Development of a bi-directional standing wave linear piezoelectric actuator with four driving feet.

•A bi-directional standing wave linear piezoelectric actuator with four driving feet is proposed.•Two transducers are set parallel: the first transducer push the runner forward, while the other drives backward.•The frictional force between the interfaces is reduced by using ultrasonic vibration.•The prototype achieves maximum no-load speed of 244 mm/s and maximum thrust force of 9.8 N.•The prototype achieves very close output speeds for the bi-directional driving.