Within the limbs of typical animals, there exist bi-articular muscles crossing two joints. It is known that the bi-articular muscles of the felid play an important role in the locomotion. Also the muscle-tendon complex, composed of the gastrocnemius muscle and the Achilles' tendon that cross the knee joint and the ankle joint contributes much to the movements such as running and jumping particularly. Besides, because the muscle-tendon complex has the function for absorbing shocking, it is utilized for soft landing from high places. To achieve high performance for jumping and landing motion like cats, we are developing robot legs inspired by the bi-articular muscle-tendon complex of cats. The leg consists of hip, knee and ankle joints. For the knee and ankle joints, a four-bar linkage mechanism with one elastic linkage, in which the knee joint is driven by an electric rotary motor and the ankle joint is passive, is applied. By this mechanism, basic functions of the bi-articular muscle-tendon complex of felids like cats can be realized and the performance for jumping and landing can be improved. In this paper, the new leg mechanism is described. Moreover, a prototype of a pair of the hind legs of the quadruped robot using the new mechanism has been developed. The results of jumping and landing experiments are shown to validate the effectiveness of the mechanism.

Quadruped animals realize dynamic motions such as fast running and high and long jumping with the utilization of the flexibility of their bodies. It is known that the motions of their front and rear legs couple with the flexion and extension of their spine when running or jumping. In recent years, quadruped robots inspired by cats and cheetahs have been developed. In our group, a small quadruped robot has been developed with leg mechanisms which consist of spring and damper systems inspired by bi-articular muscle-tendon complexes of animals. The robot is able to perform various motions such as walking, running and jumping. In this paper, flexibility features as in animal bodies are to be introduced to make the robot run fast and jump long (or high) by coupling the motions of their spine and legs. In our design, the flexion and extension of the spine are realized by a spine joint with a torsion spring, and the coupling to the front and rear legs has been realized by cables. Driving the scapula or hip joints causes flexion of the spine and stores the elastic energy in the spring. The extension of the spine with the spring assists in driving the scapula or hip joints. As a result, in the jump experiment, the robot with a flexible spine can realize longer jump distance than that by a robot with a rigid spine.

The bi-articular muscle-tendon complex (MTC) in an animal's leg allows a transfer of mechanical power between adjacent joints and exertion of a large force at the toes. It is capable of storing and releasing elastic energy efficiently during motions. In our previous studies, we proposed mechanisms of the bi-articular MTC for achieving extension of the upper and lower parts of the leg and experimentally demonstrated that each mechanism allows the higher jumping motion of a legged robot. In this study, we developed a bio-inspired legged robot by integrating the upper and lower leg bi-articular MTCs and investigated the effect of their cooperative utilization on the improvement of the vertical jumping ability. A jumping motion trajectory was generated through optimization to maximize the jumping ability based on a dynamic model of the bio-inspired legged robot with these bi-articular MTCs. The simulation results of the models with and without bi-articular MTCs were compared. Finally, through jumping experiments performed by implementing the generated motion trajectory, it was confirmed that the jumping ability was improved by the cooperative utilization of those bi-articular MTCs.

Hydraulic and pneumatic actuators are used as actuators of robots. They have large capabilities of instantaneous output, but with problems of increase in size and mass, and difficulty for precise control. In contrast, electromagnetic motors have better controllability, lower cost, and smaller size. However, in order to actuate robots, they are usually used with reducers which have high reduction ratio, and it is difficult to realize creature-like dynamic motions such as fast running and high jumping, due to low backdrivability of joints. To solve the problem, we have developed leg mechanisms, which consist of a spring and a damper inspired by bi-articular muscle-tendon complex of animals. The final target is to develop a quadruped robot which can walk, run fast and jump highly like a cat. A cat mainly uses its hind legs in jumping and front legs in landing. It implies that the hind legs play an important role in jumping, and that the front legs do in landing. For this reason, it is necessary to design different leg structures for front and hind legs. In this paper, we develop a new front leg mechanism suitable to a hind leg mechanism which was already made by our group, and make a small quadruped robot. As the result of experiments for dynamic motions, stable running trot at a speed of 3.5 kilometers per hour and forward jumping of 1 body length per jump have been realized by the robot.

Generally, large knee joint torque is required when a leg is flexed. However, the large torque motor will increase the robot size and total weight. Also, a large reduction ratio gear box to realize the large torque will decrease output speed and lower backdrivability of the joint. It makes the robot difficult to perform agile and flexible motion like animals. In this paper, we propose a robot leg with a knee joint mechanism consisting of a variable reduction ratio crossed four-bar linkage mechanism (VRRCFLM) based on cruciate ligament of an animal. The VRRCFLM is to increase the reduction ratio for large knee flexion postures without greatly reducing the total backdrivability. In this paper, we developed a robot leg with a knee joint mechanism consisting of the VRRCFLM. In order to design the link parameters of the leg mechanism, optimization design aimed at maximizing the jumping height of the robot was performed. The robot model with the designed mechanism was evaluated through the dynamics simulations. Thanks to the VRRCFLM, the required torque of the knee motor at the large flexion postures was decreased. Moreover, the vertical jumping height was improved by 24.6 % comparing with a model without the mechanism. In experiments using the prototype, the required static torque was decreased as in simulation, and the jumping height was more than one leg length.

Underwater robots are becoming increasingly important in various fields. Fish robots are attracting attention as an alternative to the screw-type robots currently in use. We developed a compact robot with a high swimming performance by mimicking the anatomical structure of fish. In this paper, we focus on the red muscles, tendons, and vertebrae used for steady swimming of fish. A robot was fabricated by replacing the red muscle structure with shape memory alloy wires and rigid body links. In our previous work, undulation motions with various phase differences and backward quadratically increasing inter-vertebral bending angles were confirmed in the air, while the swimming performance in insulating fluid was poor. To improve the swimming performance, an improved robot was designed that mimics the muscle contractions of mackerel using a pulley mechanism, with the robot named UEC Mackerel. In swimming experiments using the improved robot, a maximum swimming speed of 25.8 mm/s (0.11 BL/s) was recorded, which is comparable to that of other soft-swimming robots. In addition, the cost of transport (COT), representing the energy consumption required for robot movement, was calculated, and a minimum COT of 0.08 was recorded, which is comparable to that of an actual fish.