The Wireless Charging Closet

Printed electronics enable e-textiles and wearables powered by radio frequency wireless power.

By Charles Goetz

一种Runner投入了5K奔跑后回家。他们使用智能手机检查心率最大和峰值呼吸率,然后在淋浴前卸下健身跟踪衬衫。他们可以将其直接扔入衣服中,而不是从服装中删除电子包装,因为这些电子产品直接嵌入了电子纹理上,从而使服装机器可清洗。在洗涤周期结束时,衬衫在壁橱中悬挂着干燥,在那里它也无线充电以准备下一次跑步。

这听起来像是未来的广告,但是今天可以将无线充电系统实施到电子纹理和其他可穿戴设备中。

一眼无线充电

一种variety of wireless charging technologies are available, including magnetic resonance, ultrasound, radio frequency (RF) and Qi — inductive coupling. The longest-range option, and also the topic of this article, is RF. This technology sends a trickle charge over the air and allows a “set it and forget it” convenience where users can simply place or hang an e-textile near the transmitter without having to worry about exact alignment for charging.

一种n RF wireless power system consists of two sides:

  • 一种transmitter, Tx, placed near the e-textile such as in a closet or drawer; and
  • 一种receiver, Rx, embedded into the e-textile.

The RF transmitter, Tx, in the charging system emits radio waves at a specific frequency. The receiving antenna, Rx, and matching network then harvest that RF from the air, while a converter chip converts the RF into usable DC power(见图1)

设计RF无线电源系统时要考虑的元素

Transmit—Tx: Frequency, power level, and antenna gain are the three key elements for an RF power transmitter. First, the country where the transmitter will operate shapes its design — local frequency band allocations and power output limits dictate how much power the device can transmit and at what frequencies. For example, in the United States, the Federal Communications Commission (FCC) governs all radio equipment. Part 15 of the FCC’s rules limits power fed to the Tx antenna to a total of 30 decibel-milliwatts (dBm), or 1 Watt, and limits antenna gain to 6 decibels relative to isotropic (dBi), for a maximum of 36 dBm, or 4 Watts, of effective isotropic radiated power (EIRP). Different combinations of output power and antenna gain are allowed, but the limit of 4W EIRP remains.

This FCC rule applies to all unlicensed communication devices operating across three frequency bands — 902-928 megahertz (MHz), 2.40-2.483 gigahertz (GHz), and 5.725-5.875 GHz. Transmitters used in the United States must comply with these limits.

每个国家都有自己的规则和限制频率分配,功率输出限制以及无线电传输设备的排放要求。

接收-rx:幸运的是,无线电源网络的接收器端更容易设计,因为无线电源接收器通常不需要本地通信认证。有三个主要变量需要考虑:频率,天线增益和与发射器距离有关的输入功率范围。

Figure 2

让我们从频率开始。设计人员可以使用Friis方程计算在远场中给定的距离内接收的RF功率的水平。该方程式表明,在相同情况下,较低的无线电频率倾向于比较高的频率更有效地传递功率(see Figure 2)

The chosen frequency impacts the charging distance in an RF wireless power network (WPN). At lower frequencies, like those in common RFID industrial, scientific and medical (ISM) bands, receivers can generally operate further away from the transmitter and tend to offer superior charging time. On the other hand, WPNs operating at higher frequencies, such as 2.45 GHz, can accommodate smaller antennas on both the Tx and Rx ends, appealing to both engineers and chief technology officers in today’s quest to achieve increasingly-smaller devices.

接下来,接收器需要多少功率?弗里斯方程式表明,接收的功率下降是一个超距离平方。这意味着渴望渴望的可穿戴设备需要在接近TX源的位置充电,而传感器等较低的功率设备可以在更大的距离内充电。

Lastly, let’s discuss antenna gain. Higher gain antennas capture more RF power but inherently become more directional. Lower gain antennas are omni-directional, meaning they can receive RF power from almost all directions.

