Acoustic Balance: Weighing in Ultrasonic Non-Contact Manipulators

Acoustic traps and levitation systems can lift, translate and manipulate a wide range of objects and materials without contact. This enables new manipulation capabilities for robots that may not be possible otherwise. This paper presents an acoustic balance, a contactless method for weighing acoustically trapped objects in air. The method works by measuring a step response: the system commands a change in the phase of the acoustic emitters, which results in a sudden change in the equilibrium position of the trap. The object held within the acoustic trap undergoes damped oscillation as it settles into the new equilibrium point. The mass of the trapped object can be determined from the frequency of oscillation. Combined with methods for adding and merging materials in the trap, the method presented here can potentially enable a robot to operate a closed-loop process to acquire or maintain a desired quantity of material. Using weight as an error signal, material could be added by the acoustic system until the required quantity is in the trap.