1 INTRODUCTION
Quadruped robots appear to possess great potentials to be
used in cluttered environments for performing outdoor tasks,
such as military transportation, scientific exploration and
disaster rescue. They can be stable and efficient when move
through rough terrain with obstacles such as rocks, sands
and steep slopes. In performing such tasks, these robots
exhibit agile and dexterous locomotion capabilities in a
compliant manner and show their strong environmental
adaptability and load capacity.
The research of quadruped robots began in the 1960s, so far
researchers have produced many typical quadruped robot
prototypes. Developed by the Boston dynamics, BigDog is
one of the most representative results [1]. BigDog has high
load capacity and good adaptability to the environment.
Stable and robust locomotion has been demonstrated on this
platform. The LittleDog quadruped robot, manufactured by
Boston Dynamics achieves traveling through a wide variety
of rough terrain of varying difficulty levels [2]. Italian
Institute of Technology has developed quadruped robot
HyQ which combines the hydraulic device with electric
motor. HyQ is famous for its mechanism and control
flexibility and it shows trotting robustly by employing a
simple virtual model control approach for each leg [3-4].
South Korea Institute of Industrial Technology developed
quadruped robot qRT which driven by hydraulic motor and
realize the stable walking in rough terrain [5]. Quadruped
robot Scalf1 developed by Shandong university combines
the shape of a half elliptical with straight line, realizing the
load weight 80 kg and trotting on soft terrain [6].
Quadruped robots have difficulty in traveling through rough
terrain because when facing obstacles or hollows, the
unexpected collisions with the ground of the large impact
will cause the robot posture instability [7]. So an
appropriate compliance control is necessary to achieve
This work is supported by National Nature Science Foundation of
China (Grant NO. 61473304) and National Hi-tech Research and
Development Program of China (Grant NO. 2015AA042202).
stable locomotion in complicated environment. Raibert
used spring loaded inverted pendulum model (SLIP)
achieve the stability of a hopping monopod control, and
promoted the concept of virtual leg to the quadruped robots
[8]. The control of BigDog is developed on the basis of the
model [9]. HyQ’s virtual model control method is used to
maintain the stability of the body. Each leg of HyQ is
simplified into a linear spring damping to realize the active
compliance control. Jaehwan designed a variable
impedance control strategy, aiming at changing the
impedance parameters in different stages, so that the
quadruped robot achieve smooth passing on the ground with
multiple irregular obstacles [10-11].
In view of the compliance problem of quadruped robots
trotting in complex terrain, this paper proposes a
compliance control method based on force control. As
shown in Fig1, internal and external loop control strategy is
used. The external loop is the high level control of the robot,
using a sliding mode controller to control body’s position
and posture to avoid interference of environmental
disturbance and unknown model characters. By eliminating
the interaction force of diagonal stance legs, a force
distribution module is used to calculate desired contact
force. The internal loop is the low level control of the robot,
using the force controller make the foot contact force track
the desired force to realize the foot soft contact with the
ground and reduce the impact to the body. Simulations and
experimental results show that the compliance control
strategy based on force control allows for the quadruped
robot trotting over rough terrain in a robust and compliant
manner.
The rest of this document is organized as follows: Section II
details the kinematics model and dynamics model. We
present the sliding mode control method in section III. The
following section IV we discuss the force distribution
strategy. Then, section V shows a force controller. Section
VI and VII show results of simulations and experiments.
Finally, we conclude our remarks in section VIII.
Contact Force Based Compliance Control for a Trotting Quadruped Robot
Jiaqi Xu, Lin Lang, Hongxu Ma, Qing Wei
College of Mechatronics Engineering and Automation, National University of Defense Technology, Changsha 410073
E-mail:
xujiaqi0426@sina.cn
Abstract: In this paper, a compliance control strategy based on force control for quadruped trot gait locomotion over
unperceived rough terrain is proposed. To avoid interference of environmental disturbance and unknown model
characters, a sliding mode based controller is designed to control the position/posture of the robot torso. The force
distribution and control modules calculate desired contact force by eliminating interaction force between the stance legs.
Simulations and experimental results demonstrate the effectiveness of our compliance control strategy as the quadruped
robot successfully trotting over rough terrain.
Key Words: Quadruped Robot, Trot Gait, Compliance Control, Force Control