The challenges of time synchronisation in the industry sector

The specific needs of the industry sector

Industrial systems are often based on a sequence of synchronised events, which requires accurate timestamping shared by all machines involved in a task. There are different needs depending on the type of system.

In a production site, there are three types of systems available.

• First of all, production lines that require more or less accurate synchronisation between machines (for example: filling containers with liquids) or even a fine timestamping capacity (for example: time-tagging of impulsive events as zero crossing detection).
• Then, there are inspection and monitoring systems (SCADA systems - Supervisory Control and Data Acquisition). These systems collect, aggregate and return all data relating to factory operations. To have a coherent and sincere view of reality, it is paramount to have the most reliable time distribution service possible.
• The third type of system is the overall control system of the factory. This is a centralised system using a primary reference clock which ensures that all operations are carried out in due course and with a correct traceability. Operation here is broadly defined and includes production planification, OEE performance analysis as well as flow, personnel, quality, maintenance management, and so on.

Eventually, any organisation needs time synchronisation for day-to-day operations, such as logging of information systems, budget monitoring, telecommunication, access control, video protection, and so on.

Which time synchronisation protocol to use according to your needs?

Although the main types of industrial systems are still the same, they have evolved substantially these last few years. Initially purely mechanical, they became electromechanical and then electronic. The future of the industry now lies in production robots transferring their capacity for analysis, information storage and communication to the cloud, as well as in the use of drones for inspecting, monitoring and transporting hardware, and finally in the use of smart grids.

The ideal time synchronisation protocol for the industry sector must take several aspects into account:

• Relative or absolute synchronisation. In many cases, relative synchronisation is sufficient. To do so, a time server will act as reference clock and all industrial devices will synchronise with its time. In the case where an absolute reference is required, a clock with a GPS source is generally used. However, the issue here is the robustness of this system beyond the quality of synchronisation protocols.
• Synchronisation via wireless network or wired Ethernet network. Wireless industrial networks have specific issues. For example, these are sensor networks which will timestamp events to communicate them to a SCADA system. These sensors rarely have an internal clock and have energy connection and management dynamics that require the use of specialised protocols such as IEEE 802.14.4 (known commercially as zigbee). Synchronisation on a wired IP network is more clearly defined, and the protocols for achieving it are well known (such as NTP). This also applies to everything based on Ethernet.
• Clock availability. Many industrial systems need an unfailing availability of time servers. Some protocols such as PTP offer mechanisms to improve fault tolerance (via PTP profiles) and alternative reference clocks to reduce switchover times. Other protocols such as IEEE 802.1AS separate the sending of synchronisation and data frames in order to reduce switchover time between clocks.

What is to be used?

Whether for relative or absolute synchronisation, choosing a time server is paramount. Depending on the application, accuracy and interoperability criteria determine the most suitable solution to use.

As for accuracy, the PTP protocol enables to synchronise devices down to tens of nanoseconds, especially for critical industrial applications. Its implementation requires specific, dedicated hardware which can restrict large-scale deployment.

With an accuracy down to milliseconds, the NTP protocol is used for general purposes. It is suitable for applications involving several systems or production units.

As a result, there is no single solution that does it all. A hybrid time synchronisation architecture must therefore be considered to obtain the best possible ratio between accuracy when necessary, and flexibility of use when possible.