Encoder types or how to select the right encoder

Types of encoders

There are two most common types of rotary encoders: incremental and absolute.

Incremental (quadrature) encoder

Incremental encoders generate pulse signals when a wheel or shaft turns a certain angular distance. This type outputs two separate signals in a quadrature arrangement, so it can indicate both distance and direction of shaft rotation.

In addition to indicating direction and distance, some incremental encoders also have an indexing function at a single point of rotation. The indexing function allows the shaft to return to a known point. Other encoders have an enter button, adding a new user interface.

The main advantages of incremental encoders are:

1. reliability,
2. ease of installation,
3. relatively low cost.

In order to select an incremental encoder, it is necessary to determine the following main points:

• the required measurement accuracy,
• the shaft diameter and its model variety,
• the type of cable output connector and its length,
• electrical characteristics.

Incremental encoders are one of the most common sensors in modern complex technical systems. Their use is expedient in any powerful electric drives of precise movement systems or systems that are critical to high-speed and starting-braking modes.

Here are just a few examples of their uses:

• Medical equipment
• Automated test and diagnostic equipment
• Self-propelled robotic devices
• Access control systems

Absolute encoder

For the most part, incremental encoders inherently don’t “know” the position of the shaft (other than the index point). Absolute encoders, on the other hand, use an internal readout to measure the angular position directly and maintain this measurement capability even if the power is turned off.

In addition to determining the shaft position, multi-turn absolute encoders can also determine how many revolutions the encoder has made in one direction or the other.

Absolute encoders are an important link between the mechanical part of the machine and its control unit due to their precise and unambiguous measurements, which they can transmit instantly. This quality is successfully used in modern enterprises of mechanical engineering and robotics.

Absolute encoders are divided according to the following parameters:

• Single turn or multi turn
• Number of pulses per revolution or number of bits
• With shaft, with hollow rotor or with through hollow rotor
• Shaft or shaft bore diameter

When choosing an absolute encoder, you should pay attention to the following parameters:

• The number of bits per revolution. The accuracy of the system depends on this indicator.
• Type of shaft connection to the encoder.
• Encoder output type (The most common types of signal outputs are parallel code, SSI, DeviceNet, Profibus-DP, CANopen, LWL interfaces).
• Electrical characteristics. The operation of the system and the accuracy of signal removal depend on this indicator.
• Degree of encoder protection against dust and moisture penetration.

The absolute encoder is designed to be a versatile, configurable sensor for a wide range of applications. The following areas of use are distinguished: the medical field, alternative energy, telecommunications systems, packaging and food industries, and many others.

Optical and magnetic encoders

Depending on the application, the principle of determining the rotation is important. It can affect the price or how well an encoder can handle external interference or contamination.

The main types are:

1. Optical encoder. For optical reading, encoders have a light source that is progressively interrupted by a disc or other means attached to the shaft. This light transmits pulses for incremental encoders and transmits position data for absolute encoders.
2. Magnetic encoder. They are much cheaper than optical ones and are more compact. Most of them use analog Hall effect devices mounted on a printed circuit board. Hall sensors are driven by a two-pole magnet mounted on the end of a shaft. It produces two AC signals in antiphase with one cycle per revolution of the shaft.

Additional functions, features and characteristics of encoders

Among encoders that have the same class and rotation definition, there can be a large discrepancy between capabilities. Encoder prices often reflect these differences. As a result, the following parameters must be taken into account:

Encoder resolution.

Number of pulses or encoder positions per revolution.

The resolution of a rotation encoder corresponds to the maximum number of points it can measure in one revolution.

In the case of an incremental encoder, the resolution is directly related to the number of pulses it delivers per revolution.

In the case of an absolute encoder, resolution measures the number of revolutions the device can write before “rolling” to the zero position. For example, a 16-bit encoder would have a resolution of 65,536 points per revolution.

Encoder PPR is to be chosen according to the expected resolution, which must match the application and the accuracy of the mechanical components of the measuring circuit.

