SnakeFighter - Development of a Water Hydraulic Fire Fighting Snake Robot Pål Liljebäck1, Øyvind Stavdahl2, Anders Beitnes3 SINTEF ICT, Dept. for Applied Cybernetics, NO-7465 Trondheim, Norway 2 NTNU, Dept. of Engineering Cybernetics, NO-7491 Trondheim, Norway 3 SINTEF Building Research Institute, Coast and Harbour Research Laboratory, NO-7465 Trondheim, Norway
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Abstract – This paper presents the SnakeFighter concept and describes the generic element within this concept in the form of a water hydraulic snake robot. Applications of a SnakeFighter system are presented with focus on fire intervention tasks. The development of a water hydraulic snake robot that demonstrates the concept is described. The robot is the first water hydraulic snake robot ever constructed. The paper identifies design challenges of a complete SnakeFighter system and describes future research on this concept. Keywords – Snake, Robot, Water hydraulics, Fire fighting
I. INTRODUCTION There is a growing need for robotic mobility in unknown and challenging environments. A fire inside a tunnel, a human being trapped under the ruins of a collapsed building or a burning propane tank threatening to explode are all dangerous situations that today require human intervention in order to be resolved. This calls for the rise of intelligent and robust robots that can both support and replace human intervention in these situations and minimize human exposure to risk. Wheeled and tracked robots are today the most mature technologies for robotic mobility. The drawback of these systems, however, is their limited mobility in environments with challenging obstacles. This drawback has led to the development of new propulsion mechanisms that are now starting to emerge outside the laboratory. These new forms are typically biologically inspired. Snake robot locomotion is one such form of mobility. Several agile and impressive snake robots have been constructed so far, such as the robots developed by Prof. Shigeo Hirose at Tokyo Institute of Technology. Hirose developed the world’s first snake robot in 1972 [1]. Hirose has also developed the Active Cord Mechanism ACM-R3 [2], Helix [3], and the Slim-Slime robot [4]. The joints of the robots, except the Slim-Slime robot, are all actuated by small electrical motors. The Slim-Slime robot is pneumatically actuated. Several interesting American snake robots have been developed. The most prestigious robots include the JPL serpentine robot [5] developed by NASA’s Jet Propulsion Laboratory (JPL) in 1994, several snake robots developed at Carnegie Mellon University, one of which is described in [6], and some very lifelike robots developed by Dr. Gavin Miller [7]. These robots are based on joint actuation by small electrical motors.
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An interesting aim of current research on snake robot design involves the development of stronger and more compact joint actuation systems. Compactness and strength is crucial for the agility and flexibility of a snake robot. Research of particular interest includes the work on new types of joint design led by Howie Choset at Carnegie Mellon University [8]. A wide range of mathematical formulations have been proposed in order to better understand the principles behind snake locomotion. Some models focus purely on the kinematic aspects of snake motion [9], [10], [11] while others include the dynamics of the snake [12], [13]. The control strategies for snake robots developed so far tend to try to resemble motion patterns displayed by biological snakes [10], [12], [13], [14], [15]. This paper presents the SnakeFighter concept, which is based on the idea of enabling a fire hose to move like a biological snake and perform fire intervention on its own. The resulting system is a snake robot where water is used as: the hydraulic medium in the joint actuation the fire extinguishing medium launched through a nozzle in the front of the snake robot the cooling medium for cooling the robot in environments with extreme temperatures The system is based on being connected to a stationary water supply through a flexible hose. It should be noted that the scope of the concept extends beyond merely fire intervention tasks. The proposed system will be applicable in any other application that requires mobility in a challenging environment and where a connection to a water supply is available. The paper presents potential applications of a SnakeFighter system, some of the major design challenges and the development of a demonstrator robot named Anna Konda. Prof. Shigeo Hirose has investigated a similar concept through the development of the robot Genbu [16]. The robot is, however, merely a technology demonstrator in the form of a wheeled fire hose actuated by electric motors. The demonstrator presented in this paper is the only known snake robot with a water hydraulic actuation system. The paper is organized as follows. Section II describes the SnakeFighter concept in more detail, section III describes the mechanical design of the demonstrator, while section IV describes the control system design for the demonstrator. The implementation of the design is described in section V and a
