project specification

Soft-legged wheel-based robot

A terrestrial robot based on two soft-legged wheels. The wheels of the robot can passively climb over stairs and adapt to slippery grounds using two soft legs embedded in their structure. The soft legs are fabricated by integration of soft and rigid materials and mounted on the circumference of a conventional wheel, are able to enhance its functionality and easily adapt to unknown grounds. The robot has a semi-stiff tail that helps in the stabilization and climbing of stairs. An active wheel is embedded at the extremity of the tail in order to increase the robot maneuverability in narrow environments. Two parallelogram linkages let the robot to reconfigure and shrink its size allowing entering inside gates smaller than its initial dimensions.

Specifications

Length arm100 mm
Manufacturing techniquesLaser cutting (VERSALASER-VLS3.5),
MaterialDelrin
Plexiglas
DC gear motor Pololu Inc., 3079, gear ratio 298:1, 6 V
Motor drivers8 LV8548MC from ON Semiconductor
ControlCustom-developed Android App
MicrocontrollerPIC32MX150F128B from Microchip Inc
ConnectivityBluetooth module (RN42 from Microchip Inc.)
Inertial module LSM9DS0 from STMicroelectronics
Magnetic encoders Magnetic Encoder Pair Kit

Overview

The soft legs are fabricated by integration of soft and rigid materials and mounted on the circumference of a conventional wheel, are able to enhance its functionality and easily adapt to unknown grounds. The robot has a semi-stiff tail that helps in the stabilization and climbing of stairs. 

An active wheel is embedded at the extremity of the tail in order to increase the robot maneuverability in narrow environments. Two parallelogram linkages let the robot to reconfigure and shrink its size allowing entering inside gates smaller than its initial dimensions. 

The robot has three main features: 

  • a reconfigurable body
  • soft-legged enhanced wheels
  • a tail with multi-functionalities

Body

The body of the robot is made of two arms and a flexible tail. It stays regularly in the format of a “T,” but “T,” or “Y” shapes can also be made. Two main wheels are located at the end of the arms as well as the flexible tail hosts a small active wheel which assists the robot in changing its direction, particularly in backward movements. 

The body has a central part where the battery and control unit are located. Left and right arms and tail are separated modules assembled to the central part by means of two parallelogram linkages. Due to these linkages, each arm (left and right) can have a planar motion parallel to the surface of the central part. These planar motions permit the body to transform and shrink its configuration from “T” to “Y” shape: if we define α as the angle between two arms, in our robot, α can vary from 180° to 0°. Two links of the left and right arms are endowed with spur gears coupled together that guarantee to preserve a symmetric shape of the robot during transformation. 

The opening of arms is actuated by a cable-driven mechanism located in the central part. The closing process is passively actuated by the pulling force of an extension spring mounted between the two arms, which occur when the pulley/motor releases the cable. The parallelogram linkage is used to ensure the coaxiality of wheels in all the steps of reconfiguration, which is necessary for an accurate control and the effective motion of the robot.

Wheels

The wheels have two sides with two different properties for passively climbing the stairs and passing sandy and slippery grounds. Each wheel is an assembly of three main parts: one central quadrilateral part and two flexible legs that are installed to the sides of the central part. Each leg is a flexible belt that on one side hosts a series of rigid cubic segments. These segments provide rigid constrains for reducing bending on one side of the belt, while, from the other side, it is free to bend with almost any desired curvature. In the center of each cubic segment is present a through hole parallel to the surfaces of the belt and wheel. The holes can be used to pass a cable through all segments, and by putting the cable on a certain tension it is possible to generate in each leg a desired shape and stiffness.

Tail

Two main wheels provide two contact points to the ground, and the tail provides a third contact point for robot stabilization, and by letting the active wheels rotate respect to the ground, forward motion is generated. The tail also help in stairs climbing: the longer the tail and more easily and stably the robot climbs stairs. 

The tail of the robot is constructed by a series of rigid and soft segments. Each segment has four holes, one in the center and three out of the center, used for passing cables for tuning a desired stiffness. An extendable rubber tube passes through the central holes of all segments to assemble them and to provide an axial compression force that sticks all the segments together. Also inextensible cables passes on the other holes in order to limit the bending orientation of the tail. The tension provided by the wires, together with compression tension of the axial flexible tubes, can define the stiffness of the tail. A tail almost rigid helps the robot to climb the stairs by using its stiffness, while its partial flexibility permits us to perform a crawling motion (used for backward locomotion). The central flexible tube is also used to pass the electrical wires of a small DC motor to drive a wheel mounted at the end of the tail. This wheel can be used as a steering mechanism assistive for the wheels in both forward and backward motions.

Fabrication and Control

The robot is made using a laser cutting process. The gear-coupled links are made by Delrin that after laser cut has an acceptable quality and demonstrates a good resistance as gear. 

Each robot arm has 100 mm length, while the distance between the left and right wheels is 260 mm when the arms are totally open (180°). The duty of compression spring was done by a peace of waistband elastic. All the rigid components of wheels were fabricated by Plexiglas. Each resulted quadrilateral parts was a cube of Plexiglas with 60 mm × 25 mm × 24 mm. The flexible belt was fabricated out of polychloroprene rubber sheet with 2 mm thickness, and all the rigid segments were mounted on its surface by screws. The resulted assembly was a belt with surface of 80 mm × 24 mm and 7 mm thickness after mounting the rigid segments. 

For the pulling cable that passes through the holes of segments, a fishing wire is used with a diameter of 0.35 mm. Each driving wheel is directly mounted on the shaft of a DC gear motor, located at the end of the arms in a box of Plexiglas called motor base. The side view of each wheel could fit inside a rectangle with 240 mm length and 25 mm height. The wheel demonstrates two radiuses varying from 120 to 12.5 mm: one useful to pass narrow gates and the other useful for climbing the obstacles. The maximum radius of 120 mm was achieved by the assembly of semi-stiff hook. 

The rigid segment of the tail is fabricated by Plexiglas of 8 mm, while the soft segments were the same material of the legs (polychloroprene rubber sheet of 2 mm). A tube of silicone rubber is used as axial extendable tube, while for pulling cables was used a fishing wire with a diameter of 0.35 mm. The tail after assembled to the robot was a cylinder with a diameter of 20 mm and length 170 mm. 

The DC gear motor used in the tail is the same used for the active wheels. The control unit and battery are mounted on the central part of the body. The robot is fully untethered and driven by a user through a custom-developed Android App. A controller board is designed to manage the four motors of the robot and to have a Bluetooth link with the App. This board is composed by a microcontroller, a Bluetooth module, an inertial module able to supply acceleration and magnetic field on three axes, and eight motor drivers. The wheels are equipped with two magnetic encoders installed on the shaft of the motors in order to permit position or speed control.

References

Describes the project, materials and methods, the soft-Legged enhanced wheels, and tail. Goes into the fabrication and control, experiments and results, and the discussions and lessons from the challenge.

Ali Sadeghi, Alessio Mondini, Emanuela Del Dottore, et al. 2016

Wevolver 2022