What Is a Pneumatic Cylinder?
Cylindrical metal parts that guide the piston to linearly reciprocate in the cylinder. Air in the engine cylinder expands to convert thermal energy into mechanical energy; gas in the compressor cylinder receives piston compression to increase pressure.
- The cavity of the piston is placed on the cylinder of the internal combustion engine. It is the orbit of the piston movement, in which the gas is burned and expanded, and a part of the explosive residual heat transmitted by the gas can be dissipated through the cylinder wall, so that the engine maintains a normal operating temperature. The types of the cylinder are monolithic and single-cast. Single casting is divided into two types: dry and wet. When the cylinder and the cylinder block are cast into a whole, it is called a monolithic cylinder; when the cylinder and the cylinder block are separately cast, a single-cast cylinder barrel is called a cylinder liner. The cylinder liner that is in direct contact with the cooling water is called a wet cylinder liner; the cylinder liner that is not in direct contact with the cooling water is called a dry cylinder liner. In order to maintain the tightness of the contact between the cylinder and the piston, and reduce the friction loss of the piston moving in it, the inner wall of the cylinder should have high machining accuracy and precise shape and size. [1]
- Converting pressure energy of compressed gas into machinery in pneumatic transmission
- The air cylinder is composed of a cylinder barrel, an end cover, a piston, a piston rod and a seal. The internal structure is shown in the figure:
- The cylinder principle is derived from the cannon.
- In 1680, Dutch scientist Hoins was inspired by the principle of cannons. Wasn't it good to use the power of cannonballs to propel other machinery? He still used gunpowder as a burning explosive at first, changed the shell to a "piston", the gun barrel as a "cylinder", and opened a one-way valve. He injected gunpowder into the cylinder. When the gunpowder was ignited, the gunpowder violently exploded and burned, pushing the piston upward and generating power. At the same time, the huge pressure of the explosive gas also pushed open the check valve to exhaust the exhaust gas. Then, the residual exhaust gas in the cylinder gradually became colder, and the air pressure became lower, and the atmospheric pressure outside the cylinder pushed the piston downward to prepare for the next explosion. Of course, due to the long journey and too low efficiency, he did not succeed in the end. However, it was Hoynes who first proposed the idea of an "internal combustion engine", and it was on this basis that later generations invented the engine for automobiles.
- Early cars used single-cylinder machines
- When car ancestors Carl Benz and Daimler designed and manufactured cars in those days, they used only one-cylinder engines. Just as we thought it was impossible for a car to use two or more engines, it is estimated that people at the time would not have imagined an engine that would also use two or more cylinders. But now it s different. Let s not talk about developed countries. If you look at the domestic car ads, you will find that many manufacturers always take the number and arrangement of the cylinders of the engine. The cylinder engine is not a three-cylinder engine. The v6 engine must make the v eye-catching. Advertising has indeed played a great role. Many fans have agreed that "4 cylinders are better than 3 cylinders." The "4-cylinder is better", "V-type is better than inline", "V-type engines are advanced engines" and other concepts. Almost 20 types of domestic cars have been equipped with v6 or v8 engines.
- The crankshaft of a single-cylinder engine can generate combustion work once every two weeks of rotation, so that its sound does not sound continuous and smooth, just listen to the sound of a small displacement motorcycle. What is most unacceptable is that it runs extremely unevenly, the speed fluctuates greatly, and the shape of a single-cylinder engine is not suitable for mounting on a car. For this reason, single-cylinder engines are no longer seen in cars, and two-cylinder engines are hard to find, at least three-cylinder engines. The domestically produced Huali vans, old Xiali cars, Geely Haoqing and Alto and Flair are all equipped with 3-cylinder machines.
- 3-cylinder engines are commonly used on mini-cars below 1 liter, and 4-cylinder or 5-cylinder engines are generally used for engines from 1 to 2 liters. Engines with more than 2 liters are mostly 6-cylinders, and engines with more than 4 liters use 8-cylinders.
- In the case of the same displacement, increasing the number of cylinders can increase the engine speed, so that the output power of the engine can be increased. In addition, increasing the number of cylinders can make the engine run more smoothly and make its output torque and output more stable. Increasing the number of cylinders can make the gas car easier to start and the acceleration response is better. In order to improve the performance of gas cars, the number of cylinders must be increased. Therefore, the number of cylinders for high-performance gas cars such as luxury cars, sports cars, and racing cars is above 6 cylinders, and the maximum has reached 16 cylinders.