The sensor is connected to an electronic device, controller or meter that allows a maximum input frequency to be taken into account. Indeed, a high resolution incremental encoder generates more pulses per revolution than a low resolution incremental encoder. Depending on the speed set by the application, the encoder output may be at a higher frequency than the device connected to the encoder can handle. In this case, you will need to use a lower resolution encoder.

Speed

Encoders have a maximum mechanical speed in rpm as well as a maximum frequency response. Both must be considered, as well as the frequency response capabilities of the readout electronics.

Reliability

While many encoders require a relatively clean environment, others are hardened for industrial use. This characteristic may be related to the rotation detection method or device case type. Thus, when choosing, you need to take into account the working environment.

Eltra offers a wide range of heavy duty encoders. Also in some series it is possible to order a reinforced metal case.

Encoder electronic output types

Here some the most common output types of Eltra encoder.

1. Line Driver and RS422 (Differential output). The Line Driver can actively force the output low and high, allowing it to source and sink current from the load. As a result, a higher current is generated while maintaining longer transmission distances. As the name suggests, this type of encoder output is popular for a range of industrial applications, especially where the noise level is quite high.

RS-422 (TTL) output provides a constant 5 V signal level independent of the supply voltage.

2. Open Collector output. This type of output allows to receive a signal with a voltage level determined not by the encoder supply voltage, but by the voltage of the additional power source. To do this, it is necessary to connect an external resistor between the power circuits and the output, the resistance value of which is determined by the voltage value of the additional power source.

Eltra uses in its models NPN open collector and PNP open collector

3. Push-pull (HTL). This type of output is built on r-p-p and p-p-p transistors and has a low output impedance, which allows to increase the current delivered to the load. Thus, the load capacity is increased and the charging and discharging processes of the load capacity are accelerated, and hence the speed is increased as well.
4. Analog encoder. This is an absolute position sensor that has a 0-10 V signal at the output; 4 – 20 mA; 0 – 5V; 0.5 – 4.5 V; 0 – 20 mA. The device is used to measure angle, tilt, linear position and displacement. They have gained popularity in industrial automation, now they are being actively replaced by encoders with digital signals. But analog sensors continue to hold their niche due to high reliability, ease of signal processing, and unified dimensions.
5. SSI (Synchronous Serial Interface). It allows transmission of the absolute encoder position data by a serial line synchronized by a clock. The principle of operation of an encoder with an SSI interface is very similar to the standard one. Its main parts are a light source, disc with transparent and opaque windows, compare/trigger circuits, photoelectric receivers, parallel/serial converter, monostable circuit, input circuit for a clock signal, and an output driver for a data signal.

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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.

Сообщение Absolute optical and magnetic encoders появились сначала на Eltra-encoder.com.

Linear encoders are pivotal in applications requiring high precision and accurate measurement of linear displacement. They are employed in a wide range of industries, from manufacturing and robotics to medical devices and metrology, ensuring precise control and measurement in various complex systems.

Importance of Linear Encoders in Modern Technology

In today’s technology-driven world, the need for accuracy and precision in measurements and movements is paramount. Linear encoders play a crucial role in meeting these needs by providing exact position feedback, which is essential for the smooth operation of numerous high-tech systems.

Robotics is another field where linear encoders are vital. Robots rely on these devices to move accurately and perform tasks with high precision. Whether it’s assembling tiny components in electronics manufacturing or performing delicate surgical procedures, the accuracy provided by linear encoders is critical for successful outcomes.

In metrology, the science of measurement, linear encoders provide the high-resolution feedback necessary for accurate measurements. This is crucial in quality control processes where even the smallest deviation can lead to significant issues.

Medical devices also benefit significantly from the precision of linear encoders. Imaging systems, for example, require exact positioning to produce clear and accurate images, which are essential for diagnosis and treatment planning.

