An integrated low cost sensor for the direct torque control of brushless DC motors

Control of brushless, variable-speed drives by the use of stator field orientation techniques has received much attension in recent years and has been extensively documented. Such schemes require stator current and rotor speed or position information as inputs to a machine model which is solved for the electromagnetic torque, in real time generally via a digital signal processor. Torque control by implementation of the above technique requires the effects of machine parameter variations due for example, to changes in temperature, magnetic saturation etc. and loss mechanisms due to windage, friction and parasitic effects, are taken into consideration. These non-linearities increase the processing complexity and can result in a drive-specific controller. Also, in many industrial processors, the torque down stream in the mechanical system needs to be controlled, particularly where coupled system inertias give rise to mechanical torsional oscillations. The alternative approach of using a discrete torque sensor as the feedback element in a torque control loop has, to date, been prohibitive on the grounds of size and cost. However, this paper presents a technical description of a new, potentially low cost, non-contact, torque transducer utilizing surface acoustic wave (SAW) technology for shaft torque measurement. The sensor has a short axial length making it ideally suited to mounting integral with the motor housing or unobtrusively in the mechanical drive train. The operation, construction and application for torque measurement will be discussed together with results from a preliminary study into the feasibility of using such a device as a transducer for control of brushless dc traction motor torque.