Linear drive mechanism
1. Gear and rack assembly
Normally, the rack is stationary. When the gears are in operation, the gear shaft, along with the slide, moves linearly along the rack direction. Thus, the rotational motion of the gears is converted into the linear motion of the slide, as shown in Figure 1. The slide is supported by guide rods or guide rails. This device has a relatively large hysteresis.
Figure 1 Gear and rack assembly
2. Ordinary lead screw
A conventional lead screw drive uses a rotating precision lead screw to move a nut along the lead screw axis. Because conventional lead screws have high friction, low efficiency, and high inertia, they are prone to creeping at low speeds. Furthermore, they have low precision and large backlash, therefore they are rarely used in robots .
3. Ball screw
Ball screws are frequently used in robots because they have very low friction and a fast motion response. Since numerous balls are placed in the helical grooves of the screw nut, the friction experienced during transmission is rolling friction, which greatly reduces friction, resulting in high transmission efficiency and eliminating crawling phenomena at low speeds. Applying a certain preload during assembly can eliminate backlash.
As shown in Figure 2, the balls in the ball screw emerge from the steel sleeve and enter the ground guide groove. After rotating 2-3 revolutions, they return to the steel sleeve. The transmission efficiency of the ball screw can reach 90%, so only a very small driving force and a small drive connector are needed to transmit motion.
Figure 2 Ball bearing screw pair
Rotary drive mechanism
gear chain
A gear chain is a transmission mechanism consisting of two or more gears. It can transmit not only angular displacement and angular velocity, but also force and torque. Taking a gear chain with two gears as an example, we will illustrate its transmission conversion relationship. One gear is mounted on the input shaft, and the other gear is mounted on the output shaft, as shown in Figure 3.
Figure 3 Gear chain mechanism
Two issues should be noted when using gear chain mechanisms:
First, the introduction of a gear train changes the system's equivalent moment of inertia, thereby reducing the response time of the drive motor and making the servo system easier to control. The output shaft's moment of inertia is transferred to the drive motor, and the decrease in the equivalent moment of inertia is proportional to the square of the number of teeth on the input and output gears.
Secondly, the introduction of gear chains will increase the positioning error of the robot arm due to gear backlash error; moreover, if no remedial measures are taken, backlash error will also cause instability of the servo system.
Generally, gear chain rotation has the following types, as shown in Figure 4.
Figure 4 Commonly used gear chains
(a) Cylindrical gear; (b) Helical gear; (c) Bevel gear;
(d) Worm gear; (e) Planetary gear train
Among them, cylindrical gears have a transmission efficiency of about 90%. Due to their simple structure and high transmission efficiency, cylindrical gears are the most common in robot design. Helical gears have a transmission efficiency of about 80%. Helical gears can change the direction of the output shaft. Bevel gears have a transmission efficiency of about 70%. Bevel gears can make the input shaft and output shaft not be on the same plane, but their transmission efficiency is low. Worm gears have a transmission efficiency of about 70%. Worm gear mechanisms have a large transmission ratio, smooth transmission, and can achieve self-locking, but their transmission efficiency is low, their manufacturing cost is high, and they require lubrication. Planetary gear trains have a transmission efficiency of about 80%. They have a large transmission ratio, but their structure is complex.
2. Synchronous belt
Synchronous belts are similar to fan belts and other drive belts in factories, except that they have many teeth that mesh with the teeth of synchronous pulleys, which also have teeth.
When in operation, they function like flexible gears, offering the advantages of high flexibility and low cost. Additionally, synchronous belts are used when the input and output shafts are not aligned.
At this point, as long as the timing belt is long enough to minimize its torsion error, it will still function normally. In servo systems, if the output shaft position is measured using an encoder, the timing belt for the input drive can be placed outside the servo ring without affecting the system's positioning accuracy and repeatability, which can reach within 1mm. Furthermore, timing belts are significantly cheaper and easier to manufacture than gear chains. Sometimes, combining gear chains and timing belts is even more convenient.