在像健身衬衫这样的电子纹理上,可以实施多个接收天线并获得两全其美的最佳状态 - 多个天线捕获更多的RF功率,例如高增益天线,但也可以具有诸如低收入天线的全向方向图案。

WPN,频率,天线增益和输入功率都交织在一起。设计师不能在不影响另一个的情况下调整一个,因此在设计无线电动输送网络和一个或多个接收设备时,应平均考虑所有人。

图3:整个纺织可以作为加热器的tronic circuit board material for mounting or printing electronic circuitry, screens, chips, antenna(s) and more. Using printed electronics, the garment can still bend and flex. In this prototype, the printed circuitry and mounted components were applied to the inside of a smart athletic shirt.

改变游戏的印刷电子产品

Luckily, a shirt or pair of shorts provides a large footprint for printing electronics, unlike small earbuds, fitness trackers, or hearing aids that take up less physical space. The entire garment can serve as an electronic canvas for implementing circuitry, screens, buttons, and antenna(s)(见图3)

For recharging, consumers will simply place a wireless power transmitter in the closet or drawer near where they store or hang their smart wearables. Close-range charging and a large physical footprint make wireless charging a natural solution for smart garments.

快速充电,大足迹

智能服装中显然是刚性或刚性的电子电路板。幸运的是,当将电子设备打印到电子纹理上时,服装本身就变成了电路板,它仍然可以弯曲和弯曲。Circuitry can be directly printed onto garments using conductive materials like Pittsburgh-based Liquid X®’s particle-free metallic inks, or applied via thermal transfer using technology from companies such as England-based Conductive Transfers Ltd. These printed electronics breakthroughs — conventionally printable conductive inks and thermal trace transfer — have opened the gates for consumer smart garments.

当使用RF无线电源创建无线可充电智能服装时,整个服装可以用作画布。可以在服装上打印多种电力的天线,该天线创建了一个大面积,以捕获发射机上的空气中发出的RF能量,并将提高服装的整体充电速度。然后,可以将其他小型电子组件安装到印刷轨迹上,包括将RF能量转换为可用直流电源的微型无线电源芯片;电池以及添加所需功能的任何其他组件,例如传感器,LED,警报,GPS跟踪,甚至是Bissable显示器。最后,为了使服装清洗,封装物可以提供高强度的防水键,以密封所有电子组件。

图4:传统上可打印的导电油墨和热痕量转移突破,通过消除传统电子包的刚度,为消费者智能服装打开了大门,以便材料可以弯曲和弯曲
弯曲而不会损害嵌入式电子设备。

将电子设备嵌入灵活,耐用和可洗的智能服装,并消除传统的刚性和脆弱的电子包,这是一种改变游戏规则的产品(see Figure 4)

电池考虑

由于当今的智能服装的功能有时会被电池尺寸扼杀,并且需要拆下电池以进行充电和洗涤,因此低饮用的无线充电系统可以为日常用户提供高级的电子纹理。

当今的智能服装制造商经常使用大型电池,这些电池可以持续多个会话而无需充电,因为在洗衣服之前断开电池组并将其插入充电,对普通用户会感到烦恼。

如果用户不再需要考虑充电过程,而是可以遵循典型的洗衣服和悬挂衣服的例行程序,那么较小的单件电池就会变得可行。实施低劳用无线充电系统可以使设计人员能够部署单件电池。然后在一次会话期间可以使用最大可耐受电池大小的全部容量,因此制造商可以增强服装以执行其他功能。或者,物理收缩电池尺寸,以创造出更轻巧,较不笨重的最终产品。

无线充电壁橱的未来

先进的电子纹理已经部署在军事和工业环境中,并正在扩展到消费市场。但是一个死电池组使电子纹理毫无用处。集成RF无线电源网络以更换电池组并确保无摩擦,易于使用的充电是避免这种故障的一条途径。有线充电只会持续到绳索到达,技术界正在迅速接近该关头。


Editor’s Note: Charles Goetz is CEO of Pittsburgh-based Powercast Corp. The company specializes in RF over-the-air wireless power solutions for customers using its know-how, intellectual property and patents surrounding RF-based wireless power.


September/October 2021