Absolute Encoders

Interesting facts about these encoders

Guide for Absolute Encoders

Absolute Encoders

Magnetic rotary encoders for absolute angle measurement

Absolute encoders are rotary encoders that measure angular positions in an application, convert this angular information into electrical signals and output them as absolute values. Excellent measuring results are provided by magnetic sensor technologies based on the Hall effect with the well-known advantage of the contactless measuring principle and a virtually unlimited sensor life.

Thanks to the large selection of electrical output types, connections and mechanical designs, reliable, reproducible and accurate measurement data is provided for almost any application. The angular measuring range of an absolute encoder is of central importance. Singleturn encoders cover angular ranges up to 360 degrees and multiturn encoders cover angles beyond that. Particularly worth mentioning is the possibility of programming customer-specific output curves.

And taking into account all possible parameters, we work out the best possible product solution as part of our consultation. This is because in many cases, demanding applications require technical product adaptation. MEGATRON is your specialist for these cases and supports you from the inquiry to the realisation of series production and beyond that up to the "End of Life" of your application with assured quality products and high delivery reliability as a long-term, reliable partner.


Guide for Absolute Encoders
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What is an absolute encoder?

Absolute encoders are rotary encoders that measure angles, convert this information into electrical signals and output them as absolute values. The use of electronics for measured value processing differentiates them from potentiometers, which also provide absolute values, but are passive components without integrated electronics. Absolute encoders have a fixed reference point for angle measurement, to which the output value is referred at all times. The principle of an absolute encoder is fundamentally different from that of incremental encoders, for example, where only angle changes (relative values) are transmitted by the encoder.
Absolute encoders are divided into two categories for the angular range to be measured. Encoders that measure angles over several revolutions are called multiturn encoders, and encoders that measure angles up to 360° are called singleturn encoders.


Signal programming of absolute encoders

Absolute encoders offer numerous possibilities to display the measured angular values in the form of an electrical signal function at the output. The electronics of many encoders are programmable and allow the output curves to be customized. The following example shows the standard factory programming of an analog absolute encoder:
The encoder is programmed in the CW direction of rotation with an output signal of 0...10 V (when the shaft rotates clockwise) and measures an angle of 0...360°. 
If it is at 0°, it outputs 0 V. If the shaft is rotated clockwise by 90°, it gives a value of 90°/360° * 10 V = 2.5 V. As long as the shaft of the encoder is not moved, this value remains constant. The figure in example 1 shows the signal curve of such an absolute encoder.

Measured angle [° degrees]Output voltage [Volt]
0° = 0 V0 V
360° = 10 V10 V
45° = 1.25 V1.25 V
90° = 2.5 V2.5 V
180° = 5.0 V5 V

 

To refer to the entire signal output scale, the term "full scale" is often used with the abbreviation "F.S.". In the example above, for example, F.S. = 10 V. Now, without knowing the maximum voltage, the programming can also be agreed by means of percentage values. For example, 0° corresponds to 0% F.S. (0% of the maximum value, i.e. 0 V) and 360° correspond to 100% F.S. (100% of the maximum value, i.e. 10 V). In this way, a signal output function can be described precisely without using a graphic representation:

0° = 0% F.S.
90° = 100% F.S.
180° = 100% F.S.
270° = 0% F.S.
360° = 0% F.S.

Example 1

Example 2

The figure shows the signal curve for these requirements in example 2. The output signals of absolute encoders can also be output via other interfaces, e.g. as output current or by pulse width modulation (PWM).


Hall effect encoder

The Hall effect is a phenomenon in which an electrical voltage is generated in a current carrying conductor (Hall element) when it is in an external magnetic field.
The effect is shown in the adjacent picture and can be explained as follows: When current flows through an electrical conductor, charge carriers (electrons) move through the conductor. If an additional magnetic field is applied, e.g. by an external magnet, the charge carriers are deflected perpendicular to the direction of the current. The reason for this is the Lorentz force: It deflects charge carriers when they move and when an external magnetic field is applied. The electrons now accumulate at the edges of the conductor. Charge separation creates an additional voltage perpendicular to the direction of the current, the so-called Hall voltage.

A magnetic field generates a Hall voltage in a current carrying conductor

If the external magnetic field now changes due to movement of the magnet, the Hall voltage also changes - so sensors can be implemented relatively easily. If, for example, a circular diametrically magnetized (north pole/south pole) permanent magnet is positioned over a Hall element and this magnet is subjected to a rotational movement, then a sinusoidal output voltage curve can be measured. If the position of the magnet does not change, the measured value remains constant. However, a Hall sensor can only work if a current is flowing, because otherwise the Lorentz force will not work. Therefore, Hall sensors need electricity during operation, even if there is no change in the measuring position.


