High Accuracy Dynamometer Torque Calibrations: A Systems Approach

Highly accurate dynamometer torque data are essential for laboratory measurements of brake-specific fuel consumption and friction torque. In the Ford Scientific Research Laboratory, the torque calibration accuracy specification is ± 0.2 Nm for absolute values less than 40 Nm and ± 0.5 % of reading from 40 Nm up to 720 Nm. It is a significant challenge to meet the ± 0.2 Nm part of the specification. This paper describes how the specification was met for hydrostatically floated 225 and 300 kW AC dynamometers with a load cell measurement of reaction torque. A system analysis quantified all significant sources of torque calibration error, developed an error budget, and prioritized necessary component improvements. The most significant improvements were made to the lift and thrust hydrostatic bearings, the linkage connecting the dynamometer stator and load cell, the motor leads, the load cell, and the load cell signal conditioner. In addition, the load cell mounting system was designed to avoid excitation of the stator-load cell system at its natural frequency by known vibration sources. After all improvements were completed, the torque calibration accuracy was verified to be well within the specification.