Contactless direct torque measurement system for a single cylinder engine transient test cell

This paper describes the design and implementation of a low cost, contacless, reliable and sensitive direct torque measurement system applied to small single cylinder engine transient test cell. The experimental set-up consists of a three-phase induction machine, used as a transient a.c. dynamometer, and non-regenerative frequency inverter connected to an internal combustion engine. Coupling the engine to the dynamometer is by a low stiffness PA-6 nylon shaft with magnetic pickups at its ends. The sensors phase are zero when torque in PA-6 nylon shaft is null, adjusted in static conditions and engine/dynamometer turned off. Contactless torque measurement occurs by magnetic pickup phase difference during PA-6 nylon shaft torsion promoted by engine in operation. Engine speed is measurement by frequency of magnetic pickup signal. PA-6 nylon shaft torsion promotes a misalignment between sensors phase, proportional to torque application in both direction.The electronic capture unit using pulse train rising edge detection measures this phase change (time shift) in pulses. An embedded microcontroller development kit acquires the time-shifted pulses. This board has circuits for A/D signal conditioning, pulse width modulate channels, and a specific circuit for magnetic pickups signals conditioning, which can measure less then 0.09 Nm resolution, taking 96 time-shifts values. The design and implementation of all embedded electronics and data acquisition system follows electromagnetic compatibility premises, which have resulted in an adequate signal-to-noise ratio so that common and differential mode noises have not edges with amplitude approximately equal to the output magnetic sensors signals. The maximum measurement uncertainty associated with the contacless direct torque measurement system were calculated at +/- 0.54% to 99% confidence interval.The performance is presented under different load conditions, defined by engine throttle position and the results are compared with the engine datasheet performance curves.