Each strategy of control varies the modality used to analyse data being fed back from the sensors, and to react by forwarding commands to the machine. Manual Mode is for direct control on behalf of the operators. All controls are fed back to the control panel by the cutting machine and all changes in state come as a direct consequence of operator interaction. A watchdog system is implemented whereby if communication is lost between the robot and the control panel, both attempt reconnection for 60 s. If connection has not been re-established, the robot will force a shut-down of all systems, reset all valves of the manifold to default status, perform a check that all operations have been completed successfully, reset itself and await incoming communication from the control panel. Operator Assisted Mode assists the operator by trimming the cutting tool during operation. As for the manual mode, all information is fed from the machine back to the control panel. The control panel then opens or closes the feeding proportional valve based on feedback from the tensioner sensor, to ensure an optimal cutting
Fig. 4. Hydraulic manifold diagram.
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Fig. 5. Class hierarchy of the CP prototype.
speed and that the wire never remains trapped in the cut. Reaction to variance is calculated based on the latest value's variation being greater than standard deviation over the last 20 s (sampling every 0.2 s), and a serial command is issued to the controller to increase proportional valve opening by 1% (increase of 0.2 l/min). Operators can supervise the operation and upon interacting with the Control Panel, control mode is automatically returned to Manual. Likewise if any of the sensors reports values which are outside of operating parameters warnings are issued to the operator and control is set to Manual. The same watchdog implemented in the manual mode is also implemented. Automated Mode takes over the entire cutting operation from the operator. Unlike other modes, information is sent back to the control software for monitoring purposes only. The firmware in the control board will open or close the proportional valve based on feedback from the tensioner, thus regulating the speed of the cut. The watchdog system is disabled in this mode. If communication is lost, the robot will continue the operation and continue cutting until the tensioner detects that the cut is complete or the cutting module is fully extended. Likewise if a sensor issues a warning, based on the criticality of the sensor, the
controller will either slow the machine down for several minutes, attempt to compensate for the problem, or stop all together and reset, issuing warnings to the operator. 4. Prototype construction Two prototypes were constructed. The first to test the motor with different gear, wire and pulley configurations, the second to test the capabilities of the system whilst performing an actual cut on a mockup steam dryer assembled by a nuclear power plant maintenance company. The first prototype (Fig. 7) was constructed out of carbon steel frame able to be embedded with three to seven pulleys. This allowed testing of different motors with various wire configurations including a variable numbers of idle pulleys onto the frame immersed in water connected by variable lengths of wire and driven by a single powered pulley. During the tests the following data was recorded: • Motor speed • Motor absorption
Fig. 6. Function control hierarchy.
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from aluminium but then redesigned to use cheaper and commercially available 220 mm diameter stainless steel tubes. A 0.75 kW AC asynchronous motor was purchased and coupled with a 1.9 cm3 gear pump and filters (Fig. 8-A). The hydraulic manifold (Fig. 8-B) was machined out of a single aluminium block and incorporated suitable cartridge valves, thus allowing it to fit within the constraints of the cylinder. All hydraulic elements (pistons, 12.9 cm3 feeding motor, hoses etc.) were assembled. The printed circuit board (PCB) of the controller was developed inhouse utilising a commercial microchip. The chip was due to the presence of two serial ports for communication and sufficient pins to connect nine analogue in sensors (six active plus three spare), nine digital i/o pins to control the manifold (six active plus three spare) and two PWMs to control the proportional valve and thus the cutting speed (one active, one spare). On the PCB are also present voltage controller, LEDs, resistors and capacitors. For the sensing, absolute optical encoders were mounted on the feeding motor (Fig. 9D) and a magnetostrictive sensor was installed on the frame to track the position of the cutting module. Finally, different types of diamond wires were sourced from an undisclosed supplier.
Fig. 7. Cutting module prototype.
• • • •
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Supply frequency Motor consumption Temperature of motor Temperature of water
5. Prototype testing The cutting robotic tool prototype was tested in an enclosed pool. Motor absorption was monitored as well motor temperature, overall pool temperature and cutting rate. The objective of the test was to test various motor/gear combinations, the absorption of the motor under a load comparable to that of a cutting wire and the consequential change in temperature of the reactor pool. The load was simulated by using three additional idle pulleys, for a total of six idle and one driven. In order to reduce motor heating, the diameter of the pulley was increased from 280 mm to 320 mm, thus giving the same linear speed to the cable but with a lower motor speed. Two wire configurations were tested. Both wires used the same steel-cable core, same diameter beads and medium to large sized
Tests based comparative evaluation suggested to use a submersible brushless, permanent-magnet small servo motor which outputs 21 Nm of torque at 4000 rpm in conjunction with a right-angle 3:1 gear which resulted in a final torque on the pulley shaft of 66 Nm and an ideal output speed of 1300 rpm. Once a suitable motor was identified, fabrication of the second prototype began. The frame was constructed out of stainless steel with the main plates for the cutting modules made out of aluminium. The communication, hydraulic and control cylinder was initially machined
A
B
C
D Fig. 8. A. Hydraulic motor/pump assembly. B. Hydraulic solenoid-valve manifold. C. PIC embedded controller on test bench. D. Feeding motor with absolute optical encoder.
