Beckhoff TS5066, Руководство пользователя, Страница: 7 / 31

Overview
TS5066 7 Version: 1.0
2 Overview
Due to the increasing use of decentralised controllers, time synchronisation of different systems is becoming
an increasingly important issue. In systems without identical timebase, cyclic transfer of information can lead
to beat effects, which can manifest themselves as periodic disturbance in the synchronisation of drives for
which axis information is transferred via a network. The TcRemoteSync.lib library offers options for general
time synchronisation of information in distributed systems, and special techniques for synchronising NC axes
("distributed axes").
The problem
Notwithstanding nominally identical task cycle times, different systems will have a small phase difference
(drift) that is caused by differences in production, different temperature characteristics and different
environmental conditions of the oscillators used. The phase difference causes a continuous change in the
temporal interrelationship of the system task cycles. Figure 1 shows the situation in schematic form. The
sending system S and the receiving system R operate with slightly different cycle times T. T
S
>T
R
for the
case shown. The phase difference causes a shift in the task cycles of the two systems. Periodically, two
cycles of system R will occur during one cycle of system S. This situation is indicated by a circle in Figure 1.
Fig. 1: Figure 1: Change in the temporal interrelationship between the task cycles of two systems S and R
with slightly different cycle times (TS>TR).
Distributed systems are synchronized through cyclic sending of information from the sender (S) to the
receiver (R). Up to the point indicated in Figure 1 the sent information is receive sequentially, then the
information sent is read during two consecutive cycles, and subsequently the system returns to sequential
reception. If the cycle time relationship is reversed (i.e. cycle time T
R
> cycle time T
S
), information sent would
be missed on the receiver side.
The solution
If there is no option for physically synchronizing the cycle times of the systems, e.g. for two PCs operated
with SERCOS cards, beat effects can be compensated through correction of the information received. The
required correction time Δt is determined from the drift of the two systems such that the corrected data S'
correspond to temporally equidistant and sequential reception. Figure 2 shows the time response of the
original data S to receiver R, and data S' corrected by the correction time Δt. At the point indicated with a
circle, the data originally read identically by two receiver cycles are initially corrected with Δt
1
for the first
receiver cycle and subsequently with Δt
2
for the second cycle.
Fig. 2: Figure 2: Corrected information S' after extrapolation of the originally sent information S with
correction time Δt. The sign and magnitude of the time correction are indicated by arrows.
In order to be able to correct the data, the variation of the information over time must be known. For the
typical application of synchronization of distributed NC axes, for example, set position and velocity can be
corrected through extrapolation with the set velocity and acceleration.