Zhejiang University: Research on a New Inductive Hand Rehabilitation Pressure Sensor Based on Magnetic Stress Impedance and Magnetoelastic Coupling Effect
Flexible electronic technology has attracted much attention due to its widespread applications in human physiological signal monitoring, flexible robots, wearable devices, and other fields. Its common sensing mechanisms mainly include resistive, capacitive, frictional, and piezoelectric. However, inductance, as one of the fundamental electrical parameters, is rarely used as a response signal for flexible sensing devices, because compared to other sensing mechanisms, inductance lacks corresponding force sensitive materials.Due to its special magnetic domain structure, amorphous wire is highly sensitive to external magnetic and stress fields in terms of impedance, making it a force responsive material for inductive signals. However, its hard and brittle characteristics are difficult to meet the requirements of soft and tough flexible electronic devices. Therefore, how to make amorphous fibers flexible and meet the performance requirements of flexible sensors has become a technical challenge in expanding into the field of flexible electronics.
Recently, a research team led by Qin Faxiang from Zhejiang University has developed a new type of inductive sensor for hand rehabilitation monitoring, which comprehensively utilizes the giant magnetic impedance (GMI)/stress impedance (GSI) effect of amorphous wires and soft magnets（ NdFeB@PDMS ）The Giant Magnetoelastic Effect (GME) brings new mechanisms for flexible sensors.The sensor adopts a double-layer structure design. The lower layer is PDMS (SSAW&PDMS) embedded with square spiral structure amorphous wire, and the upper layer is PDMS foam containing NdFeB particles（ NdFeB@PDMS ）And magnetize along the plane direction. The introduction of this amorphous wire configuration and its composite with the soft matrix cleverly avoid the weakness of amorphous wires, solve the problem of monotonicity in the output signal, and broaden the sensing range.The addition of a soft magnetic elastic layer improves the sensitivity of the sensor. Subsequently, the operating frequency of the sensor was adjusted from the GHz range to the MHz range through an LC resonant circuit containing a standard capacitor to compensate for the performance degradation caused by frequency adjustment. When the sensor is integrated into a commercial finger board, it can dynamically monitor the effectiveness of hand rehabilitation.
The researchers used glass coated Co68.7Fe4Si11B13Ni1Mo2.3 amorphous wire prepared by Taylor Ulitovsky method as an inductive sensing element. Its high magnetic field/stress sensitivity, excellent soft magnetic performance, high mechanical strength and other characteristics can meet the requirements of high sensitivity and robustness of the sensor.Compared to low modulus flexible materials, this rigid sensing material has the advantage of mechanical reversibility and no hysteresis. In the study of the stress impedance characteristics of amorphous wire under uniaxial tension, it was found that the sensitivity of its imaginary part (inductance) is much greater than that of the impedance modulus and real part, further verifying the feasibility of using inductance signals as sensing signals.
However, this hard and brittle mechanical property makes it difficult to be used as a flexible sensor component alone. Embedding amorphous wires into a soft matrix to form composite materials is an effective strategy for its flexibility. However, it was found in the study. Directly embedding amorphous wires into PDMS does not meet the requirement of monotonicity in the sensing signal, which brings difficulties to the analysis of the sensing signal.The trend of first increasing and then decreasing is due to the changes in the magnetic domain structure under compression conditions. Under compressive stress, the magnetic domains of the outer shell of the amorphous wire will deflect axially and their motion will be blocked under high pressure stress. On the contrary, under tensile stress, the outer magnetic domains of the amorphous wire will deflect in the circumferential direction, and its corresponding circumferential magnetic permeability will monotonically decrease.Therefore, we have designed a planar spiral structure that embeds amorphous fibers with this structure into PDMS. By utilizing the positive Poisson's ratio pressure expansion characteristics of PDMS, the pressure acting on the surface of the composite material is converted into tensile stress acting on the amorphous fibers, solving the problem of single-mode sensing signals.However, relying solely on this method has not yet utilized the sensing performance of amorphous fibers in mechanical applications. Considering its magnetic sensing characteristics, we further designed a double-layer structure, which includes a PDMS layer (SSAW&PDMS) embedded with helical amorphous wires and a PDMS foam layer filled with NdFeB particles（ NdFeB@PDMS ）.
In the study of the magnetic sensitivity characteristics of SSAW&PDMS, it was found that their GMI curves exhibit typical bimodal characteristics. Therefore, when utilizing its GMI effect, it is important to note that its signal must also satisfy the condition of monotonic decrease in order to achieve an improvement in sensing sensitivity under the monotonic decrease in signal variation induced by the GSI effect in the stacked SSAW&PDMS layers.Due to the decrease in magnetic flux density of the magnetoelastic body under compression, it is necessary to adjust the initial magnetic flux density of the magnetoelastic body to be lower than the magnetic field strength corresponding to the turning point of the GMI curve. By adjusting the dosage of NdFeB, it was found that the sensitivity of the sensor is highest when the mass fraction of NdFeB particles is 8 wt.%. By integrating the magnetoelastic layer into the system, the sensitivity is improved by about 178% compared to SSAW&PDMS. The sensor has small signal drift and good loading unloading consistency under step loading (maximum 115 kPa).At the same time, the sensor has excellent performance such as fast response/recovery (40 ms), stable signal under different loading intensities and frequencies, and high robustness for long-term cyclic loading (15000 cycles).
In order to further elucidate the mechanism of optimizing sensing performance, the GME effect of the magnetoelastic layer was studied.In a compressed state, the relative position of the micro magnet will change, thereby altering the dipole dipole interaction and demagnetization field within it. With the increase of magnetic particle content, the initial magnetic flux density of the magnetoelastic layer and the difference before and after compression will both increase, which explains the variation pattern of sensing sensitivity increasing first and then decreasing with the increase of content.
Finally, by using an artificially constructed LC resonant circuit, the stress sensitivity of the sensing device was further improved. After connecting a standard capacitor of 680 pF in series, the sensitivity of the sensor reached 6.6%/kPa and exhibited extremely high linearity in the stress range of 0-100 kPa (R2=0.99717).After integrating the sensor onto a commercial fingerboard, it can dynamically monitor the changes in hand muscle strength of patients with hemiplegia, providing data support for the development of rehabilitation training plans.
These research results have successfully extended the traditional material of amorphous wire to the field of new flexible sensing, while fully utilizing the unique properties of amorphous wire, including conductivity, high mechanical strength, soft magnetism, and GMI and GSI effects, providing a new perspective for the functionalization of amorphous wire. Meanwhile, due to the previous dependence of GME effect signal reading on the principle of electromagnetic induction, it is only sensitive to dynamic loading. The magnetic sensitivity of amorphous wires also endows GME effect with the ability to detect static pressure.
Source: Sensor Expert Network