Case study on force compliant robot arm controller for nasopharyngeal swab insertion

Abstract

The nasopharyngeal (NP) swab sample test, commonly used to detect COVID-19 and other respiratory illnesses, involves moving a swab through the nasal cavity to collect samples from the nasopharynx. While typically this is done by human healthcare workers, there is a significant societal interest to enable robots to do this test to reduce exposure to patients and to free up human resources. The task is challenging from the robotics perspective because of the dexterity and safety requirements. While other works have implemented specific hardware solutions, our research differentiates itself by using a ubiquitous rigid robotic arm. This work presents a case study where we investigate the strengths and challenges using compliant control system to accomplish NP swab tests with such a robotic configuration. To accomplish this, we designed a force sensing end-effector that integrates with the proposed torque controlled compliant control loop. We then conducted experiments where the robot inserted NP swabs into a 3D printed nasal cavity phantom. Ultimately, we found that the compliant control system outperformed a basic position controller and shows promise for human use. However, further efforts are needed to ensure the initial alignment with the nostril and to address head motion.

Fig. 1

Nasal cavity apparatus arranged next to the Franka Emika Robot arm with the proposed force sensing swab end-effector attached. Two GoPro cameras are used to observe the outcome of the insertions and are not part of the control loop.

Fig. 2

Comparison of external force estimation methods evaluated with the arm moving through free space without any load. The underlying Franka Emika (FR) system estimates task space load by projecting the measurements from its seven torque sensors and has an unsuitable amount of noise and drift. The proposed loadcell has much lower noise and is much more suitable for responding to forces transmitted by the swab.

Fig. 3

Components of the custom end-effector. Left: Wheatstone bridge amplifier and digital conversion circuit. Right: the tri-axial loadcell interfaced with its housing and swab mount. Note that the red stripes on the swab are labelled to gauge distance in experiments, but do not have any functional purpose related to the controller.

Fig. 4

Nasal cavity phantom used for validation experiments. The red line highlights the path to reach the nasopharynx from the nostril.

Fig. 5

Block diagram for the proposed NP swab insertion system.

Fig. 6

Side and top frames of a successful insertion taken by the two GoPro cameras.

Fig. 7

Comparison of total observed forces and the accompanying displacement along the insertion axis for both versions of controllers recorded from the start of motion until the termination condition is reached, which is marked by the impulse of force as the swab reaches the nasopharynx. (a) forces encountered during a nominal insertion angle. (b) forces encountered during a misaligned insertion. Notice how the proposed controller is able to adjust and minimize a collision that occurs early in the insertion.

Fig. 8

Boxplots showing the difference in the final end-effector position and angle between the proposed and baseline controllers on trials where both controllers successfully reached the nasopharynx. The black line indicates the median, while the box and whiskers occupy 1 and 1.5 times the interquartile range.

Fig. 9

Two types of failure states observed in the trials: (a) The elevation angle for the insertion is too high, resulting in it travelling through the wrong passage in the nasal cavity. The proposed compliant controller prevented these states. (b) The swab becomes wedged before entering the nasal cavity. This state occurred with both controllers.