Biorobotics Lab Sensor Based Planning Laboratory Carnegie Mellon Robotics Institute

Gaits, Locomotions, and Behaviors

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Central to the modsnake mission is the ongoing development of motions and behaviors. The snakes locomote using cyclic base functions called 'gaits', which can be run by a computer and drive the snake indefinately. Additionally, combining these gaits and other control techniques leads to the development of 'behaviors', in which the snake robots can perform a higher-level task such as stair climbing. Ultimately, the goal is to develop these snake robots' behaviors to allow them to perform useful tasks in the field tasks.

Snake Robot Gaits

Linear Progression
Sine waves are sent through the length of the snake robot, propelling it either forwards or backwards. Linear progression may be used in tandem with bending or turning motions to navigate space. This gait is particularly suited for fitting into tight areas, such as in pipes or through chain link fence. Sinusoidal

Strafing was inspired by sidewinding biological snakes, and is currently one of the fastest ways for these snake robots to travel. One vertical and one horizontal sine wave interact to locomote the snake sideways. Strafing the front half of the snake to the right, and the back half of the snake to the left has the effect or spinning the snake robot in place. This gait is ideal on rugged or uneven terrain. Strafing

A unique gait, corkscrewing, causes the snake robot to spiral its body, and propegating these spirals back through its body, propelling it forwards. It is useful for traveling forwards or backwards in the presence of obstacles, when linear progression cannot easily propel the robot. Corkscrewing is also valuable when manuvering through a small hole in a wall or a chain link fence. Corkscrewing

Rolling is a quick and reliable way to move the snake robot sideways. The snake curves slightly into a 'C' shape and then rolls sideways. Rolling is also one of our most energy efficient gaits, since the snake's momentum is conserved. It is useful for uneven terrain and excels at climbing hills. Rolling

Provided the snake is housed in a waterproof skin, the snake robots are able to traverse the top of the water. Since the robot is buoyant, sending sine waves backwards through the body of the robot propels it forward. The snake is also able to turn. Swimming with wireless control is more efficient since drag from a power line is not an issue. Swimming

Channel Climbing
Channel climbing is a variant of linear progression, where the amplitude and period of the sine wave are adjusted to fit the chosen channel or pipe. Channel climbing snake robots are often fitted with a protective skin or rubber to provide additional friction and compliance. No adhesives are used to achieve snake robot channel climbing, only outward pressure provided by the sine waves. Climbing

Pipe/Tube Climbing
Similar to channel climbing, the snake can also climb up the inside of pipes or tubes. The snakes can climb tubes as small as they will fit into, and research is underway to allow them to climb pipes 8" in diameter or wider. A variant of linear progression can be used to climb a pipe or tube, as in a channel, but since a pipe is cylindrically symetrical, the snakes can also use a modified version of corkscrewing to climb a pipe. Since corkscrewing up a pipe uses all 16 servos, as opposed to just 8 as in linear progression, corkscrew climbing is faster (15 ft/min vs 12 ft/min with Hercules in 4" PVC pipe). Corkscrew Pipe Climbing

Pole Climbing
Pole climbing refers to moving upwards on clylinders whose perimeter is less than the length of the snake robot. The robot moves upwards without the aid of any adhesive by spiraling its body around the pole, gripping it, and using the rolling gait to travel up or down the pole. Snake robots are able to transition directly from another gait to pole climbing. Pole Climbing

Cornering is a motion to maneuver around tight corners, such as those encountered in pipe systems or at the intersection of ibeams. A bend in the snake matching the bend of the corner is generated, and the bend is maintained relative to the corner as the snake moves forward. Once the snake robot’s body is around the corner, the bend is removed. Cornering

Pipe Rolling
Pipe rolling moves the snake robot efficiently & reliably across the top of a pipe. The snake robot wraps its head and tail around the pipe and uses the rolling motion to move forward. Stability comes from its relatively low center of gravity and balance on the pipe and the energy efficiency originates in the energy efficiency of the rolling gait. Pipe Rolling

Snake Robot Behaviors

Snooping is the motion of picking up the front region of the snake robot off the ground and move the head around. The robot uses the back and middle regions of the body to create a footprint to balance, bends the front section upward and ‘snoop’. This behavior is powerful when a snake robot has a camera attached; operators can use it to see into and over obstacles in the snake robot's path. Snooping

The scanning behavior programs the snake robot to cover a certain area with its head. It is a combination of the snooping behavior and rotation. This behavior is used when mounting an infrared sensor or camera on the front of the snake. The scanning behavior will cover entire region of rotation with the sensor. Scanning

Climb Scanning
Climb scanning is, as its name suggests, a combination of the scanning behavior and the channel climbing gait. During the channel climb, at the operator’s command, the back of the snake locks into the channel while the head of the robot falls backwards. The head section proceeds to move upwards, downwards, as well as sideways. Operators have the ability to get data or images in multiple directions from great heights or hard-to-reach positions. Climb Snooping

Hole Thru
Hole Thru is the name given to the behavior of moving up to a raised hole or pipe and transverse though it. The snake robot typically uses linear progression to move up to the area below the raised hole, snooping motion to push its head into the hole and linear progression to propel its body forward into the hole. HoleThru

Stair Climbing
Stair climbing is a composite behavior of several simpler gaits and behaviors. The front section of the snake robot is snooped onto the stair, and the snake robot transfers as much weight as it needs to lift itself onto the next stair. The snake robot is able to climb multiple stairs in this manner. Stair Climbing

Breaching a Chain Link Fence
The metal module snake robots have cross sections which allow them to fit through chain link fences. The snake robot uses another gait to approach the fence and stabilize itself. The operator then chooses a hole, and hole thru behaviors follow to successfully cross through the chain link openings. Chain Link Fence

Gap Crossing
Gap crossing is the behavior of moving the snake over a discontinuity in the surface, such as a channel, a hole, or a gap between boards. The behavior of crossing gaps uses different mechanisms based on the size of the gap, the type of snake robot, and external factors. Further research is being conducted to increase the size and regularity of the gaps that can be traversed. Gap Crossing

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© Copyright 2005 Biorobotics Lab, Carnegie Mellon University. All Rights Reserved.
© Howie Choset, 2005