Article
A Piezoelectric Tactile Sensor for Tissue Stiffness
Detection with Arbitrary Contact Angle
Yingxuan Zhang, Feng Ju * , Xiaoyong Wei, Dan Wang and Yaoyao Wang
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics,
Nanjing 210016, China; zhangyingxuan@nuaa.edu.cn (Y.Z.); weixiaoyong@nuaa.edu.cn (X.W.);
wangdan_053@nuaa.edu.cn (D.W.); yywang_cmee@nuaa.edu.cn (Y.W.)
* Correspondence: juf@nuaa.edu.cn
Received: 19 October 2020; Accepted: 16 November 2020; Published: 18 November 2020
Abstract:
In this paper, a piezoelectric tactile sensor for detecting tissue stiffness in robot-assisted
minimally invasive surgery (RMIS) is proposed. It can detect the stiffness not only when the probe is
normal to the tissue surface, but also when there is a contact angle between the probe and normal
direction. It solves the problem that existing sensors can only detect in the normal direction to ensure
accuracy when the degree of freedom (DOF) of surgical instruments is limited. The proposed senor
can distinguish samples with different stiffness and recognize lump from normal tissue effectively
when the contact angle varies within [0
◦
, 45
◦
]. These are achieved by establishing a new detection
model and sensor optimization. It deduces the influence of contact angle on stiffness detection by
sensor parameters design and optimization. The detection performance of the sensor is confirmed by
simulation and experiment. Five samples with different stiffness (including lump and normal samples
with close stiffness) are used. Through blind recognition test in simulation, the recognition rate is
100% when the contact angle is randomly selected within 30
◦
, 94.1% within 45
◦
, which is 38.7% higher
than the unoptimized sensor. Through blind classification test and automatic k-means clustering in
experiment, the correct rate is 92% when the contact angle is randomly selected within 45
◦
. We can
get the proposed sensor can easily recognize samples with different stiffness with high accuracy
which has broad application prospects in the medical field.
Keywords: tactile sensor; piezoelectric sensor; stiffness detection; arbitrary contact angle
1. Introduction
With the development of medical treatment and the advancement of robot technology,
robot-assisted minimally invasive interventional surgery (RMIS) is becoming popular. It has the
advantages of small trauma, quick recovery and high accuracy. Compared with traditional surgery,
doctors cannot touch tissue directly in RMIS, which may cause misdiagnosis or wrong resection.
Since the lumps are often harder than the surrounding normal tissues, doctors can usually directly
distinguish the lump by the difference in stiffness felt. [
1
,
2
] In order to distinguish different stiffness in
RMIS, a tactile sensor for tissue stiffness detection is essential.
There are many kinds of tactile sensors that can be used to detect force and stiffness.
C. H. Chuang [3]
designs a minimized tactile sensor which can be mounted on an endoscope to
detect tissue with abnormal stiffness. A special piezoelectric system mounted on conventional biopsy
needles proposed in [
4
] is designed to evaluate abnormal tissues when penetrating tissue. A force
sensor proposed in [
5
,
6
] is used to measure instrument interaction forces in a beating heart. A unique
structure of the Fiber Bragg Grating (FBG) force sensor proposed by Lv [
7
], composed with a central
optical fiber, can detect axial force with high resolution and small error, but the structure will deform
when detecting, which will easily lead to fatigue damage of the probe. The FBG three-dimensional force
Sensors 2020, 20, 6607; doi:10.3390/s20226607 www.mdpi.com/journal/sensors
