Several factors significantly influence the decision to use pneumatic or electric control in a linear motion control application. The obvious considerations include the technology preference in use at the location, the precision of the motion required and cost containment. There are several other important factors that need to be considered in order to select the best performing solution for an application. First, letís discuss the pros and cons of both technologies.
Pneumatic systems offer simple motion with high force and high speed capacities. They are a good choice for basic end-to-end continuous motion applications. They are also easy to understand and maintain. Through the use of valve controls, air control can be achieved for end-to-end motion. Since air is compressible, it can be very difficult to achieve midstroke positioning and controlled velocity.† In some applications, air lines may not be an option or be readily available.
Simple single-axis operations such as pushing parts off a conveyor, moving parts from one point to another, straight cutting of materials (flying knife) or opening and closing devices are good choices for pneumatic control. All require fast end-to-end motion with little or no variation in speed control.
Pneumatic actuators have very low initial costs compared to electric, typically around 1/3 the cost.†However, the overhead costs of compressors, air line maintenance and the cost of air itself need to be accounted for. The costs of all these factors over time can end up more than installing electrical systems.
Electric systems provide true motion control: multi-positioning, synchronized motion and speed and force control. From a performance standpoint, electric systems offer much higher levels of accuracy and repeatability. Control flexibility can be achieved using either brushed DC, micro stepping or, for more sophisticated control, servo. The ability to create flexible motion profiles for position, velocity and thrust force allows electric systems to perform in many applications that are not possible with pneumatic systems. Feedback also ensures a precise motion is achieved. Electric control offers clean, smooth operation and, depending on the motor choice, quiet operation. Along with the increase in performance, there is also an increase in cost.†Implementation of electric control can also require a higher level of expertise to operate, maintain and troubleshoot.
When an application requires multi-axis coordination (for example, interfacing RFID or bar codes to coordinate the flow of products within a distribution center), electric systems provide a better solution, as they offer programming flexibility, high repeatability, precise motion and feedback control.†
Itís All About the Application
In all cases, the application will ultimately determine whether pneumatic or electric control should be used and any reputable supplier of linear actuators can help determine which technology and what actuator style makes the most sense. As engineers become more familiar with electric controls and programming software gets easier to use, many are migrating toward electric solutions when it makes sense.
Does the application require quick changeovers in the manufacturing process?
Electric systems allow users to implement multiple program profiles, allowing machines to automatically adjust position without stoppage. With an air cylinder, a technician would typically have to change out the cylinder or mechanically adjust the end stop positions. †
Are there static or dynamic loads being placed on the actuator?†
Most actuator manufacturers' catalog literature provides specifications for static moment loads placed on the actuator. Once the actuator is set in motion, however, static loads become dynamic, subjecting the actuator to a whole different set of forces when accelerating or decelerating. Both pneumatic and electric actuators are subjected to the same set of dynamic force characteristics, but how they affect the actuator and how the load is stopped once it is in motion is dramatically different. With electric systems, the load is controlled during accel and decel. Deceleration parameters can be set and the motor itself becomes a generator to help stop the load and absorb the energy. With pneumatics, decelerating high kinetic energy (large mass or high speed) loads may require shock absorbers placed at the center of gravity of the load. Servo pneumatic valves can also be used to decelerate loads.
A commonly held practice in pneumatics sizing is to apply a safety factor of two-to-one. For example, if an application has a 50-lb load requirement, an actuator capable of 100 lbs is sized to be sure the application will achieve the necessary speed requirements due to air leaks, pressure spikes, etc. inherent in the air supply system. If that same principle is applied to electric systems, the result can easily be an oversized motor and drive resulting in unnecessary added costs. Once a big motor or gear reducer is in the system to increase torque output, there is also a size penalty that needs to be considered.
So, what is the best choice ó pneumatic or electric linear actuators? The choice is easy when the application requirements, dynamic loading, costs, efficiencies and sizing requirements have been carefully evaluated.