Control application: pneumatic, hydraulic, or servo system
as an important automation discipline, pneumatic and hydraulic technology has been a part of the prospects of machine manufacturers for a long time. In factories around the world, a large number of such machines work well
for many automation leaders, electronic servo motors and digital control seem increasingly outdated, but those solutions are not fully applied as some people think
by using modern position sensors and closed-loop control methods, servo pneumatic control realizes more functions than traditional push applications, while servo hydraulic control system can accurately and accurately locate heavy loads. They all have some disadvantages, but the advantages outweigh the disadvantages
hydraulic power solutions are very conducive to heavy lifting or applications requiring extreme pressure, such as powder metal stamping operations (see Figure 1). Usually, the powder metal in the mold is used for stamping dies. An upper cylinder pushes a punch into the die area and compresses the powder. "The pressure depends on the size of the machine, but it can be as high as 750 tons," said Scott Green, President of alpha Lehigh tools and machines. The dense powder particles are made into a mold shape and taken out of the mold. Then bake in an oven similar to a pottery kiln. The combination of particles/molecules of metal particles is a subtle "welding" process called hot pressing
"we will re trim the die to meet modern standards, and use the closed-loop control system to operate the proportional valve. This valve is used for low precision doors to control a large amount of oil to reach the upper and lower impact cylinders," Green said, "The magnetic sensors on the upper and lower impact cylinders feed back the position to the computer. The computer system makes the movement of the upper and lower impact cylinders more flexible, and also obtains greater flexibility and synchronization due to the feedback system."
Figure 1: through the closed-loop control of the proportional valve, alpha Lehigh
tool and machine company has reconditioned the powder metal stamping under the action of hydraulic powerSteve Nylund, President of delta computer company, said that through servo hydraulic control, the system can reduce the accuracy of the sensor in the micron range, and can repeat and accurately do so. "You can apply huge power, and it is safer than using pneumatic method," he said. "Hydraulic actuators are smaller than their electric servo counterparts. Hydraulic actuators are very useful when machine builders have limited space but must accurately apply huge power."
here are some sensors that are very suitable for hydraulic applications. For example, the magnetostrictive sensor is inserted into a hydraulic cylinder to form a good execution feedback system. However, industry professionals believe that special control of hydraulic system should be considered. Hydraulic cylinders are usually built as single ended equipment. The forces at the side and reverse ends of the connecting rod are different, so the gain of the controller must work in proportion to the hydraulic system
servo pneumatic: challenge control
in some heavy applications, hydraulic mode is usually considered a good solution. But some machine manufacturers are getting rid of this method. Bill schap of schap specialty machine said that servo pneumatic is more suitable for the base test equipment built by his company for the automotive industry and large device manufacturers (see Figure 2). "We have used the servo loop for more than 15 years, and we are working together to get rid of the hydraulic mode," schap said. "It is noisy, leaky and expensive. For those large and fixed loads, it works well. But for most simple low-end hydraulic applications, it is easy to be replaced by pneumatic and servo pneumatic systems."
schap said that since the appearance of Proportional Pneumatic Valves ten years ago, he worked at Enfield Technology (). When it was first used, it tried to maintain the position of the pneumatic cylinder through position feedback. "In our application, we use a load unit to close the loop instead of using position sensors. In the past 10 years, we have made more than 100 such systems. Many of those systems use up to 16 cylinders - all closed-loop, all running independently on a single PC, and the effect is good."
Enfield's LS series servo pneumatic control system can operate in open-loop or closed-loop state. It includes a special device control and a five port, three position middle closed positioning valve. Ed Howe, President of the company, pointed out that this centralized control line can provide real-time command and feedback signal monitoring through a response time of less than 100 microseconds. When the system is designed as a proportional pneumatic control, the cycle time of full opening and closing of a port takes only 5 milliseconds - it can also be used for speed applications
like the hydraulic cylinder, the pneumatic cylinder can work with various position sensors to provide feedback to the controller. Generally, potentiometer is a low-cost instrument, and its cost is one third of that of other sensors. Because they want to have entities with the actuator, if the machine is in contact with the standby state, the life cycle is shorter than that of the non-contact device. Magnetostrictive sensors provide absolute position information and non-contact services. According to the length and application, the price ranges from $300 to more than $1000. Inductive sensors cost less than magnetostrictive sensors and do not have to be connected to the moving part, but they are not as accurate as magnetostrictive devices
according to Nylund, the challenge comes from the controller itself. Today, most pneumatic systems still use non intelligent controllers. "We found an opportunity for intelligent pneumatic controllers, but they must deal with the nonlinear aspect of the compressibility of media changes," he said: "The controller will need to have various sensors input to the monitor, and not only control displacement, but also control force and pressure.