- However, the increase in the number of cylinders cannot be unlimited. Because with the increase in the number of cylinders, the number of engine parts also increases proportionally, which complicates the engine structure, reduces the reliability of the engine, increases the weight of the engine, increases manufacturing costs and operating costs, increases fuel consumption, and makes The volume becomes larger. Therefore, the number of cylinders of a gas car engine is an appropriate choice made after weighing various advantages and disadvantages according to the purpose and performance requirements of the engine.
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- Traditionally, cylinders and electric actuators have always been considered as automation products that belong to two completely different fields. However, in recent years, with the continuous improvement of the degree of electrification, electric actuators have slowly immersed themselves in the field of pneumatics. Applications are both competitive and complementary. In this column, we will compare the respective advantages of cylinders and electric actuators in terms of technical performance, purchase and application costs, energy efficiency, application occasions, and market situation.
Cylinder technical performance
- As we all know, compared with electric actuators, the cylinder can work reliably under harsh conditions, and the operation is simple, which can basically achieve maintenance-free. The cylinder is good at reciprocating linear motion, and is especially suitable for the most demanding conveyance in industrial automation-linear handling of workpieces. Moreover, simply adjusting the one-way throttle valve installed on both sides of the cylinder can simply achieve stable speed control, which also becomes the biggest feature and advantage of the cylinder drive system. Therefore, for users without multi-point positioning requirements, the vast majority prefer to use the cylinder from the perspective of convenience. At present, most applications of electric actuators in industrial fields require high-precision multi-point positioning. This is because it is difficult to achieve with a cylinder, and the next best result.
The electric actuator is mainly used in rotating and swinging conditions. Its advantage is that the response time is fast, and the speed, position and torque are accurately controlled by the feedback system. However, when the linear motion needs to be completed, the transmission conversion needs to be performed by mechanical devices such as toothed belts or screw rods, so the structure is relatively complicated, and the working environment and the professional knowledge of operation and maintenance personnel have high requirements.
Cylinder advantage
- (1) Low requirements for users. The principle and structure of the cylinder are simple, easy to install and maintain, and have low requirements for users. Electric cylinders are different. Engineers must have certain electrical knowledge, otherwise it is very likely to be damaged due to misoperation.
(2) The output force is large. The output force of the cylinder is directly proportional to the square of the bore diameter; and the output force of the electric cylinder is related to three factors, the bore diameter, the power of the motor, and the screw pitch of the screw rod. A cylinder with a cylinder diameter of 50mm can theoretically output 2000N. For electric cylinders with the same cylinder diameter, although the products of different companies are different, they basically do not exceed 1000N. Obviously, the cylinder has an advantage in terms of output force.
(3) Strong adaptability. The cylinder can work normally in high temperature and low temperature environment, and has dustproof and waterproof ability, which can adapt to various harsh environments. The electric cylinder, because of its large number of electrical components, has higher environmental requirements and poor adaptability.
The advantages of electric cylinders are mainly reflected in the following three aspects:
(1) The system configuration is very simple. Because the motor is usually integrated with the cylinder, plus the controller and cables, the entire system of the electric cylinder is composed of these three parts, which is simple and compact.
(2) There are many stop positions and high control accuracy. Generally, there are low-end and high-end electric cylinders. There are 3, 5, 16, and 64 stop positions for low-end products, which vary depending on the company. High-end products can reach hundreds or even thousands of positions . In terms of accuracy, the electric cylinder also has an absolute advantage, and the positioning accuracy can reach ¡. 0.05mm, so it is often used in precision industries such as electronics and semiconductors.
(3) Strong flexibility. There is no doubt that the electric cylinder is much more flexible than the cylinder. Because the controller can be directly connected to the PLC, the motor's speed, positioning, and forward and reverse can be accurately controlled. To a certain extent, the electric cylinder can move freely according to needs; due to the compressibility of the gas and the Inertia, even if the cooperation between the reversing valve and the magnetic switch is as good as possible, the accurate positioning of the cylinder cannot be achieved, and the flexibility is impossible to talk about.
- In terms of technical performance, I think that electric and pneumatic have their own strengths. First, the advantages of electric actuators include:
(1) Compact structure and small size. Compared with pneumatic actuators, electric actuators have a relatively simple structure. A basic electronic system includes an actuator, a three-position DPDT switch, a fuse and some wires, which is easy to assemble.
(2) The drive source of the electric actuator is very flexible. Generally, the vehicle power supply can meet the needs, while the pneumatic actuator needs the air source and the compression drive device.
