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October 22, 2015

EtherCAT Based Technology for Measuring and Testing

1. Overview

EtherCAT is an Ethernet-based fieldbus system. It was introduced in 2003 and is internationally standardized since 2007. The protocol is disclosed under IEC61158 and may be used by everyone for dedicated purposes. EtherCAT is used worldwide.

EtherCAT is a very fast industrial Ethernet technology (up to 100 Mbaud). EtherCAT nodes read the telegram sent by the EtherCAT master “on-the-fly”. They filter data addressed to them and load their input data into the forwarded frame. The telegram is only delayed by a hardware latency of 3 bits per slave. The last node sends the telegram back to the master.

The EtherCAT master is being the only node allowed to send an EtherCAT frame actively. Any other node (slaves) may transmit frames only. The master has a standard Ethernet Media Access Controller (MAC). Therefore, it can theoretically be installed on any hardware platform providing an Ethernet port. Depending on the provider, however, hardware restrictions may apply in real time scenarios.

For “on-the-fly” processing, the EtherCAT slaves feature an EtherCAT Slave Controller (ESC) – this may be an ASIC, FPGA or on-board microcontroller.

EtherCAT uses standard Ethernet frames with embedded EtherCAT user data.

Photo: www.ethercat.org/en/publications.html / EtherCAT Technology Group ETG Nuremberg

At the physical level, EtherCAT uses an LVDS-based transfer (Low Voltage Differential Signal) which is processed by I/O components. A head station (coupler) converts the electrical connection from Ethernet to LVDS.

EtherCAT features the Distributed Clocks System (DC) for exact synchronization of all nodes in a cluster. Each node has a synchronized clock with automated alignment. This allows all nodes for setting outputs or capture input signals simultaneously. Synchronicity is better than 1 μs.

For configuration of the nodes, EtherCAT provides several acyclic protocols. Here, the CAN application protocol over EtherCAT (CoE) and the File Access over EtherCAT (FoE) are mainly used.

The CAN application protocol over EtherCAT (CoE) provides communication following the CANopen standard. Configurations are made via SDO (Service Data Objects) while process data is handled via PDO mapping (Process Data Objects).

File access over EtherCAT (FoE) allows for file transfers. Therefore, firmware updates or a distribution of configuration files may be done via FoE.

The EtherCAT master comes with a system configuration tool and is responsible for the online transfer of the process data. By standardized slave instruction files (ESI – EtherCAT Slave Information) and or a subsequent query via CoE for module functionality, the configuration tool provides the master a complete network configuration (ENI – EtherCAT Network Information). The system may also alter slave settings via the configuration tool by Service Data Objects (SDO) or distribute completely new configurations using File Access over EtherCAT (FoE).

Beckhoff’s XFC Technology (eXtreme Fast Control Technology) allows for fast cycle times of up to 100 μs and oversampling of the process data.

Oversampling means that a slave records not just one but several readings within a polling cycle. Recorded data enters the communication cycle as a packet. A communication cycle of 200 μs and a 10-time oversampling for example, induces respective data being sent as a package of 20 values with a time resolution of 20 μs.

Photo: www.beckhoff.de/XFC/

2. EtherCAT test and measurement from Gantner Instruments

EtherCat based technology for measuring and testing by Gantner Instruments supports EtherCAT in two different topologies. On the one hand, Gantner Instruments offers I/O modules as standalone EtherCAT slaves which is the most common EtherCAT structure of distributed, individual EtherCAT devices with a fixed I/O capacity. On the other hand, Gantner also provides the possibility to operate any amplifier cluster as a single EtherCAT slave. A secondary controller manages the single I/O modules and reports back to the EtherCAT master as a single slave. In this case, capacity depends on the cluster.

