Development of ALACRANE: A Mobile

Robotic Assistance for Exploration and

Rescue Missions

A. García-Cerezo, A. Mandow, J. L. Martínez, J. Gómez-de-Gabriel, J. Morales, A. Cruz, A. Reina, and J. Serón

Dept. de Ingeniería de Sistemas y Automática, Universidad de Málaga, Plaza de El Ejido, 29013 Málaga (Spain)

gcerezo@ctima.uma.es

Abstract— The paper presents ALACRANE, a new mobile robot assistant for exploration and rescue missions with dexterous load manipulation capability. ALACRANE consists of a tracked vehicle with a 4-DOF articulated arm, whose end-effector is an independent pair of 3-DOF manipulators (LR-Arms) plus a common rotation on the main arm wrist. All actuators are hydraulic in order to provide a high power-to-size ratio for both traction and manipulation. The system is equipped with CCD and IR cameras and a 3D-laser scanner for victim detection and environment perception. Three operation modes have been envisaged for the robot: navigation, main arm positioning, and LR-Arms operation. The control architecture provides different levels of autonomy and tele-operation for each mode. Preliminary tests with the actual system are presented. Keywords: Mobile robots, Search and rescue, Mobile manipulators.

I. INTRODUCTION

The application of robotics to search and rescue operations in disaster scenarios poses very demanding problems from the standpoint of design and implementation: unstructured and unknown environments, mobility on uneven terrain, detection and manipulation of victims and hazardous material, rubble removal, and specific tele-operation capabilities, just to name a few. These problems imply addressing different levels of the robotic solution: • Locomotion and mechanics. Small robots have been

used in mine [1] and victim searching [2]. These light low-cost devices can be deployed in groups and are useful for exploration of collapsible structures. However, more powerful and rugged vehicles that are able to climb slopes or to move on rubble are required in many exploration and rescue missions [3], [4]. In this sense, tracked locomotion provides better traction than wheels for vehicles on natural and uneven terrains due to a larger contact area [5].

• Manipulation. Dexterous manipulation is required for handling or to pick away unexploded ordnance or hazardous materials. Moreover, a high strength-to-size ratio is necessary to afford large load capability required for lifting and moving rubble or victims. Some mobile

manipulators based on a single arm have been proposed [4]. However, the use of mobile dual manipulators can reproduce more closely human handling capabilities [6].

• Control and tele-operation. A certain degree of autonomous navigation capacity is necessary for these applications. However, local goal selection and manipulation requires human intervention through tele-operation [7], [8], [9].

• Perception. Different requirements appear for victim (or hazardous materials) localization and navigation. In both cases 3D techniques are useful in these types of non structured and poorly modelled scenarios. For instance, [10] uses 3D scans to construct a volumetric map for a mine detection mobile robot. Infrared thermal cameras are useful for victim search [9].

This paper reports on the development of ALACRANE, a

new mobile robot assistant for exploration and rescue missions. It consists of a rugged tracked vehicle with a main articulated arm, whose end-effector is a dual manipulator. All actuators are hydraulic in order to provide a high power-to-size ratio for both traction and manipulation. The vehicle can navigate through rough terrain. The work envelope of the main arm with dual manipulator allows dexterous manipulation around the base, both at ground level and above the robot base. The control architecture sets a frame for addressing the different operation modes of the system with appropriate levels of autonomy and tele-operation. Moreover, the sensor arrangement has been designed taking into account, both autonomous operation and tele-operation feedback. The paper is organized as follows: Section II describes the

mechatronic systems of ALACRANE. Section III is devoted to discuss the operation modes of the robot. The functional control architecture is presented in section IV. Some preliminary experiments with the actual system are introduced in section V. Finally, section VI is devoted to the conclusions.

