The sense of touch plays a critical role in traditional open surgeries since it could provide tactile feedback to surgeons and is also used to acquire intrinsic properties of tissues through palpation. However, it is partially or completely lost in most existing robot-assisted minimally invasive surgeries. To solve this problem, a miniature tactile sensor with diameter less than 8 mm suitable for catheter robot-based tissue hardness palpation is presented in this paper. The stringent size constraint of minimally invasive surgery (MIS) is met by a unique spiral shape as well as a vertically configured piezoelectric transducer. The spiral shape also helps it achieve a low operating frequency suitable for testing biological tissues. The relationship between electrical impedance of the sensor and mechanical impedance of a load is derived based on the transduction matrix model, which forms the basis of the unique simultaneous actuation and sensing (SAS) technique. As a result, hardness of the load could be sensed from the sensor’s electrical impedance by extracting the resonant frequency, with simple instrumentation. The proposed sensor and SAS technique are verified numerically on a finite element model and experimentally on a prototype. After properly choosing the vibration mode and operating frequency range, the sensor is able to perform hardness sensing in a wide range of 0–1.7 MPa. In addition, both simulation and experiment results indicate that the sensor has high sensitivity and low variance in the low-hardness region, and relatively lower sensitivity and higher variance in the high-hardness region, suggesting that the sensor can be used in two different sensing modes (quantitative measurement and qualitative classification) in the two regions, respectively. An ex vivo experiment confirms that the sensor could detect the presence, shape and location of an embedded lump from spatial distribution of tissue hardness acquired through grid-based palpation, followed by an improved k-means clustering algorithm. Compared with traditional hardness sensors, this tactile sensor is developed with a unique spiral shape which reduces the operating frequency for enhancing the interaction with biological tissues while keeps the overall size of the sensor as small as possible. And the proposed unique simultaneous actuation and electrical impedance sensing mode helps simplify the instrumentation, making it easier to integrate the sensor into MIS equipment.

This paper presents a novel piezoelectric tactile sensor for tissue hardness sensing, featuring online tunable sensing performance (resonant frequency, sensitivity, measurement range) through varying its mechanical impedance. The variable-impedance mechanism is achieved with a unique double-cantilever structure, which also enables the sensor to operate at a low frequency suitable for testing biological tissues. A simultaneous actuation and sensing technique is used which allows detecting resonant frequency from electrical impedance of the piezoelectric actuator without using additional motion sensors, therefore simplifies the sensor’s structure and instrumentation. The basic working principle and the variable-impedance method for performance tuning are explained mathematically and verified by numerical and experimental studies. The test results on a prototype show that it has an operating frequency (<300 Hz) much better than those in the literature. With the variable-impedance mechanism, sensitivity improvement ratios of 1.7, 2.6 and 3.8 times are achieved in three test ranges. And measurement range improvement ratio of 6.2 times is achieved. Ex vivo experiment confirms its effectiveness in detecting embedded lump. The proposed sensor can find wide applications in robotics, biomedical engineering, aerospace, etc, and may eventually inspire the designs of a new class of tactile sensors with online performance tuning capability.

Purpose This paper aims to introduce an aircraft engine inspection robot (AEIR) which can go in the internal of the aircraft engine without collision and detect damage for engine blades. Design/methodology/approach To obtain the position and pose information of the blades inside the engine, a novel tactile sensor based on electrical impedance tomography (EIT) is developed, which could provide location and direction information when it contacts with an unknown object. In addition, to navigate the continuum robot, a control method is proposed to control the continuum robot, which can control the continuum robot to move along the pre-planned path and reduce the deviation from the planned path. Findings Experiment results show that the average error of contact location measurement of the tactile sensor is 0.8 mm. The average error relative to the size (diameter of 18 mm) of the sensor is 4.4%. The continuum robot can successfully reach the target position through a gap of 30 mm and realize the spatial positioning of blades. The validity of the AEIR for engine internal blade detection is verified. Originality/value The aero-engine inspection robot developed in this paper can replace human to detect engine blades and complete different detection tasks with different kinds of sensors.

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