First, linear transmission mechanism
The linear transmission mechanism commonly used in industrial robots can be directly generated by the cylinder or hydraulic cylinder and piston, and can also be converted by rotating motion by using pinion and rack, ball screw nut and other transmission elements.
1. Prismatic joint guide
The prismatic joint guide can play a role in ensuring position accuracy and guidance during motion.
There are five kinds of prismatic joint guides: ordinary sliding guide, hydraulic dynamic pressure sliding guide, hydraulic static pressure sliding guide, air floating guide and rolling guide.
At present, the fifth kind of rolling guide is the most widely used in industrial robots. As shown in Figure 2-15, the structure of the inclusive rolling guide is supported by the support seat, which can be easily connected to any plane. At this time, the sleeve must be open and embedded in the ram, which not only enhances the stiffness but also facilitates the connection with other components.
2. Rack and pinion device
In the rack and pinion device (Figure 2-16), if the rack is fixed, when the gear rotates, the gear shaft and the drag plate move in a straight line along the rack direction. In this way, the rotational motion of the gear is converted into the linear motion of the drag plate. The drag plate is supported by a guide rod or a guide rail, and the return difference of the device is large.
3, ball screw and nut
Ball screws are often used in industrial robots because of their low friction and fast motion response.
Because many balls are placed in the screw groove of the ball screw nut, the ball screw is subject to rolling friction in the transmission process, and the friction is small, so the transmission efficiency is high, and the creeping phenomenon can be eliminated at low speed. The backslip can be eliminated by applying a certain preload during assembly.
As shown in Figure 2-17, the ball in the ball screw nut circulates through the grinding guide groove to transfer motion and power, and the transmission efficiency of the ball screw can reach 90%.
4, liquid (gas) pressure cylinder
Liquid (gas) cylinder is the hydraulic pump (air compressor) output pressure energy into mechanical energy, do linear reciprocating motion of the actuator, the use of liquid (gas) cylinder can easily achieve linear motion. The liquid (gas) cylinder is mainly composed of cylinder, cylinder cover, piston, piston rod and sealing device and other components. The piston and cylinder adopt precision sliding cooperation, and the pressure oil (compressed air) enters from one end of the liquid (gas) cylinder, and pushes the piston to the other end of the liquid (gas) cylinder, so as to realize linear movement. By adjusting the flow direction and flow rate of hydraulic oil (compressed air) into the liquid (gas) cylinder, the direction and speed of motion of the liquid (gas) cylinder can be controlled.
Two, rotating transmission mechanism
Generally, the motor can directly generate rotating motion, but its output torque is smaller than the required torque, and the speed is higher than the required speed. Therefore, it is necessary to use gear, belt transmission device or other motion transmission mechanism to convert the higher speed into a lower speed and obtain a larger torque. The transfer and transformation of motion must be done efficiently and without compromising the desired characteristics of the robotic system, including positioning accuracy, repeated positioning accuracy, and reliability. The transmission and conversion of motion can be achieved by the following transmission mechanisms.
1. Gear pair
The gear pair can not only transmit angular displacement and angular velocity, but also transmit force and torque. One gear is mounted on the input shaft, and the other gear is mounted on the output shaft. It can be obtained that the number of teeth of the gear is inversely proportional to its speed [Equation (2-1)], and the ratio of output torque to input torque is equal to the ratio of output teeth to input teeth [Equation (2-2)].
2. Synchronous belt transmission device
In industrial robots, synchronous belt transmission is mainly used to transfer the motion between parallel axes. The contact surface of synchronous conveyor belt and pulley is made of corresponding tooth shape, and power is transferred by meshing. The pitch of the teeth is denoted by the circular pitch t when enveloping the pulley.
Where: n1 main wheel speed (r/min); n2 is the passive wheel speed (r/min); z1 tooth number of main wheel; z2 is the number of passive wheel teeth.
The advantages of synchronous belt transmission: no sliding transmission, accurate transmission ratio, stable transmission; Wide range of speed ratio; Small initial tension; Shaft and bearing are not easy to overload. However, the manufacturing and installation requirements of this transmission mechanism are strict, and the material requirements of the belt are also higher, so the cost is higher. The synchronous belt transmission is suitable for the transmission between the motor and the high reduction ratio reducer.
3. Harmonic gear
At present, 60%~70% of the rotating joints of industrial robots are driven by harmonic gears.
Harmonic gear drive consists of three main parts: rigid gear, harmonic generator and flexible gear.
When working, the rigid gear 6 is fixed, and all the teeth are distributed on the circumference, and the flexible gear 5 with outer gear ring 2 rotates along the inner gear ring 3 of the rigid gear. The flexible gear has two less teeth than the rigid gear, so the flexible gear rotates the corresponding Angle of the two teeth in the opposite direction along each revolution of the rigid gear.
The harmonic generator 4 has an oval profile, and the ball mounted on it is used to support the flexible gear, and the harmonic generator drives the flexible gear to rotate and cause plastic deformation. When turning, only a few teeth of the elliptical end of the flexible gear are engaged with the rigid gear, and only in this way can the flexible gear freely turn a certain Angle relative to the rigid gear. Usually the rigid gear is fixed, the harmonic generator is used as the input, and the flexible gear is connected to the output shaft.
Where: z1 is the number of teeth of the flexible gear; z2 is the number of teeth of the rigid gear. Assuming that the rigid gear has 100 teeth and the flexible gear has two teeth less than it, when the harmonic generator rotates 50 revolutions, the flexible gear rotates 1 revolution, so that the reduction ratio of 1:50 can be obtained by only taking up a small space. Usually, the harmonic generator is installed in the input shaft and the flexible gear is installed in the output shaft to obtain a large gear reduction ratio.
4, cycloid pin wheel drive reducer
Cycloid pinwheel transmission is a new type of transmission mode developed on the basis of needle pendulum transmission. In the 1980s, Japan developed a cycloid pinwheel transmission reducer for robot joints. Figure 2-21 shows a simplified diagram of cycloid pinwheel transmission.
It consists of involute cylindrical gear planetary reduction mechanism and cycloid pinwheel planetary reduction mechanism. The involute planetary wheel 6 is connected with the crank shaft 5 as an input to the cycloidal pinwheel transmission part.
If the involute center wheel 7 rotates clockwise, then the involute planetary gear rotates counterclockwise at the same time and drives the cycloid wheel in planar motion through the crank shaft. At this time, the cycloid wheel is constrained by the needle wheel with which it is engaged, and its axis rotates around the needle wheel axis while it also rotates in the opposite direction, that is, clockwise. At the same time, it pushes the planetary frame output mechanism clockwise through the crank shaft.