Xianbao Chen

Trot Gait Design and CPG Method for a Quadruped Robot

Developing efficient walking gaits for quadruped robots has intrigued investigators for years. Trot gait, as a fast locomotion gait, has been widely used in robot control. This paper follows the idea of the six determinants of gait and designs a trot gait for a parallel-leg quadruped robot, Baby Elephant. The walking period and step length are set as constants to maintain a relatively fast speed while changing different foot trajectories to test walking quality. Experiments show that kicking leg back improves body stability. Then, a steady and smooth trot gait is designed. Furthermore, inspired by Central Pattern Generators (CPG), a series CPG model is proposed to achieve robust and dynamic trot gait. It is generally believed that CPG is capable of producing rhythmic movements, such as swimming, walking, and flying, even when isolated from brain and sensory inputs. The proposed CPG model, inspired by the series concept, can automatically learn the previous well-designed trot gait and reproduce it, and has the ability to change its walking frequency online as well. Experiments are done in real world to verify this method.

Gait planning for a quadruped robot with one faulty actuator

Fault tolerance is essential for quadruped robots when they work in remote areas or hazardous environments. Many fault-tolerant gaits planning method proposed in the past decade constrained more degrees of freedom(DOFs) of a robot than necessary. Thus a novel method to realize the fault-tolerant walking is proposed. The mobility of the robot is analyzed first by using the screw theory. The result shows that the translation of the center of body(CoB) can be kept with one faulty actuator if the rotations of the body are controlled. Thus the DOFs of the robot body are divided into two parts: the translation of the CoB and the rotation of the body. The kinematic model of the whole robot is built, the algorithm is developed to actively control the body orientations at the velocity level so that the planned CoB trajectory can be realized in spite of the constraint of the faulty actuator. This gait has a similar generation sequence with the normal gait and can be applied to the robot at any position. Simulations and experiments of the fault-tolerant gait with one faulty actuator are carried out. The CoB errors and the body rotation angles are measured. Comparing to the traditional fault-tolerant gait they can be reduced by at least 50%. A fault-tolerant gait planning algorithm is presented, which not only realizes the walking of a quadruped robot with a faulty actuator, but also efficiently improves the walking performances by taking full advantage of the remaining operational actuators according to the results of the simulations and experiments.

A quadruped robot with parallel mechanism legs

Summary form only given. The design and control of quadruped robots has become a fascinating research field because they have better mobility on unstructured terrains. Until now, many kinds of quadruped robots were developed, such as JROB-1 [1], BISAM [2], BigDog [3], LittleDog [4], HyQ [5] and Cheetah cub [6]. They have shown significant walking performance. However, most of them use serial mechanism legs and have animal like structure: the thigh and the crus. To swing the crus in swing phase and support the bodys weight in stance phase, a linear actuator is attached on the thigh [2, 3, 5, 6], or instead, a rotational actuator is installed on the knee joint [1, 4]. To make the robot more useful in the wild environment, e.g., the detection or manipulation tasks, the payload capability is very important. To carry the sensors or tools, heavy load legged robot is very necessary. Thus the knee actuator should be lightweight, powerful and easy to maintain. However, this can be very costly and hard to satisfy at the same time.

A Simplified Control Method to Achieve Stable and Robust Quadrupedal Quasi-passive Walking with Compliant Legs

The principle of passive dynamic walking has drawn lots of attentions in the field of robotics for it provides a possibility to realize natural walking. However, stabilizing the quadrupedal passive walking remains challenging. In this paper, a novel control method is proposed to stabilize the quadrupedal quasi-passive walking. Inspired by biological concepts, this method treats the foreleg pair and hindleg pair as two bipedal walkers, and a virtual model controller is designed to maintain the quasi-passive walking of each bipedal walker independently. This control method was then verified by a planar quadrupedal model with compliant legs, which successfully achieved stable periodical walking gaits. It was found that although being controlled independently, the movement of fore and hind leg pairs still formed a time-invariant phase shift, showing remarkable resemblance to that of a walking horse. We further analyzed the influences of varying factors on the gait characteristics and stability. These analyses show the control method is robust since it can stabilize the gaits within a wide range of leg compliance parameters and resist considerably large disturbances. In addition, the optimal ranges of the leg compliance parameters for the largest stability margin were also found in this study.

Reaction forces identification of a quadruped robot with parallel-serial leg structure

Interactions between feet and environment influence the stability and mobility of legged robot. This paper proposes a model to indirectly identify 3 degrees of freedom feet reaction forces for a quadruped robot with parallel-serial legs. The research platform is called Baby-Elephant: a heavy-duty four-legged robot designed for nuclear plant maintenance and disaster relief purposes. Each leg has three hydraulic actuators. With the pressure data from pump and hydraulic actuators, a double-chamber model with experimental derived friction is used to obtain the actuated force. The reaction forces model, including joint and foot forces, is simplified into an explicit function. Comparison between CAD simulation and analytical results shows the effectiveness of the model. A walking experiment with load cells proves the model is validate in practical application. The proposed model is used to identify the foot contact phase and the zero momentum point during crawling gait walking.

Fault-tolerant gait a quadruped robot with partially fault legs

Legged robots have greater capability to traverse irregular terrains. However, one of the most common problems is the failure of the actuators when the robot is working in remote. Fault-tolerant gait about one fault actuator can be found. This paper proposes another algorithm for more than one fault actuators. The degree-of-freedoms (DOFs) of the robot body are divided into two parts: the major DOFs, which are critical in performing a gait; and secondary DOFs. The idea of the method is to find a proper kinematic resolution to perform major DOFs by controlling the secondary DOF of the robot. Simulations and experiments are presented here on a hydraulic quadruped robot.

Energy Storing Mechanism for a New Hydraulic-Motor Actuated Robot

A quadruped robot named “Baby Elephant” with parallel legs has been developed. It is about 1 m tall, 1.2 m long and 0.5m wide. It weighs about 130 kg. Driven by a new type of hydraulic actuating system, the Baby Elephant is designed to work as a mechanical carrier. It can carry a payload no less than 50 kg. Operating outdoors using wireless remote control, the robot will be able to traverse uneven terrain applying walking and trotting gaits and can walk up and down 10 degree inclines. The Baby Elephant carries a lithium battery fixed at the belly to supply all the power. This paper describes the structure of the legs and the application of the energy saving mechanisms on the leg. Simulations and experiments are carried out to testify the efficiency of the spring system in terms of energy saving.Copyright

Energy Consumption of Trotting Gait for a Quadruped Robot

Battery driven robots have many advantages over combustion robots. They are clean, quiet, and can work in the airless or flammable environment. However, the limitation of the battery endurance is a great challenge. In order to increase the working hours of the battery driven quadruped robot, the energy expenditure of trotting gait under different gait parameters is studied. Firstly, the kinematic model of the quadruped robot and its gait planning method are introduced. Secondly, the dynamic model of the leg and the robot body are presented and the energy expenditures in the stance phase and the swing phase during trotting are analyzed. It can be proved that for any given trotting speed, the combination of the stride frequency and the stride length has great influence on the energy expenditure. Finally, experiments are presented to validate of the theory. The results show that by properly choosing the gait parameters the energy expenditure in trotting can be efficiently reduced.