Sigma's HSM (HyperSonic Motor) tech

This is a fast, virtually silent, compact autofocus drive motor system for lenses.
A conventional autofocus system would use the drive by a DC motor with gear and shaft mechanism.

Ultrasonic motor

Ultrasonic motors, generate rotational force from ultrasonic vibrational energy.
The basic ultrasonic motor consists of an elastic Stator and a rotating Rotor.

The Stator is made of a piezoelectric ceramic elements and a metallic elastomer.
This elastic Stator is in friction-contact with the Rotor.

When energized by an AC voltage, the ceramic elements in the elastic Stator, vibrate at ultrasonic frequencies.
The amplitude width of the elastic stator is only around 1 micron.
Resonance amplifies the vibration of the stator in contact with the rotor, creating a high-torque drive.
It lets the drive offer arbitrarily large rotation distances.

That vibrational energy is used to continuously rotate the rotor through the pressure contact between the rotor and stator.

Piezoelectric AF Motor

An ultrasonic motor typically uses a piezoelectric material, most often: lead zirconate titanate and occasionally lithium niobate or other single-crystal materials.

Lead zirconate titanate is an inorganic compound with the chemical formula Pb[ZrxTi1-x]O3. 0≤x≤1).
A.k.a PZT, it is a ceramic perovskite material that shows a marked piezoelectric effect.
It finds practical applications in the area of electroceramics.
It is a white solid that is insoluble in all solvents.

Compared to DC motor lenses, lack of reduction gears gives it quicker start/stop response.
Ultrasonic motors are replacing conventional micro-motors in camera lens autofocus systems.

They are now used in many consumer and office electronics requiring precision rotations over long periods of time.

Besides quick responsive auto focus, most ultrasonic motorized lenses allow full-time manual focus in the AF mode. That is, AF can be overridden without disengaging the AF mode altogether.