II. THE MECHATRONIC SYSTEM OF ALACRANE

A. Mechanical Design.

The ALACRANE robot is depicted in Fig. 1. This is a fully hydraulic robot that has been developed from a modified small demolition machine by Brokk®. Originally, this is an open-loop remote-controlled device. The vehicle has two motorized outriggers to provide stability when lifting or manipulating weights. The robot consists of three main parts: the mobile base, the

main arm, and the LR-Arms dual manipulator. 1) The Mobile Base: Its goal of the mobile base is to

provide traction on rough terrain, such as rubble and moderate slopes. Tracked skid-steer traction is controlled by the speeds of two independent hydraulic motors (VL and VR) with encoders for dead-reckoning (see Fig. 2). The rubber belts are 130mm wide, with a longitudinal contact surface of 735mm and a distance between belt centerlines of 470mm. The performances of the robot base are related in Table 1. A power cable socket provides tethered three-phase AC power supply. Besides, for greater autonomy, a petrol-fed generator can be carried on a passive trailer towed by the mobile platform with a king-pin hitch. 2) The Main Arm: It has 4 DOF with 5 hydraulic cylinders

(see joints q1 to q5 in Fig. 2). This redundant configuration increases its reachability of the end-effector (see Fig. 3). Characteristics of the main arm are summarized in Table II. Its payload is 120kg when it is fully extended, and 450kg in the vicinity of the arm base for the outriggered vehicle. 3) The LR-Arms: A specific hydraulic end-effector that

reproduces some human handling capabilities has been fully developed for ALACRANE (see Fig. 4). It consists on a dual manipulator configured as left and right Arms (LR-Arms). Its characteristics are summarized in Table III. It has 7 DOF: it adds an additional DOF to the main arm (q6), and 3 DOF for each manipulator (qL7 to qL9 and qR7 to qR9, for the left and right arms, respectively) as shown in Fig. 2. Both arms can be equipped with different end effectors. Two 6 DOF force/torque sensors have been incorporated for the LR-Arms end effectors. Besides, the stainless steel manipulator has been designed so that it can also be used as a large scale two-finger hand to grip greater size objects. Moreover, the LR-Arms tool is removable so that other standard tools can be connected to the main arm (e.g., on-off grippers, buckets or clamshell buckets, and grapples with an additional DOF).

TABLE I

ALACRANE CHARACTERISTICS

Weight (no effector) 380 kg Width 600 mm Height 940 mm Length 1200 mm Speed, max 1.5 km/h Slope angle, max 30º Motor power 4 kW

TABLE II

ARM CHARACTERISTICS

Range 2400 mm Base angle ±123º Slewing speed 6,5 sec / 246º

TABLE III

LR-ARMS CHARACTERISTICS

Range 1000 mm Base angle ±90º Shoulder angle +100ºL, -100ºR Slewing speed 2,6 sec / 100º

Fig. 2. ALACRANE control inputs.

q5

q2

q1

q4

q6

q3

qL7

qL8

qL9

qR8

qR8

qR7

VR

VL

Fig. 1 ALACRANE with the LR-Arms.

LR-Arms

Mobile Base

Main Arm

Outriggers

Fig.5. Hardware Architecture of ALACRANE

3D SCANNER

THERMAL CAMERA

CCD CAMERA

JOYSTICK CONTROLLER

MOBILE PLATFORM AND ARM

CONTROLLER

IMU + GPS

PC1

TCP-IP

Wireless link

MOBILE PLATFORM AND ARM

ENCODERS

LR- ARMS ENCODERS

PC3

B. Electronic Systems

The ALACRANE Electronic System is based on 3 onboard PC computers connected by Ethernet and a remote base (see Fig. 5). The low level control is assigned to a PXI PC computer (PC1). The 12 joint absolute encoders as well as the control valves for the hydraulic actuators are connected to PC1 through a CANopen bus. An Inertial Measurement Unit and a differential GPS system are also read by PC1. This low level control also supports the manual control joystick for the mobile platform and arms, and some low level functions of the navigation controller. The other PCs (PC2 and PC3) are Compact PCs based on an

Intel Core Duo microprocessor. PC2 supports the functional architecture of ALACRANE and the communication level with the Remote base. PC3 supports the high level perception system: A 2D Scanner Laser –with an additional DOF–, and two sets of video cameras.

C. Perception

The Mobile Base and equipped with CCD cameras for navigation. The one on the base is coupled to a 3D laser scanner. Another set is mounted on the common axis of the LR-Arms. It consists of a thermal and a CCD camera, whose images are fused for target detection and teleoperation. The 3D scanner device has been constructed by adding an

extra degree of freedom to a commercial 2D SICK-LMS 291 time-of-flight range finder. Maximum specification values for this sensor are: field of view Φ=180º, horizontal angular resolution ∆φ=½º, and up to 80m range, ±4cm range error, 26msec of scan time. The 2D sensor has been mounted into a mechanical

articulation so that 3D readings are provided directly in spherical coordinates. It incorporates a special counterweight to reduce the required driving torque. The vertical angular resolution is ∆ψ=0.12º with Ψ=60º of field of view. A complete scan with 361 x 181 point is obtained in an interval of 13.6sec.

