project specification

SEA Snake

A robot which consists of a series chain of 1-DOF modules capable of torque, velocity and position control. Each module includes a high-speed Ethernet communications bus, internal IMU, modular electro-mechanical interface, and ARM-based onboard control electronics. The snake robots consists of a series chain of actuated links, sometimes referred to as hyper-redundant mechanisms. Its small cross section and snake-like motions make the robot applicable to a diverse set of tasks such as urban search and rescue, mine rescue, industrial inspection, and reconnaissance. The SEA Snake robot consists of a number of identical 1-DOF modules in which the actuated axes are oriented in the lateral and dorsal planes of the robot. It also contains a series elastic actuator in each module which enables compliant motion and fine torque control on each joint.

Specifications

Diameter5.1 cm
Length module6.4 cm
Length full 16 module robot)1.174 m
Module weight205 g
Full 16 module robot weight3.657 kg
Torque7 N-m
Speed3333 RPM (max)
Voltage48 V (resting): 40 mA
Current600 mA
Connection100 Mbps Ethernet

Overview

Each module includes a high-speed Ethernet communications bus, internal IMU, modular electro-mechanical interface, and ARM-based onboard control electronics.

The snake robots consists of a series chain of actuated links, sometimes referred to as hyper-redundant mechanisms. Its small cross section and snake-like motions make the robot applicable to a diverse set of tasks such as urban search and rescue, mine rescue, industrial inspection, and reconnaissance.

The SEA Snake robot consists of a number of identical 1-DOF modules in which the actuated axes are oriented in the lateral and dorsal planes of the robot. It also contains a series elastic actuator in each module which enables compliant motion and fine torque control on each joint.

Design

Each module possesses a self-contained 1-DOF joint, allowing for a full 180◦ of rotation. Modules are interfaced together and alternately aligned in accordance with the robot’s lateral and dorsal planes. A typical robot consists of 16 modules linked together, with unique head and tail modules.

A driving design requirement of the SEA Snake robot was ease of customization. In addition to the 16 rotary DOF design, modules can be added, removed, interchanged, or replaced with novel mechanisms. Any device meeting the interface requirements can be included in the chain.

Motor-Geartrain

The SEA Snake modules are driven by a modified Maxon EC 20 flat motor with a nominal speed of 9300 RPM. The steel pinion gear on the motor’s output shaft transfers rotation through a geartrain containing 3 steel and brass compound gears. The cumulative gear ratio is 349:1 to create hightorque joints. This motor and geartrain combination provides a maximum output torque of 7 N-m and a maximum speed of 33 RPM.

The geartrain has been designed to be back drivable, and as such experiences 2-4◦ degrees of backlash. While significant, this backlash is of limited consequence to normal snake locomotion, due to the relatively low controller gains on each module, and other modifications that are presented in Section V-C.

Sealed Housing

The housing of each module is machined from 7075 aluminum and anodized red to prevent wear and corrosion. Components are densely assembled inside to minimize volume, as illustrated in the module cross-section in Fig. 3. O-rings laid in machined grooves seal the module at each interface.

The robot meets IP66 standards, meaning it is splash-proof. Future iterations will aim for IP68, or water submersible. Additionally, an effort was made to minimize external fasteners in the design. The previous snake robot, the Unified Snake, has 14 external fasteners per module, while the SEA Snake modules have only 4.

Modular Interface

The intermodular interface features a rugged, tool-less design. Modules are aligned with dowel pins and matching recesses. A freely-spinning threaded collar, held in place by a retaining ring, is turned by hand to lock adjacent modules together.

The collar is knurled to ensure that the surface is easily gripped and can be rotated by hand. An electrical connection is made between two modules with spring-pin connectors on the interface board touching target areas on the control board. O-rings seal the collar at both ends. Modules can be connected and disconnected quickly and repeatedly.

The connections are secure and resist shock and stress. Any device with matching threads, 48V and Ethernet compatibility can be interfaced with a module, allowing for freedom of design and customization.

Series Elasticity

The SEA Snake robot features series elastic actuators. A rubber elastomer bonded between two rigid plates is torsionally sheared during actuation. The elastomer’s tapered conical cross section shown in Fig. 5 is similar to the constant-shear-stress design. The spring design of the SEA Snake robot is different in that it is molded directly to the output gear of the module’s geartrain. The rigid plate on top is then attached to the output shaft of the system through a number of pins.

Another plate is swaged onto this assembly in order to keep the output gear and output shaft aligned with the rest of the geartrain. The elastomer is molded from Natural Rubber of Shore A Durometer 50. As a torsional spring, its stiffness is roughly characterized by a spring constant of 12 N-m/rad and a maximum rotational deflection of approximately 0.6 radians.

Our research is currently exploring ways to estimate the output torque from the elastomer deflection and how to calibrate its parameters online.

Head and Tail Module

Both a head and tail module were designed utilizing the custom modular interface and they are pictured in Fig. 6. In addition to providing their specialized functions, these modules demonstrate the potential for the use of other modules that could easily be integrated into the SEA Snake system.

The head module includes a high-definition camera to provide the user with a live video feed while the four LEDs are available for illuminating darker environments. The head module housing is designed with fins to increase surface area and improve heat transfer from the electronics to the surrounding environment.

The LEDs are protected by an o-ring sealed acrylic window while the camera’s lens is protected by an o-ring sealed sapphire glass window. In order to connect a tether to the snake, the team custom designed a tether connector that is sealed and load-bearing.

The connector uses keys and keyways to provide a quick and very easy blind connection. Additionally, there are springloaded pins within the connector which ensures that the connector’s housing bears all of the load. The tail has this connector as well as a slipring integrated within its design.

References

Describes the design and architecture of the snake robot, including CAD models and cross-sections. Sensors, camera, and communication are discussed in detail.

D. Rollinson, Y. Bilgen, B Brow, et al. - IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2014) September 2014.

Describes the design, fabrication, and initial modeling of the compact, high-strength series elastic element designed for use in snake robots. Discusses the mechanical design, manufacturing, molding, experiments, and validation.

D. Rollinson, S. Ford, B. Brown, et al. - ASME 2013 Dynamic Systems and Control Conference, 2013. (Paywall)

Presents the mechanical design, control architecture, and initial locomotion experiments using the Snake Monster platform, which is based on the SEA Snake project. Discusses the capabilities, pertaining particularly to search and rescue applications.

S. Kalouche, D. Rollinson, H. Choset, et al. - IEEE International Symposium on Safety, Security, and Rescue Robotics (SSRR) Oktober 2015.

Describes three methods of achieving compliant motion with a snake robot. Two strategies command joint torques based solely on the robot’s local curvature (i.e. joint angles). A third strategy commands joint angles, velocities, and torques based on the recorded feedback from the robot while executin

D. Rollinson, K. Alwala, N. Zevallos, et al. - EEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2014), September 2014.