Yixiang Liu

High performance DSP/FPGA controller for implementation of HIT/DLR dexterous robot hand

The paper presents hardware and software architectures of the HIT/DLR Hand. The hand has four identical fingers and an extra degree of freedom (d.o.f) for the palm. In each finger, there is a re-configurable Field Programmable Gate Array (FPGA) for data acquisition, Brushless DC (BLDC) motor control and communication with the palms FPGA by Point-to-Point Serial Communication (PPSeCo). The kernel of the hardware system is a PCI-based high speed floating-point Digital Signal Processor (DSP) for data processing, and an FPGA for high speed (up to 25 Mbps) real-time serial communication with the palms FPGA. In order to achieve high modularity and reliability of the hand, a fully mechatronic integration and analog signals in-situ digitalization philosophy are implemented to minimize the dimension, number of the cables (5 cables including power supply) and protect data communication from outside disturbances. Furthermore, according to the hardware architecture of the hand, a hierarchical software architecture has been established to perform all data processing and control of the hand. The software structure provides basic Application Programming Interface (API) functions and skills to access all hardware resources for data acquisition, computation and teleoperation.

Design of a biped robot actuated by pneumatic artificial muscles

High compliant legs are essential for the efficient versatile locomotion and shock absorbency of humans. This study proposes a biped robot actuated by pneumatic artificial muscles to mimic human locomotion. On the basis of the musculoskeletal architecture of human lower limbs, each leg of the biped robot is modeled as a system of three segments, namely, hip joint, knee joint, and ankle joint, and eleven muscles, including both monoarticular and biarticular muscles. Each rotational joint is driven by a pair of antagonistic muscles, enabling joint compliance to be tuned by operating the pressure inside the muscles. Biarticular muscles play an important role in transferring power between joints. Walking simulations verify that biarticular muscles contribute to joint compliance and can absorb impact energy when the robot makes an impact upon ground contact.

Embedded FPGA-based control of the HIT/DLR hand

In this paper, we developed a performance-enhanced, stand-alone dexterous robot hand with effective mechanical structure and lightweight control hardware. In the context, the paper shows the design methodology of HIT/DLR dexterous robot hand II controller using FPGA (field programmable gate array). Lower level controller is implemented in an FPGA and higher level controller is implemented in a DSP. Instead of a conventional architecture, a FPGA-based soft processor core is utilized. It includes a set of custom peripheral cores, such as data collection, brushless DC motors control and communication with palms FPGA by point-to-point serial communication (PPSeCo). FPGAs make modular fingers more versatile, adding some new features to the design of hand like real-time control, hardware reuse, lower cost, fault-recovering, and software/hardware co-design. Finger control system use the NIOS soft CPU as hardware platform and uC/OS II real-time operating system as software platform to improve the efficiency of the system and short the responding time of a task. The experiment results clearly illustrate the high performance of the control system

Multisensory HIT/DLR dexterous robot hand

This paper describes the current work progress of HIT/DLR dexterous hand. Based on the technology of DLR hand II, HIT and DLR are jointly developing a smaller and easier manufactured robot hand with multisensory system. The prototype of one finger has been successfully built. The finger has three DOF and four joints; last two joints are mechanically coupled by a rigid linkage. All the actuators are commercial brushless DC motors with integrated analog hall sensors. DSP based control system is implemented in PCI bus architecture and the serial communications between the hand and DSP needs only 2 lines. The fingertip force can reach 10N.

Influence of the swing ankle angle on walking stability for a passive dynamic walking robot with flat feet

To achieve high walking stability for a passive dynamic walking robot is not easy. In this article, we aim to investigate whether the walking performance for a passive dynamic walking robot can be improved by just simply changing the swing ankle angle before impact. To validate this idea, a passive bipedal walking model with two straight legs, two flat feet, a hip joint, and two ankle joints was built in this study. The walking dynamics that contains double stance phase was derived. By numerical simulation of the walking in MATLAB, we found that the walking performance can be adjusted effectively by only simply changing the swing ankle angle before impact. A bigger swing ankle angle in a reasonable range will lead to a higher walking stability and a lower initial walking speed of the next step. A bigger swing ankle angle before impact leads to a bigger amount of energy lost during impact for the quasi-passive dynamic walking robot which will influence the walking stability of the next step.

