System-integrated Measurement Enables Automotive Hardware-in-the-Loop Test System
Akka DNO uses efficient acquisition of measurement data via EtherCAT analog terminals from Beckhoff to optimize steering unit testing
Reliable steering is one of any vehicle’s most important safety-relevant elements. Correspondingly high demands are placed on system testing that accompanies the development process. Together with ZF Friedrichshafen AG, the Cologne, Germany-based Akka DNO GmbH has therefore developed a high-performance Hardware-in-the-Loop test system that acquires necessary data via high-end measurement technology directly integrated in PC-based control technology from Beckhoff.
With 230 locations around the world, ZF Friedrichshafen AG is a global leader in driveline and chassis technology as well as active and passive safety technology. Akka DNO GmbH (formerly Gigatronik Köln GmbH) specializes as an engineering partner for the automotive industry and develops modern Hardware-in-the-Loop (HiL) test systems with ZF, among others. Sören Ole Kuklau, Team Management Function Development at Akka DNO, explains: “We provide support with our long-standing expertise in the creation of test specifications, test automation and in the construction of the test infrastructure in particular. A good example of this is the HiL test system for the electronic power pack (EPP) in a vehicle steering unit, which was developed in close cooperation with ZF. We began development in 2016 and even at that time we were already using measurement I/O terminals from Beckhoff, such as the EL3751 EtherCAT input terminal. With oversampling, additional filters and the adjustable measuring range, this multi-function terminal offered us many options to reliably acquire certain analog values that are particularly critical in our industry.”
Dr. Michael Moczala, Team Leader Software/System Test Tooling in the area of Active & Passive Safety Technology, Steering from ZF in Düsseldorf, outlines further details of this HiL test system: “In the EPP-in-the-Loop (EPPiL) testing, the EPP – in other words, the combination of the electronic control unit and the electric motor that it controls to drive a car’s electric steering system – is tested in a simulation environment. In practice this means that all components that interact with these two hardware elements in the vehicle are mapped as virtual models and simulated on a special real-time system.”
PC-based control is universal, open and scalable
Test systems in the HiL area can also be automated completely and universally with PC-based control – extending to seamlessly integrated measurement and safety technology. Sören Ole Kuklau outlines the specific benefits for the EPPiL test equipment: “In the EPPiL system, the PC-based control technology from Beckhoff provides the complete I/O infrastructure. The actual simulation runs on the separate real-time system. With regard to computing power, the CX5140 Embedded PC has proven to be ideal for data handling. Not only that, the Beckhoff hardware can, if necessary, be scaled very easily according to individual application requirements – even up to high-performance multicore systems.”
According to Sören Ole Kuklau, the impressive depth of the I/O range and openness through the globally established EtherCAT standard offers further key advantages: “We placed very high demands on the I/Os with regard to the resolution of time and values and we paid attention to additional functions available in EtherCAT, such as oversampling. The EtherCAT measurement terminals from Beckhoff offer the ideal solution here, especially as the widespread use of EtherCAT allows the simple integration of third-party components in order to use automotive-specific bus systems, for example. Furthermore, we benefit in the test benches from the useful diagnostic options with EtherCAT, among other things, for monitoring the data connection between the test console and the control cabinet. In addition to that, there is the high reliability and performance of the data transmission that we rely on.”
Dr. Michael Moczala adds a further aspect: “With the very high data quantities, it helps that we can acquire and transmit the information at two different sampling rates – the critical signals such as position specification and torque measurement at 4 ksamples, in other words, with a 250 μs cycle, and the rest of the data at 1 ksamples or with a 1 ms cycle. In this way, the typical bottleneck can be avoided in data transmission to the separate real-time simulation system, which is connected to the Embedded PC as an EtherCAT slave.”
Analog I/Os form backbone of the EPPiL test system architecture
Dr. Michael Moczala explains the operating principle of the test system: “The hardware interface of the EPPiL simulator – the so-called test console – includes a load motor and a torque sensor for detecting the state of the connecting shaft. Connected to that is the actual test specimen, i.e. the electronic power pack consisting of a control unit and support motor. The EtherCAT I/O system forms the connection to the simulation technology, to the models representing the real-world mechanical steering components, different types of vehicles and roads through to the vehicle communication, among others. Communication between the modeling technology and the load motor as well as the torque measurement shaft is also implemented via the EtherCAT I/Os.
"From the simulation technology we obtain the angle of the shaft of the EPP motor and transmit it to the converter of the load motor. The torque sensor of the measurement shaft supplies the resulting torque. With this information, the movement equations are in turn solved in the simulation, from which we obtain the new position value to be transmitted to the load motor to close the HiL loop.”
The EtherCAT I/O system encompasses five EK1100 EtherCAT Couplers as well as 57 different EtherCAT Terminals. For the analog value processing, these include three EL4732 eXtreme Fast Control (XFC) output terminals, seven EL3702 XFC input terminals and 18 EL3104 input terminals. Sören Ole Kuklau explains their specific tasks: “The EL4732 output terminals transmit the setpoints to the programmable power supply units of the test system, and the EL3702s read their actual values back. The 64 channels of the so-called Fault Insertion Unit (FIU) for the interspersing of electrical faults, such as short-circuit or open circuit, are checked for plausibility via the 18 EL3104 terminals.” Dr. Michael Moczala adds: “The correct FIU function is thus monitored and the time of the respective switching procedure determined. The speed of the data acquisition is particularly important here, as switching times in the millisecond range are typical.”
Three EL3751 multi-function input terminals are additionally implemented. The torque sensor is read via one of the terminals. The other two inputs read the voltages of the power supply units. Sören Ole Kuklau comments: “Their flexibility with regard to the voltage range considerably simplifies our work, because the terminals can set to +/-10 V for the torque sensor or to +/-30 V for the power supply units.” For Dr. Michael Moczala, the quality of the measured data acquisition via the EL3751 is of crucial importance: “The torque sensor signal is fundamentally important to achieve stable system behavior in conjunction with the simulation model. Therefore, the values from the torque sensor must be transmitted with the smallest possible delay, very low noise and the greatest possible accuracy.”
PC-based control offers exciting future potential
Since the start in 2016, the third generation of EPPiL test systems equipped with Beckhoff technology is now available, as Dr. Michael Moczala explains: “Our experiences with Beckhoff technology have been very good, so we will be retaining it in coming test system generations and we will also use new developments in the EtherCAT Terminals wherever possible.” Sören Ole Kuklau also sees further development potential.
“We already have pre-development projects for automotive-specific test systems that are based completely on Beckhoff technology. These also include the servo drive technology, which is advantageous due to high performance and speed as well as the convenient configuration in TwinCAT software," he says. "Another pre-development project is concerned with the integration of MATLAB®/Simulink® in TwinCAT to be able to relocate the real-time simulation to the Embedded PC and reduce costs as a result. Apart from that, we are working on using the ASAM standard XiL-API to decouple test cases, test automation tools and HiL test hardware with TwinCAT."
Interested in optimizing testing with EtherCAT measurement terminals and advanced simulation tools? Contact your local Beckhoff sales engineer today.
Daymon Thompson is the Software Product Manager for Beckhoff Automation LLC.