Design of a Climbing Robot for Nuclear Environmental Monitoring

: As nuclear science and technology develop, robotic applications are becoming common and important in nuclear islands and beyond. Climbing robots designed to replace human workers for nuclear environmental monitoring at high altitudes with dangerous working conditions offer security and efficiency. This paper proposes a feasible design of a ladder-climbing robot, including the mechanical structure, the control system and a human-machine interface, and a feasible gait planning based on the designed robot.


Introduction
With the enormous development of the nuclear power industry, the demand for nuclear robot applications in NPP (nuclear power plants) is constantly rising.Numerous specially designed robots have been developed according to different working purposes, which cover the operation, the inspection, the maintenance and the decommissioning either inside or outside of nuclear islands [1][2][3][4].
Inside nuclear islands, robots are often required to operate under water, in high temperature, or under strong radiation.For this kind of operation, the 'Pegasys' robot has been designed by Westinghouse Electric Company for plugging tubes in PWR steam generators [5].For instrument inspection, in nuclear reactor vessels, the Korea electric power research institute developed an underwater robotic system, which is already applied in the PWR in Yonggwang nuclear power plant [6,7].In primary circuit feeder pipes, a snake-arm robot system designed by OC Robotics in Britain is applied in CANDU reactors in Canada [8,9].For instrument maintenance, an underwater welding technique, including an underwater welding system developed by Toshiba [10] and the YAG-laser repair welding robot developed by Ishikawajima-Harima Heavy Industries Company [11], is important to use to fix stress corrosion cracking [12].For instrument decommissioning, the excavator-like robot "Brokk 180" designed by Brokk Company is used in decommissioning nuclear facilities at Dounreay [13].
Outside nuclear islands, robots have fewer requirements connected radiation and temperature, but more with the architecture.For environment inspection under a normal nuclear environment, the robot "Warrior" [14] uses tracks to obtain mobility.Under a nuclear accidental situation, the "Quince" robot [15] was also equipped with crawlers designed with tracks to overcome obstacles and steps.
Structure climbing is one of the common actions required for the operation, inspection and maintenance both inside and outside the nuclear island.A climbing robot with efficiency performance and high accuracy could serve as a good platform to replace a human's manual operations.The design of a climbing robot is specialized according to the features of different working conditions.Robots equipped with suction cups, such as the "ROMA" robot [16], can move omni-directionally on slick wall, while a high dryness, slickness and flatness is required for the wall.On the contrary, robots equipped with spines, such as the "CLIBO" robot [17], can climb on a rough wall, but a high roughness of the wall is needed.Robots with clampers usually use a paralleled structure to climb tubes or rods, such as the "TREPA" robot [18], however, crossing obstacles is difficult for these robots.As shown by "Quince", and "Warrior", robots with track can easily cross obstacles.Moreover, this kind of robot can also climb stairs with a large load [19], even though the stairs will not have a great slope.Robot with limbs and claws, such as the "ASTERISK" robot [20], can also afford a large load to climb ladders, while the motion is complicated so that the climbing efficiency will not be as high as the other types of robot.
Another structure that can serve as a potential consideration for climbing robot design is the ladder.A ladder is a common auxiliary tool that stands everywhere especially for vertical architectures with high altitudes, such as nuclear environmental monitoring towers.A well designed robot which is able to climb a ladder will have a wide application range to assist humans working in high altitude architecture.In this paper, a ladder-climbing robot is designed as the working platform for automatic instrument maintenance and decommissioning to replace humans working in such a dangerous working environment.In consideration of the ladder structure, a climbing strategy and the corresponding mechanical structures, a remote control system and the human machine interface are proposed together with a performance analysis.

System configuration
The robot system includes a mechanical structure, a control system and a humanmachine interface.

Mechanical Structure
The robot has a car-like structure for the feasibility of ground moving with two degrees of freedom.More precisely, using a structure with two front unidirectional driving wheels and two back omni-directional wheels, the adjustment to the moving direction of the robot can be controlled by the rotational directions and the difference between the rotational speeds on the driving wheels.A DC (direct current) motor is chosen as the driving motor for the front wheels.
When the robot climbs on the ladder, these four wheels act as the supporter that keep the robot hands grabbing on the rungs of the ladder properly.Four guiding wheels are installed to maintain the movement direction.Figure 1 shows a sketch of the robot's mechanical structure.The robot uses four grabbing hands to preserve the balance on the ladder: two front symmetric hands fixed on the frame of the robot and two back symmetric hands as mobile hands, which are mobile compared with the fixed hands.The two fixed hands grab on the same rung of the ladder, so as two mobile hands on another rung.To climb, the distance between the fixed hands and the mobile hands should change periodically.A crank-slider structure imposed with a motor can offer the mobile hands such a periodical displacement without breaking the motor's continuous rotation.A selection of a stepping motor assures accuracy for the climbing movement and a synchronous belt-pulley is used to improve the output torque of the stepping motor.
The climbing mechanism demands the hands not only grab on rungs for the robot's balance, but also cross over rungs for the augmentation or the diminution of altitude.A hand with three working positions is composed of a hooking finger, an electric magnet and a ratchet-pawl structure.With a suitable control of the electric magnet, the state of the hand is able to be switched between crossing upward a rung, crossing downward a rung and grabbing on a rung.
In order to diminish the self-load of the robot, the geometry parameters of the mechanical components are optimized by using a finite element method.Almost all the mechanical components are built with aluminum alloy except some important loading components such as the shaft and the key of the pulley.

