Absolute optical and magnetic encoders

The reliability and quality of work of both your complex equipment and production as a whole depends on the reliability of the encoder. Thus, the losses from unforeseen stops of the production line can be disproportionately high in relation to the funds saved on the purchase of encoders.

Below are the technologies underlying the Eltra absolute encoders, their differences and features.

Optical encoders

A modern absolute optical encoder is an extremely complex device. When developing a high-resolution optical encoder, designers face a large number of conflicting factors that greatly affect the accuracy and reliability of the encoder for a long time.

Principle of optical measurement

The key component of optical encoders is the encoder disk mounted on the shaft. This disc is made of a transparent material with a concentric pattern of transparent and opaque areas. The infrared light from the LED hits a series of photoreceptors through a code disk. As the shaft turns, the unique combination of photoreceptors is illuminated by light that has passed through the pattern on the disc.

For multi-turn models, there is an additional set of code discs installed in the gear mechanism. As the sensor’s main shaft rotates, these meshed discs rotate like an odometer mechanism. The rotation position of each disk is controlled optically, and the output is information about the number of revolutions of the encoder shaft.

Functionality

Eltra optical absolute encoders use highly integrated Opto-ASIC technology providing resolution up to 16 bits (65536 steps) per revolution. For multi-turn models, the measuring range is increased by mechanically engaged code discs up to 16384 (214) revolutions.

Optical encoder design

The main problem is the presence in one design of a large number of optical, mechanical and electronic components that are completely different in nature. So, the mechanics are prone to mechanical wear.

The quality of optical elements is primarily affected by follow factors:

• pollution,
• tarnishing,
• changes in radiation intensity.

The high resolution of the encoder requires the use of an optical disc with a high stencil density. For optical/physical resolution (and not interpolated!) of 12 bits, a disk with sectors dividing the circle into 4096 parts/marks is required.

The more compact the encoder and the smaller the disk diameter, the higher the requirements for the encoder optics.

To recognize such a pattern density on a disk, it is necessary to place the reading matrix in close proximity to the disk. The minimum gap between the rotating disc and the readout array places very high demands on the mechanics. Minimal runout/play of the shaft will cause the disc to contact the reading matrix during rotation and, as a result, to damage the stencil applied to the disc.

Wear of the mechanical parts of the encoder or leakage of the housing also leads to contamination of the optics with wear products and dust entering from the outside and, as a result, distortion of the measurement results.

The optical disk is an important part of the encoder. Under the influence of time, temperature changes and many other factors, the material properties of the disc may change over time, such as tarnish and deform.

1. Tarnish, in combination with the loss of LED backlight intensity, can drastically reduce the reliability of operation and/or cause a complete failure in operation.
2. Deform can cause the danger of contact between the disk and the matrix during the rotation of the encoder shaft with the same ensuing consequences.

Benefits of optical encoders

Main advantages of optical encoder are:

• providing high resolution and accuracy;
• excellent dynamic performance;
• suitability for use in areas with high magnetic fields.

Because the rotation of the encoder discs is a purely mechanical process, these devices cannot lose their absolute position information if the instrument is temporarily powered down. Backup batteries are not required!

Eltra optical absolute encoders.

The company produces both multiturn and singleturn optical encoders. The electronic interface can be Profibus, SSI or Profinet.

The main series are:

• Optical multi-turn: AAM58B, AAM58C, AAM58F, EAM58A, EAM58B, EAM58C, EAM58D, EAM58E, EAM63A, EAM63B, EAM63C, EAM63D, EAM63E, EAM58F, EAM63F, EAM63G, EAM63AX, EAM63DX, EAM90A, EAM90B, EAM90C, EAM90D, EAM90E, EAMX80A, EAMX80D.
• Optical single-turn: EA58F, EA63F, EA63G, EA58B, EA58C, EA63A, EA63D, EA63E, EA63AX, EA63DX, EA90A, EA115A, EAX80A, EAX80D.

Magnetic encoders

Magnetic encoders determine the angular position using magnetic field technology. A permanent magnet mounted on the encoder shaft creates a magnetic field that is measured by a sensor that generates a unique absolute position value.

Innovative multi-turn technology

Eltra multi-turn magnetic encoders use innovative technology to keep track of the number of revolutions, even if the revolution occurs when the system is powered off.