Types of Linear Encoders

Linear encoders come in various types, each designed to meet specific application needs. The two primary categories are incremental linear encoders and absolute linear encoders. Understanding the differences between these types is crucial for selecting the right encoder for your application.

Incremental Linear Encoders
Incremental linear encoders work by generating pulses as the encoder moves. These pulses are counted to determine the position relative to a starting point. The basic principle involves a scale with evenly spaced lines and a readhead that moves along the scale, generating signals as it detects these lines.

The key features of incremental linear encoders include:

• Relative Positioning: They provide information about changes in position from a reference point, rather than an absolute position.
• Simplicity and Cost-Effectiveness: Due to their simpler design, incremental encoders are generally less expensive than absolute encoders, making them a popular choice for applications where cost is a significant factor.
• High Resolution: Incremental encoders can offer very high resolutions, providing precise control and measurement capabilities.

Incremental encoders are widely used in applications such as CNC machines, where they provide the necessary feedback for precise control of tool movement. They are also common in various automation and robotics applications, where accurate relative positioning is sufficient.

Absolute Linear Encoders
Absolute linear encoders, on the other hand, provide a unique position value at every point along the scale. This means that each position along the scale has a distinct signal, allowing the system to know the exact position immediately upon startup without the need for re-referencing.

The key features of absolute linear encoders include:

• Absolute Positioning: They offer true position information at all times, even after power loss or system restart. This makes them highly reliable and ensures consistent performance.
• Higher Complexity and Cost: Due to their more complex design and the need for more sophisticated electronics, absolute encoders tend to be more expensive than incremental encoders.
• Enhanced Safety and Reliability: The ability to provide absolute position data enhances safety and reliability, especially in critical applications where knowing the exact position at all times is essential.

Absolute encoders are ideal for applications where maintaining position information is crucial, such as in medical imaging devices, precision measurement equipment, and high-end automation systems. They ensure that the system always has accurate position data, reducing the risk of errors and improving overall performance.

How Linear Encoders Work

Understanding how linear encoders work involves delving into their basic working principles and the processes of signal generation and processing. These elements are fundamental to the operation and utility of linear encoders in various applications.

Basic Working Principle
At the core of a linear encoder’s operation is the interaction between the scale and the readhead. The scale, which can be a strip of material with evenly spaced markings or patterns, acts as the reference for position. The readhead moves along this scale, detecting the markings to determine the position.

Here’s a simplified breakdown of the basic working principle:

• • Scale Movement: The scale, or ruler, is fixed to the object whose position is being measured. As this object moves, so does the scale.
  • Readhead Detection: The readhead, which contains sensors, moves along the scale. These sensors detect the markings on the scale.
  • Signal Generation: As the readhead detects the markings, it generates signals that correspond to the position of the scale. These signals are often in the form of light pulses (optical encoders), magnetic fields (magnetic encoders), or electrical changes (capacitive or inductive encoders).

This interaction ensures that any linear movement of the object is accurately captured and translated into electronic signals that represent the position.

Signal Generation and Processing

The process of signal generation and processing in linear encoders is crucial for converting the physical movement into precise digital data that can be used by control systems.

Signal Generation:

• • • Optical Encoders: Use a light source and a photodetector. The scale has transparent and opaque sections, and as the readhead moves, light passes through or is blocked, creating light pulses that are converted into electrical signals.
    • Magnetic Encoders: Use a magnetic strip with alternating north and south poles. The readhead contains magnetic sensors that detect changes in the magnetic field as it moves along the scale, generating corresponding signals.
    • Capacitive Encoders: Utilize a scale with varying capacitance patterns. As the readhead moves, changes in capacitance are detected and converted into electrical signals.
    • Inductive Encoders: Use coils and conductive patterns. Movement of the readhead changes the inductance, generating electrical signals.

