BECKHOFF DK9222-0711-0051 User Manual

XFC

Application Note DK9222-0711-0051

XFC technology microincrements

Microincrements | IP67-related solutions

Keywords

microincrements
Distributed Clocks
EtherCAT
EtherCAT Box
XFC
IP 67
EP5101
encoder

This application example describes how an EP5101 EtherCAT Box can be used in a harsh industrial

environment (IP 65/67) to maximize the physical resolution of an incremental encoder. The number of

counted encoder segments can be output in more detail with a data width of just 8 bit, i.e. 256 times.

The resilient IP 67 I/O system from Beckhoff

The Beckhoff EtherCAT Box line delivers EtherCAT I/O technology without requiring a control cabinet. All modules from the IP

67 series have an integrated direct EtherCAT interface, so that the protocol’s high performance is retained right down to each

module. This opens up new options in the IP 67 world: fast process data communication with eXtreme Fast Control (XFC),

high precision measurement technology and drive functions integrated into I/O solutions directly in the field. With dimensions

of only 126 x 30/60 x 26.5 mm (H x W x D) the modules are exceptionally small and are, therefore, particularly suitable for

applications where available space is limited.

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Application Note DK9222-0711-0051

XFC technology microincrements

Technical background

The incremental encoder is the main link between the mechanical system and the control system for monitoring mechanical

movements. Incremental encoders convert linear or rotary movements into signals that can be analyzed electrically. For rotary

movements, a certain number of light/dark segments applied to a pulse disc are scanned with a light beam. A scannable scale

arranged in the direction of motion is used for capturing linear movements. The accuracy of the returned position is limited by

the encoder resolution. For rotary movements, the resolution corresponds to the quotient of revolution (360°) and number of

segments. It indicates the smallest possible measurable difference between two positions. The more segments, the higher the

resolution and the more precise the position information. A standard encoder has 1000 lines, resulting in an accuracy of 360° /

1000 = 0.36°. This means a rotary movement can be monitored with a precision of ±0.36°. In many cases, this is adequate for

simple positioning tasks, although a finer resolution is required in order to monitor axis synchronism in addition to the position.

Fig. 1 Encoder signals with different resolutions

Physical improvement of the resolution through maximization of the encoder segments is only feasible to a certain degree,

since manufacturing tolerances and operating conditions increase the costs of the encoder. A simple and effective way of

maximizing the resolution is to use a second detection point. With two signals that are offset by 90°, three additional edges

are available for detection. They can be used to detect the direction of rotation in addition to the position, and an additional

reference signal for zeroing is output once per revolution. Analysis of these additional edges can refine the resolution by a

factor of 4 (360° / 4 * 1000 = 0.09°), which is why this principle is referred to as quadrature encoder.

Axis synchronism monitoring

Axis synchronism is monitored through cyclic position polling and interpolation of these values within the PLC. The timebase

for the interpolation is provided by the strict cycle-linked processing of the instructions in the PLC. With a cycle time of 1 ms

(which is common for motion applications), the positions are scanned with a timebase of 1 ms. However, the real encoder

scanning intervals are not as rigid as those of the PLC and vary. The reason for the irregularity is inherent to the functional

principle variation of the fieldbus transfer times (jitter) and the encoder inaccuracy with ±½ edge. Since the PLC does not take

this discontinuity of the polling intervals into account and assumes a constant interval duration, the position representation

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Application Note DK9222-0711-0051

XFC technology microincrements

in the process image of the PLC may be unsteady even if the axis is, in fact, synchronous. This only virtual deviation can have

three different effects:

Diagram 1 Asynchronism according to process image

1st case:

Although, in reality, the axis runs absolutely uniformly, the process image shows a non-uniform movement (see Diagram 1)

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