T
he system proposed in this work is the Alicia 3climbing robot. The aim of this project is to develop a system
that can be adopted in a variety of applications, such as mainte-nance, building, inspection, and safety, in the process and con-struction industries. Generally speaking, the system could be adopted in many places where direct access by a human opera-tor is very expensive because of the need for scaffolding or very dangerous due to the presence of a hostile environment.
Typical operating environ-ments are the external or internal
surfaces of aboveground or underground petrochemical stor-age tanks and concrete walls, like those of dams and bridges pillars. All these structures have a very high economic value,and it is very important to perform periodic inspections for maintenance purposes, as standardized by the respective con-trolling bodies. Among the most important factors to be inspected are the rate of corrosion, the potential risk of air or water pollution, and the detection of leaks, cracks, crumbling parts, or rust on exposed concrete reinforcement, and so on. While these kinds of inspection are very useful to prevent ecological disasters and to reduce the risk for the individuals working or living around the plant or structure, they are very expensive because scaffolding is often required. Moreover, for safety reasons, plant operations must be stopped and special pre-cautions must be taken when human operators are conducting inspections.
One possible solution is to carry out automatic nondestruc-tive inspection (NDI) on the target surface only [1], [2].Then, when repairs are needed,a traditional method of access-ing the plant is applied. The most common NDI uses small sensors like eddy current probes,
ultrasonic heads, and charge-coupled device (CCD) cameras. For these operations, a small automatic vector (climbing robot) able to climb verti-cal walls can be very useful for carrying sensors to the desired position on the target wall.
In Figure 1(a)–(c),typical operating environments for climbing robots are shown. An in-depth description of this system will be the aim of the following sections.
Several other systems for similar targets have been devel-oped by our group in the past [3]–[6]. Devices based on the principle of sliding suction cups are currently under con-struction by other universities and research centers. One of
BY DOMENICO LONGO AND GIOVANNI MUSCATO
1070-9932/06/$20.00?2006 IEEE MARCH 2006
?ARTVILLE & DIGIT AL VISION, LTD.
The Alicia
3
Climbing Robot
A Three-Module Robot for Automatic Wall Inspection
these is the robot developed under the Robosense project [7], [8]. The main target of this system was the inspection of concrete walls like those of highway bridges and dams. Other robots are being developed at Fraunhofer IPA [2] and the T echni-cal University of Kaiserslautern [9]; these are very small and light and use a highly efficient sealing system. However, the architecture only allows the system to climb on very flat surfaces like glass walls.
The new robot presented here can have a wider range of applica-tion fields thanks to its pneumatic adhesion system with onboard vacu-um generation as well as its very flexible sealing to cope with differ-ent wall shapes and types while allowing high payload capabilities. The Alicia Project
T o build a system capable of per-forming some of the NDI operations mentioned above, the following requirements were specified:
◆It must be very light and have
a suitable payload in order to
carry mission-specific instru-
mentation.
◆It must move over a vertical,
nonporous, flat, or quasi-flat
wall at a suitable velocity, with
a wide range of surface materials and cleanliness levels.
◆It must be able to inspect vertical walls 25–30 m in
height in a semiautonomous or remote-controlled manner.
◆It must pass over small obstacles (about 1 cm) without
stopping itself.
◆It must be able to pass over bigger obstacles at a lower
velocity.
◆It must have very few connections with the remote
console.
T o achieve these targets, some prototypes were built. The following subsections give a brief review of the two systems built in order to clarify all the choices made in the final system.
Alicia I Prototype
T o keep the operation of the system independent from the surface material (nonporous), pneumatic-like adhesion must be used. The most common way to do this is by using a set of suction cups and a vacuum generator. Such a gripping system is normally mounted on a structure that is able to generate steps because traditional suction cups cannot easily slide over the surface while attached. The structure requires complex kinematics and actuators system and generally leads to slow and heavy robots.
