HBM on the test bench: Allison Transmission puts HBM to the test

Allison Transmission, a division of General Motors, is a world-class supplier of medium and heavy-duty automatic transmissions. The company also markets Allison Electric Drives™, a line of hybrid electric propulsion systems being developed for various transportation markets.

Test-benches for hybrid electrical drives ...

By combining AC ventor-controlled, electric motor power to the conventional mechanical powertrain, hybrid electric propulsion systems are able to reduce the demand on the engine, thereby lowering emissions and improving fuel economy. By using electric motors as generators during deceleration, vehicle braking energy is recovered and stored as battery enery to be re-used during peak load periods. Brake life is extended and maintenance costs and downtime are reduced.

Using HBM Technology

Allison primarily uses HBM products to handle dynamometer driveline torque and speed measurements for hybrid electric development, powertrain development test stands. They currently use the T10F torque flange, T32 FNA torque transducer, and PME MP60 measuring amplifiers on their test systems.

Allison´s hybrid transmission test stand consists of a dynamometer or engine input to the test transmission and another dynamometer to load the output.

The input driveline has an HBM T10F torque flange mounted in-line between the dyne (or engine) and the unit under test. This transducer was customized by HBM to achieve 12,000 rpm operation. The output driveline has a custom T32 FNA torque transducer which incorporates the telemetry antenna of a T10F to incorporate the higher resolution speed sensor. Using PME MP60 measuring amplifiers, the torque and speed signals are conditioned and fed as scaled analogue DC signals to the test cell’s data acquisition system.

Fig.1:Allison automatic transmission looking from output end of test setup

Advantages of HBM Technology: Easy installation – no alignments, no bearings.

With HBM’s telemetry-style torque transducers, there are no bearings to wear out and no transducer specific alignments that are expected with traditional foot-mounted rotary transformer or slip ring units.

Fixturing is simplified, since a generous air gap makes antenna mounting easy. And T10F Torque Flange conserves valuable test length with its narrow profile.

High stiffness minimises dynamic loading

The job of any measurement engineer is to minimize the effect on the undisturbed system in which a measurement is to be acquired by introducing a sensor.

With dynamometer test stands, the torque transducer/sensor is only part of a more complicated measurement system intended to deliver dynamic torque to the unit under test without influencing its response to the torque input or load. The primary factor in minimizing dynamic loading of the torque measurement system is stiffness. The T10F is particularly rigid, providing excellent torque sensitivity and linearity with minimal deflection.

High accuracy while maintaining necessary torque overload ranges

Transmission test stands encounter large transient torque loads, particularly on the output driveline. During development, shift tuning events can ring the driveline resonances. The peak torques generated can be as much as 8 to 10 times the initial loading. Inadequately sizing the transducer for these overloads can result in driving the signals into the nonlinear range of the sensor, causing erroneous transient signals and measurement errors, and potential damage to the sensor. HBM’s T10F and T32 FNA products provide the high accuracy required over full range, while maintaining the overload capability Allison need.

Multiple torque ranges for increased resolution and accuracy of low measurement ranges Transmission development is frequently characterized by spin losses of the unit under test. These drag torque measurements are minuscule compared to the nominal torques of the input and output.

Traditionally, a low range torque transducer would be installed, or a separate test stand would be used to measure these spin losses. Setups on the test stand are time consuming and costly. Therefore, a transducer with high resolution and accuracy over both high and low ranges is desirable.

Using HBM’s PME MP60 measuring amplifier, Allison is able to have the transducer calibrated for two (or more) ranges and scale the output for increased resolution to their data acquisition system. Thus, the low torque range accuracy is improved for precise determination of spin losses, without the need for a secondary transducer change or alternate test setup.

Speed resolution and excellent noise immunity

Testing automatic transmissions and hybrid powertrains yields high dynamic responses on the input and output drivelines. A T32 FNA torque transducer was specified for the output driveline to accommodate the large torque loads. However, the standard speed sensor did not have adequate bandwidth for the application. HBM provided a custom speed sensor solution using the T10F’s speed sensor design and antenna.

Fig.3. T32 FNA on the output to the transmission

Hybrid electric systems are generally powered by DC to AC converters that switch high voltage DC power on and off to produce an AC signal to the motors. This high frequency, high voltage switching can generate EMI noise that disrupts transducers and corrupts the measurement process for sensors in the test system (i.e. torque transducers). HBM’s torque transducers and conditioners offer superb signal integrity despite the inherent measurement noise.

Terry Ledgerwood, Allison Transmission

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