Hi Mechatronitians!

Welcome to another chapter of our blog, this time about staying competitive through better positioning.

May 2021

Since we started this blog half a year ago, we’ve dealt with many topics, from general concepts about motors to much more technical elements like encoders and their function. Through this time, we’ve always had linear axes with linear motors as the central topic, we also feel the need to give our audience a straight vision on how to solve a concrete problem or simply how to improve the performances of their machines. The market grows competitive by the day and so do the customers’ requirements. Therefore, those who don’t innovate are left behind. In the old Continent, we can’t wind the prices’ race against the Asian giant, but even so, many still try to manufacture the same things as 20 years ago.

Being innovative and creative when designing one’s machine has become essential and therefore, engineers and R&D technicians got to know all novelties the market offers to them. From a simple mechanical piece to complex subsystems, assembly methods or control software, all of them may become a big advantage for the machine manufacturer.

In today’s chapter, we want to talk about how to improve the precision and repeatability o fan application. In case these concepts are not clear, check this past post in the blog which goes deeper into both concepts. In today’s post, we’ll deal about how the user may improve the machine’s movement in order to make it more competitive and so get more selling points – all of this with at almost no extra cost. In the next sections, several kinematic drawings are presented explaining how the control loop of a machine gets closed.

Control loop of a machine with linear positioning through a spindle classical transmission

On Drawing 1 we can see a classical linear positioning system which Works with a servomotor and a spindle ball axis transmission. The rotative servomotor (1) turns the spindle axis (3) and via a connected screw (4) generates a rotative movement which has the stage (5) performing a linear movement. The stage’s position is checked via the encoder (7) which communicates the position to the servo driver (6). This is a very reliable system and also a well-known one nowadays. It is a simple solution, its components are affordable and it offers a fair relationship price-performance.

Drawing 1

positioning through a spindle classical transmission

The precision in this configuration depends on the sum of the precision of each single component plus slacks, misalignments, wearing, vibrations, etc. The error in the main position may come from the coupling (2) with slacks and/or misalignments

  1. Spindle (3): motion error, absolute precision error
  2. Screw (4): slack and wearing
  3. Stage (5): misalignment between the guides and the spindle
  4. Vibrations and torsion of the spindle, in case of a dynamic movement

The precision and repeatability of this configuration may be improved by using components with a superior degree of precision and by improving the alignments when mounting the system.

Control loop of a machine with linear positioning via a classical spindle transmission and an external encoder

The following Drawing 2 shows the same configuration as in the previous case (Drawing 1) but with a slight difference: we add a linear encoder (8) which directly read the position from the stage. This means that the precision and the repeatability of the position don’t depend on gears, slacks or wearing. At every moment of the working cycle, we may get the real position of the stage with the desired precision. For it to happen, we just got to choose the resolution and the precision of the linear encoder which are required by the project.


Drawing 2

positioning via a classical spindle transmission and an external encoder

Linear encoder (8)

This type of configuration has been widely used in the machining industry for the last 70 years and has proved to be a very reliable one. The servomotor’s encoder is used to control the speed and the linear motor encoder controls the position. All numerical controllers and servo drivers on the market are able to decode the position of both encoders so this loop’s configuration is easy and more than tested.

In order to offer an outstanding absolute precision of these systems, there are calibration methods (mapping) through an interferometer (9). The mapping method is also a well-tested one and it is quite popular with high precision machines (Drawing 3)

Control loop of a machine with linear positioning through a linear motor and an external encoder

Drawing 3

positioning through a linear motor and an external encoder

Laser interferometer (9)

When applying the mapping methodology, the positions generated by the linear encoder and the interferometer are compared and the differences are entered into a table in the CNC control. This way, the the numerical controller corrects the absolute precision errors which might have the linear encoder, the mounting errors and the geometrical precision errors. The usual precision values are in a rank up to +/- 3 micron per meter.

Some of de disadvantages of this system are the increase in costs due to the linear encoder, its mounting and alignment – the latter may pose technical problems in some instances. The calibration process in itself is a complex one and its success depends on the length and the precision the manufacturer requests.

Control loop of a machine with linear positioning through a linear motor and an internal encoder

The positioning with a linear motor (10) and a lane of permanent magnets (11) sorts out many of the problems of precision and repeatability we have spoken about in the previous sections. First of all, thanks to this technology, slacks and component-related errors may be discarded from the outset; second, we improve on precision and repeatability via a linar encoder and third we reduce both the number of components and the mounting time (Drawing 4).

Drawing 4

positioning through a linear motor and an internal encoder

Linear motor (10), Permanent magnets (11)

By using linear motor technology, the machines’ manufacturer not only improves on precision and repeatability, but this novel design offers the following advantages as well:

Higher added value

Better selling points

Design and mounting made easy

Longer working life

Maintenance reduction

You may check out this video, which describes in detail the concept of ‘linear motor’. From a mechanical point of view, it is easy to understand, it is simple and the advantages of its implementation are more than evident. Drawing 4 shows how the number of components is dramatically reduced, allowing thus for shorter design and mounting times.

From a kinematic point of view, the reduction of components also means the reduction of vibrations, bouncing, resonances and natural frequencies. The stiffness loop may be increased and with that an overall smoother movement of the stage may be achieved. This all simplifies the control loop and avoids superfluous adjustments of the software. The commissioning technicians are happy to have a ‘straightforward’ machine which significantly shortens commissioning times.

So, back to the main subject of this post: the improvements in precision and repeatability will be felt as the linear direct encoder will be incorporated into the systems. Any wrong movement, no matter how small, will be detected and corrected by the controller. The control loop will be quicker, as there are no gears sending back the position’s reference to the servomotor’s encoder. The improvements become then very apparent.

At SINADRIVES we invite you to test this technology in order to see for yourself its advantages. With improved precision and repeatability, you will get machines much more appealing to your customers.

So, that’s all for today, see you next time.


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