3. Harmonic gears
Although harmonic gears have been around for many years, they have only recently begun to be used extensively. Currently, 60% to 70% of the rotary joints of robots use harmonic gears.
The harmonic gear transmission mechanism consists of three main components: a rigid gear, a harmonic generator, and a flexible gear, as shown in Figure 5. During operation, the rigid gear is fixedly installed with its teeth evenly distributed around the circumference, while the flexible gear, which has external teeth, rotates along the internal teeth of the rigid gear.
The flexible gear has two fewer teeth than the rigid gear, so for every revolution of the rigid gear, the flexible gear rotates in the opposite direction by the corresponding angle of two teeth. The harmonic generator has an elliptical profile, and the balls mounted on the harmonic generator support the flexible gear. The harmonic generator drives the flexible gear to rotate and causes 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.
Assuming the rigid gear has 100 teeth and the flexible gear has 2 fewer teeth, then when the harmonic generator rotates 50 times, the flexible gear rotates 1 time. In this way, a reduction ratio of 1:50 can be obtained with only a small amount of space.
Because of the large number of teeth meshing simultaneously, the harmonic generator has a strong torque transmission capability. Although any two of the three components can be selected as input and output elements, the harmonic generator is usually mounted on the input shaft and the flexible gear on the output shaft to obtain a larger gear reduction ratio.
Figure 5 Harmonic Gear Transmission
Selection and braking of linear drive and rotary drive
1. Selection of driving method
Before the advent of inexpensive computers, one of the main difficulties in controlling rotary motion was the large amount of computation required. Therefore, linear drive was considered preferable at the time. DC servo motors were a relatively ideal rotary drive element, but precise control required expensive servo power amplifiers. For example, in 1970, reliable high-power transistors were not yet available, and many high-power transistors needed to be connected in parallel to drive a high-power servo motor.
Today, the cost of motor drives and controls has been greatly reduced, and high-power transistors are widely used; a high-power servo motor can be driven with just a few transistors. Similarly, microcomputers are becoming increasingly cheaper, and the proportion of computer costs in the total cost of a robot has decreased significantly. Some robots even use a microprocessor for each joint or degree of freedom.
For the reasons mentioned above, many robotics companies have chosen rotary joints when manufacturing and designing new robots. However, linear drives are more suitable in many situations. Therefore, linear cylinders remain the cheapest power source among all drive devices, and should be used wherever possible. Additionally, linear drives are also necessary for applications requiring high precision.
2. Brake
Many robotic arms require brakes installed at each joint. Their function is to maintain the position of the robotic arm when the robot stops working and to protect the robotic arm and surrounding objects from collisions in the event of a power failure.
If components such as gear chains, harmonic gear mechanisms, and ball screws have high mass, their friction is generally very low, and they cannot withstand loads when the actuator stops working. Without some external fixing device, such as a brake, clamp, or stop, the robot's various parts will slide down under gravity once the power is turned off. Therefore, designing a braking device for the robot is essential.
The brake usually operates in a fail-safe mode, meaning that power must be supplied to release the brake; otherwise, the joints cannot move relative to each other.
The main purpose of this method is to provide protection when the power supply fails. Its disadvantage is that the brake needs to be continuously energized during operation.
If necessary, a power-saving method can be used. The principle is as follows: when the joints need to move, first turn on the power to release the brake, then turn on another power source to drive a stop pin to lock the brake in the released state. In this way, the power required is only the power spent to put the stop pin into place.
To ensure accurate joint positioning, the brake must possess sufficient positioning precision. The brake should ideally be placed at the drive input end of the system. This utilizes the transmission chain speed ratio to reduce system vibration caused by slight brake slippage, ensuring high positioning accuracy even under load. Brakes are employed in many robots across numerous practical applications.
Figure 6 shows the installation diagram of the shoulder brake of the Mitsubishi assembly robot Movemaster EXRV-M1.
Figure 6. Installation diagram of the shoulder brake on the Mitsubishi assembly robot.
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