With traditional pneumatic motion, the linear or rotary actuator appears in one of two states — fully extended or fully retracted. Variations in force and velocity are extremely limited. In the early 1990s, the first servopneumatic motion systems were developed and the technology has progressed ever since. Today’s servopneumatics use either external or integrated feedback to allow control of position, velocity, and force from a variety of linear or rotary actuators.
Choosing between electromechanical motors and pneumatics
When there is a choice between servomotor or servopneumatic, how does one select the most appropriate solution? Start with force requirements: Servopneumatic systems deliver higher forces than electromechanical systems of comparable physical dimensions. In fact, servopneumatic systems deliver higher force density than all other positioning technologies besides hydraulics. For applications involving space constraints, but requiring high feed forces, servopneumatic solutions are quite suitable. Likewise, when handling loads greater than 110 lb, servopneumatics are cost effective.
Servopneumatic technology is also well suited to positioning applications requiring repetition accuracies in the range of 200 µm. Further, in installations where low operating voltages are mandated, servopneumatics offer an alternative solution to high-voltage ac-driven servomotors, as servopneumatic systems operate from standard 24-Vdc power supplies.
What’s more, servopneumatic systems are not prone to the same kind of thermal buildup as electromechanical systems and are therefore well suited for continuous operation in high-temperature environments. For example, in volumetric filling applications of viscous substances where filling systems can become clogged, air-actuated servos exhibit none of the thermal buildup and shutdowns associated with electromechanical systems.
Commissioning of servopneumatic systems is not harder than what is required for electromechanical systems, but it is different. Built-in features such as “auto identification” facilitate error-free hardware configuration, while a commissioning procedure known as “identification” determines the system’s dynamics. For superior positioning performance, servopneumatics require air of higher quality than that used for traditional pneumatics. For example, a 5-µm filter may be required in addition to the industry-standard 40-µm filter.
Combining servopneumatics with other electromechanical technologies provides machine builders and OEMs with a spectrum of solutions to meet a variety of motion control needs. For example, servopneumatic and electromechanical actuators can be combined to form an integrated hybrid solution — allowing designers to provide end-users with optimized systems.
Types of servopneumatic actuators
Servopneumatic actuators fall into three primary categories:
• Piston rod cylinder
• Rodless linear actuator
• Semi-rotary drive
The new generation of servopneumatic systems can easily be integrated with PLCs from many suppliers. Further, widespread availability of a broad spectrum of fieldbus modules, including EtherNet/IP, DeviceNet, Profibus, and CANopen, facilitates the design of distributed architectures. Each networked station can accommodate multiple servopneumatic axes for multi-axis applications; each axis is comprised of an actuator with integrated feedback, digitally controlled proportional valve, and control module. Comprehensive diagnostics enable improved operation and maximum uptime. Servopneumatic control modules can be used in conjunction with digital and analog I/O modules as well as PLC or fieldbus modules to form either standalone or distributed solutions.
Servopneumatics are suitable in myriad industrial settings. Consider the actual applications described here.
In one system, wooden planks of various thicknesses are cut into slats of different widths using high-speed saws. Piston-type actuators are used.
During the sawing operation, three rollers press down on the plank; the pressure they apply can be adjusted to suit the wood type. In addition, the inherently low stiffness of servopneumatic actuators makes them more suitable than ballscrew-based electromechanical actuators.
Once a plank is through the machine, the rollers return to their initial position without contacting the conveyor. Data for each plank fed into the machine is transmitted to the system via the fieldbus. The servo controller switches between position and force control on an event-controlled basis — an advanced system enabling fast and flexible movement with sophisticated safety functions and diagnostics.
Volumetric filling application
Consider an installation where bags must be filled with 1.7 to 5 oz of fluid in just a few seconds. The outlet port of the dosing valve must be flexibly adjustable between 0% and 100%. Precision of 0.0197 in. is sufficient for this task. Another important requirement: The liquid is sensitive to heat and requires a continuously operated positioning axis with minimal thermal generation. Suitable here is a servopneumatic piston actuator used as an advanced control module.
The positioning stroke of each such cylinder is set via fieldbus, based on volumes to be filled. Encapsulation on all of the cylinders protects them against cleaning agents. Both the valve terminals and proportional valves are mounted in machine areas shielded from washdown. Due to the need for flexible dosing of a wide range of media and high cycle rates, each filling cylinder is controlled individually.
Metal sheets with temperatures higher than 212° F must be reliably stopped and centered at the discharge of an annealing furnace before being transferred to a removal robot. The metal sheets typically measure 1.64 to 3.28 ft x 1.64 to 3.28 ft² and weigh between 11 and 44 lb. Speed reaches 6.56 ft/sec and moving mass is approximately 15.43 lb. The complex contour of a sheet requires up to 32 centering fingers, while a rodless linear actuator acts as a centering drive. The control module provides end positioning. Employing a servopneumatic system facilitates continuous operation in a relatively high-temperature environment.
Labels must be glued to packages of different sizes and heights in one-second cycles, as packages travel past the labeling unit on a conveyor. The work stroke of the label picker is 1.97 to 19.68 in. Extension and retraction of the axis must take place within one second.
The distance to the packet is determined using an analog distance sensor, and the label ejection position is located 0.59 in. above the package.
Either a piston rod-style actuator or a rodless linear actuator may be used. This system can operate as a standalone unit or may be networked through fieldbus — for compact, fast, and flexible motion.
For more information, contact Festo at (800) 993-3786 or visit festo.com.
When to consider servopneumatics
Closed-loop servo control of pneumatically driven piston, rodless, and semi-rotary actuators expands the toolkit of motion system design engineers. It pays to explore servopneumatic options for applications requiring one or more of the following attributes:
• High force density and speed
• Repetition accuracy in the range of +0.008 in.
• Flexibility for standalone or distributed architectures
• Low-voltage operation
• Cost-effective solutions for handling large loads
• Continuous operation with minimal heat generation
• Compact and lightweight