Hello Mechatronitians!

Welcome to another issue of SINADRIVES’ blog.

December 2020

This time we’ll have a look at the different types of encoders and to their most important features.

We won’t get at length into matters such as precision, repeatability, etc. as we find here a strong linear correlation with the price: as it happens with most products, if we want superior performances, we’ll have to dig into our pockets. Save for tooling machines, the choice of the encoder has been viewed as a minor subject. Since in classical transmissions this element wouldn’t influence the system’s mechanics, it was an understated choice which came along with the servomotor and the driver.

With the upcoming of axes moved by linear motors, this matter has radically changed. When designing the system’s mechanics, the decision about the encoder type must be carefully taken, as factors like the work environment or the compatibility with the driver are determining, influencing in turn the final configuration of the linear axis.

1. Encoder technology

Be the encoders lineal or rotative, there is a clear-cut regarding the reading technology used.

Magnetic encoders

Using this physical principle to read a series of previously magnetized marks. The measuring ruler is polarized in order to get a north-south continuous sequence. Thus, the reader moving above the measuring rule is able to detect the positions’ increments.

This principle allows for a very economic encoder, which also accepts wide mounting tolerances. However, it gets affected by external magnetic fields and by the internal properties of magnetism, such as hysteresis, all of which lowers the incremental econder’s robustness and repeatability.

Optical encoders

Optical ones, use the refraction of a light beam on the measuring ruler. The said ruler consists of different stretches, some of them with reflecting surfaces, some others are opaque. These may reflect the light beam of the diode’s head, or not, as it is the case. Thus, the diode’s head is able to count the positions’ increments.

Optical technology allows for very high precision rates and it is immune to magnetic fields. But it is very sensitive to dirt and, since if offers superior performances, the price tends to be a high one.

 

Inductive encoders

The third option are the inductive encoders. The working principle of these does nor differ in their form from the previous two, but what changes is the detection principle. In this case, the measuring ruler is a metallic one with slots marked on it at regular intervals. The encoder’s diode detects the inductance, a magnitude taking different values depending on whether the ruler is metallic or there are slots on it.

This system is a highly robust one because it is immune to magnetic fields and resistant to bumps and to dirt. It is not as precise as optical systems are, but close enough to them so that it is a favourite candidate in many automation applications in general.

Comparison

The following table displays a comparison among the three types of encoders

Magnetic

Optical

Inductive

Solidity

0

+

Precision

+

0

Price

+

+: Positive feature

0: Neutral

-: Negative feature

2. Encoder types

Encoders are classified into two main groups: incremental and absolute.

As the name says it, an incremental encoder only counts increments. It its not capable of rendering a real position on the axis, but just allows for an increment or decrement of the starting position.

It goes without saying that real-world machines don’t work like this. That’s why the control system first executes a zero sequence, also called “homming”. This basically consists in moving the axis into a direction until a sensor is found, which will mark this position known as “zero” and from there we can count either positively or negatively.

In most cases, the application demands a reference position more exact than the one rendered by a sensor or a limit switch. That’s why incremental encoders generally include a zero-point indicator. Once found, the axis will invert the sense of the direction and will carry on moving until it finds the encoder’s zero pulse.

This function allows for a very low repeatability, around a few microns, or even nanometers in more precise encoders.

There might be situations in which the zero pulse search is not possible, due to possible collisions or long strokes requiring a much longer referencing time. In such cases, it is necessary to use an absolute encoder.

The absolute encoder communicates with the driver via a digital bus, and transmits the exact position without having to execute any movement. This is possible thanks to the fact that the measuring ruler includes a code, which the diode reads in order to know the position.

Another advantage of these encoders, when put to work in axes with linear motors, is that they avoid the execution of an initial “phasing”. Thanks to this feature, some problems might be avoided, such as in the case of vertical axes or whenever the load is quite heavy.

The market’s current trend is towards a growing use of absolute encoders. The fact that their price is coming closer to the incremental encoders, and the advancement of electronics allowing drivers to read different protocoles with the same hardware, has made it that many manufacturers introduce interfaces of absolute encoders by default, leaving the incremental encoder interface as an option.

3. Encoders and protocols: A complicated relationship?

Following up on what has been said above, absolute encoders use a digital communication bus, which has given rise to a whole host of communication protocols. Each encoder or driver manufacturer promotes its own protocol, so the compatibility between encoder and driver is an issue which must be taken into account in the design phase.

Here below we offer an overview of the most usual protocoles:

SSI Protocol:

“Synchronous Serial Interface”. This is a free communication protocol so it is accepted by most equipments. The drawback is that, as it is open, accepts certain variations in the length of the stretches as well as in the code, so that the correct configuration must be carried out if the position is to be recorded correctly. Morevover, as it is a standard from the 1980s, it has limitations as to the possible transmission rates it can handle and so it is not very adequate for highly dynamic applications. To compensate for this, the absolute signal is sided by a parallel, incremental one. Thus, the absolute position is reported by the SSI when the encoder is switched on and from there the incremental encoder reports the position during the moving sequence.

BISS / C:

This protocol is an evolution of SSI. It keeps the “open system” philosophy, and allows for a higher transfer rate, however it does not reach the standards of current protocols. As is the case with SSI, this one is also sided by an incremental signal which allows for good performances under highly dynamic conditions.

EnDat Protocol:

This one is a proprietary product by Heidenhain. The current 2.2 version is completely digital and may incorporate the required safety functions. The degree of popularity this manufacturer enjoys has made it that their protocol is read by the drivers of the market’s main manufacturers. One of its main features is its electronic label: once switched on, it reports all its parameters to the driver, avoiding the need for a manual configuration and thus saving possible errors.

Hyperface:

This protocol is property of SICK, and it has been widely used for years. Though in more recent times it has lost some of its popularity, it is still accepted by most drivers. It is used when a project’s priority is to ensure the compatibility with existing systems.

Drive-Cliq Protocol:

This is a SIEMENS’ protocol. Is is the most “closed” of them all, as only SIEMENS equipment may read it. Even if some manufacturers incorporate this protocol into their encoders, it remains under a strict surveillance in order to ensure full compatibility, both at the level of hardware and software. This reduces the offer of available electronics which are capable of working with this protocol, but also makes life easier to SIEMENS technicians by avoiding them the configuration of new systems.

Besides those protocols, there are many others, normally associated with a certain manufacturer, either from drivers or encoders, or from both. This is the case of Panasonic, Fanuc or Mitsubishi.

If you want further information about the different types of encoders our linear stages use you may visit out webpage or contact us using the “Comments” form here below.

See you in the next chapter!

The SINADRIVES Team

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