In this project, the main idea was to build a robot that is a suction cup itself, which is able to slide over the wall surface while staying attached by using two wheels, as shown in Figure 2. This type of suction cup will have unavoidable vacuum leakage; therefore, a standard vacuum pump cannot be used to provide the cup with enough pneumatic power. An air aspirator with very high airflow capabilities can be used instead. However, this kind of aspirator has a very low vacuum level compared with a vacuum pump; this can be compensated for by using a cup with a large diameter. T o avoid using a very-large-diame-ter hose for low-pressure airflow connection with a base station, the aspirator must be mounted onboard, and a very light one must be used.
T o test this idea, a very simple robot was built using a PVC cup, a low-cost air aspirator, two modified servomotors, and two wheels. The lateral sealing for the cup was a T eflon-coated foam cushion. The wheels were coated with soft
rubber, but several other materials have also been tested.
MARCH 2006Figure 1.Possible operating environments for the Alicia3climbing robot.
(a)(b)
(c)
This simple structure, shown in Figure 2, is a fully func-tioning climbing robot; it was completed with a set of infrared sensors and an inclinometer, so the onboard microcontroller unit (MCU) allows the system to climb in a vertical direction, upward or downward, avoiding obstacles in front of it.
Some of the most important problems related to this kind of structure are internal pressure requirements, sealing, fric-tion between the sealing, wheels, and wall. The maximum allowed payload must also be taken into account. The struc-ture is fully autonomous except for the electrical power sup-ply and weighs only 2 kg. However, it has no payload available to the user and can move only on a very flat surface without obstacles.
Alicia II
T o solve some problems related to the sealing and the available payload, a significantly more robust structure was built, with a larger cup and a much more powerful aspirator.
The Alicia II module, shown in Figure 3, currently com-prises three concentric PVC rings held together by an alu-minium disc. The bigger ring and the aluminium disc have a diameter of 30 cm. The sealing system is allocated in the first two external rings. Both the two rings and the sealing are designed to be easily replaceable, as they wear off while the robot is running. The third ring (the smallest one) is responsi-ble for semirigid contact between the robot and the wall, using a special kind of spherical ball bearing. This ring is also used as a base for a cylinder in which a centrifugal aspirator and its electric motor are mounted. The aspirator is used to depressurize the cup formed by the rings and the sealing, so the whole robot can adhere to the wall like a standard suction cup. The motor/aspirator set is very robust and is capable of working in harsh environments.
While the system has to move over the target surface, gen-erally a rough metal surface or a concrete wall, the cup must not adhere with a high friction; thus, a particular kind of seal-ing is required between the wall and the robot. The sealing must guarantee negative internal pressure and should allow the robot to pass over small obstacles (less than 1 cm in height) like screws or welding traces. Using the trial results obtained with the Alicia I prototype, a sandwich of T eflon/bristle sealing (see Figure 4), 2 cm in height, was built and has given good results over a wide range of rough surfaces (Figures 3 and 5). The bristles, mainly used as a support for the T eflon sheet and to produce some friction, are fitted in the two removable rings. This configuration allows the robot to pass over small obstacles and to climb a vertical surface with a minimum curvature radius of 1.8 m.
The whole structure was designed to contain two onboard wheels with two independent dc motors/ gearboxes/encoders, as can be seen in Figure 5. The total weight of the module is 4 kg. The dc motors/gearboxes used are able to move a mass up to 15 kg in a vertical direc-tion at a maximum speed of 2 m/min.
The Alicia3System
While the Alicia II module is moving over a surface, it may encounter some obstacles on the path. Obstacles that are smaller than 1 cm can easily pass through the cup sealing. When obstacles higher than 1 cm are encountered, the base module fails to overcome them (this limitation is due to the maximum height of the flexible cup sealing, which cannot be higher than a few centimeters). The basic idea for the Alicia3
robot is to use three of the Alicia II modules linked together
MARCH 2006 Figure 2.
The Alicia I Robot.
Figure 3.
The Alicia II Robot.
in series by means of two rods, allowing the whole system to deal better with obstacles on the target surface. In this case,the Alicia 3robot can pass over the obstacle in a few steps by detaching the three modules one by one, although it does this at a lower velocity (Figure 6).