ICARCV 2006
discussion related to future work and future design challenges is given in section VI. II. THE SNAKEFIGHTER CONCEPT A. Applications of a SnakeFighter system A major advantage of using a robot in a fire fighting operation is its potential ability to cope with high levels of heat and heat radiation. A SnakeFighter robot may be of use during a fire in many ways. In addition to fighting the fire with water, it could help analyzing the situation by providing camera vision to fire fighters on the outside. Locating victims trapped inside a burning building is essential before the fire fighting begins. A SnakeFighter robot equipped with suitable instrumentation could act as an effective smoke diver by detecting and providing oxygen to these victims. An ideal way to fight a fire is to do so without bringing new oxygen into the situation. This is generally contradictive to the need of fire fighters to open doors or windows in order to get to the fire. A SnakeFighter, on the other hand, could get to the fire by drilling or punching a hole through a wall or a door, just large enough for its own passage. Tunnel fires are particularly relevant for a SnakeFighter system since they are extremely dangerous and challenging. It is virtually impossible for human fire fighters to perform active fire intervention inside a burning road tunnel due to the extreme heat and smoke developed. One possible scenario will be to install a permanent SnakeFighter system inside the tunnel and mobilize the robots when needed, such as after a car accident. This scenario is visualized in Fig. 1.
A SnakeFighter system has many potential applications besides fire fighting. The generic element within all these applications is a water hydraulic snake robot capable of reaching and operating in a hostile environment. A specific application is achieved by equipping the snake robot with an appropriate set of tools. B. Functional requirements of a SnakeFighter system The authors believe that a complete SnakeFighter system should comply with the following requirements:
A SnakeFighter robot should be an articulated mechanism with joints operated by a water hydraulic actuation system. The mechanism should be able to propel itself forward through synchronized movements of the joints. The robot should have a high degree of traversability and be able to reach and operate in inaccessible and hostile areas. The robot should be covered by a wear resistant skin with tactile sensing capabilities, the latter being essential to effective snake locomotion. The robot should be equipped with tools and sensory capabilities in accordance with the given task (camera, radar, sonar, temperature/gas/pressure/noice/vibration sensor, gripper, welding tool, fire hose nozzle, etc.). The fire fighting application will require the robot to be resistant to heat and heat radiation. Sensors, tools and internal components must be heat resistant or protected by a cooling system (e.g. water-based cooling). The robot should be controllable through an intuitive man-machine interface. III. MECHANICAL DESIGN OF A SNAKEFIGHTER DEMONSTRATOR
This section presents the design of a SnakeFighter demonstrator called Anna Konda. A. Requirements of the demonstrator Anna Konda was developed in order to demonstrate the generic element within the SnakeFighter concept, that is, a snake robot with a water hydraulic actuation system. The hydraulic medium should be plain water without additives pressurized to approximately 1450 PSI. This is the pressure utilized in modern fire protection systems based on water mist [17]. Anna Konda was required to display biologically inspired gait patterns and the ability to spray water through a nozzle in its front. The robot was, however, not required to perform actual fire fighting, nor operate in a hazardous environment, and is therefore not yet intended for use in an actual fire fighting situation.
Fig. 1 Several SnakeFighter robots inside a road tunnel visualized during a fire fighting operation.
B. Snake skeleton and articulation mechanism The skeleton of Anna Konda consists of 11 identical skeletal modules interconnected by 10 cardan joints. Each joint has two degrees of freedom. A CAD model of the articulation mechanism is shown in Fig. 2. The skeletal modules are designed to have a small mass, but at the same time enough strength to withstand the forces
induced by the water hydraulic actuation system described below. The outer diameter of each skeletal module is 159 mm and the distance between the axes of rotation at each end of the module is 269 mm. The maximum angle in both the yaw and pitch direction with respect to each module is ±33.5°. This angle is limited by the stroke of the water hydraulic cylinders.
