Edited by Robert Repas
ThinGap motor cross section
Semiconductor-wafer production, servo writers, and other close tolerance applications need smooth velocities without any variations in rotational speed. Typically, eight-pole slotted brushless dc (BLDC) motors work well in these situations. However, deviations displayed in a recent application were unacceptably high prompting a search for a smoother motor with lower cogging effects.
All motors exhibit some form of cogging, an effect created by nonuniform magnetic-field densities that surround the magnets within the motor and the motor's iron construction. Conventional slotted BLDC motors feature stators with slotted steel laminations that form a uniform stack with a series of teeth. Inserted into each slot is a wound copper coil that produces an electromagnetic field. The laminated stack and copper coils form the stator assembly. Laminated material outboard of the copper windings in the stator, including the motor housing, completes the return path for the magnetic circuit.
As permanent magnets in the rotor pass the stator teeth, they're attracted to the iron in the teeth. As the magnetic attraction jumps from tooth to tooth, the magnetic force aids or hinders rotation to systematically-speed up or slow down the rotor. The uneven magnetic pull creates cogging. The effect is similar to rolling over rumble strips in a car. The loss of smooth rotation ultimately leads to velocity and torque ripple, efficiency loss, vibration, and noise. Velocity ripple describes variations or oscillations of rotational speed over time while torque ripple describes similar behavior in torque over time. Copper wires wound through the slots increase this effect. The unbalanced pull from tooth to tooth also produces a radial deflection in the rotor.
Slotless BLDC motors overcome many iron-coremotor weaknesses by using magnet-wire coils without the iron core. In place of copper wires wound through slots in a laminated-iron stack, washer-shaped silicon-steel laminations support the exterior of the wire windings. The washer-shaped laminations provide iron for the magnetic path but eliminate the teeth. Windings hold their orientation to the stator laminations through encapsulation in a thermoplastic material.
The configuration eliminates stator teeth and minimizes cogging; but the stationary laminations possess eddy-current losses. Magnetic drag on the spindle reduces efficiency and creates heat. Copper occupies only half of the winding space and so creates a large magnetic gap. The large gap limits magnetic-flux density and, thus, motor torque. In addition, the wide air gap between stator and rotor further reduces overall magnetic-flux density.
The copper packing density of circular wire, larger air gaps, wirewinding structure, and higher electrical resistance limit performance of traditional slotless BLDC motors. While they reduce cogging, magnets sweeping past the toothless laminations can create velocity and torque-ripple errors.
Feedback encoders with high data counts can measure velocity over time letting controllers adjust drive currents to compensate for velocity ripple. However, speed limitations keep these encoders out of some kinds of applications. Also, corrective actions for velocity ripple based on encoder data with cogging errors may excite vibrations that amplify the errors rather than damp them.
ThinGap BLDC motors replace the wire windings used by conventional iron and ironless-core BLDC motors with a freestanding coil. The coil is a structure made of a precisely configured copper sheet and glass-fiber composite. This configuration allows significantly higher copper packing density than that of round or even square magnet wire. The lower resistance in the copper sheet creates a more-efficient motor with higher power density with low L and I2R losses.
ThinGap BLDC motors eliminate relative motion between the rotor and outer laminated return path by rotating all of the iron with the magnets. Hysteresis losses are virtually eliminated. The only remaining torque fluctuation is caused by harmonics in the back-EMF and current waveforms.
Inductance in the ThinGap motor is extremely low. That means a standard, off-the-shelf amplifier won't provide the best low-error correction. Adding an inductor between the motor and amp does little to improve the situation where ultralow velocity errors are required. The inductor creates propagation delays that make precision speed control of 0.0001% or less unmanageable. Any deviation of the back-EMF voltage or current waveforms from a perfect sinusoidal signal causes torque ripple in conventional BLDC motors. The ThinGap motor with its sinusoidal back-EMF waveform generates constant torque when using a sinusoidal current. Matching the motor with a sinusoidal-drive amplifier produces the proper coupling of waveforms that results in extremely low torque ripple.
The unique construction and configuration of ThinGap motors produces an inherently low harmonic distortion in the back-EMF. A sine-wave input provides very smooth operation. Smooth operation is not guaranteed with a conventional BLDC motor with sinewave input.
A recent design problem in a servo-writer application that used an air bearing demonstrated the effects of cogging even when using the more common slotless BLDC motor. Servo writers are used in disk-drive production to write digitally generated magnetic reference patterns on the disk surface at very high speeds. The reference patterns are used to precisely locate the magnetic read/write heads during normal operation of the disk drive. The spindle nose within the air bearing had a deviation error that was only 0.15 in. (3.81 nm); but that still exceeded the limits for this specific application.
Motion errors in these systems are classified as either synchronous or asynchronous. Synchronous-motion errors occur in every revolution. The repetitive nature of the error makes them easy to strip out. Asynchronous-motion error is irregular and, therefore, more difficult to address. As the nose of an airbearing spindle revolves it creates a measurable asynchronous error.
In this case, the eight poles of the BLDC motor were visible in the error motion of the spindle nose. Technicians determined the major source of the error came from the small amount of cogging still present in the slotless BLDC motor used in the initial design. Cogging-torque influences disrupted shaft rotation; and that affected the encoder, its output, and position feedback. Their combined actions introduced the unacceptable error motion at the spindle nose of the air bearing. By switching to the ThinGap BLDC motor, the spindle-nose error was reduced to well within acceptable limits in the servo writer due to a motor torque ripple of less than ±0.045%.
Surface Engineering, (408) 734-8810, www.surfeng.com-9
ThinGap Corp., (805) 477741, thingap.com