Smart AGVs go with the flow
We opted for a flow manufacturing process to build our 5000-series tractors, but we needed it to operate in a fully integrated and networked assembly system. We chose ‘smart’ automated guided vehicles (AGVs) that are precisely routed through assembly steps and mesh with cross-trained operators in workstations to create a ‘smart’ device network.”
That’s Chip LaPole, project manager for John Deere’s Augusta, GA, plant, explaining the rationale behind the material handling system that helped the plant master flow manufacturing. “There are nearly 3500 steps in the assembly process for our 5000-series tractors, but we can complete the entire job in less than 4 hr,” he says. “To accomplish this, we are breaking away from some assembly line traditions.”
A recent expansion at the Augusta plant and diversion from traditional powered assembly line handling technologies is certainly a new approach to tractor assembly. Deere now builds its 5000-series utility tractors on top of wire-guided AGVs. The assembly line is more than 1/4 mile (0.4 km) long with 106 workstations, and the plant’s AGV fleet consists of 62 vehicles upon which work-in-process (WIP) tractors ride.
“The line is laid out much like a racetrack,” explains LaPole. “It has two spur or feeder lines for engine and transmission assembly. Six AGVs work the engine line, six more work on the transmission line, and the remaining 50 loop around the main racetrack line. The AGVs then mesh with cross-trained operators equipped with smart tools in workstations.”
Deere operations managers follow the progress of WIP on computer monitors. Assembly operators get their tractor build instructions and quality control information from the computerized system, which is accessible from touchscreen monitors positioned throughout the plant. System software uses radio communication to directly interact with the AGVs’ onboard computers and transceivers to precisely control and track AGV movements.
AGVs will not move from a specific assembly station until all work is complete and verified. The control system then processes quality control parameters to guarantee acceptance. Vehicles bypass stations where no assembly is needed.
“Depending on the specific tractor under assembly and its options, there is wide variability in the labor performed on each vehicle,” LaPole says. “For example, twice as many labor hours can be expended on a high-option tractor as on a basic model. Therefore, variable routing is important in juggling production scheduling.”
Operators perform up to 70 steps, often involving the exact application of the right amount of torque. Computerized ‘smart’ tools help complete these jobs, and verify that the tool itself has performed the job within the control limits. In some tasks the tool even counts out the number of bolts needed.”
Supplied by AGV Products Inc. (Charlotte, NC), each of the plant’s 62 AGVs can carry 7200 lb (3240 kg) gross weight. Vehicles are outfitted with one of three custom fixtures depending on whether they carry a tractor chassis, engine, or transmission. An onboard, microprocessor-based system called Trace 2000 controls each vehicle and communicates by radio signals with a base control unit. For most routing operations at Deere, the vehicles travel at a speed of approximately 40 ft/min (12 m/min).
AGVs are equipped with a mechanical bumper, infrared object detection sensors, audible and visual warning devices, a loss-of-guidance braking function, emergency-stop pushbuttons, and an auto/manual key switch. If needed, they can be operated manually by hand control devices that enable an operator to control speed, direction (forward or reverse), and steering. Circle 226.
Copyright Society of Manufacturing Engineers Mar 2002
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