Tracked robot with underactuated tension-driven RRP transformable mechanism: ideas and design

Abstract

Robots with transformable tracked mechanisms are widely used in complex terrains because of their high adaptability, and many studies on novel locomotion mechanisms have been conducted to make them able to climb higher obstacles. Developing underactuated transformable mechanisms for tracked robots could decrease the number of actuators used while maintaining the flexibility and obstacle-crossing capability of these robots, and increasing their cost performance. Therefore, the underactuated tracked robots have appreciable research potential. In this paper, a novel tracked robot with a newly proposed underactuated revolute‒revolute‒prismatic (RRP) transformable mechanism, which is inspired by the sit-up actions of humans, was developed. The newly proposed tracked robot has only two actuators installed on the track pulleys for moving and does not need extra actuators for transformations. Instead, it could concentrate the track belt’s tension toward one side, and the unbalanced tension would drive the linkage mechanisms to change its configuration. Through this method, the proposed underactuated design could change its external shape to create support points with the terrain and move its center of mass actively at the same time while climbing obstacles or crossing other kinds of terrains, thus greatly improving the climbing capability of the robot. The geometry and kinematic relationships of the robot and the crossing strategies for three kinds of typical obstacles are discussed. On the basis of such crossing motions, the parameters of links in the robot are designed to make sure the robot has sufficient stability while climbing obstacles. Terrain-crossing dynamic simulations were run and analyzed to prove the feasibility of the robot. A prototype was built and tested. Experiments show that the proposed robot could climb platforms with heights up to 33.3% of the robot’s length or cross gaps with widths up to 43.5% of the robot’s length.

Key words： mechanical design; tracked robot; underactuated mechanisms; RRP mechanism; obstacle crossing strategy

Nomenclature

Abbreviations
COM	Center of mass
DOF	Degrees of freedom
RRP	Revolute‒revolute‒prismatic
PWM	Pulse-width modulation
UTMTR	Underactuated tension-motivated tracked robot
Variables
a, b, \gamma	Positions and postures of the UTMTR in the environment
Ax, Bx	Intermediate variables, which shows the relationships between xcf and l_{\mathrm{T}} (or l_{\text{Tr}})
Ay, By	Intermediate variables, which shows the relationships between y_{\text{cf}} and l_{\mathrm{T}} (or l_{\text{Tr}})
l1, l2	Lengths of Links 1 and 2, respectively
l1m	Distance between the center of rear pulley and COM of Link 1
l2m	Distance between revolute joint B and COM of Link 2
l3	Distance between revolute joint A and the axis of the front pulley
l3m	Distance between revolute joint A and COM of Link 3
l4m	Distance between the center of front pulley and COM of Link 4
lT	Length of the tensioned segment of the track belt in steps 1‒4
lT0	Initial value of lT
lTr	Length of the tensioned segment of the track belt in step 5, which is reversed with the ones in steps 1‒4
lY, lZ	Intermediate variables for the convince of formulation
L	Total length of the track belt
mfp, mrp	Masses of front and rear pulley, respectively
mi (i = 1,2,...,4)	Mass of Link i
p0, p1, p2	Homogeneous coordinates of a support point under robot coordinates
pf0	Homogeneous coordinate of the front pulley’s center under the base coordinate of the robot
pG	Homogeneous coordinate of a support point under ground coordinate
r	Dividing radius of pulleys
R	Transformation matrix between OR0 and ORi (i = 1,2)
R01, R12	Transformation matrixes between robot coordinate systems
RT	Transformation matrix between the ground coordinate and the base coordinate of the robot
Tfront, Trear	Driving torques of front and rear pulleys according to results of dynamic simulations, respectively
xcf, ycf	Coordinate values of the front pulley’s center under the base coordinate of the robot in the x and y directions, respectively
xsp, ysp	Coordinate values of the support point under robot coordinate in the x and y directions, respectively
XCOM1	Distances between COM and the shaft of the rear pulley along the moving direction before climbing actions
XCOM2	Distances between COM and the shaft of the rear pulley along the moving direction after climbing actions
ΔXCOM	Moving distance of COM while transforming
{\omega }_{\text{rear}}	Angular speed of the rear pulley
θA	Revolve angle of revolute joint A
θAcal, θBcal	Revolving angles of revolute joints A and B according to calculation, respectively
θAsim, θBsim	Revolving angles of revolute joints A and B according to results of dynamic simulations, respectively
θB	Revolve angle of revolute joint B
θlim	Maximum value of θA
Δθ	Two pulleys’ differential-rotate angle
Δθ (i = 1,3)	Values of Δθ at the end of step i

Acknowledgement

This work was supported by the Fundamental Research Funds for the Central Universities, China (Grant No. 2022JBZY026).

Electronic Supplementary Material

The supplementary materials can be found in the online version of this article at https://doi.org/10.1007/s11465-023-0777-8 and is accessible to authorized users.

Conflict of Interest

The authors declare that they have no conflict of interest.

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