EtherCAT sensor enables real-time measurements

03 July 2012

EtherCAT is growing in importance for enabling the latest laser displacement sensors to transfer measurement data in real-time in automated industrial environments, says Chris Jones, managing director at Micro-Epsilon UK.

There is a growing trend in industrial automation technology towards replacing existing first generation fieldbuses with industrial Ethernet. As a result, Micro-Epsilon is integrating new, real-time Ethernet technology into many of its laser displacement sensors and controllers. Unlike conventional bus systems which often require a great deal of installation effort, industrial Ethernet is faster and easier to integrate.

Due to its larger bandwidth and real-time communication capabilities, sensors and controllers connected via Ethernet enable high speed data transfer, simplified read-out and analysis of process and measurement data, as well as location-independent configuration of the sensors. Unlike conventional analogue output signals that suffer from inherent ‘noise’ on the output circuit due to grounding, electrical resistance from connectors and cable quality, measurement data sent via digital networks does not degrade even long sensor cable lengths. Any increase in ‘noise’ on a circuit effectively reduces the measurement resolution and results in reduced accuracy of measurements.

In addition, manufacturing processes are becoming higher speed and more automated and so these industrial networks require sensors and measurement systems that support these faster, dynamic processes. As a manufacturer of precision sensors, Micro-Epsilon is at the forefront of developing sensors and controllers that cater for these future industrial requirements.

In recent years, EtherCAT has established itself within the market as a communications standard (protocol) that can offer true real time capabilities in automation technology. In line with this, Micro-Epsilon has developed sensors and controllers with EtherCAT capability, enabling them to be smoothly integrated into existing control and automation networks.

EtherCAT explained
EtherCAT is a fully Ethernet-compatible and is an ‘open’ standard which is able to overcome the system limitations of other Ethernet solutions. The Ethernet data ‘packet’ is no longer received, then interpreted and copied as process data at every connection. Instead, the Ethernet frame is processed ‘on the fly’. The newly developed fieldbus memory management unit (FMMU) in each slave node reads the data addressed to it, while the telegram is forwarded to the next device or sensor. Similarly, input data is inserted while the telegram passes through. This means the telegrams are only delayed by a few nanoseconds.

On the master side, inexpensive, commercially available standard network interface cards (NIC) or any onboard Ethernet controller can act as the hardware interface. The common feature of these interfaces is data transfer to the PC via direct memory access (DMA). In other words, no CPU capacity is taken up for network access.

Due to the FMMU in the slave nodes and DMA access to the network card in the master, with EtherCAT the complete protocol processing takes place within hardware and is independent of the run-time of protocol stacks, CPU performance or software implementation.

In industrial automation environments, accurate synchronisation is critical. This is particularly important if coordinated movements are necessary or where multi-channel displacement measurements need to take place. The most powerful approach for this is the accurate alignment of distributed clocks. In contrast to fully synchronous communication, where synchronisation quality suffers immediately in the event of a communication fault, distributed aligned clocks have a high degree of tolerance from possible fault-related delays within the communications system.

With EtherCAT, data exchange is completely hardware-based on ‘mother’ and ‘daughter’ clocks. Each clock can determine the other clocks’ run-time offset because the communication utilises a logical and full-duplex Ethernet physical ring structure. The distributed clocks are adjusted based on this value, which means that a precise network-wide timebase with a jitter of significantly less than 1 microsecond is possible.

However, high resolution distributed clocks are not only used for synchronisation, but can also provide accurate information about the local timing of data acquisition. For example, controls frequently calculate speed from sequentially measured positions. Particularly with very short sampling times, even a small temporal jitter in the displacement measurement leads to large step changes in velocity. With EtherCAT, new expanded data types are introduced. The local time is linked to the measured value with a resolution of up to 10ns, which is made possible by the large bandwidth offered by Ethernet. The accuracy of a velocity calculation then no longer depends on the jitter of the communication system. It is orders of magnitude better than that of measuring techniques based on jitter-free communication.

EtherCAT controllers
For many years, Micro-Epsilon has provided customers with displacement sensors that offer synchronous master-slave capability. However, EtherCAT now provides customers with genuine real-time measurement performance, particularly in support of high speed processes and automated production technologies.

All new laser and optical sensors and controllers developed by Micro-Epsilon have EtherCAT protocol integration as standard, allowing integration with EtherCAT/Ethernet networks.

For example, Micro-Epsilon recently launched a powerful, flexible programmable universal controller, the CSP2008, which enables the real-time processing (up to 100kHz) of up to six input and output signals from the company’s displacement sensors. The controller is suitable for thickness, height, roundness, flatness or coplanarity measurements, where signals from multiple displacement sensors need to be synchronised and processed in real-time. The controller is able to process up to six digital or analogue input signals: two internal (via the front panel), plus four external signals via plug-in EtherCAT modules. EtherCAT can also be used as an external interface for connecting further sensors and I/O modules.

EtherCAT laser displacement sensors
Laser displacement sensors are also being equipped with Ethernet interfaces, allowing connectivity with EtherCAT networks. Micro-Epsilon’s optoNCDT 2300, for example, is a self-contained laser displacement sensor that requires no separate controller and provides a measuring speed of up to 50 kHz and resolution of 0.0015% Full Scale Output (FSO).

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