This force is extremely large and corresponds to more than 700 kg in the gravity field. The distance from the centre of each joint to the mounting of the cylinder is rt = 44 mm. The maximum torque produced by a cylinder is therefore Tmax = rt ⋅ Fmax = 311Nm
(2)
Small water hydraulic valves were needed to control the pressure applied to each cylinder. These were custom-built based on a simple valve design principle illustrated in Fig. 4. The direction and the magnitude of the water flow in/out of a cylinder are controlled by rotating a spool inside a valve block.
Fig. 2 CAD model of the articulation mechanism of Anna Konda. The robot has 11 identical skeletal modules interconnected by 10 cardan joints.
C. Water hydraulic actuation system The main challenge encountered during the design of Anna Konda was the development of a compact water hydraulic actuation system. There are virtually no components available in the market for mobile or miniature water hydraulic applications. For this reason, the small water hydraulic valves and cylinders needed for the robot had to be custom-built. Water used as a hydraulic medium represents a challenge due to the corrosiveness and the poor lubricating properties of water compared to oil. However, water has advantages over oil since it is cheap, easily available, environmentally friendly, and fire resistant. Water may be tapped from and drained to almost anywhere. The low compressibility of water compared to oil is advantageous since it allows for higher stiffness and accuracy in the hydraulic actuation. Each joint of Anna Konda is actuated by two doubleacting water hydraulic cylinders as illustrated in Fig. 3. The linear motion of these cylinders causes the corresponding joint to flex in the yaw and pitch direction, respectively. The cylinder that operates the pitch direction of joint i is mounted inside the same skeletal module as the cylinder that operates the yaw direction of joint i+1. The linear motion of the piston inside each cylinder is induced by the pressure difference across the two openings of the cylinder.
Fig. 3 Mode of operation for the water hydraulic cylinders. The pressure difference between input A and B decides the motion of the piston.
The cylinders used in Anna Konda were designed with a piston radius of rp = 15 mm. With a supply pressure of PSupply = 1450 PSI = 9 997 398 Pa , the maximum achievable linear force from a cylinder in the robot is Fmax = π rp2 ⋅ PSupply ≈ 7067 N
(1)
Fig. 4 Mode of operation for the water hydraulic valves. Left: The valve openings are closed by the spool. Middle: Output A of the valve is pressurized while output B is connected to PAtm. Right: Output B of the valve is pressurized while output A is connected to PAtm.
The pressure is balanced across the spool so that the rotation of the spool is not counteracted by the pressure. Counterclockwise rotation of the spool causes output A of the valve in Fig. 4 to be pressurized while output B is connected to the drain pressure, which is equal to the atmospheric pressure. Clockwise rotation causes output B to be pressurized while output A is connected to the drain pressure. By connecting the two openings of a cylinder to the two outputs of a valve, the motion of the piston inside the cylinder may be controlled by the valve. D. Snake skin Existing snake robot designs tend to focus less on the skin structure that covers the robot and very often it is completely omitted from the design. The lack of a skin structure is not favourable since the utilization of external contact forces from the ground and external objects is important for efficient snake locomotion. Many existing snake robots have passive wheels along the body [2] in order to achieve a larger friction in the lateral direction than the longitudinal direction with respect to the snake body. This frictional property is favourable for certain types of gait patterns. The skin covering Anna Konda was designed based on the desire to achieve a smooth outer shell that will enable the robot to glide forward when it is curved around external objects. Each skeletal module is covered by four identical smooth plates, or scales. These plates need to have a radius of curvature equal to the outer radius of the skeletal module in order to fit around the module. The design is very simple and does not cover the articulated areas of the robot completely. However, when a joint flexes it will produce more coverage of one side of the articulated area since the skin structure of
neighbouring modules will overlap to produce a smooth exterior. The side with the most coverage is generally the area where contact forces from external objects will be produced. Contact force sensors were mounted beneath the skin plates in order to enable the robot to sense external forces applied to its body. The implementation of the skin structure is described in section V. IV. CONTROL SYSTEM DESIGN FOR A SNAKEFIGHTER DEMONSTRATOR
This section describes the motion planning and control strategy used in Anna Konda. The aim is not to outline a control system for a complete SnakeFighter system, but merely to present the control strategy used to demonstrate snake locomotion with Anna Konda. The theory is based on results previously presented in [13].