2.1 I/O Modules from Gantner Instruments as Standalone EtherCAT Slaves

In the following, I/O modules from Gantner Instruments featuring EtherCAT at module level are referred to as Q.bloxx EC. In the Q.bloxx EC Series, all module types of the classical Q.bloxx are available. Inputs for voltage, current, resistance, potentiometer, Pt100, Pt1000, thermocouple, measuring bridge, LVDT, IEPE, status, frequency, PWM and counter are provided – partly also for high voltage applications in electric vehicles and batteries with 1200V isolation voltage. Outputs for voltage, current, status, frequency and PWM are also given. The test and measurement characteristics follow the standard Q.bloxx system. More about Q.bloxx…

All Q.bloxx EC modules support the EtherCAT protocol according to IEC61158 and have a LVDS interface with a bus coupler. A single bus coupler may address up to 10 Q.bloxx EC modules. The bus coupler provides an additional port for configuration via USB. All Q.bloxx EC modules feature their own EtherCAT Slave Controller with Distributed Clock (DC).

In order to use the maximum sample rate of the Q.bloxx EC modules (up to 100 kS/S, device-dependent), Gantner supports the XFC technology (eXtreme Fast Control Technology). Up to 100 values may be transmitted per polling cycle. Respectively, the module records them equidistantly with a high sampling rate and transmits the data with the subsequent cycle. The oversampling factor may be varying from module to module. Thus, even different sampling rates may be realized in a single polling cycle.

Unlike traditional EtherCAT “terminals”, the Q.bloxx EC series also offers multi functional modules. These modules may be reprogrammed for different modes of operation – from voltage measuring device to thermocouple, resistor or bridge (etc.). Within these modes, subcategories may be given as for example resistance measuring with 2-, 3- or 4-wire technology. In addition, switchable measuring ranges, linearization and various filter functions (etc.) are available. The set-up is done alike classical EtherCAT slaves via SDOs. Unfortunately, a great variety of combinations of settings is possible. The traditional tools of the EtherCAT system configurations help users only with very little support for the configuration via SDOs. Therefore, Gantner Instruments delivers a configuration tool with a graphical user interface granting full access in cluding syntax check (etc.). This software makes the complete functionality of the Q.bloxx EC available easy and fast, preventing misentries at the same time. The configuration files may either be transmitted by the EtherCAT system configuration tool via FoE or  directly via the bus coupler’s USB port to the Q.bloxx EC device.

The multi-functionality of the Q.bloxx EC series also affects the ESI (EtherCAT Slave Information) file. Hence each configuration requires a dedicated ESI file for a full set-up; the Q.bloxx EC modules provide a simplified ESI file. After initiation, the master may read the actual configuration (SDOs and PDO mapping) via CoE and go online. In case a master does not support this functionality, the configuration software of Gantner Instruments may also generate an individual ESI file with the according configuration.

2.2 Amplifier Cluster by Gantner Instruments as Single EtherCAT Slave

In a classical configuration, the decentralized devices from Gantner Instruments may be managed by a controller reporting back as a single device. The controller provides different interfaces for communication with your test bench environments, including EtherCAT. Also, each controller features an additional Ethernet interface. Both interfaces may be used independently. Process data for example may be, parallel to the test bench control signals, transmitted via EtherCAT at 1 kHz and additionally via Ethernet at 50 kHz for more demanding measurement tasks. The controller may also collect further data on predefined events, save data to a local storage device or send data via FTP (etc.). Thus, two or more applications are possible simultaneously with no cutback on EtherCAT functionality.

3. Summary

Gantner Instruments measuring devices provide full support for the EtherCAT standard. Depending on the application, the two concepts of the I/O modules, either standalone EtherCAT slave or as amplifier cluster reporting back as single EtherCAT slave, offer individual advantages. Both show the excellent accuracy and long term stability Gantner Instruments devices are known for alongside solid electrical isolation (even up to 1200 V), multi-functionality and flexibility. Leading manufacturers of test bench environments trust in Gantner Instruments.

Measurement Systems Used

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