(3) The electric actuator has no danger of leakage and high reliability, and the compressibility of air makes the stability of the pneumatic actuator slightly worse.
(4) No need to install and maintain various pneumatic pipelines.
(5) The load can be maintained without power, and the pneumatic actuator needs a continuous pressure supply.
(6) Since no additional pressure device is required, the electric actuator is quieter. Generally, if the pneumatic actuator is under a heavy load, a silencer is added.
(7) Electric actuators are superior in terms of control accuracy.
(8) Pneumatic devices usually need to convert electrical signals into gas signals, and then into electrical signals. The transmission speed is slow, and it is not suitable for complex circuits with too many component stages.
- The advantages of the cylinder are in the following 4 aspects:
(1) The load is large and can be adapted to applications with high torque output (however, the current electric actuators have gradually reached the current level of pneumatic load).
(2) Quick action and quick response.
(3) The working environment has good adaptability, especially in harsh working environments such as flammable, explosive, dusty, strong magnetism, radiation and vibration, which is superior to hydraulic, electronic and electrical control.
(4) The motor is easily damaged when the stroke is blocked or the valve rod is tied.
Comparison of cylinder purchase and application costs
Generally speaking, the electric servo drive is more expensive than the pneumatic servo drive, but it also depends on the specific requirements and occasions. Some low-power DC motors constitute electric slides (electric servo systems) which are actually cheaper than pneumatic servo systems.
For example: when the load is 1.5kg, the working stroke is 80mm, the speed is between 2 ~ 170mm / s, and the accuracy is ¡. Under working conditions such as 0.1mm and acceleration of 2.5m / s2, FESTO uses an electric servo system consisting of a small electric slide table, controller, motor cable, control cable, programming cable and power cable, which is more expensive than a pneumatic servo system 25% cheaper. The same is true for electric cylinders with piston rods. It should be noted that if an AC motor is used, the price of the formed electric servo system is about 40% higher than that of the pneumatic servo system.
From the perspective of purchase and application costs, the current cylinder still has obvious advantages. For the pneumatic system, the control system and actuator are very simple. Each cylinder only needs to be equipped with a solenoid valve to complete the switching of the gas path for motion control. The probability of cylinder failure is relatively small. Maintenance is simple, convenient and cost. Also low.
For electric actuators, although the acquisition of electrical energy is relatively simple and the energy cost is low, the purchase and application costs are relatively high. Not only need to configure the motor, but also a set of mechanical transmission mechanism and corresponding drive components. At the same time, the use of electric actuators requires a lot of protection measures. Incorrect circuit connections, voltage fluctuations and overloads of the load will cause damage to the electric drive. Therefore, protection systems need to be installed on the circuits and machinery, which adds a lot of extra costs. In addition, because the electric actuator drive unit has many parameter settings and high integration, it should replace the entire component if it fails. And when the driving force required by the system increases, it can only be achieved by replacing components in sets. Therefore, a comprehensive comparison shows that the cylinder has a great advantage in terms of purchase and maintenance costs.
Comparison of cylinder energy efficiency
The results of our research show that in horizontal reciprocating motions with short reciprocating cycles (less than 1 min), the operating energy consumption of electric actuators is usually lower than that of cylinders, which means more energy saving. When the reciprocating cycle is longer (more than 1min), the cylinder becomes even more energy efficient. This is firstly because the controller of the electric actuator usually consumes about 10W of power when the terminal is stopped, and the cylinder only has the power consumption of the solenoid valve and gas leakage, which is generally lower than 1W, that is, the longer the terminal stop time, the better the cylinder; Secondly, the rated efficiency of the motor under continuous rotation can reach more than 90%, but the average efficiency under the condition of table acceleration and deceleration in linear reciprocating motion (screw conversion) is less than 50%. During the vertical reciprocating movement, the holding action of holding the workpiece requires continuous supply of electric current to the electric actuator to overcome the gravity, and the cylinder only needs to close the solenoid valve, which consumes less electrodes. Therefore, the energy consumption advantage of electric actuators compared to cylinders during vertical reciprocating motion is not great.
As can be seen from the above, the motor itself is very efficient, but in the linear motion of reciprocation, considering the decrease in efficiency and the power consumption of the controller, the electric actuator may not necessarily be more energy-efficient than the cylinder. Exercise cycle and load factor.
Comparison of cylinder applications
Pneumatic and electric systems are not mutually exclusive. Rather, it's just a matter of asking different things. The advantages of pneumatic actuators are obvious. When faced with harsh environmental conditions such as dust, grease, water or cleaning agents, pneumatic actuators are more adaptable to harsh environments and are very rugged. Pneumatic actuators are easy to install, can provide typical gripping functions, are inexpensive and easy to operate.