III. OPERATION MODES

Three different standard operation modes (plus a special maintenance mode) have been devised for ALACRANE. Each mode requires that only some of the robot drives are actuated, and has specific operational constraints. The rationale for these modes lies on the following: • Autonomy levels. Different kinds of operations allow

different degrees of autonomous control (see Table IV). • Tele-operation sustainability. Only a reduced number of

Fig. 4. Dexterous manipulation in a car trunk (computer simulation).

Fig. 3 Main Arm reachability around the base.

LR- ARM CONTROLLER

CANopen

PC2

TCP IP Cameras

variables can be under the attention of the operator. • Mobile base stability. Manipulation requires that the

mobile base is stabilized by the outriggers. • The hydraulic system. Proportional valves can be bulky

and expensive. They are optimized to control more than one actuator, which cannot be operated in unison but in sequence. For instance, the same valve is used for the right track motor and the q3 joint.

TABLE IV

AUTONOMY LEVELS FOR OPERATION MODES

Autonomous Tele-Autonomous Tele-operated

Mode 0 enabled

Mode 1

Mode 2 enabled

Mode 3

disabled

A. Maintenance and development mode (Mode 0)

This mode is an “unprotected mode” for maintenance and development, but also for singular operation. This mode allows actuation over all drives and reading all system variables. The security restrictions are disabled.

B. Navigation mode (Mode 1).

This mode involves the coordinated control of the track drives (VL, VR) and allows controlling the angle of the main arm base (q1). Navigation of the mobile platform involves differential control of the track drives. Low level control is performed by the PC1 onboard computer, which receives curvature and speed references from the control architecture in PC2. In skid steer vehicles, pure rolling does not apply, so the differential-drive kinematic model that relates these references to track speeds has to be characterized from experimental data [5]. When towing the optional power generator trailer, path

tracking and path planning methods have to run with additional curvature limitations [11]. Regarding the arm base rotation (q1), this is expected to be

usually in rest mode, with its yaw angle aligned with the forward motion direction. Nevertheless, rotation of this articulation changes the centre of mass of the vehicle, so it could be useful to balance the system when driving on slopes. Moreover, this rotation also adapts the shape of the arm projection on the plane (as in an actuated trailer joint), which could prove interesting when accessing to narrow areas.

C. Arm mode (Mode 2).

This involves articulations q1, q2, q3, q4, and q5. This mode refers to control of the main arm for approaching the LR-Arms to their workspace. Besides, the CCD-IR camera

set is attached to the LR-Arms, so this mode also allows visual exploration of the environment from the tele-operation station. Security constraints require that the anchoring base is deployed when switching to Arm mode from Navigation mode. Actuator redundancy requires establishing kinematic criteria for selecting the appropriate actuation sequences, in tele-autonomous operation.

D. LR-Arms manipulation mode (Mode 3).

This mode involves control of the LR-Arms joints (q6, qR7, qR8, qR9, qL7, qL8, qL9). It requires that the main arm has attained the manipulation pose and that the anchoring bases are deployed. The LR-Arms are controlled through the tele-operation station, using haptic devices.

HIGH LEVEL

SUPERVISORY

LEVEL

CONTROL LEVEL

Supervisor

Aggregation

Activation/Deactivation

Weights

Mission

Planner i-th task

Sequenced tasks � Human

Operator

Vehicle Motion

Policies Set

Execution

Data

K, ωωωω

ALACRANE ROBOT

Perception Arms Control

References

Behaviours

Manoeuvres

B1

B2

… Bn

M2

… Mj

P1

… Pr

P2

Path tracking methods

Aggregation

M1

Sensor Policy

Arm + LR-Arms

Motion Policies Set

Behaviours Ba1

Br

P1

… Pr

P2

Planning methods

Pj

Sensors Data and

Odometry

Fig. 6. Functional Architecture of ALACRANE.

Fig. 7. 3D mapping.

Fig. 8. 3D Terrain mapping.

Fig. 9. Gripping an object with LR-Arms.

