The mechanical properties of a certain material are usually acquired experimentally. The typical method is to apply specific sensors on the surface of the material, observe measurement changes under certain external influences, such as compressing it or applying temperature change, and deduce the mechanical properties based on related physics models. Such observations, unlike “feelings” by human which are purely qualitative, are quantitative measurement in a form of digitized data. They are more objective than qualitative approaches and can be further analyzed with mathematic tools. The type of sensors which make direct contact with the material are called tactile sensors, as they are sensors performing measurement by “touching” the samples.
One of important properties of a material is elastic modulus (or Young’s Modulus). The property describes the object’s resistance to being deformed elastically when a stress is applied to it. Experimentally, it is not uncommon to use a force-displacement method. The deformation and the force relation determined in the tests can be further processed to calculate the modulus. However, such measurement method has strict definitions on the compressor head, shape of the sample, etc. to allow the physics model to work. The method can also be disastrous to the material under test, especially for the soft material whose deformation can be large even with small amount of force being applied.
Therefore, in order to acquire the material properties in a less invasive manner, vibrations, or more specifically, resonance phenomena were employed in developing a new type of tactile sensor. Due to the physics involved, these sensors are called “vibro-tactile sensor”. In my project, the vibro-tactile sensor takes the form of a vibration concentrator combined with the piezoelectric transducer.
The piezoelectric transducer was a commercial product for ultrasonic machining; and the vibration concentrator was designed and manufactured by me. The vibration concentrator was made of 6061 aluminum. The design was first done analytically with vibration and resonance analysis employing electro-mechanical analogy. The 2D analytical model was then populated with more design features and numerically modelled in COMSOL Multiphysics to perform finite element simulations for a refined design. The vibration concentrator was then machined out of an aluminum rod in the student machine shop with traditional lathe and mill.
The application of this sensor relies on the analysis on the electrical terminals. An impedance analyzer was used to conduct the electrical measurement with frequency sweeps. To demonstrate the concept of measurement, a series of samples made of silicone rubber were fabricated beforehand and used as objects under test in the experiments. These silicone rubbers had elastic modulus from 14 kPa to 142 kPa. The Z-axis movement was controlled by a small tensile test machine.
Naturally, the varied stiffness of the samples resulted in different curves jotted down by the impedance analyzer. My research is to figure out how we interpret these curves into material property, or more specifically the elastic modulus, which relate to the samples.
Equivalent circuit theories were employed to run analysis on the acquired frequency spectrum and the linear relation was found between the elastic modulus and equivalent conductance of the material and the resonance frequency shifts caused by the samples.
Related publications:
Qian Y, Han S W, Kwon H J. Design of an ultrasonic concentrator for vibro-tactile sensors using electro-mechanical analogy[J]. International Journal of Precision Engineering and Manufacturing, 2019, 20(10): 1787-1800.
Qian Y, Salehian A, Han S W, et al. Design and analysis of an ultrasonic tactile sensor using electro-mechanical analogy[J]. Ultrasonics, 2020, 105: 106129.
