Torque Measurement on Electric Vehicles

The gradual change in mobility concepts from the combustion engine toward the electric motor has had impacts on test-bench measurement technology that will become more pronounced in the near future. Compared to classic combustion engines, electric drive systems have a significantly higher power density due to their smaller dimensions and lower weight. Heat loss of electric motors has been reduced to only about 10 percent, while over 90 percent of the electric energy is converted into mechanical energy. On top of that, electric drives in vehicles run at considerably higher rotational speeds, a fact that, at the same time, makes e-mobility test benches such a challenge for torque measurement technology.

Higher Rotational Speeds and Multiple Interfaces

HBM’s T11 torque transducer set new standards in terms of the nominal (rated) rotational speed in 2004. The T11 was long considered the standard in motorsport, and it could reach a rotational speed of up to 30,000 rpm due to its small rotor mass and low mass moment of inertia. The sensor was continuously developed and was replaced by the T40 series in 2016.

Alongside an increased nominal (rated) rotational speed of up to 45,000 rpm, the latest transducer generation has EtherCAT and PROFINET interfaces; it is thus ideal for dynamic applications and considerably facilitates the integration of torque and rotational-speed measurements into higher-level automation and control systems. 

In addition to their increased rotational speed, the dynamic behavior of electric drives places high requirements on future e-mobility test-bench concepts. Accordingly, the further reduction of mass moments of inertia and weight ranked high on the list of development targets. However, the optimization of electric power trains places further demands on the measurement systems. In view of an energy conversion efficiency of over 90 % and in contrast to the situation with combustion engines, very high requirements must be placed on the measuring equipment’s accuracy to enable differences between the respective variants to be implemented.

New Challenges: Accuracy and Freely Selectable Measurement Ranges

The T12HP precision torque transducer is available to accomplish such optimization tasks. With a specified maximum permissible speed of up to 22,000 rpm, this series is available with an accuracy class of 0.02. This transducer’s high basic accuracy allows for a FlexRange™ function over the entire measuring range of up to 10 kNm. Therefore, users do not need to toggle between defined measurement ranges, but they can look closely at freely selectable measurement ranges.

A generally growing environmental awareness and the trend toward increased sustainability will impact the further development of e-mobility. Strain gauge-based torque sensors will continue to play an important role in the future and be an integral part of the process of optimizing machine and vehicle components.

Embedded in a Holistic Measurement Concept

This optimization process aims at acquiring and analyzing not only the mechanical quantity torque but also many other mechanical and electrical quantities in the development of electrified drive trains. This is exactly where HBM’s eDrive system for testing electric inverters and machines comes in: It acquires physical signals (e.g. torque), electrical signals (current and voltage), and digital bus signals (CAN) simultaneously, allowing for data to be calculated and analyzed in real time. The eDrive system provides you with a holistic system from a single source to enable you to analyze the drive train and its losses with the highest precision.

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