the primary gait pattern for Anna Konda in order to demonstrate snake locomotion. B. Motion control The control strategy developed for the robot is based on abstraction. At the highest level, a motor intention arises. This intention propagates down through the control hierarchy and results in the joints being actuated in a manner that seeks to fulfil this intention. The control strategy that takes place in the brain of the snake is illustrated in Fig. 5. An intention arises in the cognition layer and propagates down to the motor control layer. This layer decides which gait pattern that should be utilized and assigns values to the parameters characterizing the desired motion. These parameters are received by the CPG layer (central pattern generator), which generates motivation-based reference setpoints that are sent down the “spine” to all the segments.
A. Motion planning The motion planning algorithm is based on a partitioning of the snake movements into a horizontal and a vertical part. Consider joint i of the robot and denote the setpoint for the angle about the vertical and horizontal axis of rotation by i q1,ref and i q2,ref respectively. A general expression for characterizing the different forms of snake locomotion may then be written as i
q1,ref = Ahor sin(ωhor t + (i − 1)δ hor ) + ψ hor
i
q2,ref = Aver sin(ωver t + (i − 1)δ ver + δ 0 ) + ψ ver
(3)
where i ∈ [1,10] and joint 1 is the foremost joint. The first equation relates to the generation of the horizontal wave, and is characterized by the amplitude of the wave Ahor, the angular frequency ωhor, a phase offset δhor, and an angular offset ψhor. The second equation characterizes the vertical wave in a similar manner, but has an additional parameter δ0 that represents the phase difference between the horizontal and the vertical wave. Different gait patterns are achieved by setting each of these parameters in a particular way. Sidewinding is a common form of snake locomotion and is a sideways rolling kind of motion consisting of alternating waves of lateral bending. Results previously presented in [13] have shown that sidewinding motion with a snake robot composed of two DOF joints may be achieved by setting
Fig. 5 Control scheme for the brain of the snake robot.
The control scheme taking place locally in each joint is depicted in Fig. 6. Motivation-based reference setpoints from the brain are received and affect the control of the two joint angles. The angular control may also be affected by reflexbased reference setpoints induced by contact forces on each segment measured by the contact force sensors. Inclusion of reflex-based reference setpoints enable the robot to curve around external objects in order to achieve greater propulsion forces. The angular control scheme (PD control) in the feedback loop generates control signals to the valves that control the pressure applied to the two cylinders in each joint. This loop represents the lowest layer in the control hierarchy.
3π rad , δ = −70° , ψ hor = 0° (4) s hor 4 3π rad , δ = −70°, ψ ver = 0° , δ 0 → Directional control = s ver 4
Ahor = 30° , ωhor = Aver = 30° , ωver
Sideways locomotion of the snake robot to the left and right with respect to the direction the head is pointing in is achieved by setting the phase difference between the horizontal and the vertical wave to δ0 = 90° and δ0 = -90° respectively. Counterclockwise and clockwise rotational motion is achieved by setting the phase difference to δ0 = 0° and δ0 = 180° respectively. Sidewinding motion was chosen as
Fig. 6 Control scheme for local control of each segment.
V. IMPLEMENTATION OF A SNAKEFIGHTER DEMONSTRATOR The implementation of the design presented in section III and IV is now described. All mechanical parts were custombuilt at a local workshop in Trondheim, Norway. A. Snake skeleton and articulation mechanism The skeletal modules were cut from a steel pipe. An assembled joint is shown to the left in Fig. 7. The modules were galvanized in order to improve their appearance as shown to the right in Fig. 7. The figure also shows the ring that interconnects two skeletal modules, parts needed to assemble the joints and mounting brackets for the cylinders. Two rotary potentiometers were used to measure the pitch and yaw angle in each joint.
Fig. 7 Left: Two skeletal modules connected to form a cardan joint. Right: A galvanized skeletal module, the cardan joint ring, parts needed to assemble the joints and mounting brackets for the cylinders.
Fig. 8 Left: One of the 20 water hydraulic cylinders installed in Anna Konda. Right: A valve block used to operate the two cylinders in each skeletal module.