IV. THE FUNCTIONAL ARCHITECTURE OF ALACRANE

A general outline of the control architecture proposed for the vehicle is presented in Fig. 6. The high level determines the mode and level of autonomy for each task. The Motion Control for the vehicle, main arm, and LR-Arms is implemented as a set of Policies. Polices include reactive behaviours, tracking of planned paths and manoeuvres (i.e., predefined motion patterns). The actual motion references for the robot low level control are the result of the aggregation of all active policies. The supervisory level establishes what policies should be active for executing a task. Finally, sensor fusion policies are an important basis of this

architecture, as different motion policies need an anchoring approach to process environmental and internal information coming from the different sensors.

V. PRELIMINARY TESTS

This section illustrates some of the ALACRANE capacities by experiments developed with the actual robot. These experiments correspond to different aspects of the project.

A. Perception in Navigation Mode.

The laser rangefinder offers three main functions: • 2D motion estimation, using a hybrid GA-ICP (Genetic

algorithm and Iterative Closest Point) technique [12]. • 3D depth map generation (see Fig. 7). Segmentation and

feature extraction of the scene are obtained from CCD camera images and 3D depth maps.

• Geo-referenced mapping (see fig. 8). In this case, the scanner generates consecutive scans of the ground surface, obtaining an approximated surface model. The mapping procedure generates uniform grids. Farther points with greater uncertainty are matched by new scans as the robot approaches them. The map is geo-referenced using calibrated GPS points.

B. Arm mode Experiments.

The manoeuvrability and load capacity of the main arm are

tested by the manipulation and lifting of persons (see fig. 9). In this case, an auxiliary tool, based on a claw with safety karabiners, is used. The load, on the left, is a person with a harness. The experiments have been carried out with two volunteers of 61kg and 93kg, operating at 1m from the main arm vertical axis.

C. LR-Arms mode Experiments.

The experiment shown in Fig. consists on gripping a relatively heavy object (a CRT monitor) from a difficult access area. First, the vehicle approaches the operation area and is outriggered, then the main arm is positioned, and finally, the LR-arms are used to grip the object. The figure shows the moment when arm mode is regained so as to lift the object.

D. Tele-operation in Navigation Mode..

In navigation mode an experiment of tele-operation is implemented with a load of 93kg. Fig. 10 shows a screenshot of the current tele-operation menu. The choice of an operation mode displays different sets of drive controls and status indicators (left side). In this example, the navigation mode is active, with the DGPS trajectory on-screen. An image from onboard cameras can be seen on the right.

E. Perception in LR-Arms Manipulation Mode.

The CCD and thermal cameras are used together to determine the possible targets (see Fig. 11). The cameras have different

Sensor

position

X (m)

Y(m)

Z (m)

Fig. 7. 3D scanner map.

resolution and field of view (due to the lens focal distances) and a calibration procedure is required to to match both images (the thermal camera image limits are shown by a dashed line on the figure). Besides, a threshold can be specified so that only the warmest areas of the thermal image are matched onto the CCD image. A threshold of 28ºC has been considered in the figure, so only the robot and the lifted person are highlighted with the thermal information.

Fig.10. Tele-operation menu for the Navigation Mode.

IV. CONCLUSION

The paper has introduced ALACRANE, a new mobile robotic assistance for exploration and rescue missions. The robot and its control architecture are the keystone of a project to develop new methods and techniques for mobile robotic assistants in missions of exploration, search and rescue. ALACRANE consists of a tracked vehicle with a 4-DOF

articulated arm. A special end-effector has been designed, which consists of is a pair of 3-DOF manipulators (LR-Arms) with an additional 1-DOF common base. Hydraulic actuators provide a high power-to-size ratio for both navigation and manipulation. This design favours exploration of unstructured environments, such as disaster areas, as well as dexterous manipulation in workspaces of difficult access. The system is equipped with CCD and IR cameras and a 3D-laser scanner for victim detection and environment perception and force/torque sensors.

Three operation modes, with different levels of tele-operation and autonomy levels, have been envisaged for the robot: Navigation, Main Arm operation, and LR-Arms operation. The control architecture provides different levels of autonomy and tele-operation for each mode. Preliminary tests have shown perception and loading capabilities of the system for search and rescue operations. This robot presents some new interesting challenges as:

navigation with active trailers in natural terrain, dexterous manipulation, redundant arms control, 3D slam, and cognitive architectures, that will be addressed in future works.

ACKNOWLEDGEMENT

This work was supported by the Spanish project DPI 2005-00207.

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Fig. 11. Matched thermal and CCD image.