Absolute Encoders

• What is an absolute encoder?
• Signal programming of absolute encoders
• Hall effect encoder
• Gradient based Hall sensors
• Resolution
• Update rate and sampling rate
• Accuracy - Absolute linearity
• Singleturn and multiturn absolute encoder

Absolute encoders offer a wide range of options for displaying the measured angular values as an electrical signal function at the output. The electronics of many encoders are programmable, allowing the output waveforms to be customized. The following example shows the standard factory programming of an analogue 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°.
When it is at 0°, it outputs 0 V. When the shaft is rotated 90° clockwise, it gives a value of 90°/360° * 10 V = 2.5 V. This value remains constant as long as the encoder shaft is not moved. The figure in example 1 shows the signal curve of such an absolute encoder.

The term "Full Scale" is often used with the abbreviation "F.S." to refer to the entire signal output scale. In the example above, F.S. = 10 V. Without knowing the maximum voltage, programming can also be done using percentage values. For example, 0° corresponds to 0% F.S. (0% of the maximum value, i.e. 0 V) and 360° corresponds to 100% F.S. (100% of the maximum value, i.e. 10 V). In this way, a signal output function can be accurately described without the use of a graph:

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

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 diagram opposite 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. This is called 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. The charge separation creates an additional voltage perpendicular to the direction of the current, called the Hall voltage.

In principle, external magnetic fields can interfere with Hall technology if no precautions are taken. Today, so-called gradient-based Hall sensors are used, which are largely insensitive to such interference.
The principle of this particular variant is that two or more Hall sensors are placed close to each other. The measuring magnet, which is very close to these two sensors, creates a difference in the signals of the two sensors because the curvature of the field is relatively strong. However, an external interfering field, which usually has a slight curvature, is 'seen' by both sensors in the same way. If only the difference between the two sensors (the gradient) is evaluated, then virtually only the measurement magnet is perceived and the measurement system is therefore very robust to external interference fields.

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 between the acquisition of the measured value and the output of the signal in the angle encoder. It is usually between 96 µs and 600 µs for magnetic 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.

Calculating the possible angular deviation of an encoder is complex and depends on many factors, such as environmental influences (temperature), mechanical factors (bearing play), component tolerances of the electronics, etc. In order to determine the angular error of an absolute encoder reliably and quickly, a calculation based on absolute linearity has proven to be practical. Absolute linearity describes the maximum possible percentage deviation of the signal output function (the measurement result) from 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

Analogue outputs are still very important in the encoder market. For this reason, most series are offered with these outputs. Absolute encoders from MEGATRON with analogue signal outputs are always designed in 3-wire technology, unless they 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 galvanically isolated and therefore offer separate supply voltages, ground and signal outputs for each signal branch.

External wire break detection
In order to generate an external wire break detection via an evaluation unit, the output signal of the angle encoder must not be zero during operation, regardless of the angle, as 0 V output voltage or 0 mA output current are the indicators of a wire break. For all MEGATRON absolute encoders with current output, an external wire break detection can be realized by 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 programme that offer this function ex works. However, this is not possible on all models. Please contact us if you are not sure.

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 (EMI), since EMI usually affects the signal amplitude rather than the frequency (e.g. AM/FM radio). However, the signal must be detected in an external evaluation unit designed for PWM.
The pulse width on MEGATRON encoders varies between 10% (0% F.S.) and 90% (100% F.S.). The carrier frequency is 244 Hz.

Microcontrollers often provide inputs for digital signals based on the SPI and SSI formats. Absolute encoders are offered with these digital interfaces to ensure high compatibility with many microcontrollers on the market. It is beyond the scope of this manual to describe the digital interfaces and their specific characteristics in detail. The following explanation is therefore only intended as a brief overview.

The SPI interface is based on a serial master/slave BUS protocol developed by Motorola. Communication is via 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 exceed 0.6 m. For further information on MEGATRON magnetic encoders with SPI interface, please contact us.