Two industrial worlds have merged

December 6th, 2013, Published in Articles: EngineerIT

Traditionally, the worlds of PLC, motion control and control technology on one hand, with measurement technology and “lab applications” on the other, have diverged when it comes to data entry and processing in an industrial environment.

Yet divergence always means having to use additional interfaces, which leads to restrictions on efficiency and operating convenience. But why have this separation in the first place? For some time now, PC-based control has offered sufficient performance reserves to manage both tasks seamlessly and scalably – from the computer to the high-speed fieldbus EtherCAT and to the wide range of I/O terminals for measurement in particular.

Fig. 1: Scientific automation unites control and measurement technology, thanks to high-performance measurement terminals

 

The reason for the traditional separation of the two data processing worlds is precisely why PC-based control is of such an enormous benefit today: Until just a few years ago, the computing power of control systems was far too limited for handling automation and measurement technology. Thanks to Moore‘s Law and its predicted rapid advancement of PC technology, high-performance automation and sophisticated measurement data processing, as well as functions such as condition monitoring and energy monitoring, can now be united.

Taking the step from conventional programmable logic controller (PLC) technology to PC-based control must be accompanied by a corresponding change in thinking – in other words, integration of measurement technology aspects – during development and design.

Measurement technology from standard to high-end functionality

Fig. 2: TwinCAT Scope is an easily configurable software oscilloscope offering modern graphical signal representation including additional functions for measuring processes, e.g. a clear overview of oversampling values captured by EtherCAT measurement terminals.

 

This shift in thinking is extremely simple thanks to a unique approach where measurement technology is no longer restricted to a “black box” that is difficult to integrate. Rather, it can be implemented directly in a familiar and easily upgraded engineering environment that utilises standard I/O components.

The main advantage is in the underlying system structure: The measured data is captured in a simple, cost-effective and optimally scalable manner directly at the machine, using measuring terminals and is then transmitted for processing to high-performance Industrial PCs via extremely fast EtherCAT communication. All of the software modules, for example PLC, measurement technology, and visualisation, are combined on a single platform – for the purpose of consistently integrating the two worlds of control and measurement technology.

High-end measurement applications and standard measurement technology tasks are capably covered thanks to a comprehensive range of measurement I/O terminals. The numerous analogue input terminals with standard resolution will suffice for many applications: For example, the KL30xx bus terminals as well as EL30xx EtherCAT terminals with 12 bit resolution record analogue signal voltages in a wide signal range from -2 to +2 V, -10 to +10 V, 0 to 2 V, 0 to 10 V, 0 to 500 mV, 0 to 20 mA and 4 to 20 mA. Standard EL31xx analogue input terminals with 16 bit resolution are already suitable for high-precision control processes. In addition, they also support time-critical applications thanks to the extremely fast A/D conversion time – approx. 40 to 100 µs depending on the type – and provide support for EtherCAT distributed clocks.

Fig. 3: The thermocouple input terminal enables high precisiontemperature measurement to be integrated in the controller while the analogue input terminal allows direct connection of a resistance bridge or load cell.

 

High-end technology is further supported by terminals with 18 and 24 bit resolution and the corresponding measurement accuracy for standard current and voltage signals. In addition, highly accurate models are available for connecting thermocouples and resistance thermometers as well as versions for especially complex signals, such as performance measurement or condition monitoring based on IEPE acceleration sensors. The EtherCAT terminals enable the development of technically demanding solutions through the support of XFC technology (extreme fast control) and with it, advanced functions like oversampling.

XFC delivers high-speed data acquisition for measured values

XFC technology is based on an optimised control and communication architecture comprising an industrial PC, I/O terminals with extended real-time characteristics, EtherCAT, and TwinCAT automation software. I/O response times of under 100 µs can be achieved with this finely tuned system. XFC in particular uses functionalities such as distributed clocks, time stamping and oversampling.

Fig. 4: The EL3773 terminal is a power monitoring terminal for status monitoring of 3-phase AC voltage systems with a high resolution of up to 100 µs.

 

The distributed clocks in EtherCAT represent a core XFC technology and are a fundamental component of EtherCAT communications. This is because, in addition to minimum response times, the crucial factors for the control process are deterministic actual value acquisition (i.e. an exact temporal calculation must be possible), and a corresponding deterministic set value output. A deterministic behaviour must therefore be supported by the I/O components, but not in the communication or calculation unit. All EtherCAT devices therefore have their own local clocks, which are automatically and continuously synchronised with all other clocks. Different communication runtimes are compensated, so that the maximum deviation between all clocks is generally less than 100 ns and the system time is synchronised with extreme precision.

Time-stamped data types actually contain a time stamp in addition to their user data. This time stamp – together with the synchronised distributed clocks – enables the provision of temporal information with significantly higher precision for the process data. EtherCAT terminals with time stamp latch the exact system time at which edge changes occur. Digital values can likewise be output at predefined times.

Oversampling data types enable multiple samplings of process data within a single communication cycle and the subsequent collective transfer of all data. The oversampling factor describes the number of samples made within a communication cycle. High sampling rates can be achieved easily, even with moderate communication cycle times. Analogue EtherCAT input terminals with oversampling thereby enable signal conversion up to 100 kHz and time resolution up to 10 µs; in case of digital input terminals, these values reach 1 MHz and 1 µs.

