More and more state-of-the-art robots have employed hydraulic actuating systems. It has a high power-to-weight ratio. Robots with these actuators can bear more payloads and achieve highly dynamic performance. However, the energy consumption is also very high and the system is very complicated comparing to the electronic motor actuated robot. A lot of research has been done to save the energy. Among which the application of springs is one of the most commonly used methods. This paper presents another use of the spring to save the energy by reducing the hydraulic system pressure of a newly built robot called the “Baby Elephant.” The configuration of the spring is designed according to the leg mechanism. The spring gives an assist force in the stance phase of the leg and exerts a passive payload in the swing phase. The maximum cylinder force is then reduced so as to bring down the pump pressure. The energy to be saved depends on how much the hydraulic pressure can be reduced. In this paper, the Baby Elephant is briefly introduced, the design of the springs on saving the energy are described. Simulations and experiments are carried out to confirm the effect.
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 body's 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.This video introduces our lately developed quadruped robot. It is named as "Baby Elephant" because of the heavy load capability and the elephant like appearance. The leg is a serial-parallel hybrid mechanism that is one novelty of our robot. On the sagittal plane, it is a parallel mechanism consisting of two symmetrical crossed limbs. Each leg has three active DOFs, driven by three hydraulic actuators mounted on the hip. To guarantee the sufficient workspace of the leg (i.e., the spatial range that the foot tip can reach) during the walking, the leg structure parameters are optimized. Springs are attached to the legs to reduce the impact and save energy. Using the pressure sensors on hydraulic actuators, it is possible to estimate the ground forces without extra foot force sensors. Comparing with its serial mechanism counterpart, this leg has neither actuator nor sensor on the lower part of the leg. Thus the leg's inertia becomes lower. Moreover, it is easy to protect electronic devices when the robot is walking on marshy terrains. Two hip actuators on the sagittal plane support the body's weight together, and thus the load capability is also increased.A type of hydraulic actuator named the "Hy-Mo" was developed that is another novelty of our robot. Its principle is that the hydraulic cylinder's motion is controlled by the motor while its power is supplied by hydraulic system. The servo motor controls the valve opening, which is proportional to the piston displacement via an inner mechanical feedback. The advantage is that it does not need servo valve, cooler, filter and accumulator. It will result in a more simplified structure, better heat dissipation, and lower leakage.Currently, two control schemes are employed. When the robot is in a static walking gait, a ZMP-based online planning algorithm [7] is employed to generate the trajectory of the body's COG. The ZMP is guaranteed to be located inside the support polygon. The ground reaction force (GRF) is estimated from the driving force of the actuators using a force esti...
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.
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