Gradient based Hall sensors

In principle, external magnetic fields can interfere with Hall technology, unless precautions are taken against them. Today, so-called gradient-based Hall sensors are used, which are largely insensitive to such interference.
The principle of this special variant is that two or more Hall sensors are placed in proximity to each other. The measuring magnet, which is very close to these two sensors, creates a difference in the signal of the two sensors because the curvature of the field is relatively strong. However, an external interference field, which usually has a slight curvature, is "seen" by both sensors in the same way. If now only the difference between the two sensors is evaluated (the gradient), practically only the measuring magnet is perceived and the measuring system is therefore very robust against external interference fields.


Resolution

Most hall encoders are digitally operating encoders and process measuring signals with a certain resolution. The information is processed with a precision corresponding to the number of bits. The higher this value is, the finer signals can be processed. Analogue output curves of digital devices therefore always have a fine gradation, the height of which is determined by the resolution. Typical resolutions are 10 bits, 12 bits or 14 bits, depending on the encoder model. For example, the angular resolution is 0.088° for 12 bits and 0.022° for 14 bits. The following simple observation helps to determine these values::

  • The resolution is used to calculate the number of state changes that can be displayed: 1 bit corresponds to 2 state changes (since 21 = 2), 12 bits correspond to 4096 state changes (since 212 = 4096)
  • The number of state changes is divided over the entire range of the electrical angle of rotation

So to calculate the angular resolution, one has to divide the effective electrical angle of rotation by the number of possible states:

\(\text {Angular Resolution in degrees} =\frac {360°} {2^\text {number of bits}}\)


Update rate

Since many Hall encoders are equipped with digital integrated circuits (ICs) that always send their signals with a certain delay, the update rate in milliseconds must be taken into account in the application. The update rate is the time period between the acquisition of the measured value and the signal output in the angle encoder. The time span is usually between 96 µs and 600 µs for magnetic angle encoders with digital signal processing, but can be up to 3 ms for some multiturn encoders.

If the update rate is increased, this results in a higher current consumption of the angle encoder. Some angle encoders can also be ordered with a reduced update rate, for example 600 µs instead of 200 µs, to reduce power consumption. These angle encoders are then particularly suitable for use in a battery-powered application with low power consumption.
If a different update rate of the angle encoder is required, it must be ordered ex works. This feature cannot be changed in the field. The update rate should not be confused with the sampling rate.


Accuracy - Absolute linearity

The calculation of the possible angular deviation of an encoder is complex and depends on many factors, such as environmental influences (temperature), mechanical factors (bearing clearance), component tolerances of the electronics, etc. In order to be able to determine the angular error for an absolute encoder reliably and quickly, a calculation based on absolute linearity has proven to be practical. The absolute linearity describes the greatest possible percentage deviation of the signal output function (of the measurement result) compared to an ideal straight line. However, these specifications apply under the following conditions:

  • Operation of the shaft in one direction of rotation
  • Operation at room temperature
  • Reference to an electrically effective angle of rotation specified in the data sheet
  • The absolute linearity is a "worst case" consideration
  • In practice, the actual angular error will be lower

Absolute linearity describes the deviation of the signal (red) from an ideal straight line (green) passing through the zero point.

The angular error is reproducible for the single encoder. This means that the error is always largely the same for a given angle of rotation. The high repeatability of a contactless absolute encoder therefore allows the signal output function to be offset against a calibration function stored in an evaluation unit in order to reduce the angular error of the encoder. The information on absolute linearity is a fixed value in the data sheet of absolute encoders.
The information of the absolute straight lines in the data sheet of a kit encoder (without shaft) is provided under the condition that the centre axis of the magnet is aligned with the centre axis of the encoder. Some data sheets of magnetic kit encoders also provide information on how the value of the absolute linearity changes when the magnet is positioned eccentrically relative to the centre axis.


Singleturn and multiturn absolute encoder

Singleturn absolute encoder

Singleturn encoders are absolute encoders which are only suitable for measuring angles <= 360°, since their output signal shows the same value as for 0° after one full revolution. Most contactless singleturn absolute encoders therefore measure an angular range from 0° to a maximum of 360°. This category also includes models that have a limited angular range, such as ±45°.