The structure has been designed in such a way that only two modules at a time can support the weight of the entire robot, while the third is raised up about 10 cm with respect to the wall. The two links between the three modules are actuated with two pneumatic pistons, as can be seen again in Figure 6.Each of the two external modules has a passive rotational joint between itself and the link, so it can rotate with respect to the entire structure. This rotational joint also allows a degree of freedom (DOF)of ±3?around the vertical direction on the center of rotation;this is to accommodate nonflat surfaces or mechanical misalignment of the system.
The Alicia 3system was first simulated in the VisualNastran 4D and Simulink environment to analyze its mechanical behavior. Using the simulation results and testing experience gained on the Alicia II system, the robot was fully designed with a total weight of 20 kg and a total length of 1.3 m [10].The Alicia 3Architecture and Kinematics
From Figure 7it is possible to distinguish the main blocks
that compose the Alicia 3system architecture. These blocks can be grouped mainly in three kinds of tasks for the con-
trol system:
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Figure 4.
Sealing detail.
Figure 5.
A test over an indoor surface and dc motor detail.
Figure 6.
Alicia 3
main idea.
◆
adhesion task ◆motion task
◆obstacle avoidance task.
In addition to these tasks, the system needs suitable user interface support for teleoperation purpose and for prepro-grammed trajectory generation.
The adhesion task has been implemented in three separate processors, one for each module, and is based on a simple 8-b embedded MCU with an industry-standard 8051 core. The reason for this choice is that this task is very critical for avoid-ing damage to the system and to the operation area; therefore,a dedicated control system is needed.
The main purpose of this board is to read the value of the vacuum level inside the cup and regulate it using a ref-erence signal. Through an I 2C bus, the board receives the reference value for the internal pressure and some control parameters, and it can send back to the main CPU the pressure value to visualize it in the user interface.
The reference signal will depend on system payload (which can be mission specific) and operating surface (very flat or rough). Nevertheless, because of some surface irregu-larity or little obstacles, the internal vacuum level may vary from the desired one and this can led to a system fault or very
dangerous conditions. Through the same I 2C bus, it is possi-ble to send position commands to the pneumatic pistons.
All the control tasks for system motion and obstacle avoid-ance have been centralized in a single board computer based on a 32-bit RISC processor. Motion tasks for the Alicia 3sys-tem include reading the six wheel encoders, reading some inertial sensors (like some inclinometers), generating six pulse-width modulated (PWM) signals for the dc motor con-trol, controlling the two pneumatic pistons, and reading a set of infrared sensors for obstacle avoidance and some other sen-sors for homing the entire system.
By using the information from the six encoders, a pro-portional-integral-derivative (PID) controller, and a suitable power amplifier, velocity and position control has been implemented for all six wheels. By using two independent motors/wheels in a differential drive configuration, each Alicia II module is able to move in all directions in a differ-ential drive configuration. While the module moves on a vertical surface, using an inclinometer it is possible to com-pensate the rotation for possible wheel slippage.
In Figure 8,the Alicia 3system is shown changing its direc-tion of movement. This sequence requires coordinate motion of the two pneumatic pistons, the six wheels, and the aspira-tor; it is summarized in T able 1.A sequence that allows the Ali-cia 3system to translate in a direc-tion orthogonal to the robot main axis is shown in Figure 9and explained in Table 2.
In Figure 6, the Alicia 3robot is passing over an obstacle. A set of infrared sensors for distance measurement in the range 10–80cm located in the front side of
each module allows the system to avoid collisions with those obsta-cles that cannot pass through the sealing (obstacles greater than 1cm in height). In this situation,the robot enters a special motion sequence to pass over the obsta-cle. This is explained in more detail in Table 3.
Software Architecture and System Simulator
In this section, we discuss the soft-ware architecture that is implement-ed in the microcontroller firmware as well as the development of a sys-tem simulator.