Signal Processing:

• • Initial Conversion: The raw signals generated by the readhead are analog in nature. These signals are first converted into digital form through analog-to-digital conversion.
  • Interpolation: To achieve higher resolution, the digital signals are often interpolated. This means dividing the basic signal period into finer increments, allowing for more precise position measurement.
  • Filtering and Conditioning: The digital signals may undergo filtering to remove noise and conditioning to ensure they are in the correct format and level for the receiving control system.
  • Output Generation: Finally, the processed signals are outputted as digital data, which can be used by the machine’s control system to determine the exact position of the object being measured.

Advantages of Using Linear Encoders

Linear encoders offer a multitude of benefits that make them indispensable in many high-precision applications. Let’s explore some of the key advantages:

High Precision and Accuracy
One of the most significant advantages of linear encoders is their ability to provide high precision and accuracy in position measurement. This is crucial in applications like CNC machining, where even minor deviations can affect the quality of the final product.

Reliability and Durability
Linear encoders are designed to be robust and reliable, making them suitable for demanding industrial environments. Many are built to withstand harsh conditions such as dust, vibration, and temperature variations.
Ease of Integration
Linear encoders are designed to integrate seamlessly with existing systems. Whether you’re upgrading an old system or installing a new one, linear encoders can be easily incorporated, providing immediate improvements in precision and control.

Enhanced Performance
By providing precise position feedback, linear encoders help enhance the overall performance of the systems in which they are used. This leads to improved efficiency, higher quality outputs, and better resource utilization.

Immediate Position Feedback
Unlike some other types of position sensors, linear encoders offer immediate and continuous feedback. This real-time data is essential for dynamic systems that require constant adjustments and monitoring, such as robotics and automation systems.

Disadvantages and Limitations

While linear encoders offer numerous advantages, they also come with certain limitations that need to be considered:

Cost Considerations
High-precision linear encoders, especially absolute types, can be quite expensive. The cost includes not only the device itself but also the associated installation and integration expenses. For budget-sensitive applications, this can be a significant limitation, making it necessary to balance the need for precision with cost constraints.

Environmental Sensitivity
Certain types of linear encoders, particularly optical encoders, can be sensitive to environmental factors such as dust, dirt, and temperature changes. These conditions can interfere with the encoder’s ability to accurately detect position, potentially leading to errors. Protective measures, such as enclosures or regular maintenance, are required to mitigate these issues.

Installation Complexity
Installing linear encoders can be complex and requires precise alignment to ensure accurate measurements. Any misalignment can lead to errors and reduced performance. This means that skilled personnel are often needed for installation and calibration, adding to the overall cost and complexity.

Maintenance Requirements
Regular maintenance is necessary to ensure the optimal performance of linear encoders. This includes cleaning the scale and readhead, checking alignments, and ensuring that the encoder is operating within its specified environmental conditions. Failure to perform routine maintenance can lead to degraded performance and eventual failure.

Limited Range
In some applications, the linear range of the encoder might be a limiting factor. While many linear encoders can cover significant distances, there are practical limits to their range. For extremely long-distance measurements, alternative solutions might be required.

Electrical Noise Interference
Linear encoders, especially those with analog signal outputs, can be susceptible to electrical noise interference. This noise can distort the signals, leading to inaccurate position data. Shielding and proper grounding are essential to minimize the impact of electrical noise.

Conclusion
Linear encoders are indispensable tools in modern technology, providing the precision and reliability needed for a wide range of applications. From CNC machines and robotics to medical devices and metrology, these devices ensure accurate position feedback, enhancing the performance and efficiency of countless systems.

By understanding the types of linear encoders, their working principles, and their signal processing methods, one can appreciate their critical role in various industries.

Choosing the right linear encoder involves balancing these factors to meet specific application requirements. Whether you need the simplicity and cost-effectiveness of incremental encoders or the absolute position accuracy of absolute encoders, there’s a linear encoder solution to fit your needs.

As technology continues to evolve, so will the capabilities of linear encoders, promising even greater precision, durability, and smart integration in the future. By staying informed and selecting the appropriate encoder for your applications, you can harness the full potential of these remarkable devices, ensuring optimal performance and innovation in your field.

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