Test & measurement

APPLICATION REQUIREMENTS Solar Team Twente had two main uses for its power measurement system:

To validate the accuracy of the ‘fuel gauge’, the car’s on-board sensor measuring the battery’s state of charge. This system measures current flowing into the battery (from the solar panels) and flowing out of the battery (to the motor). By subtracting output from input, it can calculate the residual charge in the battery. This required extremely accurate continuous measurement of current flows.

To measure the power output of the motor system, including the inverter and the motor itself, at a range of power input values, to enable the team to refine the design and incrementally improve its efficiency. This called for extremely accurate power analysis at high sampling frequency.

The team’s race strategy also called for precise regulation of the battery’s state of charge. Every team’s goal is to finish the race with zero energy left in the battery to maximize total energy usage, and thus achieve the highest possible speed for the longest possible distance without running out of power. The more accurate the state of charge measurement, the more confidently the race team can set the optimum speed of the car’s cruise control, taking into account, the weather, the capacity of the battery, the performance of the competitors and other factors.

THE CHALLENGE RED E’s engineering design team needed to repeatedly perform power (concurrent voltage/current) measurements to measure the efficiency of the motor system under various operating conditions. The development process involved multiple iterations of the system design, each one validated to assess the effect on efficiency. The benchmark for the team’s design was the off-the-shelf motor that it, and other teams, had used previously – already itself a highly efficient motor system. As electrical engineer Rob Kräwinkel says,

“We were looking for that extra one per cent efficiency to give us an edge over other race cars. When you are already at better than 95 per cent efficiency, eliminating any remaining losses is really hard to do. You have to be able to look at a detailed level at tiny deviations in voltage or current. That means you need a really accurate and sensitive power measurement system.” The same requirement for measurement

accuracy applied to the battery management system: even tenths of a percentage point of extra accuracy in state of charge measurement can make the difference between winning and losing.

Instrumentation Monthly June 2020

THE MEASUREMENT SOLUTION To achieve the accuracy and precision required for system design and validation, the RED E team chose a WT5000 Power Analyzer from Yokogawa. Measurement accuracy is rated at ±0.03 per cent in the instrument’s datasheet. Measurement bandwidth is 10MHz for voltage and 5MHz for current. The maximum sampling rate of 10Msamples/s exceeds the data refresh rate required to validate the fuel gauge system. WT5000 users can make simultaneous measurements on up to seven inputs, and view them on its high- resolution 10.1” touchscreen. Solar Team Twente’s Rob Kräwinkel says that when the electrical engineers first installed the WT5000-based test set-up, they discovered that the current sensors in the fuel-gauge circuit had a previously undiscovered offset which had been making the state of charge measurements inherently inaccurate. “The current sensing circuit we had previously was already better than 99 per cent accuracy, but we were looking for better than that – it was only when we analyzed the sensor with the WT5000 that we were able to compensate for the offset in the current sensor, and so configure the measurement outputs to achieve optimal accuracy. That crucial extra confidence in our state of charge measurements can give the driver a vital extra 1km or 2km of range at a given speed that we would not otherwise have been sure of getting from the battery,” says Kräwinkel.

PRODUCING OUTSTANDING RACE PERFORMANCE The ultimate test of the engineering development work done by Rob Kräwinkel and his team came when RED E competed in the 2019 World Solar Challenge in October 2019. The race was a test of whether Solar Team Twente had produced a more efficient motor system than competitors. The race would also tell whether the battery’s fuel gauge was as accurate as the team believed it was. With over half the race completed, RED E was in

the lead and on course to win, with an average speed of 89.7 km/h, when mother nature intervened. The car was irreparably damaged when a very strong cross-wind gust forced the car off the road and down a slope, where it rolled over. In the event, the race was won by the Belgian

team, Agoria, with an average speed of 86.6 km/h. Without the accident, and if RED E had maintained its superior performance, not only would the car have won, but it would also have achieved an average speed nearly 19 per cent faster than it managed in 2017. This improvement in 2019 was in part due to better weather for solar-powered racing before RED E’s accident. Nevertheless, the engineering developments implemented in the 2019 car were responsible for a substantial improvement in performance.

WT5000 PRECISION POWER ANALYZER Offering the best in isolation, noise immunity, current sensing and filtering in a modular architecture, the WT5000 is an extensible measurement platform that provides precision power analysis backed by the world’s leading in- house calibration facility for power analyzers, in Amersfoort, The Netherlands. Users can make simultaneous measurements on up to seven inputs and compare them in split- screen mode on the high-resolution 10.1” touchscreen. The modular architecture of the WT5000 provides seven slots supporting various types of input modules, providing a flexible measurement system. The WT5000 also offers advanced filtering options, including: synchronization source filter ; enhanced frequency filter ; and digital parallel path filters. Operable by a touchscreen and hardware controls, the WT5000 offers an intuitive measurement experience. As Rob Kräwinkel says, “The WT5000 is a very nice instrument to use, it’s intuitive, and it’s easy to find your way around the controls. It’s also easy to tweak the display so that it shows exactly the measurement outputs you are interested in, like when it clearly demonstrated that our new inverter design outperformed the equivalent off-the-shelf model by a significant margin.”

Yokogawa 53

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