The Truth about Accuracy in Linear Actuators
When specifying a linear actuator, the first question most people ask is: “How accurate is it?” There is no simple answer and it is possible that it’s not even the right question to ask as accuracy may not be the most important parameter for a given application. While accuracy is often viewed as the most crucial criteria, without a thorough understanding of what accuracy means when specifying a linear actuator, this approach may lead to selection of an actuator with unnecessarily high levels of accuracy, as well as higher costs and greater degrees of complexity. Below we will explain why this is so.
What is a linear actuator?
Linear actuators create motion that moves loads in a straight line by converting energy into force. They can be powered by pressurized air, fluid or electricity. Here’s a breakdown on how each type of linear actuator functions:
- Pneumatic linear actuators
In this type of actuator, pressure from an external compressor moves a piston inside a cylinder and, as pressure increases, the cylinder moves across the axis of the piston to create a linear force. Once it reaches its commanded position, the piston returns to the starting point via a spring-back force or a fluid supplied to the other end of the piston.
- Hydraulic linear actuators
These operate like a pneumatic linear actuator, but liquid from a pump is used to move the cylinder rather than pressurized air.
- Electric linear actuators
This type of linear actuator converts the rotational motion of an electric motor into linear motion by rotating the actuator’s screw. The screw turns either clockwise or counterclockwise, causing the shaft to move in a straight line to move the load.
How is Accuracy Measured?
Before discussing the importance of accuracy, let’s define the key parameters of a linear actuator: accuracy and repeatability. The accuracy of a linear actuator refers to its ability to hit, as close as possible, its intended, or commanded, position. Repeatability is the ability to hit a given position each time. Clearly, accuracy and repeatability are closely related and are both desirable, but they are distinct concepts and, depending on the requirements of the application, one characteristic may take precedence over the other.
A dartboard or target is often used to illustrate the difference between the two concepts. Using this visual, accuracy would be depicted as how often you can come close to hitting the bullseye, while repeatability would be depicted as the ability to hit the same location on the target each time. The ability to hit the bullseye (or commanded position) repeatedly is referred to as “precision.” In other words, the total precision of motion systems is a combination of the accuracy and repeatability of the linear actuator within the system, which is more likely what system designers and engineers really want to know when they inquire about the accuracy of a linear actuator.
It’s also important to know that there are several factors that can impact the accuracy, repeatability and overall precision of a linear actuator. First are the mechanical components, such as the rod, bearings, bushings and pulleys, and their tolerances, which are highly dependent upon the manufacturing process. For example, if a thread on the actuator’s rod is out of spec, it may impact the accuracy and/or repeatability. In addition, the tooling or components used to mount the actuator to the carriage can create a wobble effect that can also impact accuracy and repeatability. Mounting and orientation can also influence accuracy and repeatability.
Before moving on, there is an additional factor that can affect the overall precision of a linear actuator and that is resolution, which is defined as a small increment of movement. Higher precision requires higher resolution, which is dependent upon the encoder that provides positional feedback.
Accuracy is commonly specified in units of microns or thousandths of an inch, while repeatability is quoted as a plus/minus tolerance, meaning that according to the defining statistical standard, the true position of the actuator will fall within the specified range 99.8% of the time.
How Important are Accuracy, Repeatability and Precision?
So, rather than asking how accurate a linear actuator may be, a better question might be: “What level of accuracy and repeatability or overall precision is needed for my application?” To properly answer this question, it’s important to have a good grasp on the application requirements, as well as how potential tolerances, mounting and orientation may impact the precision of the system. And, keep in mind that it is possible – and often acceptable – that a linear actuator may have a low degree of accuracy and a high degree of repeatability.
Actuators with high levels of both accuracy and repeatability would be considered high-precision actuators and are typically employed in critical applications such as the manufacture of printed circuit boards and medical devices where precision is crucial. In more traditional applications, high levels of precision may not be needed and will only serve to drive up costs and complexities of the system. In applications that aren’t critical, a linear actuator with lower levels of precision or accuracy but higher levels of repeatability may be a better choice because it may be more important to ensure that the actuator can reach the same position each time it is required to do so. A material handling robot equipped with a gripper that sits atop a linear actuator is a good demonstration of when repeatability may be the more important parameter. In this example, the actuator must move the robot into a variety of positions so the gripper can pick up boxes and load them onto pallets. In this application, it is more important that the motion is repeatable and moderately accurate, rather than highly precise.
While there are many linear actuators available with varying degrees of precision, remember that the tolerances of the components, as well as the way the actuator is deployed and mounted can influence accuracy, repeatability and overall precision in the application. Further, the best method of selection should not be based solely on the rated accuracy, but rather by gaining an understanding of what is required of the application. Not only will this help avoid excess costs and system complexities, but it will ensure that the performance of the linear actuator is most suitable for the specific application.
For assistance finding a linear actuator that best suits your application, please contact an expert at JHFOSTER