Updated May 2018 || Pneumatic cylinders and actuators are commonly known as “bang-bang” devices, making quick moves from one end of their stroke to the other, with limited regulation of the force or the move profile.
On the other hand, electromechanical actuators with servo controls offer high levels of refinement in positioning, force-torque, and accuracy. Generally speaking, pneumatics are a cost-effective solution for rather crude point-to-point moves, while electromechanical actuators provide high precision at a higher cost.
There’s a spot between these two solutions where a relatively high level of control is needed, but without the complexity and cost of electromechanical servo driven systems. Bridging this gap are pneumatics that operate in a closed-loop system — in other words, servo pneumatics.
Servo pneumatics versus traditional pneumatics
A servo system is one that uses a feedback device and a controller to monitor and correct the system’s error (in position, speed, or torque-force). Hence, integrating a pneumatic cylinder or actuator with a feedback system and a controller that can issue commands based on that feedback, gives us a servo pneumatic device. Another key component of a servo pneumatic system is a proportional valve, which precisely regulates air delivery to ensure that the commanded position or force is achieved.
Traditional pneumatics enable rapid, high-force, point-to-point motion. Servo pneumatics provide the same speed and force capabilities, with the advantage of higher accuracy positioning, not only at the ends of the stroke, but also at intermediate points along the travel. In addition to obtaining feedback on position, servo pneumatics also monitor and regulate air pressure, which enables precise control of the force that’s produced.
One drawback (real or perceived) to traditional pneumatics is air consumption. Air preparation and delivery costs money, and pneumatics can use a significant amount of air even when they’re not working. Servo pneumatics, on the other hand, control air flow based on the required position and force. This leads to less air consumption than standard pneumatics, by as much as 30 percent. It’s important to note that servo pneumatics require higher quality air than standard pneumatics. In addition to industry-standard filtration, a 5 micron filter is typically recommended for servo pneumatic systems.
Why servo pneumatics instead of electromechanical actuators?
Of course, electromechanical actuators can provide accurate position and force control, but servo pneumatics have a much higher power density—that is, force capability for a given size. A servo pneumatic cylinder or actuator typically provides many times the force capability of an electromechanical actuator of a similar body size, which is a significant advantage in pressing, inserting, and tightening applications. Servo pneumatics also operate with 24 Vdc power supplies, which allows them to be used in low-power applications. Lower power also reduces heat generation and thermal build-up, so they perform well in continuous-duty applications and high-temperature environments.
While electromechanical servo systems have been in use for decades, the adoption of servo pneumatics in industrial applications hinged on advancements in controls and software. Air is compressible, and this variable is much more difficult to define and model than the “compliance,” or backlash, in an electromechanical system.
Before servo pneumatics could be commercialized, pneumatic control algorithms capable of taking into account this non-linearity had to be created. But the past ten years or so have seen the development and integration of high-response valves and digital signal processors (DSPs) that can preform high-speed computations, making servo pneumatic systems a reality, capable of providing accurate, highly responsive positioning and force control.
For a demonstration of a servo pneumatic actuator with both velocity and position control, check out this video from Enfield Technologies.
Feature image credit: Festo