Last Updated on April 14, 2023 by You Ling
PVDF is a new type of piezoelectric material, and sensors made of PVDF have been widely used in recent years. So we wrote this article to introduce the relevant knowledge of piezoelectric film sensors.
- What is a pvdf piezo sensor(piezoelectric film sensors)?
- Characteristics of piezoelectric film sensors
- History of piezoelectric film sensors
- Working principle of piezoelectric film sensors
- Preparation method of piezoelectric film sensor
- Application of Piezoelectric Film Sensors
1、What is a pvdf piezo sensor(piezoelectric film sensors)?
PVDF piezoelectric film is a new type of high polymer piezoelectric material, which has the advantages of light weight, small volume, simple structure, good compatibility, etc. Therefore, using it as a sensitive element has little influence on the structural and mechanical properties of the sensor. The use of PVDF piezoelectric film to make various sensors has always been a hot topic of concern.
2、Characteristics of piezoelectric film sensors
Advantages and disadvantages of PVDF film sensors. Compared with other piezoelectric crystals, PVDF piezoelectric film has many advantages such as a charge piezoelectric constant more than ten times higher than quartz, good flexibility and processing performance, low acoustic impedance, wide frequency response, high chemical stability and fatigue resistance, low moisture absorption and good thermal stability, high dielectric strength, light weight, and easy processing and installation. At the same time, PVDF piezoelectric film also has some disadvantages, such as high internal resistance of the sensor made from it, and relatively small output energy. At zero frequency, it is difficult to maintain a weak static charge for a long time, so its low-frequency characteristics are poor and it can only be used for dynamic or quasi-dynamic measurement. Due to the polarization characteristics of piezoelectric materials, the piezoelectric system can only meet approximate linearity within a certain range and is easily affected by various environmental factors. The linear temperature range of PVDF film is relatively small, generally between -40°C and 60°C.
3、History of piezoelectric film sensors
In 1880, J. Curie and P. Curie first discovered the piezoelectric phenomenon in crystals. In the mid-1940s, the United States, the former Soviet Union, and Japan successively discovered the piezoelectric effect of titanium steel ceramics and used them to make piezoelectric ceramics. Later, it was found that wood, wool, bones, and other materials also have piezoelectricity, and research on piezoelectric polymers began. However, their piezoelectric properties were very low and had no practical value. It was not until 1969 that HKawai of Japan reported that PVDF can produce relatively high piezoelectricity after polarization at high temperature and high voltage, which has good industrial application value, thus making a historic turning point in the research of piezoelectric polymers. With the development of technology and the emergence of various new manufacturing processes, the piezoelectric properties of PVDF have been greatly improved, and the application range of PVDF has been expanded.
4、Working principle of piezoelectric film sensors
The fundamental reason why PVDF piezoelectric film has piezoelectric effect lies in the intrinsic piezoelectricity of the crystalline phase in its structure, which is determined by the electrostriction effect and residual polarization. Therefore, PVDF piezoelectric film needs to be artificially polarized to have an ordered residual polarization strength inside. The polarization charge of PVDF piezoelectric film is bound charge rather than free charge, so these polarization charges cannot move freely. When the PVDF piezoelectric film shows positive or reverse piezoelectric effect and produces charging and discharging phenomena, the change in polarization strength inside the PVDF film will cause the supplement and release of free charges on the film surface, resulting in a change in the surface charge of the film.
5、Preparation method of piezoelectric film sensor
Common methods include coating method, casting method, small molecule evaporation coating method, and pressing method, etc. Because the casting method is simple in process, requires low equipment requirements, and the film produced has good film density and high strength, it is often used in laboratory film preparation.
PVDF is dissolved in a fixed amount of NMP, and then filtered, vacuumed to remove bubbles, and evaporated by heating to remove NMP. This will form a layer of PVDF film on the casting dish. After stretching, polarization and other processes, a high piezoelectric PVDF film can be obtained.
6、Application of Piezoelectric Film Sensors
By utilizing the dynamic strain characteristics of piezoelectric thin films, it is easy to directly fix them on human skin (such as the inside of the wrist). The reason why piezoelectric films can detect very small physical signals and also sense large movements is that the piezoelectric response of PVDF films is linear within a considerable dynamic range (about 14 orders of magnitude). In most cases, as long as the bandwidth that can significantly distinguish the target signal from the noise is available, small target signals can be collected through a filter.
Similar sensors have been used to detect the movement of the chest, legs, eye muscles, and skin in the study of sleep disorders. In addition, the sensor can be used as an indicator to test the anesthetic effect by detecting the reaction of muscles (such as the muscles between the thumb and forefinger) to electrical stimulation (neuromuscular conduction).
Minisense100 is a standard product of Precision Electronics, which uses a cantilever beam design. One end is clamped with a rigid PCB board and has connection pins, and the other end is equipped with a mass block.
The mass block makes the sensor respond coherently to vibration. The mass block “remains motionless”, while the thin film part undergoes deformation, thus producing a very high voltage sensitivity (about 1V/g). Other life feature sensors are derived from this component, such as smart chest cards (built-in RF sensing equipment) worn by staff or patients. By periodically collecting signals from the wearer, the position and status can be determined. When the sensor senses that the chest card has been removed, it will set the chest card to sleep mode; if the sensor senses any movement of any part of the body, muscle tremors, or even pulse vibrations, it will wake up the chest card.
Many electronic stethoscopes use piezoelectric thin films as sensor components because they are durable, highly sensitive, and have a wide bandwidth range. In this application, the sensor element is usually encapsulated in a traditional metal stethoscope head because the sensor needs to form “force” with the body. Once the dynamic pressure signal is converted into an electrical signal, it can be selectively filtered or amplified, played back as an audio signal, used more complex calculation methods to determine the specific condition, or transmitted to a remote base station for further analysis and storage.
A composite acoustic sensor can monitor multiple points at the same time. DeepBreeze uses a sensor array with nearly 100 sensors to collect sound information of patients’ inhalation and exhalation. The sensors are attached to the skin surface with a vacuum hood. The collected signal is processed and converted into a “picture” of the sound, so the airflow in the trachea and lungs can be imaged like an animation. Any foreign object or abnormal phenomenon is obvious in these images. This method is more reliable and safe than X-rays.
Hospital bed monitoring
Piezoelectric thin films and cables can be installed on the mattress to detect the patient’s heartbeat, breathing, and body movements. Hoana Medical Inc.’s monitoring bed alarm system installs a set of sensors between the mattress and the sheet. When the patient sits or lies on the monitoring bed, the sensor can accurately measure and collect the patient’s life feature information through the clothes and sheets. Flexible switches are used to collect static signals, and all dynamic signals of patients are collected by piezoelectric thin films.