Printed circuit boards (PCB) technology is a mature and reliable technology that has been widely adopted in the electronic industry. Standard PCB technology employs both rigid flame retardant (FR)-4 boards and flexible polyimide (PI) films. Multiple rigid and flexible substrates can be laminated and metalized for electrodes and electrical connection. They can also be cut or milled into complex shapes for electronic manufacturing and integration. Therefore, PCB is also a good candidate as a manufacturing technology for electromechanical or mechatronic devices. Another advantage of PCB is that the typical board size of PCB is in the centimeters to tens of centimeters range. Therefore, meso-scale devices and applications can be easily implemented by this technology.
Devices fabricated by PCB technology include various sensors and actuators. Vibrational energy harvesters have also been implemented in PCB. In particular, an electromagnetic (EM) energy harvester was demonstrated in Rigid-Flex PCB structures. In further development of this device, embedded cavities were milled in the rigid PCB and filled with ferrofluid (FF) to improve the EM coupling and, thus, enhance output. Detailed device modelling and characterization of the presentation is presented in this paper.
PCB technology has also been suggested as a potential approach for lab-on-PCB and micro total analysis systems (μTAS) applications for its low cost and up-scalability. Various microfluidic devices and systems have been demonstrated. Most of the PCB-based microfluidic devices used PCB as substrates for sensing electrodes and electrical signal connection. The microfluidic chips were fabricated in other materials and bonded to the PCB substrates for the lab-on-PCB modules. In microfluidic devices based only on standard PCB materials, the channels and fluidic structures have been fabricated in the copper layers or the milled cavities in the PCB laminate. The embedded cavities/channels in PCB presented in this paper complement prior achievements and offer a more versatile tool for implementing complex microfluidic devices.
Vibrational energy harvesters harvest the kinetic energy in ambient vibration sources, such as machinery, buildings, and moving body parts of human and animals. When subjected to an external vibration, the vibrational force causes the shuttle mass in the harvester to move with respect to the rigid device frame. In EM harvesters, the shuttle mass is a permanent magnet and such displacement causes magnetic flux variation in the surrounding transduction coils. Thus, the vibrational energy is converted to electric energy by the induced electromotive force (EMF) in the coils. In electrostatic harvesters, the moving shuttle mass is one of the electrodes of the transduction variable capacitor. The displacement causes charge or potential variation on the moving electrode. Thus, the vibrational energy is converted into the electrostatic energy in the capacitor. In piezoelectric harvesters, the shuttle mass displacement causes deformation of the suspension structures which are made of piezoelectric materials. Thus, the mechanical deformation energy is converted to electricity by the piezoelectric effect. Among these different conversion devices, EM harvesters are attractive because they have simple electrical and mechanical structures, they have low output impedance, and they often use materials that are compatible to modern electronic manufacturing technology.
Harvesters with adequate output power levels can be fabricated by different technologies. Compared with semiconductor-based microfabrication and traditional precision machining, the mature PCB technology has the advantage of low costs and a fast turnaround. Typical minimum line width and feature size in PCB technology is about 100 µm and the available total area of fabricated circuit boards ranges from centimeters to tens of centimeters. Therefore, PCB technology is particularly attractive for manufacturing EM harvesters because the latter often do not have the fine mechanical structures or small electrode gaps seen in other types of harvesters. In addition, PCB can be used as a common substrate to route the coil windings for harvesters and the circuit traces for power processing and control circuits. This enables the development of low-cost and robust integrated wireless sensing modules with minimal components for emerging applications such as the Internet of Things (IoT).