does pneumatic technology have its end? For slamming applications, some will use a servo motor and ball screw, rather than a pneumatic cylinder. Schap said that some of his customers have established fatigue test devices - using a combination of servo motor and ball screw to repeat the post Move backward and forward. They are optimistic about the life expectancy of this system. So, what about linear motors? "We have observed them," schap said. "They are expensive and cannot generate much power. What is worth advocating is that they are indeed very fast and do not have the replacement of moving parts. But they cannot generate enough power in our applications."
linear motors seem to be the ideal choice for location applications. They are used in reeling machines, semiconductor chip conveying systems, PC board picking/placing machines and liquid medicine processing systems for automated experimental testing applications. When speed and accuracy are important for applications, linear motors can work in the form of open-loop stepping motors and closed-loop motors
in the application of schap, is it cost-effective to follow the servo route? When adopting the servo pneumatic scheme, we need to use a displacement sensor, a servo valve and a load unit - each part is worth thousands of dollars. When the servo motor scheme is adopted, a servo motor and an electric cylinder shall be used. Schap believes that the cost difference between the two technologies is close, but it is recommended that the discarded pneumatic cylinder be used for packaging machine, which can maintain low-cost operation for two to three years. Generally, in high impact applications, ball screws will not last longer than cheap pneumatic methods, and usually the replacement price is more expensive
Figure 2: schap specialty machine has used the servo loop for more than 15 years. They are trying to abandon the hydraulic mode and replace it with pneumatic and servo pneumatic systems
digitalization makes control easier
whether your application is hydraulic, pneumatic or electromechanical, for machine manufacturers and their customers, the real difference today is due to digital control. Maybe it's because it's easy to program, maybe it's because of its flexible electronic control. Steve Bergholt, chief electronic engineer of triangle packaging machinery company, said, "the flexible system allows us to reuse design and simplify engineering tasks. We can predict and pre build, allocate our resources, and put the machine on the market in three months instead of six months - saving thousands of dollars in engineering costs."Erik Nieves claims that digital systems provide faster upgrade times, are more robust and have no temperature sensing problems. He is a senior manager in charge of technology at Motoman, an Ohio robot manufacturer
machine makers benefit not only from digital control. "Customers want to transfer files over Ethernet," Nieves added. "They also want to be able to communicate i/o over digital networks."Green of alpha said that customers also like the function of storing and retrieving quality control records. "We can achieve the final positioning and pressure of each part," Green said. "Internet connection is another favorable factor, because troubleshooting can be solved without expensive services." It is also easy to train on a system like this. Customers like the ability to connect with the enterprise system to monitor the production process
software eliminates the complexity of customers in completing a work task, rather than playing with some knobs, switches and dials. "No matter what weird load task you want: 20 lbs and keep it for 10 minutes, 30 lbs and keep it for 10 minutes, then 400 lbs and keep it for 10 minutes - we can do that," said brain sayranian, Department Manager of schap, "This is the software. Sun Dongquan, the Secretary General of the Preparatory Committee of the battery diaphragm special committee of China Plastics Association and the Secretary General of the Preparatory Committee of the battery diaphragm special committee of China Plastics Association, believes that =100; other scales can be compensated in the software."
get ready and make a decision
please carefully consider the practical application before you decide to buy. Don't ignore any special options until you have completely weighed the costs and benefits. Although sayranian doesn't talk about hydraulic methods very well, he will tell you that his company's safety belt is dragging the test system, "where you need to continue to pull the load point of the safety belt until the safety belt is damaged - there is no other option to transmit impact and force. If you use pneumatic methods, you need a cylinder with a diameter of 8.5 inches and a length of 25 inches." (end)
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