In robot applications, the use of piezoelectric film sensors can allow robots to have human-like dexterity in hand movements and sensitive tactile feedback is crucial. Robots need to be able to sense changes in pressure when grasping objects, control grip force output effectively, and incorporate inclinometers to enhance their control capabilities. This represents an important stage in the development of robots. Piezoelectric film sensors provide sensitive pressure sensing capabilities, and when combined with inclinometers, they offer new opportunities for controlling tactile applications in robots. This article will introduce the technological development and applications of piezoelectric film sensors and inclinometers, as well as the solutions offered by Murata.
More environmentally friendly and higher sensitivity piezoelectric film materials
Piezoelectric film is made from a material called Polylactic Acid (PLA), which is a type of "biodegradable polymer" with strength and moldability comparable to conventional petroleum-derived plastics. It has gained attention since around 1995 and is now widely recognized as an environmentally friendly polymer. The lactic acid in PLA is produced through lactic acid fermentation by lactic acid bacteria from starch extracted from plants. Starch is derived from photosynthesis in plants, and besides the energy used in the manufacturing process, carbon-neutral materials are used throughout the manufacturing, disposal, and decomposition lifecycle, thus not adding to atmospheric CO₂.
PLA possesses high transparency, a feature not found in other biodegradable polymers, with a light transmittance even surpassing that of acrylic at 93%. PLA is commonly used in everyday items such as egg cartons and tomato boxes found in supermarkets. While PLA is typically used as a low-cost, plant-based polymer for environmentally friendly purposes, its piezoelectric properties offer unique characteristics.
The piezoelectric constant (piezoelectric d constant) of PLA is around 7 to 12 pC/N, which is relatively small compared to materials like PZT. However, PLA has a very low relative permittivity, approximately 2.5, resulting in a relatively large piezoelectric output constant (equal to piezoelectric g constant, where g=d/εT). Therefore, PLA exhibits high sensitivity in terms of piezoelectric output. Comparatively, when considering the piezoelectric output constant, PLA is roughly equivalent to polyvinylidene fluoride (PVDF), which has a piezoelectric constant four times larger than PLA.
Piezoelectric films made from PLA generate polarization proportional to the distortion (stretch, contraction) of the film surface due to compression or deformation. The polarization generates charges that are converted into voltage by an I/V converter, resulting in an analog signal output. This allows for the detection of pressure changes experienced by the piezoelectric film.
Flexile thin piezoelectric film sensors for high-sensitivity pressure detection
Murata's "Picoleaf" is a flexible thin sensor developed using their proprietary piezoelectric technology, designed for high-sensitivity pressure detection. It is smaller and thinner than traditional sensors, saving installation space and improving assembly performance and durability. The Picoleaf aims to enhance the functionality, ease of assembly, and durability of products requiring human-machine interface (HMI) sensing. To function as a sensor, electrodes are printed or laminated onto a thin and highly flexible organic piezoelectric film. This film can be mounted onto devices using double-sided tape, eliminating the need for a bonding process.
Picoleaf contributes to device slimming, allowing space savings even when combined with displays and touch panels. With its high sensitivity, Picoleaf can detect pressure across large displays using just one sensor. It can also be applied to detect minute displacements in the micrometer range, involuntary muscle tremors, hand grip maintenance during actions, and physiological signals such as human pulse.
Picoleaf possesses non-pyroelectric properties, meaning it does not polarize due to temperature changes, thus minimizing sensitivity variations and interference caused by factors like body temperature, sunlight, or semiconductor heating. Additionally, Picoleaf exhibits low power consumption, with zero power consumption for the sensor device itself and the possibility of designing the driving amplifier for low- power consumption current (around 10μA). Its flexible structure allows it to be bent and attached to curved surfaces of highly designed devices.
Utilizing its proprietary piezoelectric technology, Murata's Picoleaf can be mounted in tight spaces. Compared to previous sensors, it achieves thinness while enhancing assembly and durability. Picoleaf combines detection circuits to obtain outputs based on the displacement speed of the piezoelectric film. Leveraging this output characteristic, Picoleaf serves various sensor applications such as pressure detection, grip detection, and biological signal detection.
