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We have applied MEMS (Micro Electro Mechanical Systems) technology to develop an ultra-compact tactile sensor. The figures below shows the principle of sensing. This tactile sensor consists of several micro-cantilevers fabricated on a Si substrate and covered entirely with an elastic material (elastomer). When an external force is applied to the top of the elastic material, the elastic material deforms and the tilt angle of the cantilevers changes simulteniauly. By electrically measuring this set of tilt angle and performing appropriate calculations, the external force acting on the top of the elastic body can be measured in three axes (pressure + shear force in two axes). The current prototype sensor has a diameter of 1 mm and a height of 1 mm.
Measurement Principle
By arranging three cantilever structures in a MEMS tactile sensor, external force applied to the tip of the sensor can be measured. The next figure shows the measurement results on an actual sample. By multiplying the output of each sensor as ei (i=1-3) by the configuration matrix M to be fabricated for each sensor in advance, Fk (k=x,y,z) is obtained as the pressure perpendicular to the sensor and the shear force in the two axes along the sensor. As shown in the figure, it can be seen that the operated forces in each axial direction can be accurately separated and measured. The figure shows an external force of about 80 gf in pressure, but the linearity is maintained up to about 500 gf. The PDMS is a transducer that converts external force into deformation, and the hardness of the current PDMS is designed to be about the same as that of human skin. By changing this hardness, it is possible to change the dynamic range without changing the structure of the MEMS part.
Measurement Sensitivity
The sensitivity of this prototype sensor is extremely high. Japanese bill have a slit-like structure with a height of 30 µm and a width of 500 µm printed with ink to prevent counterfeiting. When we traces the surface of the slit, we can esaly find the authenticity of the bill. Similarly, our MEMS tactile sensor allows to detected as it is. Thus, this sensor combines high intensity and sensitivity.
Proximity Detecting Function of Tactile Sensor
This tactile sensor is an MEMS structure sealed in elastic resin. Making use of this electrical circuit of MEMS structure, the detection of an object before touching was made possible. When functioning as a tactile sensor, the direct current resistance of strain gauge resistor implemented in MEMS structure will detect the variation in tilting of cantilever. On the other hand for proximity measurement, the capacitance of Si depletion layer under the strain gauge resister will vary in accordance with the photoconductive effect of incident light. Making use of this effect, the intensity of incident light can be measured with high frequency impedance of strain resistor. If a light source is placed near the sensor, the intensity of light will increase in accordance with the approach of an object, and as a result the measurement of distance from the sensor to an object will be possible.
Mounting Inherently
The MEMS mechanism is microscopic and can be easily damaged mechanically. In this sensor, this fine structure is sealed with elastomer (PDMS) to increase the mechanical strength of the MEMS part.
In addition, a flexible substrate is flip-mounted for signal extraction, and a mechanism was developed to achieve both electrical connection and mechanical strength.
Tomato Grasping Experiment
As an application for industrial robots, we implemented a mechanism for gently gripping mini-tomatoes of different sizes. Here, a microcontroller independent of the arm body is mounted on the gripper part, and the gripper motion is performed by commands from the arm body. A tactile sensor mounted on the inside of the gripper measures the pressure applied to the tomato and stops the gripping when the tomato is fully grasped.
As an application for industrial robots, we implemented a mechanism for gently gripping mini-tomatoes of different sizes. Here, a microcontroller independent of the arm body is mounted on the gripper part, and the gripper motion is performed by commands from the arm body. A tactile sensor mounted on the inside of the gripper measures the pressure applied to the tomato and stops the gripping when the tomato is fully grasped.
Publication (English Jouranl only)
Tactile sensor with microcantilevers embedded in fluoroelastomer/PDMS for physical and chemical resistance , │ Electrical Engineering in Japan │ 2022.10
Tactile sensor using a microcantilever embedded in fluoroelastomer with resistance to cleaning and antiseptic solutions , │ Sensors and Actuators A: Physical , │ 301、2020
Fabrication and Normal/Shear Stress Responses of Tactile Sensors of Polymer/Si Cantilevers Embedded in PDMS and Urethane Gel Elastomers, │ Joural of EE Japan │ 128 (5),193-197 , 2008
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