In recent years, thanks to the rapid development of the industrial robot market, China's motion control industry has entered a stage of rapid growth. The further release of downstream demand has also driven the rapid development of upstream industries, with core transmission components such as linear guides, ball screws, gear racks, hydraulic (pneumatic) cylinders, gears, and reducers experiencing a significant increase in orders. The entire motion control industry market is showing a vigorous upward trend.
As we all know, the drive source of an industrial robot is the transmission component, which drives the movement or rotation of its joints, thereby enabling the movement of the body, arm, and wrist. Therefore, the transmission component is a crucial part of an industrial robot. Based on the type of transmission, transmission components can be divided into two main categories: linear transmission mechanisms and rotary transmission mechanisms. Today, we will delve into these categories together.
Linear transmission mechanism
The linear transmission mechanism commonly used in industrial robots can be generated directly by cylinders or hydraulic cylinders and pistons, or it can be obtained by converting rotary motion using transmission elements such as gears, racks, ball screws and nuts.
1. Moving joint guide rail
Moving the joint guide rail during movement can ensure positional accuracy and provide guidance.
There are five types of sliding guide rails: ordinary sliding guide rails, hydraulic dynamic sliding guide rails, hydraulic hydrostatic sliding guide rails, air-bearing guide rails, and rolling guide rails.
Currently, the fifth type of rolling guide is the most widely used in industrial robots. As shown in Figure 2-15, it is an enclosed rolling guide. It is supported by a support seat and can be easily connected to any plane. In this case, the sleeve must be open and embedded in the slide, which not only enhances rigidity but also facilitates connection with other components.
2. Gear and rack assembly
In a rack and pinion device (Figure 2-16), if the rack is fixed, when the gear rotates, the gear shaft, along with the slide, moves linearly along the rack direction. Thus, the rotational motion of the gear is converted into the linear motion of the slide. The slide is supported by a guide rod or guide rail, and this device has a relatively large backlash.
3. Ball screw and nut
Ball screws are frequently used in industrial robots because they have very low friction and fast motion response.
Because the ball screw nut has many balls in its spiral groove, the screw is subjected to rolling friction during transmission, which is relatively small. Therefore, the transmission efficiency is high, and the creeping phenomenon during low-speed movement can be eliminated. Applying a certain preload during assembly can eliminate backlash.
As shown in Figure 2-17, the balls in the ball screw nut circulate and transmit motion and power through the ground guide groove. The transmission efficiency of the ball screw can reach 90%.
4. Hydraulic (pneumatic) cylinder
A hydraulic (pneumatic) cylinder is an actuator that converts the pressure energy output from a hydraulic pump (air compressor) into mechanical energy, enabling linear reciprocating motion. Linear motion can be easily achieved using a hydraulic (pneumatic) cylinder. A hydraulic (pneumatic) cylinder mainly consists of a cylinder barrel, cylinder head, piston, piston rod, and sealing devices. The piston and cylinder barrel are precisely slidingly fitted. Pressurized oil (compressed air) enters from one end of the hydraulic (pneumatic) cylinder, pushing the piston to the other end, thus achieving linear motion. The direction and speed of the hydraulic (pneumatic) cylinder's movement can be controlled by adjusting the flow direction and flow rate of the hydraulic oil (compressed air) entering the cylinder.
Rotary transmission mechanism
While most electric motors can directly generate rotational motion, their output torque is lower than the required torque, and their rotational speed is higher than the required speed. Therefore, gears, belt drives, or other motion transmission mechanisms are needed to convert the higher speed into a lower speed while obtaining a larger torque. The transmission and conversion of motion must be completed efficiently without compromising the characteristics required by the robot system, including positioning accuracy, repeatability, and reliability. The following transmission mechanisms can achieve the transmission and conversion of motion.
1. Gear pair
Gear pairs can transmit not only angular displacement and angular velocity, but also force and torque. One gear is mounted on the input shaft and the other gear is mounted on the output shaft. The number of teeth of the gear is inversely proportional to its rotational speed [Equation (2-1)]. 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 drive device
In industrial robots, synchronous belt drives are mainly used to transmit motion between parallel shafts. The contact surfaces of the synchronous conveyor belt and pulleys are both made with corresponding tooth profiles, and power is transmitted through meshing. The pitch of the teeth is represented by the circular pitch t when enveloping the pulley.
In the formula: n1 is the speed of the driving gear (r/min); n2 is the speed of the driven gear (r/min); z1 is the number of teeth of the driving gear; z2 is the number of teeth of the driven gear.
Advantages of synchronous belt drives: no slippage during transmission, accurate transmission ratio, smooth transmission; wide speed ratio range; low initial tension; shaft and bearings are less prone to overload. However, the manufacturing and installation of this transmission mechanism have strict requirements, and the requirements for belt materials are also high, resulting in higher costs. Synchronous belt drives are suitable for transmission between electric motors and high-reduction-ratio reducers.
3. Harmonic gears
Currently, 60% to 70% of the rotary joints of industrial robots use harmonic gear transmission.
Harmonic gear transmission consists of three main components: rigid gears, harmonic generators, and flexible gears.
During operation, the rigid gear 6 is fixedly installed with its teeth evenly distributed on the circumference. The flexible gear 5, which has an outer toothed ring 2, rotates along the inner toothed ring 3 of the rigid gear. The flexible gear has two fewer teeth than the rigid gear, so for each revolution of the flexible gear along the rigid gear, it rotates in the opposite direction by the corresponding angle of two teeth.
The harmonic generator 4 has an elliptical profile, with balls mounted on it supporting the flexible gear. The harmonic generator drives the flexible gear to rotate and cause it to undergo plastic deformation. During rotation, only a few teeth at the elliptical end of the flexible gear mesh with the rigid gear; only in this way can the flexible gear freely rotate through a certain angle relative to the rigid gear. Typically, the rigid gear is fixed, the harmonic generator serves as the input, and the flexible gear is connected to the output shaft.
In the formula: z1 is the number of teeth on the flexible gear; z2 is the number of teeth on the rigid gear. Assuming the rigid gear has 100 teeth and the flexible gear has two fewer teeth, then for every 50 revolutions of the harmonic generator, the flexible gear rotates once. This achieves a 1:50 reduction ratio in a very small space. Typically, the harmonic generator is mounted on the input shaft, and the flexible gear on the output shaft to obtain a larger gear reduction ratio.
4. Cycloidal pinwheel drive reducer
Cycloidal pinwheel transmission is a new type of transmission method developed on the basis of pinwheel transmission. In the 1980s, Japan developed a cycloidal pinwheel transmission reducer for robot joints. Figure 2-21 shows a simplified diagram of cycloidal pinwheel transmission.
It consists of two parts: an involute cylindrical gear planetary reduction mechanism and a cycloidal pinwheel planetary reduction mechanism. The involute planetary gear 6 is integrated with the crankshaft 5 and serves as the input to the cycloidal pinwheel transmission.
If the involute center wheel 7 rotates clockwise, the involute planetary gears will simultaneously revolve around the central axis and rotate counterclockwise, driving the cycloidal wheel to perform planar motion via the crankshaft. At this time, the cycloidal wheel, constrained by the meshing pinwheel, will rotate in the opposite direction (clockwise) while its axis revolves around the pinwheel's axis. Simultaneously, it will drive the planetary carrier output mechanism to rotate clockwise via the crankshaft.