The software developed for all the closed-loop controls and coordi-nation of the Alicia 3robot has a hierarchical structure with three lev-els, as shown in Figure 10.
IEEE Robotics & Automation Magazine
MARCH 2006
Figure 7.Alicia 3architecture block diagram.
Figure 8.Alicia 3kinematics: rotation.
At the lowest level, all the inter-faces with the robot hardware com-ponents are implemented. These interfaces are distributed in different microcontrollers. At the middle level,all the system coordination is imple-mented in the main CPU. At the highest level, a user interface enables control of the complete system.
Also, the firmware in the MPC555main CPU is organized into three lay-ers using a modular approach. In the lowest layer, an interface for the inter-nal peripherals of the MPC555processor—analog-to-digital convert-er, multipurpose input output sub-module (MPIOSM), PWM, and periodic interrupt timer (PIT)—has been implemented. In the middle layer, there is an interface for the external peripherals (inclinometers, encoders,motors, and I 2C bus). And, in the highest layer, the locomotion strategy that uses all the low-level interfaces is implemented. This layer has also been implemented using a modular approach.In one software module in the highest layer, speed/position control of the single wheel is implemented using a classical PID controller. A second module implements a control loop on the speed of the wheels and inclination of the Alicia II system, which allows the robot to move in a user-defined direction. A third module implements high-level robot functions, like those that allow putting the Alicia 3robot in a predefined home position or enable straight forward or backward movement or rotation.To tune PID parameters for locomotion control (speed/position control loop of the wheels), a simulator for the Alicia 3
system has been developed in Simulink.
MARCH 2006All three aspirators and six dc motors are
working; the robot is going straight forward.A direction change is needed; the robot must stop and the two external modules must turn
able 1. System kinematics: direction change.
Step Action
a
All three aspirators and six dc motors are
working; the robot is going straight forward until the first IR sensor detects an obstacle.
b
The robot must raise the first module; the first aspirator is turned off and the module raised by T able 3. System kinematics: obstacle overcoming.
Figure 9.Alicia 3kinematics: traslation.Figure 10.Alicia 3
control architecture.
Moreover, this simulator allows the designer to test various locomotion strategies, and it can be useful for training the technical operators. A graphical three-dimensional (3-D) viewer, developed in Delphi 7 using Mex-function to inte-grate it in Simulink, is used to visualize the Alicia3 sequence of operations.
The simulator is realized by basic blocks that one by one represent a realistic model of the Alicia3robot components The basic model blocks used are listed below:
◆dc motor
◆gearboxes
◆differential drive
◆wheel
◆wheel-wall contact.
For each block, a set of parameters has been defined and can be adjusted according to real system components. These parameters can be easily modified from the Simulink diagram.
The control algorithms implemented in the simulator are the same as that of the real robot. Each Alicia II module con-tains two motors with gearboxes and two wheels in differen-tial-drive configuration; the link between these subsystems is shown in Figure 11.
A Simulink block named Alicia3, built by merging three module blocks, represents the entire model of the robot. The locomotion strategies implemented for turning, going for-ward/backward, or overcoming obstacles are the same imple-mented in the real robot.
System Security and Final T ests
The safety aspect of the system has been carefully considered; due to some unexpected small obstacles, the sealing can have some leakage (reducing the vacuum level inside the cup), and in this condition the entire system can fall down, damaging itself and the operation area. On the other hand, if the vacu-um level is too high with respect to the surface type, the fric-tion between the system and the wall can increase in such a way that system motion is no longer allowed.
T o deal with this type of problem, an experimental setup was built to test the sealing and payload capabilities of the Ali-cia II module. During these tests, the maximum payload to aspiration power was measured in various kinds of walls. One such test is reported in Figure 12.
Once the system capabilities have been tested, to solve the pressure regulation problem, a nonlinear dynamic model of the motor/aspirator/cup system was identified using input/output (I/O) measurement and a neurofuzzy algo-rithm. The obtained model has then been used as a system emulator to derive a suitable control algorithm [11].
