As a key component of the robot end effector, the manipulator can realize the grasping operation of the target object and is an important interface for the robot to directly interact with the surrounding environment. Nowadays, mobile robots are constantly emerging. Light weight and energy saving will be an important indicator of robot design. As an important executive component, the manipulator also needs to save energy and reduce consumption on the premise of meeting the grasping demand. Under this condition, compared with the complicated transmission and control system of the fully actuated manipulator, the underactuated manipulator is more suitable for the mobile robot field with its advantages of fewer drivers, fewer sensors, low power consumption, simple and efficient control system, and strong adaptability.
Underactuated manipulators mainly include tendon rope type, link type, differential gear type, pneumatic type, etc. Among them, the link type and differential gear type underactuated manipulators have a large grasping force, and the tendon rope type and pneumatic manipulators have a wide grasping range. However, at present, the grasping form of underactuated manipulator is single, which is the same as that of fully actuated manipulator. The grasping form is usually fingertip grasping or envelope grasping. At the same time, some joints of the fingers of these manipulators are passive and do not have driving devices, so they have a strong dynamic coupling effect. For example, link type and tendon rope type manipulators, their passive joints need to obtain driving force indirectly through the dynamic coupling of the active joints. Although this characteristic makes the underactuated manipulator have better environmental adaptability, there are few other ways to realize the underactuated form and grasping form of the manipulator with the motor as the driving device.
In order to control the clamping force of the manipulator, it is necessary to introduce the feedback closed-loop control of the force. For example, the PID (proportional integral differential) controller is used to adjust the motor to control the clamping force, but this requires the feedback of the sensor and the handling of the controller. The layout of the sensor and the design of the motor controller should be considered in the design process, which will increase the complexity of the manipulator control. Another type of manipulator that can realize the adjustment of clamping force is pneumatic manipulator, which adjusts the grasping force of fingers through air pressure, such as pneumatic flexible five finger manipulator and pneumatic tendon underactuated manipulator, which reflects the characteristics of pneumatic manipulator that can adjust the position and posture of fingers and the size of clamping force through gas, while the only disadvantage is that the manipulator needs an air pump to provide it with gas of different pressures.
According to the problems of single grasping form and inability to control the clamping force of under actuated manipulator, an under actuated manipulator based on planetary gear mechanism was designed. The manipulator uses a planetary gear mechanism that does not fix the output shaft and internal gear ring at the same time to distribute two driving torques: one is used for the joint rotation of the manipulator finger to achieve the clamping of the target object; The other is used for belt rotation to achieve traction of target objects. The target object is pulled in through the traction form of the belt to achieve a new form of target object grasping, so that the manipulator can not only hold the object, but also pull in the object. According to the characteristics of planetary gear mechanism with single input and double output, the resistance torque adjustment mechanism is innovatively designed, and the clamping force adjustment function of the manipulator is realized without the feedback of the force sensor.