  Ultrasound scanning robots enable the automatic imaging of a patient's
internal organs by maintaining close contact between the ultrasound probe and
the patient's body during a scanning procedure. Comprehensive, high-quality
ultrasound scans are essential for providing the patient with an accurate
diagnosis and effective treatment plan. An ultrasound scanning robot usually
works in a doctor-robot co-existing environment, hence both efficiency and
safety during the collaboration should be considered. In this paper, we propose
a novel multi-modal control scheme for ultrasound scanning robots, in which
three interaction modes are integrated into a single control input.
Specifically, the scanning mode drives the robot to track a time-varying
trajectory on the patient's body under the desired impedance model; the
recovery mode allows the robot to actively recontact the body whenever physical
contact between the ultrasound probe and the patient's body is lost; the
human-guided mode renders the robot passive such that the doctor can safely
intervene to manually reposition the probe. The integration of multiple modes
allows the doctor to intervene safely at any time during the task and also
maximizes the robot's autonomous scanning ability. The performance of the robot
is validated on a collaborative scanning task of a carotid artery examination.
