In linear motion applications, pneumatics, servo pneumatics, and controlled pneumatics each deliver unique motion and force capabilities.

Contributed by By Frank Langro, Director of Product Market Management, Pneumatic Automation, Festo

Pneumatic linear actuators are fundamental for automation because of their low cost, ease of installation and operation, high yet compliant force, speed, compactness, safety in hazardous areas, and long service life in harsh environments. Advancing technology in valves, valve terminals, and remote and decentralized I/O is opening new application solutions for this tried-and-true technology.

This article discusses pneumatics, servo pneumatics, and controlled pneumatics to answer the questions of where, when, and why to apply each form of this automated linear motion technology. First, a review of the basics.

Pneumatic actuators for linear motion
The key component in all linear motion pneumatic actuators is the internal piston. By applying compressed air supply into a cylinder’s two inlet ports the piston driving a piston rod can be made to move forward and backward between two points – fully extended and fully retracted. To regulate the speed of the piston, design engineers specify a flow control valve. Opening the flow controller’s orifice allows more flow and the piston moves faster, while closing it reduces flow and slows travel between two points.

To ascertain the location of the piston at either start or end point, proximity sensors are attached to the body of the cylinder. The sensors provide closed loop feedback that the extreme positions have been reached. Measuring the time between positions allows the controller to ascertain speed. To lessen the shock of impact as the piston reaches its start and end points and to reduce vibration and wear, actuators include cushions, throttling screws, or self-adjusting cushioning systems.

Compressed air-based motion is found in most production equipment. Often pneumatic actuators are used for picking and placing in packaging applications. It is also common to use pneumatics for clamping, stamping, and positioning. Pneumatics also suit applications in hazardous areas, food and beverage, and robotics, due to limited electrical signals, which are well suited to being in washdown areas. Additionally, pneumatics technologies are light weight — a benefit in end of arm tooling.

Servo pneumatics enables multiple positioning and force control
While traditional pneumatic technology is simple with just two positions, servo pneumatics delivers infinite positioning. PLCs, proportional control valves, and actuators with linear displacement encoders attached along their lengths can position the piston anywhere along its route of travel. (The linear displacement encoder gives closed loop feedback of the actual position of that piston.) With this set up, the PLC can select a defined position and the proportional valve will change the pressure and the flow rate to ports 2 and 4 to hunt for that position. Pressure will be equalized at that point, holding the piston in place. Defined acceleration, motion profile, and deceleration are achieved through proportional control.

Along with position control, servo pneumatics also provides force control. By increasing pressure proportionally, the force on the piston is increased, and by reducing pressure, the force is decreased. In gripping applications, force control allows grasping of soft delicate objects or heavy rugged workpieces within the same set up.

Servo pneumatics’ ability to control position and force can be applied in any situation where holding an intermediate position improves cycle time. Visualize a labeling application where multiple sized boxes are coming down a conveyor. Utilizing a two-position pneumatic cylinder to apply the label would require the cylinder to completely extend and retract for every label applied. A servo pneumatic actuator could be positioned at an intermediate distance, speeding up the cycle.

In a hypothetical palletizing scenario, various sizes and weights of shipping cases are traveling down a conveyor. The cases need to be centralized before they are gripped and lifted off the conveyor. A servo pneumatic actuator would be ideal as a centralizer because it can be set at an intermediate position, boosting cycle rate. Furthermore, it offers force control to accommodate the various masses of shipping cases. In pick-and-place applications or positioning, servo pneumatics can be soft stopped with air pressure introduced against the oncoming piston. Soft stop is a key component in pick-and-place applications to control the momentum of the load.

In sawmills, operations staff benefit by recording the contour of the log during milling. An actuator pressing a guide along the log as it is sawed will extend and retract as the log passes underneath. The control system can record the position of the piston via the linear encoder. This provides an accurate record of each log’s circumference. In summary, servo pneumatics provides positioning and force control for linear motion, but not the type of interpolated motion found in servo motors.

Controlled pneumatics
Directional valves control the flow of compressed air to an actuator — either on or off for two position pneumatics or proportionally in servo pneumatics. Controlled pneumatics replaces traditional directional solenoid operated valves with proportional based valves.

One advancement in proportional valve technology has been the development of fast, quiet, accurate, and energy efficient piezoelectric-based valves.

A piezo ceramic element has capacitive properties: meaning it needs voltage to change its shape but does not require a continuous voltage to hold its position, very different from a solenoid that requires continuous voltage to remain activated. This means that a piezo valve generates virtually no heat, requires little energy input, has low wear, and operates silently.

Piezoelectric valves arranged to pilot a higher flowing 2/2-way valve are able to magnify the compact piezo valve’s functionality. For example, four 2/2-way valves in a bridge circuit, when controlled by eight proportionally controlled 2/2-way valves, can act as a standard directional control valve, a flow control valve, a pressure regulator, even so much as to replicate the functionality of a shock absorber on the motion of a cylinder. This is due to the software that controls the response of the proportional piezo pilot valves. The pilot valves can perform exceptionally fast and accurate flow control thereby managing the response of the main valves in the bridge circuit to achieve the desired pneumatic output to the cylinder or system.

A controlled pneumatic valve for linear motion consists of four 2/2-way diaphragm poppet valves that are connected in a bridge. Each diaphragm poppet valve is proportionally piloted and controlled by a pair of piezo valves. The system features embedded temperature, pressure, and position sensors.

Controlled pneumatics offers many capabilities in linear motion. Imagine, for example, an application that calls for one second travel time between position A and position B. If there is a leak in the system, the controlled pneumatics valve terminal can detect the loss and the controller can increase the flow rate to meet the one second travel time and preserve the output of the machine. With pressure detection, the system can trace leakage down to an individual cylinder for better understanding of the condition of the machine and its output. Controlled pneumatics can reduce energy consumption by applying high force to start the piston moving and then reducing pressure once it is in motion.

Controlled pneumatics are ideal for ensuring uniform web tension with high dynamics and precision. The extremely sensitive pressure control compensates for fluctuations and takes kinetic energies into account. Controlled pneumatics is applied for dynamic and flexible surface pressure over the entire machining process when grinding, brushing, or polishing. The solution can eliminate the need for gripper changes, as pre-sets ensure fast commissioning and pre-positioning.

Controlled pneumatics capabilities:

• Flow control
• Selectable pressure level
• Positioning over the entire stroke of the cylinder
• Preset travel time
• Operating actuators with minimum pressure
• Leakage diagnostics
• Soft stop
• Model-based proportional pressure regulation
• Proportional-pressure regulation

Summary
Pneumatic actuators are compact, rugged, easy to apply, and suitable for the most harsh or hazardous environments. Pneumatics is ideal for end position travel. Servo pneumatics provides gentle movements, positioning, and fast switching between position and force control. Controlled pneumatics offer high flexibility, multiple functionalities, controlled movements with more than one axis, force and stroke control, condition and process monitoring, and energy reduction. The real excitement comes when machine and process designers understand how much flexibility these three offer when fluid power is absolutely the best option for automated linear motion.

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