Design of a pneumatically driven bionic flexible robotic arm

Author: Yu Youhe, Li Jian Zhou Yan, Zhang Zhaoqi

summary:

Multi-degree-of-freedom flexibility has gradually entered modern production and human life with its superior safety and flexibility. Its main structure is often made of flexible materials. A design of a pneumatically driven bionic flexible arm is proposed. Different from the common flexible robot, it adopts a multi-joint structure, and hinges in different directions are arranged between different joints to achieve bending movement of the robotic arm in different directions; the coordinated deformation between multiple joints can achieve telescopic movement of the robotic arm. The driving part adopts a cylinder controlled by a solenoid valve, and the compressibility of the gas is used to achieve partial flexibility. A pneumatic manipulator gripper is installed at the end of the robotic arm, so that it can safely grasp objects.

0Introduction ​

Soft robots are a new type of robot with continuous deformation structure and high degree of freedom designed from the perspective of bionics based on creatures in nature, such as elephants, starfish and octopus. The robot body is usually made of flexible materials, which are generally considered to have a Young's modulus lower than that of human muscle. Generally, there are dielectric elastomers (DE), ionic metal composites (IPMC), shape memory alloys (SMA), shape memory polymers (SMP), etc. Different from traditional robot drive, the drive mode of soft robots mainly depends on the materials used. According to the physical quantity of the response, it is temporarily divided into several categories: electric field, pressure, magnetic field, light, temperature, and chemical reaction. The design and production of bionic robots involve multiple disciplines such as materials science and mechanics. At the same time, they are also combined with advanced technologies such as 3D printing technology and intelligent new material drive, and have gradually become one of the research hotspots in the field of robotics at home and abroad [1].

In this article, a flexible robotic arm is designed based on the movement characteristics of fish tentacles. Unlike common soft robots that use flexible materials to achieve deformation and multiple degrees of freedom, this design adopts a multi-joint structure, with hinges in different directions between different joints, so as to achieve bending movement of the robotic arm in different directions; the coordinated deformation between multiple joints can achieve telescopic movement of the robotic arm. The driving part uses a cylinder controlled by a solenoid valve, and the compressibility of the gas is used to achieve partial flexibility. A flexible bionic mechanical claw is installed at the end of the robotic arm, so that it can grasp objects. In order to meet the needs of flexible use of pneumatic soft robots and multi-scenario work, the flexible robotic arm is installed on an electric drive vehicle.

1. Flexible robotic arm structure design

With sports realization

1.1 Overall layout design of the robot

This study mainly focuses on the multi-degree-of-freedom flexible robotic arm and pneumatic gripper. In order to expand the working space of the robotic arm, its overall mechanism design and distribution are shown in Figure 1.

It is connected to a mobile platform, which is a remote-controlled electric car. This article will not introduce it in detail. The flexible robotic arm and the mobile platform are connected by a servo and a fixed beam, and the pneumatic gripper is fixed to the end of the robotic arm. The robotic arm, as a core component, has a total of 6 joints, each with the same structure and connected in series. There is an angle of 120° between adjacent joints. A pneumatic gripper is connected to the end of the robotic arm to grab objects. The structure of the independent joint is shown in Figure 2.

1.2 Bending motion realization

Each independent joint is a rotary joint. When the cylinder is inflated, the piston extends, causing the upper plate to flip around the hinge axis to a certain angle, thereby enabling the robot arm to bend in a specific direction. The greater the piston stroke, the greater the flip angle. Since the joints are placed at an angle of 120°, bending movements in multiple directions can be achieved.

When the joint cylinder is extended, the bending of the robot arm is shown in Figure 3. The bending of other joints is similar.

1.3 Implementation of telescopic movement

The telescopic movement is achieved by the coordinated rotation of multiple joints. The robot arm has a total of 6 independent joints, with adjacent joints forming an angle of 120°. The first 3 joints and the last 3 joints have similar spatial positions. For the first 3 joints (similar to the last 3 joints), when they make appropriate rotations relative to their respective rotation axes, the joint rotations will cancel each other out, which is manifested as linear motion in space, that is, the elongation of the robot arm (there will also be radial offsets, which are small in size and are ignored here). When the first 3 joints rotate to specific angles, the deformation of the robot arm is shown in Figure 4.