A standard PID controller has been tested over the system emulator and has been implemented on the aspirator control power interface. It has also been tested over the real system, and some of these results are shown in Figure 13.
In Figure 13(a),the output signal (the pressure inside the cup) and the reference signal, while the aspirator control board is performing a PID control on the internal pressure level, are represented. During the test, a disturbance has been introduced in the internal pressure of the cup by opening a gap in the sealing. In these pictures [in Figure 13(b),a detail is shown], 1 indicates system overshoots or undershoots while 2 and 3 indicate the start and end of disturbance, respectively.
The system time constant is about 20 s.
MARCH 2006 Figure 11.Simulink representation of the Alicia II system model.
Tr
Va
T
n
T r
Va
T
n
TIn
nIn
TOut>
nOut
TIn
nIn
TOut>
nOut
Contact
nIn
Wheel
Wheel1
VWI
nIn
VWI
Contact
Gearbox Left
Gearbox Right
Contact
3
DC Motor
DC Motor 1
Figure 12.Payload test of the Alicia II module.
It must be remarked that this control algorithm is useful only when small obstacles, passing under the sealing, could compromise system security. Apart from that, as these kinds of robots are not intrinsically secure because their wall gripping method is not passive like one based on per-manent magnets, a security steel cable is always recom-mended during operation. In fact, robot safety strongly depends on the continuity of the power supply and the regularity of the surface. This drawback is lessened by the fact that systems based on high-power permanent magnets are much more expensive and can only move over ferro-
magnetic surfaces.
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Figure 13.(a) A test of the pressure PID controller. (b) A test of the pressure PID controller: a detail.
Figure 14.A test of the Alicia II module over an indoor wall with payload.
Figure 15.A test of the Alicia 3robot over an outdoor con-crete wall.
Several tests have been performed on the base module Alicia II as well as on the entire system. In Figure 14,the Alicia II module is tested on a concrete indoor wall with a payload. Each of these trials was useful for verifying the sys-tem behavior and the maximum payload in various condi-tions. Special attention was reserved for trying the Alicia II module and the Alicia3robot on a concrete wall, because this can be considered as one of the worst-case conditions regard-ing sealant capabilities for the robot. The outdoor test surface used is composed of rough concrete and a small vertical con-crete band 0.5–1 cm high.
The Alicia3robot has been tested on an outdoor concrete wall. In Figure 15,an obstacle pass-over trajectory step sequence is represented.
Conclusions
Many service and industrial constructions have a very high strate-gic and economic value. For these structures, it is very important to perform periodic inspections to guarantee proper structural operations. An automatic vector to carry out NDI over these structures has been designed and realized through a modular approach. The Alicia3robot comprises three identical modules in an easily reconfigurable assembly. Moreover, the base module Alicia II is a fully working robot and can be used to inspect verti-cal surfaces that do not have obstacles higher than 1 cm.
T o reach the design targets, a series of preliminary proto-types were built to better analyze all the problems arising in this type of structure. T o improve system performance and reliability, a control algorithm was designed for the internal pressure of the cup and tested using a system simulator derived from some I/O measurements as well as a neuro-fuzzy algorithm.
Some simulations and experimental results have been presented. Moreover, some tests of the robot have been per-formed to verify design goodness and real system capabilities.
A friendly user interface has been designed to enable easy operation of the system.
Expertise gained during the design phase and the results of real tests of the Alicia3robot will be useful in the design of next-generation high-performance climbing robots. Keywords
Robot, automatic industrial inspection, pneumatic adhesion, pressure control, motion control.
References
[1]F.W eise, J.Kohnen, H.Wiggenhauser, C.Hillenbrand, and K.Berns,
“Non-destructive sensors for inspection of concrete structures with a climbing robot,” in Proc. 4th Int. Conf. Climb ing and Walking Rob ots CLAW AR 2001, Karlsruhe, Germany, Sep.24–26,2001, .pp. 945–952.
[2]R.D.Schraft, F.Simons, T.Schafer, W.Keil, and S.Anderson, “Con-
cept of a low-cost, window-cleaning robot,” in Proc. 6th Int. Conf.