Analogue input terminals with XFC functionality

The 1-channel EL3356 analogue input terminals enable direct connection of resistance bridges, such as strain gauges, for example. The 24 bit XFC variant is especially suitable thanks to support of distributed clocks as well as automatic self-calibration (which can be deactivated), dynamic filters, and a sampling rate of up to 100 µs for fast and precise logging of torque or vibration sensors.

The additional benefits of oversampling are leveraged, for example, by the 2-channel EL3702 analogue input terminals for signals in the ±10 V range and by the EL3742 for 0 to 20 mA. The temporal signal resolution can be increased in both cases by an oversampling factor of up to 100 compared with the bus cycle time.

The EL3773 EtherCAT terminal is a power monitoring device designed for status monitoring of 3-phase AC voltage systems and likewise supports XFC technology. For each phase, voltages up to 288 Veff/410 V DC and currents up to 1 Aeff/1,5 A DC are sampled as instantaneous values with a resolution of 16 bits. The six channels are measured simultaneously based on the EtherCAT oversampling principle with a temporal resolution of up to 100 µs and passed on to the control system. The control system has sufficient computing power for true RMS or performance calculations, and also for complex customer-specific algorithms based on the measured voltages and currents.

The 2-channel EL3632 analogue input terminal also supports oversampling. It is suitable for condition monitoring applications in which fluctuations are to be detected by means of acceleration sensors or microphones. Piezo sensors with IEPE interface (integrated electronics piezo electric) can be connected directly without the need for a pre-amplifier. The measuring signals sampled at a rate of up to 50 kHz are analysed on the PC via the TwinCAT Library or alternatively by means of custom software. The terminal can be adapted to individual requirements through configurable filters and supply currents. Through interfacing via EtherCAT and support for the distributed clocks function, the measurement results – and any detected defects – can be precisely allocated to an axis position.

High-precision measured values for sophisticated measurement technology

Higher resolution analogue input terminals are suitable for recording signals with extremely high precision. Examples include the 2-channel EL3602 input channels with 24 bit resolution for ±10, ±5, ±2,5 and ±1,25 V or the EL3612 input channels for 0 to 20 mA as well as the 2-channel EL3692 resistance measurement terminal for a measurement range between 10 mΩ and 10 MΩ.

The EL3681 EtherCAT Terminal is a digital multimeter terminal with 18 bit signal resolution. Currents of up to 10 A can be measured as well as voltages up to
300 V AC/DC. The measurement ranges are switched automatically, as is usual in advanced digital multimeters; the measurement type and range can also be adjusted with EtherCAT if required. Excellent interference immunity is achieved through the design of the EL3681, which features full electrical isolation of the electronic measuring system and dual-slope conversion.

The 1-channel and 2-channel PT100 input terminals for direct connection of resistance sensors enable high-precision temperature measurement. They are extremely precise with a resolution of 0,01°C per digit and are suitable for a temperature range of -200 to +320°C. The calibration result is confirmed in the case of the -0020 terminal versions by means of a calibration certificate. The 4-channel input terminal picks up the signals from four thermocouples and can also detect any broken wires. The high 24 bit resolution enables a scaling of 0,1 to 0,001°C per digit and 10 nV per digit. The internal high-precision temperature measurement at the cold junction therefore allows precise temperature measurement in calibrated operation also when using thermocouples.

Full utilisation of multi-core technology results in lower engineering costs

In particular, it is the new TwinCAT 3 software generation, featuring integration in Microsoft Visual Studio, which best fulfills the requirements of scientific automation – the convergence of automation and measurement technology. The real-time environment is designed in such a way that practically any number of PLCs, safety PLCs and C++ tasks can be performed on the same or different CPU cores. The new TwinCAT 3 condition monitoring library exploits these opportunities in particular: Raw data can be sampled with a fast task and processed with a somewhat slower task. This means that measured data will be continuously recorded and can be analysed independently in a second task with numerous algorithms. The individual functional modules of the condition monitoring library store their results in a global transfer tray, i.e. a type of storage table. From here the results can be copied to variables or processed using different algorithms so that an individual measurement and analysis chain can be compiled.

No specific Beckhoff modules or other modifications to the original model are required to create Matlab/Simulink modules. The Matlab and Simulink coders generate C++ code, which is then compiled in a TwinCAT 3 module.

Repeated use of the modules is possible through instantiation. The block circuit diagram from Simulink can be displayed directly in TwinCAT and used, for example, to set break points.

TwinCAT Scope enables the graphical representation of all relevant signals from scientific automation software. Its viewer component can be used for visualising signals in charts, while the server component logs the data on the picks up the signals from four thermocouples and can also detect any broken wires. The high 24 bit resolution enables a scaling of 0,1 to 0,001°C per digit corresponding target device. Scope also allows measured values to be read in the microsecond range to the exact cycle and enables visualisation of oversampling values captured by EtherCAT measurement terminals.

Contact Kenneth McPherson, Beckhoff Automation, Tel 011 795-2898,  kennethm@beckhoff.com

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