Picoleaf can be utilized in applications for pressure detection, serving as a UI sensor by detecting pressure variations. Placing Picoleaf on a stylus, for example, allows it to monitor the grip status of the user's hand. Its functionality extends beyond touch, as it can prevent inadvertent actions by detecting the hand pressure.
Furthermore, Picoleaf can be employed for biological signal detection applications, leveraging the high sensitivity of piezoelectric film sensors. It can serve as a sensor to detect biological signals such as "pulse and respiration." Several academic papers have already been published discussing the use of biodegradable piezoelectric sensors to monitor heart and respiratory rates on the human body's surface, confirming the accuracy of piezoelectric sensor-based biological signal detection.
Murata offers a variety of product formats for Picoleaf, including sensor element that can be directly attached to the main board, as well as sensor element combined with wiring, which allows for the separation of sensor placement from the board. Additionally, there are sensor element combined with part mounting wiring, which further separates the sensor placement from the board, resulting in a design with no mounting parts on the main board.
The detection circuit of Picoleaf consists of an I/V converter and amplifier circuit. When the piezoelectric film warped (due to pressure force or deformation causing stretching or contracting), it generates a polarization proportional to the applied warping amount. The charge generated by Picoleaf can be converted into voltage through the I/V converter, resulting in an analog signal output. This output signal can be amplified and adjusted as needed for processing by a general-purpose ADC or CPU, unaffected by sensor capacitance and stray capacitance. Currently, discrete circuits are recommended, but Murata is developing a dedicated ASIC for Picoleaf that includes signal processing functions, with availability expected to begin in 2025.
Picoleaf sensors possess outstanding deformation detection capabilities
The piezoelectric properties of Picoleaf sensors enable the detection of both "displacement direction" and "displacement velocity." Regarding displacement direction, when detecting "mountain-fold" deformation, the displacement direction outputs to the positive side of the Picoleaf reference voltage, while if "valley=fold" deformation is detected, the displacement direction outputs to the negative side. In addition to displacement direction, displacement velocity can be calculated based on peak voltage, which increases proportionally with displacement velocity.
Furthermore, utilizing the reversed output feature of Picoleaf sensors, the push and release actions can be seamlessly applied for toggling switches in user interfaces. If Picoleaf is mounted on an object undergoing periodic vibration, it can detect the periodic vibration and thus be used as a state detection sensor.
Picoleaf's thickness is only 0.2 mm or even smaller, making it incredibly compact with dimensions of 2x10 mm. Even when combined with displays or touch panels, it can save space. It can also be adhered to curved surfaces of devices designed with height variations, including special shapes such as wrapping around cylindrical objects. Picoleaf boasts high sensitivity, capable of detecting displacements on the order of 1 micron. A single sensor can detect pressure across the entire surface of large displays. It can be used to detect unconscious muscle movements such as tremors, gripping, and pulses.
With its non-pyroelectric properties, Picoleaf experiences minimal noise since there are no drifts caused by thermal factors such as body temperature, sunlight, or semiconductors. Consequently, algorithms can be constructed more easily, and the sensor itself has zero power consumption, while the driving amplifier circuit can be designed for low power consumption (approximately 10μA).
Picoleaf can be used for a variety of pressure detection applications
Picoleaf's transparency exceeds 90%, allowing it to be installed in transparent display panel areas. By combining the UI functionality of the touchscreen with Picoleaf's pressure detection capabilities, a Human-Machine Interface (HMI) different from ordinary touchscreens can be achieved, one that is more based on human behavior principles. In applications, it can be designed as a reliable input UI triggered by pressure detection, using deformation detection to implement input devices such as pens, and creating a real software keyboard experience by applying various pressure detection functions.