Multiturn absolute encoder

Multiturn encoders are capable of measuring angles beyond the zero point, i.e. beyond 360°. This is possible because the measuring system is capable of counting the number of revolutions. Often the signal increases continuously over the entire specified angular range. For example, the ETA25PM multi-turn absolute encoder from MEGATRON is capable of measuring angular ranges up to a maximum of 72000° (up to 200 shaft revolutions) and this range can be limited by programming. Ex works 3600° are programmed for this (10 revolutions). For measuring angles >360°, however, the sensor may be rotated by a maximum of ±179° in the de-energized state, otherwise the sensor loses its measured value.

In order to avoid this fact, there are true power-on encoders. They have the ability to output the angular position correctly in any case, even after a voltage-free state. One possible variant is to use a gear reduction so that the shaft rotates several times, but the magnet only rotates a maximum of 360° within the positioning range. Our guide for Multiturn Encoders provides more detailed information on this topic.


Electrical signal outputs

Analogue and digital signal outputs are available for the absolute encoders.

Analogue signal outputs for absolute encoders:

  • Voltage
  • Current
  • PWM (pulse width modulation)

Digital signal outputs for absolute encoders:

  • SPI
  • SER
  • SSI

Current and voltage output

Analogue outputs are still of great importance in the market for encoders. Therefore, most series are offered with these outputs. Absolute encoders from MEGATRON with analogue signal outputs are basically constructed in 3-wire technology, provided that they do not offer redundancy. The two connections for the supply voltage (VSUP) and the output signal (OUT) have a common ground. Many contactless absolute encoders with redundant signal outputs are electrically isolated and therefore offer separate supply voltages, ground and signal outputs for each signal branch.

External wire break detection
In order to produce an external wire break detection via an evaluation unit, the output signal of the angle encoder must not be zero during operation, independent of the angle, since 0 Volt output voltage or 0 mA output current are the indicators for a wire break. For all MEGATRON absolute encoders with current output, an external wire break detection can be realized with the factory programming, as the measured angle is always output in a range of 4...20 mA.
For a voltage output, we have series in our program which offer this function ex works. However, this is not possible for all models. Please contact us if you are unsure.


PWM interface

With the PWM output, the measured angle is not proportional to the signal amplitude but to the pulse width. The advantage over current or voltage outputs is that this form of signal output is largely insensitive to electromagnetic interference, since EM interference is usually interference of the signal amplitude rather than the frequency (example: AM/FM broadcasting). However, the signal must be detected in an external evaluation unit designed for PWM.
The pulse width varies with MEGATRON encoders between 10% (0% F.S.) and 90% (100% F.S.). The carrier frequency is 244 Hz.


Digital interfaces

Microcontrollers often provide inputs for digital signals based on the SPI, SER and SSI formats. In order to ensure a high compatibility to numerous microcontrollers on the market, absolute encoders with these digital interfaces are offered. It is beyond the scope of this guide to explain the digital interfaces mentioned with their specific properties in detail. The following explanation therefore only serves as a compact overview.


SPI (Serial Peripheral Interface)

The SPI interface is based on a serial Master/Slave BUS protocol, which was developed by Motorola. The communication takes place over the data lines:

  • MOSI (Master Out → Slave In)
  • MISO (Master In ← Slave Out)
  • SCK (Serial Clock) (= bus clock/shift clock)

In addition to these three lines, a line called "Slave Select (SS)" or "Chip Select (CS)" is required for each slave. Please note that this format is not suitable for field communication, as the cable length between master and slave should not be longer than 0.6 m. For further information on magnetic encoders from MEGATRON with SPI interface, please contact us.


SER interface

The SER interface is a special form of the SPI format. The format is proprietary and therefore not a standard. The difference to the SPI format is that a falling edge "CSn" is required for each measured value transfer. Like SPI, the SER format is not suitable for field communication because the cable length between Master and Slave should not be > 0.6 m. For further information on magnetic encoders from MEGATRON with SER interface, please contact us.


SSI (Synchronous Serial Interface)

The SSI interface is a widely used interface for serial data communication. It is particularly suitable for digital communication of absolute encoders in the field, where measured values have to be transmitted over long cable runs must be transmitted between the encoder and the evaluation unit. The data transmission (clock and data) is carried out via four lines, each transmitting two signal pairs (symmetrical signal transmission in phase and rotated by 180° in phase). If signals of one signal pair are superimposed by a disturbance on the transmission path between slave and master, this disturbance can be removed from the two signal pairs in the evaluation unit by subtraction. Twisted pair signal cables with individually shielded pairs are particularly suitable for reliable transmission of the measurement signals over longer distances. For further information on SSI communication with MEGATRON encoders, please contact us.

Concept of fault elimination using SSI

Data transmission on the two channels Clock and Data


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