Piezoelectric polymer sensors have become one of the fastest growing technologies in the global sensor market. There have been numerous instances where piezo film was considered for the sensor solution. Since the discovery of piezoelectric polymer in the last two decades, the technology has advanced significantly and is being adopted on a larger scale. In this application note, the development of piezoelectric polymer technology, its properties, and sensor design are discussed in detail.
Properties of Piezoelectric Films
Piezoelectric film is a strong, lightweight and flexible plastic, which comes in a wide range of thicknesses and areas. As a transducer, piezo film can be molded into unique designs and can also be glued with commercial adhesives. The material includes the following properties:
- High dielectric strength
- High voltage output
- High elastic compliance
- Low acoustic impedance
- Large dynamic range
- Wide frequency range
- High impact resistance
- High mechanical strength and stability
Among all the materials, polyvinylidene fluoride (PVDF) was found to exhibit high piezo-activity. Although PVC and nylon materials exhibit piezoelectricity, they do not match the high piezoelectric effect of PVDF and its copolymers. Similar to other ferroelectric materials, PVDF generates electrical charge in response to temperature variations. PVDF absorbs infrared energy in the 7-20 ^m wavelengths and can function as a human motion sensor. It can even be used as pyroelectric sensor and laser beam profiling sensors for more advanced applications.
Figure 1. Typical infrared absorption spectrum of PVDF film
Additionally, piezo film has a major advantage over piezo ceramic. For instance, the former has low acoustic impedance which is closer to that of human tissue, water and other organic materials. Such a close impedance match allows efficient transduction of acoustic signals in tissue and water. Piezo films also exhibit excellent sensitivity and low density. When piezoelectric polymers are extruded into thin film, they can be joined to a structure without affecting its mechanical motion. Piezo films are ideal for strain sensing applications, which demand high sensitivity and wide bandwidth.
However, when compared to ceramics, piezo film makes a relatively weak electromechanical transmitter, especially in low frequency and resonance applications. Also, PVDF is not suitable for using or storing over 100°C. Moreover, when the film’s electrodes are exposed, the sensor tends to be sensitive to electromagnetic radiation.
Applications of Piezoelectric Films
Piezoelectric films are used in a variety of sensor applications. Some of the applications are given below.
Contact and Beam Switches
Figure 2. Switch for pinball machine
Figure 3. Beam switch
Contact switches have low reliability factor because of dust, moisture and other contaminates, which tend to tamper with the contact points. Piezo films provide excellent reliability since it has a monolithic structure and is not sensitive to other standard switch failure modes. Pinball machines happen to be the most challenging of all switch applications.
Piezo film switches are generally utilized to calculate the frequency, amplitude and direction of an event and prove quite useful in detecting and recognizing objects and are also used in bidirectional encoding applications, while beam switches are often used as gear tooth counters in electric utility metering and in shaft rotation counters in natural gas meters. They do not require an external power source.
Piezo film sensor strips integrated within the printer platen help in monitoring the impact timing and force of the bank of print heads, and relay the data to the controller. In order to accommodate slight changes in the print head timing, actuator timing is automatically adjusted. The high speed of the embossed steel ribbon demands a quick switch response. However, the impact forces of the print head tend to destroy other conventional impact switches. But piezo film switches overcome these complications and have been used in this application for over five years
Figure 4. ACH-01-XX internal view
Accelerometer designs are built on conventional piezoelectric ceramic and also on piezoelectric polymer materials. The use of base materials enables the product to be customized for particular applications. Accelerometers are configured as either beam-design type or compression-design type. Beam-design accelerometers exhibit lower resonant frequencies, while compression-design accelerometers tend to have higher resonant frequencies.
Composite materials have excellent strength, lightweight, resistant to corrosion, and are non-magnetic in nature. Versatile sheets of piezoelectric polymer transducer arrays are suitable for non-destructive testing. For instance, piezo films are used for NDT of aerospace engine parts.
In addition to the above applications, piezo films are used in musical instruments, bearing wear sensors, machine monitoring, acoustic emission, fluid level sensor, and ultrasound applications, to name a few.
Piezo films are lightweight and have high dielectric strength, high mechanical strength, and excellent stability and impact resistance. They also have high elastic compliance, wide dynamic range, and low acoustic impedance. All these aspects make Piezo films suitable for a wide range of applications.