If you encounter difficulties in realizing new wearable concepts, you can use Picoleaf's lightweight, thin, short, and flexible structure to add new features while maintaining design integrity. Picoleaf's installation flexibility can easily address challenges such as minimal space, curved surfaces, and others. By applying pressure detection in confined spaces and on curved surfaces to create UIs with reliable operability, and incorporating wear and/or grip detection capabilities (utilizing high sensitivity), wearable devices can be used to detect biological signals such as pulse and respiration. Currently, fitness device equipped with Murata's piezoelectric film sensor Picoleaf is already available on the market.
When designing, Picoleaf can also be used to give robotic products a sense of touch. By utilizing the highly sensitive characteristics of these devices, Picoleaf can contribute to the development of the robotics industry that utilizes tactile information. Picoleaf can be applied to the fingertips of robotic hands to detect deformations caused by contact with soft or hard objects and adjust the grip force accordingly, allowing a single robotic hand to grasp various objects regardless of their softness. Picoleaf can be connected to remote-controlled robots to quantify tactile feedback, providing more realistic environmental information for VR pilots. Picoleaf can also be bonded to the wiring inside operating robots to detect internal abnormalities such as deterioration of the wiring.
Compared to other products such as strain-based, capacitive-based, accelerometer-based, and mechanical software-based sensors, Picoleaf has a standard size of only 3x17 mm and a thickness of only 0.25 mm (back glue 0.1 mm). It can be assembled using adhesive tapes and has force detection capabilities, with a sensitivity of up to 1μm and the ability to detect deformation speed data. In circuit design, common components can be used, with a 4-pin ZIF connector interface, a typical voltage of 3.3V for Vdd, power consumption of less than 10μA, a response time of less than 10 ms, and support for curved designs, providing more advantages.
High-performance inclinometers meet the demands of harsh environments
The SCL3300 inclinometer, introduced by Murata, is a high-performance 3-axis inclinometer. Its dimensions are only 7.6×8.6×3.3mm (width × length × height), and it offers a choice of four measurement modes to suit various applications and requirements. The sensor boasts ultra-low noise, with a resolution of up to 0.001°/√Hz. It features an internal SPI digital interface, superior mechanical damping characteristics, and an operating temperature range of -40 to 125°C. The current consumption is only 1.2 mA when powered by a supply voltage of 3.0 to 3.6V. Utilizing mature capacitive 3D-MEMS technology, the SCL3300 delivers high performance and is suitable for rugged and durable designs.
The SCL3300 is ideal for applications requiring high stability in harsh environments, including level sensing, tilt compensation, machine control, structural health monitoring, inertial measurement units (IMUs), robotics, and positioning/navigation systems.
To accelerate the development speed of products, Murata has introduced the digital accelerometer/inclinometer board SCL3300-D01-PCB. This board supports a three-axis inclinometer with a range of ±2.4g and is equipped with the MEMS digital inclinometer SCL3300 series chip carrier circuit board. The purpose of the chip carrier circuit board is to facilitate rapid prototype design. The SCL3300 chip carrier circuit board includes sensors and circuit board designs soldered onto the circuit board, along with headers and passive components.
Another inclinometer, BCGMCU, is the second-generation BCG solution with improved performance. It opens up new possibilities for monitoring the status of people sleeping in hospitals or at home. It can detect various biological signals such as pulse rate, breathing frequency, and breathing time of a sleeping person, allowing for the detection of when a person leaves the bed and analysis of sleep status. The BCGMCU solution consists of a pre-programmed microcontroller (BCGMCU-D01) and the SCL3300-D01 inclinometer, providing a component-level solution tailored for software solution providers, service providers, and OEM system integrators.
Conclusion
Murata's Picoleaf piezoelectric film sensor, known for its high sensitivity and environmental friendliness, boasts excellent characteristics such as small size, low power consumption, and high sensitivity. It can be widely used in various piezoelectric detection applications, including touch user interfaces, wearable devices, and robotic tactile sensing. Combined with the high-performance SCL3300 inclinometer, it can provide excellent control capability for robot applications, making it an outstanding choice for relevant applications.