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[3] L.Fortuna, A.Gallo, G.Giudice, and G.Muscato, “ROBINSPEC: A
mobile walking robot for the semi-autonomous inspection of industrial plants,” Robotics and Manufacturing: Recent T rends in Research and Applica-tions, vol 6. New Y ork: ASME Press, 1996,pp. 223–228.[4] G.Muscato and G.Trovato, “Motion control of a pneumatic climbing
robot by means of a fuzzy processor,” in Proc.1st Int. Symp. Climbing and Walking Robots (CLAW AR ‘98), Brussels, Nov.26–28,1998, pp. 113–118.
[5] https://www.doczj.com/doc/2c13563649.html, Rosa, M.Messina, G.Muscato, and R.Sinatra, “A low cost light-
weight climbing robot for the inspection of vertical surfaces,” Mechatron-ics,vol. 1. New Y ork: Pergamon, 2002, pp. 71–96.
[6] D.L ongo and G.Muscato, “SCID—A non-actuated robot for walls
exploration,” in Proc. IEEE/ASME Int. Conf. Advanced Intelligent Mecha-tronics, Como, Italy, July 8–12, 2001, pp. 29–33.
[7]K.Berns and C.Hillenbrand, “Climbing robots for commercial appli-
cations—A survey,” in Proc. 6th Int. Conf. Climbing and Walking Robots (CLAW AR 2003), Catania, Italy, Sep. 17–19, 2003, pp. 771–776. [8]K.Berns and C.Hillenbrand, “A climbing robot for inspection tasks in
civil engineering,” in Proc. 1st Int. Workshop on Advances in Service Robot-ics, Bardolino, Italy, Mar. 13–15, 2003, pp. 34–40.
[9] K.Berns and C.Hillenbrand, “A climbing robot based on under pres-
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[10] D.Longo and G.Muscato, “A modular approach for the design of the
Alicia3climbing robot for industrial inspection,” Indust. Robot., vol. 31, no. 2, pp. 148–158, 2004.
[11] D.Longo, G.Nunnari, and G.Muscato, “Neural network system identi-
fication for a low pressure non-linear dynamical subsystem onboard the Alicia II climbing robot,” in Proc. 13th IFAC Symp. System Identification, Rotterdam, The Netherlands, Aug.27–29,2003, pp. 383–388. Domenico Longo obtained his degrees in electronics and automation engineering in 2001 from the University of Cata-nia, Italy. In February 2005, he obtained his Ph.D. in elec-tronics and automation. Since 2000, he has contributed to many robotics activities at the Robotics L aboratory of the University of Catania. His research activities include the iden-tification and control of nonlinear dynamical systems with neuro-fuzzy methodologies, the control of multiple input multiple output thermal systems, and the control of systems for sustaining life. He has a research contract at the University of Catania and is involved in many robotics projects. Giovanni Muscato received the electrical engineering degree from the University of Catania, Italy, in 1988. Following gradu-ation he worked with Centro di Studi sui Sistemi in T urin, Italy. In 1990 he joined the Dipartimento di Ingegneria Elettri-ca Elettronica e dei Sistemi of the University of Catania, where he is currently a full professor of robotics and automatic control. His research interest includes model reduction, service robotics, and the use of soft-computing techniques in the modeling and control of dynamical systems. He was the coordinator of the European Commission project ROBOVOLC: A Robot for V olcano Explorations, and he is in the executive committee of the CLAWAR 2 Network and in the executive committee of the EUROBOT Association. He is Senior Member of the IEEE Control Systems Society and of the IEEE Robotics and Automation Society. He has published more than 190 papers in scientific journals and conference proceedings and three books in the field of control and robotics.
Address for Correspondence:Domenico Longo, Dipartimento di Ingegneria Elettrica Elettronica e dei Sistemi, Universitàdegli Studi di Catania, Viale A. Doria 6, 95125 Catania, Italy.
E-mail: dlongo@diees.unict.it.
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