The art of powering wearable electronics
Another neat way to keep wearables charged and running could be RF Energy harvesting. Research is being done in this area and a good example of a possible design solution is a millimeter-wave Ink-jet energy harvesting method that can be used in flexible electronics in wearable designs.
The design uses an inkjet-printed mmWave rectenna which is a rectifying antenna that will convert electromagnetic energy into DC power.
The design consists of capturing a 24 GHz signal via the antenna and sent on to a rectifier that has an input matching network in front of it for maximum power transfer to a harmonic termination network (HTN). The harmonic termination network (HTN) provides an open-circuit impedance seen by the diode, at odd-order harmonics and a short-circuit impedance at the even-order harmonics, as an example. The HTN also isolates the RF components from the DC component. The Via-less DC Return path eliminates signal-to-ground vias which would create high parasitic problems at mmWave frequencies. See Figure 6.
Figure 6: A block diagram of the rectenna development idea (Image courtesy of Reference 3)
The entire system is made by the use of a flexible liquid crystal polymer (LCP) substrate along with microstrip traces made by ink-jet printing of a silver nano-particle ink using a Dimax FujiFilm DMP-2831 printer.
The antenna is a planar 2x2 patch array design with four patches connected through a corporate feed network with three T-junctions which all are on the same layer as the 50 ohm input feed line. See Figure 7.
Figure 7: The antenna layout. Antenna dimensions can be found in Reference 3 (Image courtesy of Reference 3)
In the experiment with this design, an LED was successfully turned on with a 24 GHz signal through the air.
Some interesting component solutions and techniques
Powercast PowerHarvester ICs4
I recently spoke to Dr. Charles Greene, COO/CTO of Powercast regarding his company’s solution to powering wearables. Powercast’s solutions intrigued me. See Figure 8.
Figure 8: Powercast’s wireless recharging solution (Image courtesy of Powercast)
Powercast’s Powerharvester ICs enable wearables designers to embed them in wearable devices to provide wireless recharging over distance to one or more devices. By harvesting radio frequency (RF) energy in the ISM band, the ICs enable a trickle charge to the wearable device’s battery. This method enables a hermetically sealed device which can be washable.
The solutions are in a small form-factor; the Powerharvester chips provide frequent and transparent recharging allowing the use of a smaller battery which enables smaller, thinner devices.
A low-power RF transmitter, similar to that in a cell phone, can be mounted in a closet or under a dresser drawer. This transmitter creates a charging area for the wearable clothing. Whenever the wearables are in the closet or drawer, the devices’ batteries will automatically receive a trickle charge. This method is independent of being in an area where the receiver would need to depend upon random RF signals in its frequency range. See Figure 9 for RF Power categories.
Figure 9: RF Power categories as mentioned in Dr. Charles Greene’s recent presentation at the 2017 Sensors expo & conference (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)
The key to success for this innovative solution is a very efficient RF-to-DC converter design, so let’s see how this company manages their system.
The system design
I really like Powercast’s approach because relying only on ambient sources of RF energy can be so unpredictable. By providing their own source of RF Power, the system can count on a constant power flow. Granted that ambient power sources might be a better solution in remote areas where electricity is not available, but I believe that there is a major portion of this market that could use a dedicated, reliable wireless transmitter, especially for wearables.
Figure 10 shows a typical intentional broadcast of RF power over distances from inches to more than 100 feet. The transmitted power can range from microwatts to milliwatts.
Figure 10: A Powercast system would provide automatic charging of enabled devices with parameters that can be controlled by specific design needs. Those parameters are Power level, Frequency, Transmit/Receive Antenna gain and the number of transmitters, Distance, Device duty cycle, and System cost. (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)
The importance of frequency4
The Friis Transmission equation is used to calculate the power received from one antenna, with a gain of G1, when transmitted from another antenna, with gain G2, while separated by distance r, and operating frequency f or wavelength λ. Dr. Greene’s designs begin with the Friis equation.
Note: The following equations assume far-field operation (as "r" goes to zero, power received goes to infinity, therefore there is a limit to these equations)
It is a fact that Power Density (S), in W/m2, is independent of the frequency:
PT is the transmit power
r is the antenna range or distance
ΓT is the transmitter reflection coefficient
GT(θT,ΦT) is the angular dependent transmitter gain
The Effective Area (Ae) of the antenna type decreases as the square of that frequency:
So, an increased antenna size, if the particular device allows, would cause a more directional signal with a larger antenna at higher frequencies. The following is based on the Radar Equation:
PR is the received data power
GR is the maximum receiver gain
Conversely, the lower the frequency, the more omni-directional the signal and the generally allowing more throughput as well. The size of the antenna would naturally be determined by the size of the receive device (e.g. A 915 MHz dipole for a game controller or a 2.4 GHz or 5.8 GHz dipole for something like a hearing aid)
The Powercaster TX 91501 transmitter uses a 915 MHz center frequency on the unlicensed ISM band with Direct Sequence Spread Spectrum (DSSS) modulation for power which is a spread spectrum technique in which the original data signal is multiplied with a pseudo random noise spreading code. The spreading code has a higher chip rate (bitrate of the code) that enables a wideband time-continuous scrambled signal. The military uses this technique for significantly improving protection against interference/jamming signals, especially narrowband which renders the signal less noticeable to possible hackers.
For the data, an Amplitude-Shift Keying (ASK) digital modulation scheme is used that gives a sinusoid signal two or more discrete amplitude levels related to the number of levels adopted by the digital message. The modulated waveform typically looks like bursts of a sinusoid signal.
On the receive side are also the PCC110 RF to DC converter IC and a PCC210 Boost converter IC for high volume OEM designs and reference designs. See Figure 11.
Figure 11: The Powerharvester ICs for the receive side power (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)
Also, based on the PCC110 and PCC210 ICs are the P1110 and P2110 modules. See Figure 12.
Figure 12: Powercast modules are RF-in to DC-out devices with high RF-to-DC conversion efficiency---a critical parameter. They are designed for 50 ohm antennas and support multiple frequency bands in the 840-960 MHz range. (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)
Figure 13: A bulk trickle charge example (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)
Figure 14: Consumer electronic devices can be charged overnight, when not being used, via the PowerSpot Transmitter seen in the center of the diagram (Image courtesy of Wireless Power, Energy Harvesting, & Power Management Solutions for Sensors and the IoT presentation by Charles Greene, Ph.D., Sensors expo & conference, 2017)