Fig. 9 The skin plates used to cover the skeletal modules of Anna Konda.
washers and the snake robot in order to keep the supply pressure beneath 1450 PSI. The pressurized water runs through each joint of the robot inside a flexible hose. The valves and cylinders connect to this hose inside each skeletal module. The drain water is returned through a separate hose. In a SnakeFighter robot used in an actual fire fighting operation the drain water can for example be used to cool down the exterior of the snake. Anna Konda is equipped with 20 water hydraulic cylinders that operate the 10 joints of the robot. One of these is shown to the left in Fig. 8. The stroke of the cylinders is 50 mm. As described in section III, the valves were custom-built due to the unavailability of commercial water hydraulic valves. In order to save space, the two valves needed inside each skeletal module were integrated in the same valve block. One of these valve blocks is shown to the right in Fig. 8. The spool in each valve is rotated by a digital servo motor. C. Snake skin A total of 44 identical skin plates were manufactured in order to cover the robot’s 11 skeletal modules. The plates were made from aluminium. Two of these plates are shown to the left in Fig. 9. Some of the plates are assembled on skeletal modules to the right in this figure. As described in section III, the robot needs to be equipped with contact force sensors in order to enable it to sense applied forces from the ground and from external objects. The sensors utilized are so-called FSR components (Force Sensing Resistor). The electrical resistance through an FSR component is altered when pressure is applied to it. Fig. 10 shows how the FSR components are mounted beneath each skin plate. D. Control system Angular control of the joints of the snake robot is performed by a microcontroller (ATmega128) located in each skeletal module. The microcontroller reads sensor data (joint angles and external forces) and controls the valves. The highlevel control strategy for the snake (Fig. 5) is implemented in a dedicated microcontroller located in the foremost skeletal module. The snake is equipped with a wireless transceiver that enables remote control from an external computer. E. The demonstrator – Anna Konda Anna Konda is three meters long and weighs 70 kg. The assembled snake robot is shown in Fig. 11. The water hydraulic actuation system works in accordance with the predefined specifications.
Fig. 10 FSR components mounted beneath a skin plate.
B. Water hydraulic actuation system The water hydraulic pressure is supplied to the robot by two high-pressure washers connected in parallel. The total flow from these washers is limited to about 30 l/min. A pressure reduction valve is connected between the high-pressure
Fig. 11 The demonstrator assembled with skin scales.
The sidewinding gait pattern described in section IV has been implemented on the robot and is displayed in accordance with the theoretical results. Fig. 12 shows the motion of the robot during this gait.
VII. CONCLUDING REMARKS The SnakeFighter concept targets fire intervention through the use of water hydraulic snake robots. The concept is based on a combined use of water for hydraulic joint actuation, fire extinguishing and cooling. The paper has presented the major design challenges of this system and described the development of a water hydraulic snake robot that demonstrates the SnakeFighter concept. A biologically inspired gait pattern has been successfully implemented and tested on the demonstrator robot. However, several design challenges still remain before a fire fighting snake robot may become operational. These include a more compact and efficient water hydraulic actuation system and more intelligent and robust control strategies. REFERENCES [1] [2] [3]
Fig. 12 Sidewinding motion with the snake robot
The foremost skeletal module (the brain module) is equipped with two nozzles. These are connected to the pressure supply hose through a dedicated valve. The nozzles enable the snake to spray water and thereby demonstrate the fire fighting application. The robot is raising its head to put out a small candle flame in Fig. 13.
[4] [5] [6] [7] [8] [9] [10] [11]
Fig. 13 Anna Konda raising its head and spraying water.
VI. DISCUSSION AND FUTURE WORK Future use of snake robots will require them to become stronger and smarter. Their strength is needed in order to give them satisfactory payload capabilities and to enable the robots to survive in a hazardous environment. The snake robots need to become smarter in that they need to learn how to sense and exploit external contact forces for more efficient propulsion. A significant amount of research still remains before a true fire fighting snake robot may be developed. A major challenge is the development of a more compact water hydraulic actuation system with better performance and low power consumption. Another challenge is the development of more sophisticated control strategies combined with a robotic vision system that will enable the robot to navigate in unknown environments. Future research on the SnakeFighter concept will target all these challenges.
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