Control System
The control system of the robot (Figure 2) is consisted of an Arduino board, a couple of wireless RF (radio frequency) modules, DC motors with driver, a stepping motor with driver, photoelectric velocity sensors, photoelectric position sensors, electric magnets, electric relays, a WIFI camera with PTZ (pan-tilt-zoom) control platform and a computer.The computer is the host-computer and the Arduino board with necessary circuits is its slave-computer.The camera and its PTZ platform form a visual system.Data transmission is realized by two independent ways respectively: a, the realtime video and the command and feed-back signal between the camera and the hostcomputer are transmitted by WIFI through the Internet Protocol; b, the command and feed-back signal between the slave-computer and the host-computer are transmitted RF through the RS232 Protocol.The separation of visual system data from the control command guaranties the transmission bandwidth and improves the reliability of system.

RS-232 Bus
During the motion of the robot, the host-computer keeps sending orders to the slave-computer until the robot reaches its aimed position.Within each command period, which can be chosen from 0.1s to 1s, the salve-computer enables motors.Consequently, the robot will be stopped automatically without any order.It assures that the robot can still be stopped even under a bad communication environment.

Human-machine Interface
On the consideration of the Windows system, the interface is developed on the basis of MFC (Microsoft Foundation Classes) programming for the control and the inspection of the robot.The interface is composed of two modules: the real-time video module and the serial port communication module.
The real-time video module is developed to realize user login, PTZ control, realtime video display, video recording and image capture.That can support the camera integrated (DS-2CD3Q10FD-IW, HIKVISION ® ).The data transmission of video relies on the LAN (local area network) built by a portable rooter.
For the serial communication module, RS-232 communication protocol is adopted.The reading file and writing file function, the thread listening function and other files related to serial communication are considered as the fundamental part.With these basic files, the following functions are realized: -accessible port selection; -operation mode selection, including ground moving mode, ladder climbing mode and automatic climbing mode; -relative parameters set, such as speed, sensibility, etc.; -long press control by the virtual button and keyboard; -display of the robot state.
To realize remote control, a wireless communication through RS-232 communication protocol between the robot and the human control software on the computer is realized with the help of the couple of radio frequency module.A string of numbers in a length of 8 or 15, whose first and last number is the check code, is sent to give orders or to feed back information.

Ladder climbing strategies
The crank-slider structure transforms the continuous rotational movement denoted by the motor into a periodical linear to-and-fro movement for the mobile hands.With the help of switching the state of the hands for grabbing or for crossing, the robot can climb up or down on the ladder.The climbing procedure is the cycling of three primary states shown in Figure 3.The robot starts the climbing from the initial state, shown in Figure 3a.At this state, the robot hangs on the ladder by keeping a long distance between the fixed hands and the mobile hands.To climb up on the ladder, the robot decreases the distance between the fixed hands and the mobile hands while the fixed hands grab the rung, retaining its old position.Consequently, the mobile hands move across one rung to grab the upper rung and reach its transitional state, shown as Figure 3b.Next, the distance is enlarged by keeping the mobile hands grabbing on the rung and moving the fixed hands across one rung to grab the upper one.At the end of this movement, the robot reaches its climbed-up state, shown as Figure 3c   In the climbing procedure, both fixed and mobile hands are required to grab the rung and to move over either the upper rung (for climbing up) or the lower rung (for climbing down).In order to satisfy this requirement, the robot hand is designed to integrate a ratchet with a pawl driven by an electric magnet and a rolling-up spring that provides the recovery stress.The working sequences of one hand for climbing up and climbing down the ladder are presented in Figure 4 and Figure 5 respectively.By controlling the pawl and the relative position of the hands on the rungs, the robot can climb the ladder without breaking the continuous rotation of the loading motor.Consequently, the energy dissipation to change the rotation direction of the motor is avoided and the energy is used more efficiently for climbing.Furthermore, the accumulation of the rotational angle error due to the change of the rotational direction can also be avoided.

Conclusion
The ladder-climbing robot is developed with the realization of the mechanical structure.The control system is constructed to realize the required motions.The human-machine interface is composed based on the need of operation and inspection.
According to the specific design of the robot, the climbing mechanism and the gait planning are conceived.

Figure 1 .
Figure 1.Mechanical structure of the robot and finishes its climbing cycle one time.The robot climbs up one-rung distance in one climbing cycle.The distance between the fixed and mobile hands in the climbed-up state is the same as that in the initial state.The climbed-up state is the initial state of a new climbing cycle.The climbing down procedure is to cycle the climbing up process in adverse order.

Figure 3 .
Figure 3. Climbing cycle of the robot states (side-view): a, initial state on lower altitude; b, transitional state; c, climbed-up state

Wireless Radio Frequency Module Arduino Control Unit (Slave Computer) Wireless Radio Frequency Module Wireless Network Card Wireless Network Camera
Figure 2. Control system of the robot Unauthenticated Download Date | 6/23/19 6:56 AM