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A
B
Fig. 9. Wire performance cutting SS304 in water A. Forty beads per metre cut. B. Fifty beads per metre cut.
industrial diamonds. The first cable (A) was fitted with a low number of beads per metre (40), the second (B) with a high number of beads (50 +). A series of SS304 box sections were cut over a 7 hour period. The wire performance is indicated in Fig. 1. It is interesting to notice the difference in cutting behaviour of the two wires. Due to the greater distance between beads, wire A proved to be more flexible than wire B; because of the angle created between the wire outside the target material, and the wire actually shearing, the increased flexibility created a sharper angle of entrance into the material, thus forcing the beads into the target and producing higher shearing force. In the long run however the smaller number of diamonds on the cable causes the shearing capabilities to be reduced, and with the reduction in diamond size, there is an increase in diamonds rendered useless by malleable material pasting. Wire B on the other hand, although never reaching the cutting efficiency of Wire A, maintains its shearing ability for longer due to the larger number of diamond, and thus the higher number of shearing elements as well as the reducing pasting of the wire. From this data it was decided that a reduced bead configuration would be chosen for shorter cutting operations (less than 5 h) to favour speed, whilst a higher density of
beads would be chosen when dealing with long cutting operations (5 h or more) in those cases when the wire could not be changed. During the trial, the motor temperature reached a peak of 65 °C which approaches a potentially damaging temperature. This however was not considered to be a concern as the 4 × 4 × 2 m pool was tested to be significantly smaller than the pools found in reactor cores and without induced circulation, causing the water temperature to increase from 19.4 °C to 37 °C. The second prototype was tested on a mock-up steam dryer assembled to simulate as accurately as possibly the conditions in a nuclear reactor. Due to safety concerns, the fully automated system was not used in this setup; however the controller-assisted cutting was enabled for much of the operation. The cut was successful and completed in just less than 6 h. Recordings from the sensors are displayed in Fig. 10. The motor power was controlled manually, and the assisted cutting algorithm worked effectively, adjusting the feeding speed in relation to the ease of cut. It is visible from the graph that an increase in cutting power corresponds to the robot increasing feeding speed as the force acting on the tensioner decreases.
Fig. 10. Main variables trend during the cut of the steam dryer mock-up.
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The motor power was controlled manually, and the assisted cutting algorithm worked effectively, adjusting the feeding speed in relation to the ease of cut. It is visible from the graph that an increase in cutting power corresponds to the robot increasing feeding speed as the force acting on the tensioner decreases. The initial slow feeding speed is due to the wire encountering large flat surfaces of the box tubular used to assemble the mock-up, a similar slow down was detected when approaching completion of the cut. 6. Conclusions and future developments A robotic system is proposed suitable for contaminated structure maintenance and decommissioning. The system uses diamond wire for cutting. The standalone teleoperated robot is purposely developed to be safe for users and for the working environment itself. A hydraulically powered robust clamping secures the robot to the structure to be cut, a hydraulically driven feeding mechanism moves the diamond wire toward the part to be cut, and a brushless motor drives the cutting wire. A custom designed hybrid control unit encloses the communication, the control and powering of system functions. Two cutting tools prototypes have been constructed and successfully tested. Overall the robot works effectively and efficiently. All proposed objectives are met: the operator uses the robotic system to cut at a safe distance (300 m), the integrated feeding control algorithm reduces the human error. Recently the robot has successfully completed a cut of a steam dryer for upgrading purposes in an undisclosed US reactor and is in list for a similar operation in Japan in the near future. Whether nuclear power plants become a technology of the past in the search for cleaner solutions, or are maintained and modified for increased life span, civil engineering in contaminated environments is a necessity. The proposed remotely controlled robot minimises the amount of dangerous waste and polluting substances generated during the cutting process. The need of materials, resources and energy involved in the decommissioning process is also minimised. The proposed robotic tool guarantees that any human operator, working with the system or in close vicinity to contaminated environments, is fully safeguarded, in conformity with the highest level of health and safety standards. Some improvements are necessary in order to allow the robot to operate in highly contaminated environments; the feeding adjustment algorithm needs to be further developed, the embedded controller should be enhanced and radiation shielded to protect the electronic devices from damage and interference. The use of a compact new generation brushless motor will allow to reduce the cutting actuation system in size and weight; maintenance will be simplified and cost will be reduced. Acknowledgment TS Tecnospamec srl, patent holder for the Diamond Wire Cutting System, CUT Offshore and CUT Nuclear are gratefully acknowledged. The authors also thank the editors and referees for their careful review.
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