Posted May 13, 2019, 2:13:40 PM

Accuracy in linear motion system manufacturingIn today’s manufacturing and production world, there are two key factors that lead to your plant’s success: production and efficiency. The linear motion systems you implement will most likely be aimed to increase both of these. The accuracy and repeatability of the actuator you use are two of the most important measures. A previous blog post, Understanding the Factors that Determine Accuracy and Repeatability in Electromechanical Actuators, outlines how the precision of the manufacturing process used to create the cylinder components affect the accuracy and repeatability of electromechanical actuators (EMAs). In this post, we’ll explore why there is more variability, which means less predictability, in the pneumatic and hydraulic counterparts. 

Similar to electric actuators, the manufacturing processes for the cylinder components affect the accuracy and repeatability of hydraulic and pneumatic actuators; for instance, the fit between the cylinder housing and the piston. Further, the cylinder must be able to deliver a specified amount of force consistently and hold the pressure being fed into the system by air or hydraulic fluid. If the fit between the cylinder and piston is loose, leakage will occur, which will result in pressure variations that can cause inaccuracy in movements, poor control of motion, and loss of both repeatability and accuracy.

Air and Fluid Compression

A variable with a significant effect on accuracy and repeatability is the compression of air and hydraulic fluid, and the inability to compensate for this variation consistently. This inconsistency is mainly a result of variations in air temperatures and humidity, as these two variables affect air density. Pneumatic actuators rely upon compressed air to develop the necessary pressure to apply a force to move a piston in a cylinder. This compression of air, which is inherent in their design, is what prevents pneumatic cylinders from maintaining high levels of accuracy. Compressed air can act as a spring in a cylinder, and loads being positioned by pneumatic cylinders are inherently incapable of being held in a static position; most importantly not with a high degree of accuracy.

The oil used in hydraulic cylinders is far less compressible than air, but hydraulic cylinders can still have accuracy issues similar to pneumatic cylinders. Fluctuations in oil temperature will change the fluid’s viscosity, leading to variations in system pressure. Leakage can also be problematic as the seals in hydraulic cylinders wear, making it difficult to maintain consistent pressure. Without consistency of pressure in a fluid driven system, accuracy and repeatability are unattainable, and predictability of control greatly suffers.

Ancillary Component Effects

The ancillary components required for a fully operational pneumatic or hydraulic system also result in limitations to accuracy and repeatability. Parts such as valves, solenoids, and sensors must be considered when measuring the accuracy of the system. For instance, the springs in the valves and solenoids will compress and extend at different rates throughout their lifespan due to material fatigue. There are various methods for improving the accuracy and repeatability of pneumatics and hydraulics, but none of these methods can mitigate or account for positional variability in the manner that EMAs can. Each component has a control tolerance that can affect the accuracy of the system, but the stack-up has a cumulative effect that can lower the overall accuracy, and must be considered when designing the overall system.

Environmental Factors

When it comes to accuracy and repeatability in linear motion systems, electric actuators are without a doubt superior to pneumatic and hydraulic cylinders. Because the mechanical relationships between EMA system components are definable and easily calculable, they are much more predictable than fluid-driven systems. The predictability of hydraulic and pneumatic solutions is dependent on a wide range of variables: air temperature, humidity, and the total size and scope of the system among them. In an industrial environment, environmental factors can be especially difficult to control. Conversely, with EMAs, system accuracy is determined only by a mechanical drive mechanism and is much less susceptible to these environmental factors.

Predictability and Repeatability

Repeatability in screw-driven systems is dependent upon the manufacturing process, design tolerances, and fitment of the screw and nut as a combined drive element. As backlash values decrease, repeatability improves. The ability of these drive elements to be manufactured within known and predictable tolerance levels greatly mitigates variability, resulting in increased predictability. It is this predictability that gives EMAs a clear advantage.

Engineers constantly have to pursue solutions to mitigate the problems created by the inherent variability of hydraulic and pneumatic system designs. This shortcoming cannot be eliminated in these types of systems. This fact, along with the additional limitations of the necessary control devices, makes it clear that pneumatics and hydraulics cannot compete with the accuracy and repeatability provided by EMAs.

Learn more about the benefits of electric actuators. Download our full white paper, Accuracy and Repeatability in Linear Actuators: Why Electromechanical Systems Outperform Pneumatics and Hydraulics.

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