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Test bench for hybrid drive: drive hybridisation means that requirements for test bench equipment have become considerably more stringent. Klaus Lanq, Business Development Manaqer at HBM in Darmstadt, explains how the eDrive test system for inverter-fed electrical machines benefits system integrators by simplifying measurement strateqies.

For many years, drive test benches in automotive engineering have always had a very similar design. After all, the object under test was always the same: a combustion engine and possibly a transmission. The focus has been mostly on optimising test routines, shortening test times and therefore cutting costs. Technical modifications were brought about by the increasing number of bus systems in test specimens and the introduction of real-time fieldbuses for automation.

But if the more stringent requirements for hybrid drives are to be satisfied by measurement systems for different tasks, the complexity of the test bench increases dramatically. The eDrive from HBM offers turnkey test and measuring equipment for even the most complex test benches, all from a single source, including the real-time transfer of data to the automation system.

The first group of measurement signals are rather slow variables--pressure, vibration and temperature, for instance. As a rule, these are used to determine the general status of the test bench and test specimen, to ensure the correct conditions for actual testing. However, things are now rendered more difficult by the need to insulate these inputs--sometimes up to 1,000V--for technical or safety reasons. If the temperature is measured directly at an inverter of the auxiliary drive, for example, it would be advisable from a technical safety perspective to insulate these temperature channels. This protects the measuring equipment and the operators in the event of a malfunction in the inverter. Even if the winding temperature of the motor also needs to be measured on a development test bench, the sensor must be insulated, just as when measuring the temperature on the HV battery.

After the fundamental mechanical variables, mechanical power variables now come into play--first rotational speed and then torque. Here, too, hybrid test benches face more exacting requirements. For example, the rotational speed of an electric motor can be much higher than that for a combustion engine, and torque fluctuations can also lie in a higher frequency range. This is due to the number of pole pairs of the motor which, together with the magnets, is responsible not just for the rotational motion but also for the torque ripple.

This is an interfering signal that must be recorded in order to understand its effects on the test specimen, the test bench and, of course, the drive train itself. Here, measurement needs to be more dynamic than on a pure combustion engine test bench, on which so-called torque peaks are only generated by the combustion process, which is of a much lower frequency than the torque ripple. Things also get more complicated when the mechanical power needs to be measured several times--if the power produced by the combustion engine and the electric motor have to be analysed separately, for example. Then, two measurement flanges and two rotational speed measuring systems have to be used.

Measurement of the angle of rotation is another special case. If the signals of the electric motor are to be analysed later on, for example, in order to produce flux maps or MTPA (maximum torque per ampere) curves, the position of the rotor is crucial for the necessary mathematical analysis.

Electrical power measurement is a completely new field: power analysers are normally used for this purpose, but they present the test bench of a dynamic motor with an array of problems. Conventional power analysers are optimised for use in the network, or with white goods. The measurement cycles are suitably slow, so that greater accuracies can be achieved through averaging. However, it is precisely these slow data rates that stand in the way of dynamic measurement, or rapid progress through a characteristic map with thousands of measurement points. And in most cases, there is no connection to a fieldbus system.

Yet another problem is the restriction of the number of channels to three or four power channels in most cases. What may just about suffice for three-phase motors and an intermediate circuit becomes a problem when there are five- or six-phase motors or other complex systems.

One of the most frequently underestimated problems is that of complete traceability. Power analysers deliver ready calculated results, and are unable to store raw data. Therefore, comprehensive tracing of the measurement chain is not possible. Calibrated power analysers may be used, but they are normally calibrated with pure sinusoidal signals at 53 Hz. But in the hybrid drive, PWM signals in the range of several kHz have to be measured. In such cases, a calibration certificate is unable to demonstrate exactly how a power analyser can perform measurements. In the aftermath of #dieselgate, this problem is obviously going to require even more attention in future.

Another often underappreciated detail is the synchronicity required for an efficiency measurement. If we intend to compare the electrical power input with the mechanical power output--to calculate efficiency--these power values need to have been obtained and averaged in precisely the same measurement window. For dynamic measurements, even the most disparate sample rates and input filters influence the electrical and mechanical signals, and may play a considerable part in possible measurement errors. The storage of raw data provides an escape from the problem of traceability. Here, in addition to the measured values for power, small extracts of raw data--current and voltage--are stored at a higher resolution. This permits the subsequent, renewed calculation of power values, such as active and reactive power and the mechanical power values, enabling the calculated efficiency map to be verified. This verifiability extends back to the raw data and the sensors, not just to the power analysers. Of course, power analysers can also store raw data, but they have not been optimised for this task.

A new approach is offered by the eDrive GEN DAQ measurement system from HBM. Controlled from a Windows computer, this modular device records all the necessary signals synchronously and simultaneously. To forward the computer results in real time, at the same time as raw data are stored, the EtherCAT bus--virtually standard in the automotive industry--is used.

Plug-in cards for current and voltage signals, mechanical variables such as vibration or pressure, and special satellites for recording bus signals and, as a special feature, temperatures with thermocouples with insulated inputs up to 1,000V. The rotational speed, torque and angle signal can be recorded directly and simultaneously for up to six measuring points. Since temperatures and CANbus signals are also recorded, it is now possible to carry out temperature compensation.
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Author:Lang, Klaus
Publication:Environmental Engineering
Date:Dec 1, 2016
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