Hello Mechatronitians,

Welcome to the latest post on the Sinadrives blog.

August 2022

Today we will talk about three-dimensional precision. It seems as though everyone has a clear idea of what it is, but when we receive consultations from our clients, many times the determination of certain geometric errors is not well defined.

Basic errors and their definition

In total, there are over 20 basic precision errors. In this article, we will only show you the most common if you’re using a three-dimensional precision machine.

Flatness

The term flatness means a condition in which a surface has elements located on a theoretical plane comprised of at least 3 points. Any error or deviation from this plane is considered an error in flatness.

Straightness

The term straightness means a condition in which all the elements of a surface are in a straight line. Any error or deviation from this plane is considered an error in straightness.

Parallelism

The term parallelism means a condition in which the surface is parallel (equidistant in all its points) to another surface. Any error or deviation from this plane is considered an error in parallelism.

Perpendicularity

The term perpendicularity means a condition in which a surface cuts another one (it creates an intersection with it) to form a 90º angle. Any error or deviation from this plane is considered an error in perpendicularity.

Absolute precision movement

The term absolute precision means the degree to which the measurement system approaches the real value of this position.

Angle errors of an object in motion

When an object moves in space, its main coordinate axes can differ from the space where this object is moving. For example, the position of a plane with regards to the control surface (or the ground), or of a boat with regards to the surface of the sea. The main angle errors are pitch, roll and yaw. The following example shows the angles of a plane:

Pitch axis

In our case, we will talk about a linear motor stage motorised by a linear motor with a slider. The following image illustrates these errors:

Pitch error, Raw error and Yaw error

Pitch is the rotation of the slider in relation to the transverse axis, roll is the rotation in relation to the longitudinal axis, and yaw is the rotation in relation to the vertical surface axis.

Why do we talk about these angle errors and why are they important?

In precision applications, in the industries of semiconductors, optics, laser or measurement, these types of errors are used to determine the precision of equipment, machinery or a system. If the error is greater than that which has been set, in most cases, the process will not be completed, or the resulting product will not comply with the expected quality. In the case of machines in metrology, this would be a complete fiasco.
Remember that these types of errors only make sense for axes or the parts of machine that are in motion. For static objects, we use concepts such as straightness, parallelism and perpendicularity.

How can we achieve high precision?

Basically, there are 3 pillars of precision:
– Use certified precision components
– Use proper assembly and adjustment
– Use electronic means for the correction of errors

We will take a detailed look at each of these pillars:

Precision components

Using precision components is the most important pillar. If from the get-go the components do not comply with the required levels of precision, then achieving precision for the entire machine will become mission impossible. For example, linear guides must comply with a certain class of precision, encoders must offer absolute precision in conformity with requirements, and the machine base (structure) must be made of a suitable material with a suitable finish. As an example, we can compare a structure made from aluminium structural profiles with a solid granite structure that offers exceptional geometric stability and is not influenced by changes in temperature.

Proper assembly and adjustment

Even if the components applied in a machine have high precision, if we assemble them with a few bangs of a hammer, no level of precision will be guaranteed. We recommend relying on a qualified person to do this process, someone who not only knows how to assemble but who also knows how to safely use measurement and verification elements. We’re talking about micrometres, precision levels, inclinometers, laser interferometers, etc. Thanks to these measurement and verification elements, the assembly process can benefit from the correct adjustment and alignment of all components, thus guaranteeing overall precision. In our extensive experience, we have collaborated with companies whose adjusters were one of the key employees in ensuring this process. They even earned above-average salaries for the know-how they possessed. Of course, there were other adjusters, but they took hours or even days to do the same job. If you want to know more about measurement equipment, check out our blog article: Calibration of linear axes. SINADRIVES Direct Drive Experts.

Electronic equipment for correcting errors

A method exists called Mapping which is very popular among manufacturers of industrial precision machinery. It entails measuring errors at specific points on the machine, for example every 100mm of motion (or every 5mm, depending on the length of the axis) and recording these errors in the control system. Then a Controller, normally a CNC (Computer Numerical Control), runs a correction calculation to try to minimise the error between the points measured previously. This process can even be repeated various times in order to reach an approximation close to zero of the error. This method may work really well for the absolute precision of an axis (mapping encoders) or of an entire system where you can minimise to a maximum the errors of the entire system.

Other methods for improving precision exist. For example, in rotary axes, with the use of a double encoder (double rotating heads), you can obtain better absolute precision per rotation and eliminate eccentricity; in other words, errors in mechanical assembly or ball bearings.

Conclusions

Precision is not a topic you can learn about quickly. It requires time, practice and above all patience. If you need to improve the precision of your machine but you don’t know how, contact our technicians. We have a solution for you.

Over the last 10 years, we have carried out applications on granite and ground steel workbenches. We have deployed air bearings and cross roller guides. We understand optical encoders with high absolute precision, and we know how to map them. We invite you to take advantage of our experience in this field, and rest assured that your application deployed by SINADRIVES will comply with all the precision requirements you are looking for.

If you have an application in which you’re keen to improve the performance of your machine, be it in terms of speed, dynamics, precision or simply reducing maintenance needs, please contact us.

Our specialists in Direct Drive technology and Linear Modules with linear motor technology will be happy to advise you free of charge.

Draw your own conclusions. Decide which innovation you want to implement in your machine to be competitive. We can help you.

Your SINADRIVES Team.

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