Timing is an immensely important and difficult matter in distributed multi sensor systems.
From the created signal to the storage, the data package with the measured values passes through several software blocks and systems, such as measurement adapters or Ethernet switches. In the process, propagation and transmission delays (e.g. store and forward) and process-related clock jitter occur.
Due to these phenomena, the collected measurement data will not arrive and be processed chronologically with respect to their origin at the data recorder unit, which is responsible for recording and storage.
However, an exact temporal correlation of the data packets forms the basis for a meaningful fusion of sensor values. Without it, the values are not correctly assigned and cannot represent a system in its complete model. A reliable and safe statement beyond one measured value is therefore not possible.
To compensate for the effects mentioned above, it is important to time stamp each data package with the creation time in order to be able to assign it later. The timestamp has to be applied to the data package as early as possible in order to be able to consider transmission delays at the same time.
Taking the individual time stamps and the underlying time into account, the clocks in the complete system must run on a time basis and be continuously synchronized to form a time domain.
This forms the basis for demanding safety-critical applications and provides the basis for safety certification of this multi sensor system.
The Working Clock Domain provides the time base for measurement and synchronization tasks and makes optimal use of hardware structures in the AVETO platforms. gPTP (IEEE 802.1AS) is used for synchronization over Ethernet based network connections. It additionally integrates into domain 1, defined in the IEEE 802.1AS-2020 standard. The clock of this domain runs continuously interference free from global influences, such as a GPS synchronization.
The TAI clock (International Atomic Time) is taken into account as an external reference to events in the extended system network in the AVETO Toolbox. This is synchronized via gPTP(IEEE802.1AS) or optionally via PTP (IEEE1588v2-Industrial Profile). The temporal assignment of the acquired measurement data can be done possible via the TAI clock. Thereby this domain 0 is used for a global temporal correlation of measurement data.
The first type of clock is the Grandmaster Clock, the main source of time in a network. From this, time is distributed.
Clocks in PTP and gPTP work with the Best Master Clock Algorithm (BMCA).
This algorithm automatically determines the best clock in the network, designates it as the master clock, and distributes it accordingly to the other clocks, which act as slave clocks. Within devices master and slave may occur per Ethernet port.
Ordinary Clock is either the source of time with the Master role or the receiver of time with the Slave role, but not both. These clocks synchronize directly. Ordinary Clock is either the source of time with the Master role or the receiver of time with the Slave role, but not both. These clocks synchronize directly.
The distribution of time across different components is done via Boundary and Transparent Clocks.
A boundary clock is a clock that carries time information across a network boundary. It is both a master and slave clock.
It takes the timing information, adjusts it to the delay, and creates a new master timing signal that it distributes again. It can translate between signal types and manage multiple sub-networks.
The Transparent Clock (used in PTP) lacks these features. A Transparent Clock is a feature used by switches to measure the packet delay caused by them. This feature stamps the time delay of the pass in the switch into the packet to distribute it to the slave nodes. The Transparent Clock also organizes the order of packets from multiple sources. In addition, if an output is busy, then the timing information is stamped in from the time it arrives at the switch until it is dispatched at the switch.
We support with XTSS in our Building Blocks in AVETO a widely configurable,
Plug and Play capable and very accurate timing concept - ideal for any measurement application and best data quality.
| XTSS Feature | USB Dongle for 3rd Party Hardware | i210 PCIe x1 Set | IP Core FPGA | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| PTSS – Within the platform hardware synchronized clocks | |||||||||||
| PTSS | X Only with DATALynx und BRICK Families | X | X | X | |||||||
| CTSS – Clocks synchronized via Ethernet | |||||||||||
| Ordinary Clock (Master/Slave) | X | X | X | X | X | X (SW based IEEE 1588v2) | X | X | |||
| Ordinary Clock Slave Only | X | X | X | ||||||||
| Boundary Clock | X | X | X | X | X | X (SW based IEEE 1588v2) | X | ||||
| Transparent Clock | X (IEEE 1588v2) | ||||||||||
| TAI Time Source | X (GPS) | X (GPS) | X (GPS) | ||||||||
