Positioning System Precision
time：2011/4/24 click：8521
The precision of the positioning system can be easily divided into two categories: the precision of the guide rail itself and the precision of the alignment along the guide rail. The former describes the accuracy of the guide rail(ball and bar, cross roller, air bearing, etc.) to provide the ideal single-axis translation, while the latter involves the accuracy of the incremental motion along the shaft(usually related to the guide screw, linear encoder, or other feedback device).
Rail precision
Any moving object has 6 degrees of freedom. They include translation, linear motion, and rotation along any of the three mutually perpendicular axes(QX, qy, and qz). The function of the linear positioning guide rail is to accurately limit the movement of the object to a single flat axis(usually the X axis). Any deviation from the ideal line along the X axis is the result of an inaccurate guide rail device.
-Six degrees of freedom.
There are 5 possible types of guide rail inaccuracy, corresponding to 5 remaining degrees of freedom: translation in the Y axis; Translation in the Z axis; The rotation(pendulum) along the X axis; Rotation along the Y axis(longitudinal tilt); And the rotation(horizontal tilt) along the Z axis. Since these errors are related to each other(for example, angular displacements generate Translational errors at any point other than the center of rotation), it is worth carefully examining the effects of each type of error and its measurement methods. -Possible inaccurate guide rail translation error
Since all useful methods that can produce linear motion are averaged at multiple points(due to the area of multiple balls or rollers, or air bearings), the "pure" translation error from the linear motion is usually small. An exaggerated sine wave error in the rolling body guide rail can obtain a pure translation error without rotation, just as each roller in the guide rail rolls through the contaminated particles at the same time; These two situations will not be encountered in practice. If a rolling body platform is subjected to a large impact, the guide rail may be depressed in each ball or ball position; This results in a purely Translational error that occurs periodically along the stroke. However, the positioning work station will show some vertical and horizontal eccentricity(usually referred to as plane degree and straightness error, respectively), which can be measured by placing a sufficiently sensitive indicator on the work table and measuring vertical or horizontal displacements along the stroke. error. A typical high-resolution measurement technique places an optical flat glass coated with a conductive coating on the test platform and monitors the eccentricity error through a capacitometer. This will reveal errors that can be divided into three major categories:
1. A potentially larger component roughly aligned with distance.
This is due to the lack of parallelism between optical flat glass and guide rails. It can be eliminated by adjusting the optical flat glass so that it is parallel to the platform guide rail. However, it should be noted that in order to reduce the vertical eccentricity(plane error) to a lower level, the customer's components must be similarly parallel to the guide rail, but it can not be exactly parallel to the platform base or its top.
2. Low frequency components that can not be eliminated by adjusting optical flat glass.
This is rarely the result of a "pure" translation error, but the result of the basic angular error(<UNK> longitudinal, horizontal, and horizontal) îin the guide rail. Since the moving part of the platform moves(to some extent) along a curved trajectory, there is a corresponding linear deviation from the straight line direction. The correlation between the angular error and the linear error is very good and can be derived from the integral or differential.
3. Higher-frequency components from various error sources may not be from rail errors.
If the ball screw is used, the work table will rise and fall once per turn, especially near the end of each stroke. Use flexible connection nuts and/or fastening nuts to reduce this effect. Additional sources of higher plane error include fine structures in guide rails or rolling bodies, vibrations caused by drives and/or Motors, and structural resonance at the top of the platform. Since many optical positioning applications limit the depth of application, it is important to understand the size of each of the above impacts and modify the platform design to reduce the impact to a lower level. The use of air bearings and linear motors can reduce the plane degree and the total linear error to less than 0.5 microns / 250 mm and less than 20 microns / 10 mm.
Guide rail angle error
Angular errors of pendulum, vertical, and horizontal(QX, qy, and QZ, respectively) are always present in the positioning table to some extent, and the performance is reduced in several ways. Their direct effect is to change the angular direction of the user's payload; Since these errors can easily be kept low(1-50 arc seconds, depending on platform technology), the effect of the change in payload angle is not important in many applications. However, some optical positioning tasks may be directly affected by angular errors. To a large extent, people are concerned about the translation error from the basic angle error. The simple longitudinal deviation error of ± ± 16.5 arc seconds corresponding to the radius of curvature of 1 km shown in Figure 3 will produce a 20-micron Z axis translation relative to the center position of each half-meter stroke platform at each end of the stroke. Due to the outstretched nature of the load at both ends of the trip, this simple longitudinal tilt error can usually be found in the design of non-circular worktables. More complex curvature and multi-center curvature involving transverse, longitudinal, and transverse tilts may also be encountered.
The bad influence of the angle error is the Abbe error(deviation) which can influence the positioning accuracy of the straight line. Unlike the simple translation error described in the above example, the Abbe error increases with the accuracy determining the distance between the component and the measurement point. This effect will be described in detail in the Abbe error section. The guide rail angle error is easily affected by the method of installing the positioning platform(see installation problem). In general, air bearings can provide peak angular precision because they have an intrinsic average effect, and their reference surface can be done very flat. The best stars can be bold angels to as low as 1 arc-second per 250 mm. Angular errors of a way assembly bean best be a need for a use. We use a double-track optical component to eliminate sensitivity to linear translation while providing a resolution of 6.5 microarc seconds(32 nanradians) for vertical or horizontal tilting. The measurement of the transverse pendulum requires the use of a rectangular optical flat glass and an automatic collimator or a pair of differential working capacitometers.
There are various techniques for linear positioning accuracy
It can be used to position user payloads incrementally along a straight axis. Up to now, guide screw and ball screw are commonly used, although linear Motors, piezoelectric mechanisms and actuators are also used. The precision of linear positioning is simply the degree of matching between the movement controlled by the command and the defined length unit. All length measurements are measured in meters, as defined by Comitee Consulf pour Defetion du Metre. The current value of 1 meter is the distance that light travels in a vacuum in 1/299 ,792,458 seconds.
A system based on a guide screw
Low to medium precision systems usually rely on a guide screw or ball screw to provide accurate incremental motion. These systems are often open-loop driven by stepper motors; If a closed-loop drive is used, a rotating encoder is often used. In both cases, the lead screw is a major precision determining element. The guide screw has a cumulative lead error with monotonic properties and a cyclic component that changes with each rotation of the screw. In addition, there may be a side gap in the nut that will appear when the direction is reversed. Precision positioning platforms usually use a pre-loaded ball screw or a fastening nut with a side gap. Ball screw is more suitable for high-speed applications and can provide higher natural frequencies due to its inherent rigidity. The guide screw with a side gap nut can provide high repeatability at medium cost and is suitable for most applications. The NEAT guide screw has two types: commercial grade and precision grade, and precision grade has 0.0001/in. (1 micron / cm) cumulative lead error, while the commercial class has 0.0004î/in.
Linear motor