A servomotor-based alternative to traditional torque measurement systems: calibration and performance

Abstract Motor test benches require torque and speed measurement instruments alongside a loading system to characterize performance. However, Technology Readiness Level 4 (TRL4) motors often feature custom geometries, leading to costly adaptations of standard equipment. This paper introduces the Servomotor Torque Measurement System (STMS), an integrated solution that measures torque under rotation. The STMS combines a servomotor (SM), its servo drive, and a programmable logic controller (PLC). It repurposes a single SM to replace three conventional bench components: the torque transducer, speed sensor, and primary load unit. Operating in torque control mode with a speed setpoint of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>ω</mml:mi> <mml:mrow> <mml:mi>SP</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> </mml:mrow> </mml:math> rpm, the system applies a controlled load via the torque setpoint while simultaneously acquiring torque and speed data directly from the motor shaft. This eliminates the need for multiple instruments and inline mechanical couplings. The SM acts as a load generator, applying a counteracting torque without affecting the rotational speed, which is also acquired via the PLC. As the SM torque is internally calculated by the drive, the STMS was calibrated for two ranges (0–1.000 N·m and 0–9.00 N·m) using different motor models, following the Brazilian standard NBR 12240. Results showed measurement uncertainties between 0.6% and 0.8% of full scale. The system’s practicality was demonstrated by testing a TRL4 thermomagnetic motor, confirming its feasibility for potential dynamic torque measurement applications.