Smoother motion can be achieved with sinusoidal commutation where the drive current is not a square wave but a sinusoidal signal phase-shifted by 120° between each motor winding. It is possible to interpolate the position signals from the hall sensors to provide more detailed position information of the rotor and avoid torque ripples, but for the best results an additional position encoder enables the controller to precisely control the current in each winding. In this scenario, hall sensors are redundant or may be used for the determination of the phases respectively. The described operation is available for PI's C-891 3-phase motion controllers, SMC Hydra controllers, as well as all ACS motion controllers.
At high rotor speeds, the calculated currents may lag behind the actual rotor position as a result of limited control bandwidth, which means that the stator and rotor field are no longer exactly perpendicular to each other. This can be avoided by field-oriented control, also called vector control, where the current vector is controlled in the rotating coordinate system of the rotor resulting in extended speed and positioning accuracy. This type of operation is available with the PI C-891 3-phase motion controller as well as all ACS Motion Controllers.
Torque motors are permanent-magnet synchronous rotary motors providing smooth motion, high peak and continuous torque, and high efficiency. They are used in applications where the load is directly connected to the rotor, without additional components such as gear boxes.A torque motor is similar to a linear motor bent into a circle or a low profile BLDC motor, with a large diameter (fig. 11a). In precision positioning applications, torque motors are often employed in rotary stages, directly driving the platform (turn table) without play or backlash.
The large radial dimensions allow for hollow shafts and large apertures, an advantage for conducting laser beams or cables. Because of the motor design, gear boxes are not required to amplify the torque output, eliminating backlash and play, leading to higher stiffness, positioning accuracy and repeatability. The direct drive principle also provides high dynamics, important for fast acceleration and step/settle response.