# Quasi-Wireless Capacitive Power Transfer for Wire-Free Robotic Joints +
## 1 Introduction  
A robust number of wires is the typical solution when constructing a robot. Wiring is problematic and requires maintenance, as a single break can result in system failure, or worse a hazardous environment. Wireless Power Transfer (WPT) could be a possible means to remove wiring within robotic design [1, 2, 3, 4, 5, 6, 7, 8]. Researchers have investigated Inductive Power Transfer (IPT), demonstrating a 1 degree of freedom experimentally [9], which was theoretically expanded to 3 degrees of freedom [10]. A major challenge with IPT comes from lower misalignment tolerances between the transmitter (Tx) and receiver (Rx). For power transfer within a robotic appendage, developing methods to maintain IPT alignment can be difficult and lead to limitations in motion. Additionally, there is still a bulk of wires and electronics (such as an inverter) that must be present at each linkage if IPT is used [3, 11, 12]. A second method of WPT that exists is Capacitive Power Transfer (CPT), where an electric field at high frequency is used to transfer power from one electrode \"plate\" to another. CPT approaches have two primary configuration methodologies: a bipolar method, where the Tx has two physical plates with a matching two plate Rx [13], and a unipolar (or single-wire) method, which consists of a single Tx plate and a ground plane with the Rx either sharing the Tx ground plane or having a capacitive connection to that ground plane through a counterpoise (a large return plate) [14]. For design of a fullywireless robotic system, the same issues arise that were present in IPT, a need for a bulk of wiring and an inverter between each wireless joint [15; 16; 17]. Due to space limitation on most robotic systems, it is generally not possible to include a large counterpoise for single-wire CPT. Therefore, it can be concluded that there is no existing WPT methodology that can remove the wiring completely within a robotic system. Table 1 includes a side-by-side comparison of state-of-the-art WPT techniques, listing the pros and cons. It includes IPT, Magnetic Resonance Power Transfer (MRPT), CPT, and Acoustic Power Transfer (APT).  
With this in mind, the work presented here aims to create a new method of robotic design that completely removes wiring along and between joints of a robotic chassis. A quasi-wireless capacitive (QWiC) method will be utilized that replaces the large counterpoise requirement of a single-wire CPT system with a small quarter wave resonator (QWR) acting as a Rx, providing a compact method of power transfer over the surface of a robotic chassis. This approach presents a significant deviation from the standard two-wire electrical design methodology, simplifying it to only one electrical connection per joint. In other words, the robot wiring will be completely replaced by a conductive chassis that will allow the QWR and chassis surface to act as a power conduit.  
One challenge in utilizing the QWiC method is that the QWR is highly sensitive to external influences very near or touching the QWR receiver. This makes impedance matching significantly more difficult. Firstly, the impedance becomes position dependent, such that the parameters of the matching network are based on the location it is placed (similar to an antenna). Secondly, unlike an antenna and most problematic, when the matching network is connected, the internal coupling at each section of the QWR causes the entirety of the QWR's transmission line characteristics to change, making a simple smith-chart impedance calculation nearly impossible. To address this issue, a less invasive technique will be presented that offers a way to improve the impedance match of the load, without drastically affecting the QWR's transmission line characteristics.