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Battery-free AGV in use at an off-highway vehicle assembly line. (SAE technical paper 2014-01-0782)

Flexible, ‘invisible’ assembly lines gain popularity

Interest in automated guided vehicles (AGVs) to convey product from one station to another on the plant floor assembly line continues to grow as manufacturers look for greater flexibility, scalability, and sustainability in their operations, according to Rod Emery, Vice President of Integrated Assembly Systems at RedViking, an engineering, integration, and manufacturing company based in Plymouth, MI.

AGVs have been around for some time, and the vast majority of applications of the technology today involve battery-powered units. At the Society of Manufacturing Engineers’ recent International Manufacturing Technology show in Chicago, RedViking displayed a battery-free AGV that is powered inductively from a system of cables embedded just under the surface of a concrete plant floor. They amount to a “flexible, invisible assembly line,” Emery told SAE Magazines at the show.

The battery-free AGV is an equally good solution for new plant floors and for reconfigured ones, he said.

The underlying technology for RedViking AGVs is inductive power transfer. Emery described it in a paper (2014-01-0782) delivered at the 2014 SAE World Congress in Detroit: “Inductive Power Transfer uses the same fundamentals of electromagnetic induction that are key to all forms of electrical generation and transmission. At a very simple level, you can think of Inductive Power Transfer as a transformer, where an alternating current in the primary windings induces an electromagnetic field, which then induces an alternating current in the secondary windings. This solution, instead of having the primary and secondary windings closely coupled about a shared core (either iron or ferrite), has separate parts for the primary and secondary. Instead of primary windings the primary is a conductor loop consisting of cable installed just below the surface of the floor. Rather than secondary windings, the secondary is a movable power pickup separated from the conductor loop by an air gap of approximately 10 mm. This requires that the electrical supply operate differently, switching at a much higher frequency that the 50 to 60 Hz of a typical transformer. The switching frequency of the Inducting Power Transfer (IPT) power supply is in the range of 20,000 Hz. The alternating current that is induced in this field requires a control circuit to regulate the power in the loop. This is achieved through a patented control circuit that provides optimal system performance, eliminates undesirable feedback in the primary circuit, and enables the entire system to achieve very high power transfer efficiency (in excess of 95%).”

About 2.5 kW of power is transmitted wirelessly to a RedViking AGV at 95% efficiency, Emery told SAE Magazines.

IPT does not face the same challenges that batteries do, according to Emery. There are the environmental and safety hazards and potential hazards of making, using, and disposing of them. And there are the logistical issues of charging them, Emery pointed out: “What is your spare-battery situation? How many additional AGVs do you need sitting idle in a charging location?”

The advantages of IPT technology over conventional chain-based conveyor systems is even stronger, as detailed in Emery’s SAE paper: "Installation of battery-free AGVs is much quicker, less expensive and far less risky than installation of chain conveyor systems. The installation does not require any pits, trenches, rails or structural steel. The installation consists of cutting two shallow channels in a flat concrete floor with an iron-free zone approximately 70 mm deep by 300 mm wide. Each channel cut into the floor is approximately 30 mm deep by 30 mm wide. Special purpose Litz power cable and communication cable is laid into the channel and covered in epoxy. Once the epoxy cures a few hours later, it is sanded smooth. The resulting installation is a smooth, flat floor free of any obstructions or trip hazards.

"The only other installation activity remaining is to set and connect the master control panel to the cable in the floor and to a 480 V ac, 3-phase power source.

"The installation requires far less labor, material and skilled trades than a chain conveyor. As a result the installation can be completed in much less time with far fewer labor hours expended in the installation of dedicated infrastructure which is neither scalable nor salvageable. For comparison purposes, consider the installation of a conveyor loop that is approximately 800 m in length.

"For an Inverted Power & Free (IPF) conveyor system, there needs to be a pit around the entire conveyor path for the conveyor steel to reside within. This pit is approximately 2 m wide, by 1.5 m deep by 800 m long, with an additional pit for the chain drive and chain tensioning device that is approximately 3 meters wide by 3 m deep by 3 m long. The total labor hours required to perform this installation are in excess of 13,000 hours based on approximately 5000 hours of mechanical labor, 1250 hours of electrical labor, 1000 hours of pipefitter labor and the balance consisting of concrete work and supervision.”

Other advantages come in “cleanliness, low noise levels, improved sustainability, a safer work environment, improved operator access to the product, and increased load capacity.”

Regarding load capacity, he wrote: “There is no practical limitation to the carrying capacity of a Battery-free AGV system. The AGV can be designed and constructed to carry a payload well above and beyond 100,000 lb. The carrying capacity of an IPF conveyor system is limited by the structural steel, the trolley, rail, chain and drive mechanisms. The practical carrying capacity limitation is less than 10,000 lb.”

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