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Sweet spots of pneumatic and electric actuation

July 7, 2021
How to decide which actuators to use in your system design

Should you use pneumatic or electric actuators in your system design? Often, the most significant performance differentiator is precision. In this area, electric actuators have the advantage, but pneumatic actuators can also deliver very precise motion. So, one issue you need to consider is how much precision you really need. There are other factors that can’t be ignored: component costs, system costs, maintenance and repair expenses, and productivity. Taken together, we can get closer to an understanding of the true value of your entire system. 

System energy costs

Any poorly designed system is liable to increase your energy costs. It would permit air leaks in pneumatic circuits, which will cause your main component, the compressor, to run excessively. Compressors already use the most energy, idling when there is no load or a partial load. Idling wastes electric power consumption. Sometimes, oversized compressors are chosen because it is thought they will compensate for anything that isn’t working efficiently but this hurts more in the long run because such compressors use more energy and are more expensive. Simply put, properly sized compressors help to minimize costs.

By comparison, servo-driven electric actuators consume little energy at rest. If we have a system with nothing pushing against the actuator and it is just sitting still, the system is consuming very little energy. There is only a small current flowing to the drive and it simply monitors the position. It will not consume any energy unless it is either moving against the load or if the load starts to move against the actuator.

Furthermore, there is no potential energy sitting behind the actuator as is the case where you must have continuously compressed air or fluids. Servo motors do no more than deliver the required torque to perform the operation at hand.

Given the inefficiencies of compressed air and the potential for air cylinders to be oversized, a servo-driven control system appeals because it consumes approximately a quarter of the energy of a comparable pneumatic unit. For example, when the system starts to move, an air cylinder designed to push with 400 lb of force, based on the bore of the air cylinder and the pressure of the compressed air, is going to be pushing with that fixed amount of regulated pressure regardless of what it takes to move the load. Your energy requirements remain fairly fixed. Compare that to a servo system that consumes only the energy it needs to perform the operation. There is no wasted energy.

System component costs

A cost comparison of the components needed to operate pneumatic and electric actuators gives a distinct advantage to pneumatic. A small compressor for a pneumatic cylinder system costs about $200, assuming you are running just a couple of cylinders in a room and not for an entire plant. All accessories, such as fittings, tubings and switches, should cost approximately $30, an air preparation unit like a filter adds another $50, while regulators and valves cost another $20. One large actuator or a few small actuators will be about $120. Total cost per circuit in a pneumatic system should be about $420.

An electric actuator, on the other hand, which converts linear motion to rotary motion at the actuator, is quite a bit more expensive. The costs of the power supply ($300), drive ($500), cable ($100), motor ($300), actuator ($1,400) and switches ($100), bring component expenses to approximately $2,700.

Maintenance and repair costs

In a pneumatic system, air molecules are compressed, and so is anything floating in the ambient air. If water is being condensed, so too is the debris it carries. Eventually, these elements must be removed. The most important component therefore is the filter. If is should fail, contaminants will be introduced into the cylinder, eventually eroding seals and damaging the actuator from the inside.

With an electric actuator this problem is eliminated. They are designed and rated for thousands of kilometers of travel and require minimal upkeep. As long as regulated power is coming in, the motor will not need maintenance. The actuator itself, with its moving components, does need attention. A bearing system should be lubricated every few months or after a certain amount of travel. The power transmission, whether it’s a screw-driven actuator or a rack-and-pinion-driven actuator, or even if it employs pulleys on a belt-driven actuator, must be lubricated, but only a little, and infrequently.

However, there are environmental conditions that we have to account for to keep contamination out of our actuators. There are a number of ways to combat environmental hazards that can cause corrosion, such as by shielding purge ports from dust and debris or employing stainless steel.

Installation and troubleshooting with an electric system is going to require support from a skilled technician so it is imperative to focus on the initial cost of installation. The expertise and knowledge needed for an electric system will be significantly more complicated than with the fairly basic pneumatic system.

True value of your system

Pneumatic is not going away. We will always have a need for efficient, economical motion control. Pneumatic circuits are simpler, cost less and need less power to run.

On the other hand, the increasing significance of IoT and Industry 4.0 is driving us to gather much more data, whether it’s monitoring the current of a motor or watching a sensor or determining the rate of parts per minute. That’s very easy to do with an electric actuator. And, given the minimal energy consumption at standstill with electric systems, electric is environmentally advantageous.

With pneumatic, there is great force and speed in a smaller profile. For example, the power factor on a 2-inch bore is 3.1 times input pressure. To achieve that in an electric actuator requires a significantly larger profile, and that will affect cost, as well. You can easily adjust the force and speed of a pneumatic actuator. Force can be changed by changing input pressure or going to other options such as multi-power designs or by increasing bore size to deliver more force.

Overall, the pneumatic system will have lower cost per unit and a lower upfront cost. But, if you are considering long-term usage where you are going to be supplying energy to a compressor in a plant or a system, then you might want opt for an electric system where everything is environmentally friendly due to its lower energy consumption. In short, electric has a higher cost but there is greater precision, more feedback and greater control. You will always need pneumatic because there are some very simple applications that don’t need the bells and whistles of an electric actuator, but when you need to know more about what is going on or having more control, electric may be preferable.

About the authors

Ed Long is electric motion products manager, Americas Motion Control Group, at Norgren. Contact him at 724/840-1999 or [email protected].
Sarah Manuel is product marketing manager—actuators, Americas Motion Control Group, at Norgren. Contact her at 708/534-8544 or [email protected].

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