To accomplish this task, encoders convert shaft rotation into electrical energy. The technology is based on the Wiegand effect: when the permanent magnet on the encoder shaft is rotated through a certain angle, the magnetic polarity in the “Wiegand wire” changes abruptly, creating a short-term voltage spike in the winding surrounding the wire. This pulse marks the rotation of the shaft and also provides power to the electronic circuit that registers this event.

The Wiegand effect occurs in all conditions, even at very slow rotation, and eliminates the need for backup batteries.

Advantages of magnetic encoders

Magnetic encoders are:

• reliable,
• durable and

The battery-free, gear-free design provides mechanical simplicity and lower cost than optical encoders. Their compact dimensions allow them to be used in very limited spaces.

Eltra magnetic absolute encoders.

Eltra offers both singleturn and multiturn optical encoders with blind and hollow shaft.

The main families are:

• Magnetic multi-turn encoder series: EAM36A, EAM36G, EAM36F, EAMW58B, EAMW58C, EAMW63D.
• Magnetic single-turn encoders: EA36A, EA36G, EA36F, EMA22A, EMS22A, EMA50A, EMA50B (BY), EMA50F, EMA50G, EMA55A (AY), EML50A, EML50B (BY), EML50F, EML50G.

Rotary encoder – optical or magnetic?

Rotary encoders convert the angle of rotation of the shaft into an electrical signal and operate on an optical or magnetic principle of operation.

A common belief is:

• optical encoders measure more accurately, while
• magnetic encoders are more stable and durable in design

Is this really true?

Experts believe that this is not so. At present, optical encoders no longer outperform magnetic encoders in terms of accuracy.

The technology of magnetic encoders in recent years has made it possible to completely bridge the gap with optical encoders in relation to all important electrical parameters. Today’s magnetic encoders already achieve a resolution of 16 bits at an accuracy of 0.09° and thus performance that was previously only achievable with optical encoders. In 2013, there was a real revolution in the ratio of technologies, when a magnetic encoder was introduced that, in all key parameters, reaches traditional optical systems.

What made it possible to increase the capabilities of magnetic encoders so much?

The key to success was a technologically qualitative leap, in which the successful combination of the hardware and software of the magnetic system played an important role.

New generation magnetic encoders are based on Hall sensors, which analog signals are processed by a fast 32-bit microcontroller in real time.

Accurate calibration is ensured by:

• complex software algorithms,
• new high-tech chips.

All this guarantees the highest accuracy of the new series of magnetic encoders.

As for optical encoders, there is also further development here, but without significant jumps in the results achieved. In principle, this technology is used in the form it existed 50 years ago.

Today’s optical encoders are:

• smaller,
• with higher resolution,
• partly mechanically stronger,
• more stable than the previous generation of encoders.

However, the underlying issues regarding sensitivity to moisture, soiling and mechanical stress remain today.

Optical systems are inherently sensitive to anything that might interfere with reliable signal transmission from the light source on its way to sensitive photoreceptors. In this regard, magnetic encoders have always been ahead. Whether it’s dust, fog or strong shaking, nothing can break the performance of a magnetic encoder so quickly.

In terms of immunity to magnetic fields, optical encoders are preferable to magnetic ones?

The noise immunity of Eltra magnetic encoders is well controlled thanks to special shielding mechanisms against magnetic fields. Even in close proximity to strong interference sources such as electronic motor brakes, magnetic encoders work without problems.

Thus, optical encoders no longer have any advantages in terms of magnetic stability. Optical encoders can be considered as an expensive solution for applications where extremely high resolution is required, say 20 bits per revolution. In most cases, the accuracy of magnetic encoders is sufficient.

Drawing conclusions, it can be noted that magnetic encoders offer significantly more possibilities and freedom in design. They are much more compact and lighter than optical ones, which in multi-turn models are much more massive than magnetic ones due to the presence in the design of a fairly large gearbox consisting of several optical disks.

Magnetic encoders, due to their compactness, allow them to be integrated into very limited spaces in a machine or other equipment. Well, another not insignificant positive factor is a more budgetary price. In a word, it is not at all surprising that magnetic encoders are now the main trend.