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Cutting the Cord with Wireless Power Transfer
Article written by Ben Waters CEO and Co-Founder at WiBotic, Robolliance Expert
As the number of electronic devices in our lives continues to proliferate, it’s no wonder we’ve come to loathe the power cord. Not only do cords clutter our desktops and work spaces, but even battery powered devices must periodically submit themselves to ‘cord captivity’ when recharging is required. Nonetheless, the cord inevitably remains a critical component for charging most of our devices today.
So how, has charging been accomplished in the robotics market? Tethered charging systems have existed for some time, especially for stationary devices like robotic manipulators in a factory or any device that a human would physically plug in. For more mobile robots that have the freedom to drive (or fly) around, the tether has typically been replaced by a physical dock where the robots can charge via mechanical contacts. However, replacing cords with docking stations may not be an effective solution for robots to reach their true potential and become highly functional, fully autonomous systems. Both cords and docking stations restrict the robot to a fixed location, and neither provides the charging flexibility and battery intelligence today’s users need to maximize robot uptime.
Fortunately, the field of wireless power transfer (WPT) is rapidly evolving to solve this problem. In a series of posts over the next few months, we will provide information on the types of wireless power technologies available and the benefits they can provide for any autonomous system (aerial, mobile or aquatic). In this first post, we will describe the two most common forms of WPT, inductive and resonant systems, and how they differ.
Inductive Charging Systems
Whether we have recognized it or not, most of us have experienced wireless power transfer. Examples include electric toothbrushes that receive power through sealed plastic docking stations. Many smart phones can now be charged on special “power mats” that eliminate the need for plug in chargers. Some stovetops are even able to heat water without direct contact between the pot and a heat source such as an element or flame. Instead, these products use electrical induction to transfer energy from point to point – without the need for direct contact or physical wires.
How does this work in a battery charging situation? Think back to Physics 101 and you may recall that alternating electrical current creates an electromagnetic field as it flows through a conductor. If a second conductor is placed alongside the first, the electromagnetic field will induce electrical current in the second conductor as well. By coiling the wire, and changing the number of coils between the primary and secondary wire, electrical energy of one voltage can be converted into another voltage. This is the principle behind electrical transformers – and is essentially the same concept behind wireless inductive charging.
Like a strongly coupled transformer, inductive WPT systems require the receiving coil to be very close to the transmitting coil in order to achieve efficient power transfer. Typically less than one centimeter of separation is allowable. The angular orientation of the coils must also be nearly perfect, making it especially difficult to maintain charging efficiency if the robot returns to the charging area in a slightly different position each time. In short, if there are anomalies in how the robot is positioned, a great deal of energy can be lost. Due to this challenge, only very low power devices (i.e. toothbrushes and cell phones) have been feasible candidates for this type of charging in the past.
Resonant Charging Systems
A newer wireless power technology is based on magnetically coupled resonators (MCRs). MCRs are essentially matched sets of coils that use highly tuned oscillating magnetic fields to transmit energy. While similar to inductive coils in appearance, MCR coils and circuit boards incorporate a much more advanced mechanism for generating and transmitting power at high efficiency.
Unlike inductive systems, MCRs allow the transmitter and receiver coils to be placed at distances of up to several centimeters (or even meters) in most mobile robot applications. The coils also do not need to be angularly aligned, meaning robots do not have to be in precisely the same position each time they return to the charging area.
MCR systems are available today in wall or floor mounted configurations that allow the robot to approach a charging area within a few inches to begin charging. Perhaps the most exciting element of MCR technology, however, is its ability to allow for future robot recharging “on the fly”.
To understand this benefit, let’s first consider the status quo. Today, even the most advanced mobile robots must return to a “home base” for periodic recharging. The robot can be recharged and put back into service in a number of ways:
- Battery swapping – humans replace discharged battery and send the robot back into service
- Corded battery charger – humans plug the robot into a charging device and wait for recharge
- Automated contact charging station – the robot routes itself into a charging station and makes a physical connection with metal contact points
- Automated wireless recharging – this scenario is similar to #3 above, but the physical contact points are replaced with an inductive charging system
With the exception of scenario #1 (which is extremely manpower intensive) all of these options take the robot out of service for an extended period of time. Even battery swapping is somewhat time consuming and repeatedly changing batteries leads to other reliability issues from dropped/damaged batteries to worn contact points.
With MCR based systems, however, dynamic adaptation can be used to adjust for the relative position of the transmitter and receiver coils change in real time. So, what exactly does this mean? It means the robot doesn’t need to dock with absolute precision. Instead, robots simply need to approach a nearby transmitter station (typically located in the floor or wall) and charging will begin automatically. Furthermore, this capability opens up the possibility of charging robots while they are moving! In this scenario, multiple power transmitters hand off the WPT connection as the robot drives by – thus keeping the robot moving, and charging, 24/7. Only MCR systems with adaptive matching capabilities have this awesome potential.