Schematic of the integrated system, consisting of the pressure sensor, the built-in battery and the wireless Bluetooth module. Pictures of the built-in battery which is manufactured directly on the back of the pressure sensor. Change in electric current due to applied pressure (60 kPa). a) Response time (~110 ms) and (b) recovery time (~120 ms) of the single pressure sensor, which is measured by the real-time current plot.
Schematic of the operating mechanisms of the switch and the pressure sensor when external pressure is applied. Output current change of the device with the variation of applied pressure in real time (VD . = 5 V, VG = 10 V). Intensity change of the light emitted by OLED coupled in series with the variation of applied pressure.
Displayed pressure force applied to the sensors on the screen of the smartphone via wireless Bluetooth communication.
Research background and motivation
Among conventional sensors, field effect transistor (FET) type pressure sensors are the most widely used in electronic devices, which makes them easy to use with other additional components and the least crosstalk can be achieved by using the active matrix structure. . Even if the electrical signal is sent from the sensor, it is difficult to apply it to other electrical appliances apart from extra wire. Herein, we have developed a system that can minimize the volume, weight and driving voltage of devices by integrating a built-in battery, produced by a direct printing method, on the back of the sensor substrate [12,13], a pressure-sensitive switch that drives a low power by eliminating standby power consumption [14], and a wireless communication module that allows remote interaction with the external source [4,7,15].
The device transmits the pressure data to the smartphone or other receiving devices when the user presses the pixel composed of the pressure sensor, and by connecting the Bluetooth module to each other, the wireless communication with the smartphone records and stores the pressure data, thereby increasing the use of the sensor data . The converted electrical signals were transferred to the Bluetooth module and immediately displayed on the smartphone screen. Pressure-responsive OLEDs are designed in the sensor array so that the user can visually receive pressure information [15], and the visualized OLED intensity depends on the size of the device and the amount of pressure.
By building a system that will enable these wireless communications, we expect to expand the sensor field by enabling the free conversion of sensor results into digital data.
Research objectives and contents
Pressure sensor with Low-Power driving
Research background
Therefore, to overcome these shortcomings, the dielectric layer of the pressure sensor is replaced by air, so that the side wall is made with PDMS and SU8 photoresist, so that the gate electrode is placed in the upper channel in a free-standing form [10]. Due to the elastic property of PDMS, when pressure is applied to the sensor, the change in the thickness of the dielectric layer due to the pressure changes the switching effect on the channel. This pressure sensitive transistor can be used as a pressure sensor without any other additional sensor component.
To make such a device, OLED and switch are added next to the sensor so that a switch provides electrical disconnection when no pressure is applied. Also, the coupling of OLED for the visual expression of current change in pressure sensor is made for immediate feedback for human touch movement [12].
Experiments
- Experimental Methods
- Results and discussion
- Conclusion
The pressure sensor and battery were patterned on the front and back sides of the Si substrate and packaged together. Optical microscope image of each component of the pressure sensor array and a magnified image of the pressure sensor pixel showing the pressure-sensitive switch, isolated Si channel, and OLED. To prove that the pressure sensor works regardless of the presence or absence of the switch, a compression test instrument (Mark-10) was used and the measured pressure was 60 kPa, where the initial state of the electric field at a drain voltage of 5 V (VD = 5 V) and gate voltage for 10 V (VG = 10 V).
Each of the pressure sensors showed negligible differences in response time and recovery time (response time ~ 230 ms, recovery time ~ 260 ms graph in figure 2.10). To assess the quality of each component before integration, Figure 2.11 shows the contact pressure of the switch. The OLED was formed to emit a green light through the structure as shown on the right side of the figure [21,22].
As higher pressures were applied, the thickness of the air gap changed depending on the amount of pressure (thickness of the air gap in d2) and the degree of PDMS condensation [23]. Due to the variation of the thickness of the air gap, the theoretical capacity of the insulator, Ci, can be determined from equation (1). In this way, it was proved that the influence of the pressure at the same gate voltage (VG) appears as a difference in the output of the drain current (ID).
Pressures ranging from 20 kPa to 3 MPa were detected at each step in most of the areas where the pressure sensor could be used. This range of pressure can be compared to the pressure of a soft touch (the pressure when a person uses a smartphone) with the pressure of the heel. The intensity of the light emitted by the OLEDs was dependent on the amount of external pressure.
The change in the thickness of the PDMS layer is therefore directly related to the brightness of the OLED. In this way, the measuring principle of the pressure sensor and its results can be clearly demonstrated. The pressure sensor with switch shows that the base current of the sound level is 3×10-12 A, but the pressure sensor without switch shows a current leakage of about ~2×10-8 A.
Therefore, the results indicated that the existence of the switch does not affect the sensitivity of the pressure sensor, resulting in advantages for a wireless communication system.
Integrated wireless system with pressure sensor
Research background
Experiments
- Experimental Methods
- Results and discussion
- Conclusion
In ACF bonding, the ACF tape type was secured between the substrate pad and the tape using the natural adhesive strength of the ACF itself. The other side of the strap connects to the strap holder of the microcontroller unit (MCU) board. A thin band-shaped connection of the printed built-in battery is connected to the contact surface of the Bluetooth module using soldering technology.
The integration of the pressure sensor and the battery reduces the size of the entire device and maximizes the efficiency of the battery using the switch. A tape-type flat flexible cable (FFC) with a pitch length of 2.53 mm is attached to the pressure sensor contact pad by soldering to transmit the electrical signal from the sensor to the module, and a dedicated connector is attached to the PCB board attached to the module. lace. The FFC is sufficiently bent to connect the sensor substrate and the module, which consists of the double layer, resulting in the volume of the device to be small (Figure 3.5).
It can be seen that the brightness of the OLED is changed reflecting the change in transmitted current depending on the magnitude of the pressure. In addition, the numerical value of the current change value depending on the pressure force placed on the sensors is displayed on the smartphone screen through Bluetooth wireless communication and is divided into different colors on the screen. When pressure is applied to a wide area of PET, the switch does not operate under a pressure of 10 kPa or less.
And the light emitted by OLED does not appear at very low pressure range. Furthermore, it can be seen that the concentration of pressure is divided by the point where the pen presses with greater pressure. This shows that the brightness of the OLED coincides with the brightness of the smartphone screen.
Therefore, we performed the visualization in two ways according to the size of the pressure, the OLED emission and the display on the smartphone screen. In relation to the smallest size, which is one of the most important factors for the integration of a wireless communication device, the new built-in type battery has been integrated with the sensor.
Conclusion
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Acknowledgements