1.4 Rotary joint design

Since the angle between each independent joint of the robot arm is 120°, it can only bend at a specific angle. In order to realize the multi-directional bending operation of the robot arm and expand the working space, the team added a slewing joint to the robot arm. The design installs the robot arm on the servo, and the servo structure is shown in Figure 5. The 89C51 main control controls the angle and angular velocity of the servo rotation to achieve the control of the steering of the robot arm. The design of the slewing joint not only increases the degree of freedom of the robot arm, but also can replace the movement of the mobile platform in some cases to complete the handling operation, greatly improving work efficiency.

1.5 Claw design

The pneumatic five-finger gripper mainly uses a series of connecting rod hinges in the middle to convert the reciprocating motion of the cylinder rod of the small cylinder into the grasping motion of the gripper. When the cylinder rod extends outward, the gripper is in an open form; when the cylinder rod contracts inward, the gripper is in a grasping form. The gripping angle of the gripper depends on the telescopic length of the cylinder rod. Figure 6 shows the state of the gripper when it is relaxed and grasping.

2 Flexible Robotic Arm Drive Design

2.1 Comparison of common drive modes

2.1.1 Motor drive mode

Motor drive is to use the force or torque generated by various motors to directly drive the joints, or to drive the joints of the robot through mechanisms such as deceleration to achieve the required position, speed, acceleration or other indicators. This method has the advantages of environmental protection, neatness, easy control, high motion accuracy, low maintenance cost and high drive efficiency. There are 4 types of motors used: DC, AC servo motors and linear motors.

2.1.2 Hydraulic drive mode

Hydraulic actuators use liquid as a medium to transmit force and use a hydraulic pump to generate pressure in the hydraulic system to drive the actuator to move. The hydraulic drive mode is a mature drive mode. It has the characteristics of easy-to-control pressure and flow, high rigidity, incompressible hydraulic oil, simple and stable speed regulation, convenient operation and control, and a wide range of stepless speed regulation (speed regulation range up to 2000:1). It can also obtain greater power with a smaller driving force or torque. However, due to the influence of fluid flow resistance, temperature changes, impurities, leakage, etc., the stability and positioning accuracy of the workpiece are inaccurate, and it also causes environmental pollution and increases the maintenance technology requirements. Therefore, this method is often used in occasions that require large output force and low movement speed. Before the maturity of electric drive technology, hydraulic drive was the most widely used drive method [2].

2.1.3 Pneumatic drive mode

Pneumatic actuators use air as the working medium and use an air source generator to convert the pressure energy of compressed air into mechanical energy to drive the actuator to complete the predetermined movement. Pneumatic drive has the advantages of simple energy saving, short time, fast action, softness, light weight, high output/quality ratio, easy installation and maintenance, safety, low cost, and no pollution to the environment. However, due to the compressibility of air, it is not easy to achieve high-precision and fast-response position and speed control, and it will also reduce the rigidity of the drive system.

2.2 Driving of bending and extension joints

In recent years, people have used the flexibility of pneumatic drive to develop robots that collaborate with humans in rehabilitation, nursing, and assistance [3]. The bionic flexible robotic arm designed in this study requires a simple and compact structure, easy to carry, easy to disassemble, and high position and speed accuracy. Combining the advantages and disadvantages of the above three drive methods and the actual situation of the bionic flexible robotic arm, this study chooses the pneumatic drive method. The drive part mainly consists of an air pump, pipeline, cylinder, and solenoid valve.

An air pump is a device that removes air from a closed space or adds air to a closed space. There are many types of air pumps, but this design only needs to use a micro air pump to meet the requirements. A micro air pump is a gas delivery device with a small size and a gaseous working medium. It is mainly used for gas sampling, gas circulation, vacuum adsorption, vacuum pressure maintenance, exhaust, inflation, and pressurization [4]. The advantages of a micro air pump are: small size, low noise, low power consumption, easy to operate, easy to carry, maintenance-free, can operate continuously for 24 hours, and can also allow the medium to be rich in water vapor. Most importantly, the micro air pump is dry and oil-free, and does not require vacuum pump oil or lubricating oil, so it will not contaminate the working medium, will not interfere with the analysis of the medium, and is relatively cheap.