How Torque Transducers Help Optimize Hybrid Drivetrains for Buses and Trucks

Automotive OEMs commonly house test stands or special facilities to develop, characterize, and test car engines.  As hybrid technology plays an increasing role in vehicle designs, OEMs increasingly use such facilities in the development of hybrid-electric propulsion systems for the truck and bus industries.  Components of a hybrid-electric propulsion system might include a liquid cooled inverter, a high power-to-weight ratio AC induction electric motor and a reduction gear connected directly to a standard drive shaft and rear axle to provide traction power and regenerative braking.  

To ensure such systems manage power efficiently, engineers must accurately measure electrical motor/drivetrain variables such as rotational speed (rpm) and torque.  An in-house test stand setup usually comprises a battery simulator, frequency inverter, electric motor, dynamometer, data acquisition system (DAQ), communication interface and a PC in a control area. The dynamometer provides simulated resistance to the system to replicate road situations the electrical motor/drivetrain might encounter, such as when it’s pushing a heavy vehicle up a hill. This arrangement frees engineers from having to drive around in actual vehicles.

Note that while power tests on combustion engines mainly record mechanical signals, the testing of electric motors also necessitates measuring electrical signals between inverter and motor.

The inefficient method to measuring torque and speed

Previously, engineers took speed and torque measurements of motor/drivetrains by mounting the (big) electrical motor on the dynamometer using bearings so the motor could rotate. The dynamometer was equipped with a load cell and an offset torque-reaction arm. An engineer would run a pre-defined drive cycle, which made the inverter feed a certain amount of current into the motor and cause it to rotate, as if being driven in a vehicle. The offset arm pushed down on the load cell, thereby measuring force (that the motor was trying to spin) at a distance. So, for instance, say the offset arm was one meter away and was pushing down with a force of 1,000 Newtons, the system was then producing 1,000 Newton-meters of torque.  

A problem with this approach was that mechanical  inertia masked measurement subtleties. Hybrid drive trains can run at very high speeds, around 12,000 rpm as compared to the typical 6,000 rpm of a gasoline engine drivetrain, which introduces high-frequency signals not normally encountered when testing passenger cars. Signals from the load cell might therefore look nice and smooth, but in reality, the signals have lots of peaks and valleys because the drivetrain is actually encountering extreme variations.

The new and improved solution

A better solution came from putting a T12 digital torque transducer from HBM Inc. in-line with the electrical motor, the drivetrain and the dynamometer.  HBM provides measurement products including data acquisition systems, analysis and calibration software, strain gauges, transducers and sensors. 

The T12 transducers are a non-contacting design that reads torques via digital telemetry and speed via a speed sensor. An integrated measurement signal conditioner ensures high quality and reliable signals. Measuring speed and torque with the transducer gives engineers necessary information on how fast the system is going and how much torque it is using to go that fast. From this data, engineers can calculate how much power the motor/drivetrain requires. The T12 torque transducer records dynamic torque, indicates the direction of rotation, and can even compute the mechanical output power. The DAQ synchronously records the electrical signal of the inverter and motor as well as the digital signals from the torque transducer to provide voltage, for example, as an equivalent torque value. Other outputs can include frequencies and currents. An optional feature allows the output of data to the DAQ via CAN or Ethernet, which eliminates the necessity for digital to analog conversion.

The transducers can pick up the high-frequency signals that are produced when testing electrical motor/drivetrains, so engineers can get data they normally wouldn’t see. This helps engineers improve their hybrid drivetrain designs. Engineers can use the transducer to tune a hybrid system because the device lets them determine exactly how much current it takes to produce a desired amount of torque instead of approximating this value. And the robust T12 requires little maintenance, which speeds up development times.

Improve the efficiency of electric motors and inverters

In addition, HBM now offers a solution that can help improve the efficiency of electric motors and inverters using a T12 torque sensor and HBM Genesis HighSpeed data acquisition system. Together, they allow synchronous, dynamic measurement of mechanical and electrical signals. Continuous storage of raw data allows live analysis for determining active and reactive power, as well as energy conversion efficiency.  For more information on this new solution, please visit

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