As advancements in technology continue to define how pneumatic or electromechanical linear devices lift, move, or place a load, the requirements of excellent closed-loop positioning and accuracy greatly increase. Between these two automation technologies, however, there are variables that affect manufacturing cost, safety, and productivity in different ways. Moreover, recent developments in applying multiple closed-loop control and position detection to electropneumatic positioning (E/P) systems add a new level of precision and safety by using dynamic force control to maintain the position of a load, overcoming traditional E/P and electro-pneumatic system limitations.
With any positioning system, the accuracy achieved during indexing through positions can be extremely critical. This includes any point along the motion path and especially with positions where a change in load is dynamic.
How Electromechanical and Pneumatic Systems Differ
Electromechanical (EM) positioning systems use electronic servo controllers, variable speed drives, and toothed belt, gear drive, or screw assemblies to accurately move and position a load with electrical motors.
The main advantage of an EM positioning system is the ability to accommodate high-speed production assemblies and manufacturing processes with accuracy, while positioning dynamic or non-static loads. There are disadvantages:
Thermal: EM positioning systems can quickly undergo excessive temperatures related to changes in process control, resulting in significant downtime required to cool the equipment and prevent long-term damage or mechanical failure.
Electrical: If power or communication is lost, an EM system can cause unpredictable movements and create potential safety hazards as mechanical drives are started or automation processes reset. Productivity and the quality of manufactured products may suffer as a result.
Financial: Cost can be an issue. Over the long term, the need to replace failure-prone EM components can lead to higher maintenance and production costs. When an EM system has issues, it typically requires an expensive electric drive or mechanical linkage. In fact, many low-cost EM systems only deliver moderate efficiency and struggle to reach acceptable duty cycles, ultimately leaving companies to pick up the costs of wasted energy and downtime.
Traditional Electropneumatic Systems
When E/P systems are required to move an actuator and load to a specified location, the system utilizes electrically controlled pneumatic valves to regulate the airflow or pressure of the actuator and achieve the desired position.
Typical components of an E/P system include air cylinders, pneumatic valves or electropneumatic pressure regulators, and related position sensors. The pneumatic cylinders typically include both piston-rod and rodless versions. Many systems use basic flow control valves and open-loop operation for positioning, which require large amounts of airflow. Other systems use direct-acting proportional pressure control combined with analog signals and closed-loop feedback circuits to enhance positioning accuracy and overall system efficiency.
Compared to EM systems, traditional E/P systems have the advantage of providing higher productivity during process changeover, an increase in reliability of system components such as a pneumatic actuator, and more maintenance options when repairs are required. They are often more energy-efficient than electro pneumatic systems because E/P systems consume very little wattage to hold a position. Overall, E/P systems provide lower cost of ownership than EM systems. There are more advantages:
Reliability: Typically, E/P systems are more reliable, resulting in less maintenance; with pneumatic valves and air cylinders, system components are unlikely to overheat, causing premature wear. Pneumatic devices tend to stay cool as they utilize air to perform work. E/P systems deliver a high duty cycle as they provide continuous operation and can even withstand harsh environments that include temperature extremes and dusty or dirty conditions.
Electrical: If power or communication is lost, E/P systems can use standard pneumatic valves to block airflow and hold the last position reached, allowing for a smoother and easier transition when restarting the process once power has been restored. This is a key advantage for technicians and facility maintenance, as manufacturing downtime costs are greatly reduced.
Financial: When pneumatic cylinders break or wear out, they can be repaired or replaced at a low cost and are typically easy to install. These advantages, combined with the inherent low maintenance of pneumatic devices, significantly lower the cost of ownership compared to electromechanical systems. In fact, the cabling alone on an electromechanical device can cost more than an entire pneumatic system.
There is one disadvantage: E/P systems cannot be perfectly sealed and an incremental or intermittent loss of air or pressure is unavoidable. Minor leaks can cause the pneumatic cylinder to move slightly. A reduction in airflow can cause the cylinder to begin to oscillate in what is known as a “dithering” state, which negatively affects system accuracy and performance. On the other hand, E/P systems that use direct-acting proportional pressure control can reduce the effects of dithering. And with today’s sophisticated pneumatic monitoring and IoT capabilities, new pre-emptive maintenance techniques can address other issues that may lower the performance of the pneumatic positioning system.
Advanced Electropneumatic Positioning with Dynamic Force Control
One of the shortcomings of traditional E/P systems is the inability to control both position and force; for example, maintaining weld tip force in automotive body welding applications at an accurate position is very difficult to achieve. But by utilizing multiple closed-loop feedback circuits within the system, advanced electropneumatic positioning systems can apply the additional pressure required at a target position and deliver the force needed to achieve an accurate weld.
The solution is to combine E/P regulators with advanced electronic feedback signals from a high-resolution distance measurement sensor. This technology uses dynamic direct-acting proportional control valves combined with precision feedback for positioning. This combination enables higher performance and repeatability, while securing a position without the use of a mechanical cylinder rod lock and ensuring the load does not drop, move, or lunge forward.
Direct-acting dynamic E/P valves allow for high positioning accuracy and precision through the process of dynamic pressure regulation. They can achieve a resolution and accuracy of within one percent of stroke length during pneumatic positioning. Dynamic force control is achieved by using multiple closed-loop control systems with the advanced electronic pressure regulators, instead of using flow-control valves and basic open-loop command signals.
Additional closed-loop feedback signals enhance position monitoring and stability of the system, while advanced electronic pressure regulation is achieved on either side of the piston. Multiple closed loops also provide additional programming parameters for dynamic force control once the load and cylinder have reached the desired position.
Some E/P systems only offer one closed-loop parameter group, allowing for simple positioning, but lack the technology to deliver additional pressure to the piston, resulting in force control. Unlike traditional E/P systems, advanced E/P systems with multiple closed-loop control can switch from dynamic positioning to force control via a quick parameter setting. This design complements the ability of precise pressure regulation to eliminate the effects of dithering. With a properly selected actuator and optimized tuning of the machine controller, it is possible to attain an accuracy of one percent of stroke length during pneumatic positioning.
Applications for Advanced E/P Systems
Machine designs using multiple closed-loop control systems are designed to provide an economical solution for a broad range of automation and industrial applications. Examples include automotive body welding (men tioned above), electropneumatic material handling systems (such as pick and place), hopper/damper/gate control for bulk materials, and other industry applications requiring precision indexing. Basically, any application requiring force control in addition to accurate positioning is suitable for considering an E/P system.
Traditional E/P systems have provided acceptable positioning for several years. Now, an advanced E/P system using multiple closed loops allows dynamic force control and can compete with servo motor technology. Users will find that many applications do not require “ultra-precision” servo motors and can benefit from a more cost-effective electropneumatic solution with other inherent advantages.
Due to safety and structural factors, system designers should consult with the manufacturer to account for the load, bore size, and stroke length needed for a given application. Emerson, for example, has multiple closed-loop positioning systems available and compatible with a wide range of current valve systems. The modular design easily integrates into valve manifold assemblies to accurately regulate the position of loads with dynamic force control, while communicating through an extended variety of protocol types.
Advanced E/P systems are integrated with multiple PID capabilities and greatly eliminate the dithering associated with flow-based pneumatic positioners.
In summary, adding multiple closed-loop feedback control to pneumatic positioning systems can quickly enable dynamic force control for advanced applications, while delivering increased reliability and safety in securing load during pneumatic positioning.
This article was written by Jon Revlett, Application Specialist, Electro-Pneumatic Systems and Controls, at Emerson Automation Solutions, St. Louis, MO. For more information, visit here .