Position control of a single pneumatic artificial muscle with hysteresis compensation based on modified Prandtl–Ishlinskii model

BACKGROUND High-performance position control of pneumatic artificial muscles is limited by their inherent nonlinearity and hysteresis. OBJECTIVE This study aims to model the length/pressure hysteresis of a single pneumatic artificial muscle and to realize its accurate position tracking control with forward hysteresis compensation. METHODS The classical Prandtl-Ishlinskii model is widely used in hysteresis modelling and compensation. But it is only effective for symmetric hysteresis. Therefore, a modified Prandtl-Ishlinskii model is built to characterize the asymmetric length/pressure hysteresis of a single pneumatic artificial muscle, by replacing the classical play operators with two more flexible elementary operators to independently describe the ascending branch and descending branch of hysteresis loops. On the basis, a position tracking controller, which is composed of cascade forward hysteresis compensation and simple proportional pressure controller, is designed for the pneumatic artificial muscle. RESULTS Experiment results show that the MPI model can reproduce the length/pressure hysteresis of the pneumatic artificial muscle, and the proposed controller for the pneumatic artificial muscle can track the reference position signals with high accuracy. CONCLUSION By modelling the length/pressure hysteresis with the modified Prandtl-Ishlinskii model and using its inversion for compensation, precise position control of a single pneumatic artificial muscle is achieved.

Design and control of a pneumatic musculoskeletal biped robot

BACKGROUND Pneumatic artificial muscles are quite promising actuators for humanoid robots owing to their similar characteristics with human muscles. Moreover, biologically inspired musculoskeletal systems are particularly important for humanoid robots to perform versatile dynamic tasks. OBJECTIVE This study aims to develop a pneumatic musculoskeletal biped robot, and its controller, to realize human-like walking. METHODS According to the simplified musculoskeletal structure of human lower limbs, each leg of the biped robot is driven by nine muscles, including three pairs of monoarticular muscles which are arranged in the flexor-extensor form, as well as three biarticular muscles which span two joints. To lower cost, high-speed on/off solenoid valves rather than proportional valves are used to control the muscles. The joint trajectory tracking controller based on PID control method is designed to achieve the desired motion. Considering the complex characteristics of pneumatic artificial muscles, the control model is obtained through parameter identification experiments. RESULTS Preliminary experimental results demonstrate that the biped robot is able to walk with this control strategy. CONCLUSION The proposed musculoskeletal structure and control strategy are effective for the biped robot to achieve human-like walking.

Design and control of a pneumatic-driven biomimetic knee joint for biped robot

This paper presents the design and control of a biomimetic knee joint for biped robots. The robotic knee joint driven by pneumatic artificial muscles is designed by imitating the structure of the human knee. It has desirable characteristics similar with human knee joint including joint compliance, changeable instantaneous center of rotation, as well as large range of joint motion. In order to model and compensate the length/pressure hysteresis of pneumatic artificial muscles in the position control, a novel method termed as direct inverse hysteresis modeling approach is introduced. Other than deriving from the forward hysteresis model, the inversion of the length/pressure hysteresis is directly modeled by a modified Prandtl-Ishlinskii model which has two newly-designed play operators. Then a cascade position controller with forward hysteresis compensation is proposed for the knee joint. The mechanical structure and control scheme of the knee joint are validated by experiments.

Development of a passive dynamic walking robot based on mechanical structural parameters optimization

Passive dynamic walking robot can walk with low energy consumption and exhibits human-like natural gait. However, because the walking performance greatly or fully depends on the mechanical structural parameters, their walking stability is quite low compared to active walking robot. In other words, proper mechanical parameters are one of the key factors to achieve stable walking for a passive dynamic walking robot. In this paper, parametric mechanical structural parameters were used to fulfill the parameters optimization process and optimal mechanical structural parameters were obtained based on the global stability analysis with cell-mapping method by numerical simulation. A passive dynamic biped walking robot prototype with hip joint, knee joints, ankle joints and an upper body was developed based on the optimization result, both the simulation and experiments results proved that the optimization result is reasonable.

BIPED ROBOT DESIGN WITH VARIABLE ANKLE STIFFNESS

Human lower limbs have particular flexibility. Both the efficiency of bipedal walking and the ability to protect actuators with low energy loss are worthy references for the design of bipedal robots. This paper proposes a design for a biped robot with joints of variable stiffness. The robot has three degrees of freedom in the sagittal plane in each leg. The hips and knees are driven directly by the motor, while the ankles are passive joints with adjustable stiffness. After a comprehensive investigation, a variable stiffness mechanism was introduced based on lever principles, and driven by a motor that can realize real-time adjustment. Simulations verified the necessity of variable stiffness joints in the robot. The variable stiffness joint can absorb the ground impact on each joint, reduce the energy loss of the motor, and improve the efficiency of movement.