PCB-based energy harvesters have been demonstrated by using both rigid FR-4 boards and flexible PI films. The rigid boards and flexible films can be laminated in standard processes to form the Rigid-Flex PCB substrates, which have been widely used in consumer products, such as mobile phones and laptop computers which have limited internal space or irregular shapes. The advantage of the Rigid-Flex PCB is that more complicated, versatile, and compact three-dimensional structures can be realized by careful design of the rigid and flexible parts in the device. For example, the Rigid-Flex PCB technology was employed in the folded structure in an electret-based electrostatic harvester where the flexible PI was used as electrical connections and mechanical resonance structures, while the rigid board was used to house the components for power management. In another work, the Rigid-Flex PCB technology was used to fabricate an EM/piezoelectric harvester where the PI layer was used to support the piezoelectric bimorph. In a further work, a wide-band EM energy harvester with a more complex rigid-flex structure was demonstrated based on a commercial Rigid-Flex PCB process. This harvester had its transduction coil windings and support mechanical frame designed and fabricated in rigid FR-4 boards, whereas the mass platform and elastic suspension beams were constructed in the flexible PI film. The PI film was sandwiched and laminated between the top and bottom rigid boards to demonstrate the feasibility of Rigid-Flex PCB as both electric substrates and mechanical structure materials.
However, only standard PCB materials, such as copper, fiberglass cloth, and epoxy binder, which were non-magnetic, were used in that work. Therefore, the magnetic flux from the permanent magnet in the harvester could not be confined and guided efficiently through the transduction coils. To improve the magnetic circuit design and EM conversion efficiency in such devices, a novel rigid-flex-PCB-based EM energy harvester was demonstrated recently. This device had embedded cavities milled in the rigid PCB layers and filled with FF. The high-permeability FF helped guide the magnetic flux of the permanent magnet through the transduction coil and, thus, improve the overall magnetic efficiency of the device. In the following sections, more details of modelling and characterization of the device reported are presented.
The schematic view of the proposed device is shown in Figure 1. By using commercial Rigid-Flex PCB technology, the top and bottom transduction coils are routed in FR-4 boards, which are also used as mechanical supports, whereas the elastic suspension beams and the central platform for attached magnets/mass are fabricated in the PI layer. Each of the two sets of coils has three layers of windings connected by via holes (‘via holes’ are holes that connect traces in different layers). The two coils can be connected in series or in parallel in external circuits to increase the output voltage or current, respectively. The central PI layer and the FR-4 coil layers are separated by thick dummy boards which serve two functions: (1) To adjust the positions of the coils to pick up the maximum spatial flux gradient; and (2) to house the embedded cavities for FF, as shown in Figure 1b. After the PCB structure in Figure 1 was fabricated, FF was injected into the embedded cavities in the thick dummy boards. Finally, two NdFeB magnets were attached to the central PI platform and held in place by their strong magnetic force. Figure 2 shows the PCB layout of the proposed harvester. Table 1 summaries the PCB design parameters of the device.
Figure 1. Proposed harvester fabricated based on a commercial Rigid-Flex printed circuit boards (PCB) technology, (a) schematic view; (b) exploded view.
Figure 2. PCB layout of proposed harvester. FF: Ferrofluid.
Table 1. Design parameters of proposed electromagnetic (EM) harvester. Some parameters are defined in Figure 2.
In the proposed EM harvester, the magnets move perpendicularly to the planes of the transduction PCB coils. The induced EMF, V, can be expressed as: V=−n d Φ d t=−n d Φ d z z˙ (1) where Φ is the total magnetic flux enclosed by the coil, and z and z˙ are the position and velocity of the permanent magnets in the vertical direction, respectively. Therefore, it is important to achieve maximum spatial flux gradient of Φ to maximize the induced EMF. One of the methods to improve the flux gradient is to increase the total flux enclosed by the coils. This can be accomplished by proper magnetic circuit design of the device. Magnetic circuit design in magnetic actuators and sensors is crucial to enhance the coupling of magnet fields and flux in three-dimensional device structures. For example, high-permeability yokes are often used in voice coil motors (VCM) in optical pickup heads in optical disk drives to guide and concentrate magnetic flux and enhance actuation sensitivity. In another work, soft iron was sandwiched between permanent magnets to guide the magnetic flux to enhance the output of an EM energy harvester. In addition to flux guidance, high-permeability materials effectively lower the overall magnetic
