Blog: Understanding Difference between Accuracy and resolution
All mechanical devices and components have dimensional and geometric tolerances associated with them. However, expressing the accuracy and repeatability of a motion device such as a linear actuator is a more complex process than simply stating a number. The user also needs a basic understanding of the actuator’s specifications and what those specifications mean. The illustration below shows that accuracy and repeatability aren’t the same thing, although they are related. Their relative importance also varies, depending on the application.
The first step in expressing an actuator’s parameters is to define them. Accuracy refers to an actuator’s ability to achieve a commanded position, meaning that accuracy is a measurement of the difference between the commanded position and the actuator’s actual position after executing the command. Positional errors in a single-axis system have several causes, including the actuator’s motor, motor driver, encoder and actuator itself. Each of these components influences a linear actuator system’s accuracy and repeatability.
This paper specifically discusses the example of a mechanical linear actuator and how its components contribute to the actuator’s accuracy and repeatability. Future papers will discuss related attributes of a motion system’s motor and electronics.
Linear actuators can exhibit errors for each of their six degrees of freedom, which include their X, Y and Z axes in addition to the rotation about each of these three axes. This description is known as the motion control coordinate system. Machine designers must decide if they only need to eliminate the errors of one degree of freedom, or if they must accurately position the device in three-dimensional space by eliminating the errors from all degrees of freedom.
An actuator lying flat on an XY table provides a simple example for understanding motion errors. In this case, the X axis is forward and back, the Y axis is left and right, and the Z axis is up and down. These parameters can seem quite complex when you consider all the possible sources of errors and what they represent. The applications below clarify this situation by examining the basics of a sample device.
When the actuator makes a linear motion, it travels through the length of its stroke as determined by the components and methods used in its construction. Linear actuators need some type of structure upon which to attach its components. This structure is typically composed of extruded aluminum or steel, but it can also be made of granite for actuators requiring a very high degree of precision. Manufacturers also need to machine the actuator’s components to the required precision.
Manufacturers also attach driving components such as lead screws and timing belts to the actuator’s structure, along with any hardware needed to assure proper function. An actuator designed to support loads like a tooling fixture or end effector will also need load-carrying components such as bearings and bearing rails. These fundamental components form the basis of the actuator. Next, we’ll look at some possible sources of errors.
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