In the case where the force is rapidly increasing and precise positioning is required, an electric drive with a servo motor has advantages. For applications that require precise, synchronous operation, adjustable and specified positioning programming, an electric drive is the best choice. An electric drive system consisting of a servo or stepper motor with a closed-loop positioning controller can supplement the deficiencies of pneumatic systems. Office.
From the perspective of technology and use cost, the cylinder has obvious advantages, but in actual use, which technology should be used for drive control, or should be considered from a variety of factors. Various systems in modern control are becoming more and more complex and sophisticated, and it is not a certain drive control technology that can meet the various control functions of the system. The cylinder can simply realize fast linear circular motion, simple structure, convenient maintenance, and can be used in various harsh working environments, such as explosion-proof requirements, dusty or humid conditions.
Electric actuators are mainly used in applications that require precise control. Nowadays, the requirements for flexibility in automation equipment are constantly increasing. The same equipment is often required to adapt to the processing requirements of workpieces of different sizes. The actuators need to perform multi-point positioning control. The precise control or synchronous tracking of the running speed and torque of the motor can not be achieved by the traditional pneumatic control, and the electric actuator can realize such control very easily. It can be seen that the cylinder is more suitable for simple motion control, and the electric actuator is mostly used for precision motion control.
Comparison of cylinder market situation
Cylinder drive systems have gained rapid popularity in the field of industrial automation since the 1970s. Today, the cylinder has become the mainstream actuator for PTP (PointToPoint) handling in the industrial production field at home and abroad, and the pneumatic component market centered on the cylinder drive system has reached the scale of 11 billion US dollars.
Since the 1990s, the rapid development of electric motors and their microelectronic control technology has made it possible to apply electric actuators in industrial automation. Moreover, the rise of the semiconductor industry has also directly promoted the expansion of electric actuators that can achieve high-precision multi-point positioning in the industrial field.
In the late 1990s, Japan and other major industrial developed countries even once appeared that electric actuators were about to replace cylinders, and that cylinders would withdraw from the historical stage. Because it is generally believed that the energy conversion efficiency of the motor in the electric actuator is high, and the energy conversion efficiency of the cylinder is low, inefficient products will be eliminated. However, ten years later, electric actuators have not been popularized on the industrial site, and the market scale is still far from that of pneumatic. Moreover, both in industrially developed countries and emerging industrial countries such as China, the sales of cylinders have not only decreased, but also steadily increased. In China, the annual growth rate of cylinder sales has been maintained at more than 20% in recent years.
If you need to evaluate both scientifically and objectively, you must use the LifeCycleAssessment method to consider and compare the comprehensive indicators of the three phases of the manufacturing phase, the use phase, and the waste phase. Specific indicators are cost, energy consumption, and burden on the environment (mainly emissions, etc.). For example, cost, electric actuators have advantages in operating energy consumption (use stage), but maintenance costs (use stage) and purchase costs (manufacturing stage) are much higher than cylinders. The comparison on this indicator should be based on The sum of all costs.
- In terms of total cost, our research results show that the cylinder has certain advantages in most industrial applications.
Based on the above analysis, we should see that cylinders and electric actuators have their own characteristics, and we cannot simply evaluate their pros and cons based on their efficiency. With the development of electrical technology, the cost of electric actuators will further decrease, and its application fields are expected to be further expanded, but it is not realistic to completely replace the cylinders with self-priming non-clogging sewage pumps.
From the perspective of the market form, it has been mentioned earlier that if the electric cylinder is produced with reference to the shape of the cylinder and the installation and connection dimensions from the beginning, it is a good start. For rodless cylinders and pneumatic slides that do not currently have an ISO standard, the corresponding shape and installation connection dimensions are also adopted. This convenient measure can eliminate unnecessary competition between air drive and electric drive in terms of installation, addition or replacement. . FESTO's electric drive products include more than 300 freely combinable gripping modules and multi-axis systems. At Festo, electric drives are not a competitor to pneumatic drives, but a perfect complement to their performance. Electric drives are characterized by precision and flexibility. In applications where the force quickly disappears and precise positioning is required, electric drives are the ideal solution for non-blocking self-priming sewage pumps.
Therefore, the development of cylinders and electric actuators should be in a very healthy condition and complementary in the future, and they will certainly develop according to the scientific and natural development laws of these two technologies.