Design of biped robot joints based on pneumatic artificial muscles

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Design of biped robot joints based on pneumatic artificial muscles [Copy link]

　　A biped robot joint constructed by pneumatic artificial muscles is introduced. The joint uses the flexible characteristics of pneumatic artificial muscles to effectively control the impact of the landing foot when the biped robot walks or runs quickly. The working principle of pneumatic artificial muscles and the hardware architecture of the joint system composed of pneumatic artificial muscles are given in detail. At the same time, the control software system built based on this hardware joint is introduced. Compared with general mobile robots, bipedal robots have better mobility in unstructured environments, so they have attracted widespread attention from researchers. Controlling robots to achieve fast walking speed and running gait is still one of the challenging problems in the field of bipedal robots. When the robot walks or runs quickly, the swinging foot will generate a large impact force at the moment of landing. This force causes the landing foot to rebound or the zero moment point to have a large jump, resulting in a decrease in the stability margin of the robot and a fall. This phenomenon is called the impact effect, which is a factor that restricts the bipedal robot from increasing the walking speed and running. Pneumatic artificial muscles are a new type of actuator developed in recent years, and McKibben pneumatic muscles are the most widely used one. It has the advantages of flexibility, high power/mass ratio, and similar force and length characteristics to human muscles. Due to its controllable flexibility, the use of pneumatic artificial muscles as actuators can effectively solve the impact problem of the landing feet of bipedal robots. Therefore, the use of pneumatic artificial muscles as actuators for bipedal robots has a good prospect. However, artificial muscles have highly nonlinear characteristics. And accompanied by hysteresis, it is difficult to build and control them. At present, the research on bipedal robots based on pneumatic artificial muscles has just started, and only a few bipedal robot projects have studied this. This paper uses MeKibben pneumatic artificial muscles to build a single-degree-of-freedom artificial joint similar to a biological antagonistic joint. The hardware part of this system includes a pneumatic drive subsystem, a sensor subsystem, and a control subsystem. A software system is built on this hardware system to achieve tracking control of the trajectory of this artificial joint. Based on the work of this paper, the modeling and control problems of pneumatic artificial muscles and joints can be further studied and solved, laying a foundation for the design and construction of bipedal robots based on pneumatic artificial muscle actuators. 1 Software and hardware design of pneumatic artificial muscle joint system 1.1 Pneumatic artificial muscle McKibben pneumatic artificial muscle is a flexible pneumatic actuator invented by American doctor Joseph L. McKibben and named after him. The main body of McKibben pneumatic artificial muscle is mainly composed of an outer braided mesh and an inner elastic rubber tube. Its structure is shown in Figure 1. Figure 1 is a muscle structure diagram, where Pi is the input air pressure, and its size is controlled by the controller according to the actual working conditions. When the input air pressure increases, the inner rubber tube expands. Since the outer braided mesh has a large rigidity, it can hardly stretch, limiting the muscle to radial deformation (diameter increases, length shortens), generating axial contraction force; when the input air pressure Pi decreases, the artificial muscle stretches (relaxes), and the rigidity and driving force of the muscle decrease accordingly. The stiffness of the muscle can be achieved by controlling the air pressure in the rubber tube. This muscle has a variable stiffness characteristic and can be equivalent to a variable stiffness spring. 1.2 Single-degree-of-freedom joint system Since pneumatic artificial muscles can only provide unidirectional driving force, two muscles are required to form an antagonistic rotary joint in a manner similar to biological antagonistic muscles to achieve the force closure of the operating arm. This paper uses McKibben pneumatic artificial muscle as a driver to build a single-degree-of-freedom antagonistic joint system. The hardware part of this system consists of a pneumatic drive subsystem, a sensor subsystem, and a control subsystem. The system structure diagram is shown in Figure 2. 1.2.1 Pneumatic drive subsystem The pneumatic drive subsystem consists of an air source, a pressure servo proportional valve, a McKibben pneumatic artificial muscle, and a mechanism. The compressed gas with a pressure of 0.6-0.9 MPa is provided by the gas source. The compressed gas is sent to the pneumatic artificial muscle through the servo proportional valve through the catheter. Each muscle is connected to a servo proportional valve and has an outlet valve and an inlet valve. The gas pressure in the muscle can be controlled by controlling the voltage applied to the servo proportional valve. The pressurized pneumatic muscle outputs contraction tension and drives the joint rotation of the mechanism part. Therefore, the joint torque required for trajectory tracking can be achieved by controlling the muscle pressure. The McKibben pneumatic artificial muscle used in this system is the MAS-20-300N type of FESTO Company, with a working pressure range of 0-0.6 MPa, a maximum working frequency of 3Hz, a maximum contraction of 25% of the muscle length, a theoretical force of 300N at 0.6 MPa, and a repeatability accuracy of less than 1%. The pressure servo proportional valve receives the voltage input from the control end and controls the air pressure in the muscle by adjusting the inflation valve and the inlet valve. This system uses the SMC company's 丌ITVOO5C-2ML type pressure proportional valve. The input range of this valve is 0~5VDC, and the output is a pressure between 0.001~0.9MPa. 1.2.2 Sensor subsystem The sensor subsystem consists of a force sensor and a linear displacement sensor. The contraction of the muscle can be measured by the linear displacement sensor, and the muscle and joint model can be used for trajectory tracking control based on this contraction. The force sensor measures the muscle tension, and the joint torque can be calculated based on the linear relationship between this tension and the joint torque, thereby completing the servo closed-loop control of the joint. The force sensor used in this system is the BK-2F high-precision S-shaped force/weighing sensor of the 701 Institute of Aerospace Science and Technology Group Corporation. The maximum range of its measured force can reach 80kg, with an accuracy of 0.05%. After the output is amplified by the TS-2 amplifier, the output voltage range is -5V~+5V. The linear displacement sensor uses a WDL direct-slide conductive plastic potentiometer. 1.2.3 Control subsystem The control subsystem consists of an industrial control computer (IPC), an A/D acquisition card, and a D/A conversion card. The software control system runs on the industrial control computer and converts the digital control quantity into analog quantity through the D/A converter. This analog quantity is used to control the output air pressure of the pressure servo proportional valve. The A/D converter collects the data of the tension sensor and the linear displacement sensor, and provides the industrial control computer with a digital signal that can be processed by the software. The D/A converter used in this system is the PCL-726 6-channel analog output card. It provides 6 12-bit double-buffered analog output channels to meet the needs of muscle servo control. The MD acquisition card uses the PCL-813B 12-bit 32-channel A/D card, which provides 32 channels of isolated DC voltage measurement, and the accuracy can meet the system requirements.

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