Carts are an affordable, flexible, and safe method for linking two areas in a facility that are far apart and share a common traffic pattern. They excel at simple, repetitive applications.
Cost is one of the major reasons for their increasing popularity. A medium complexity full-size AVG costs an average of $100,000. On the low end, carts can cost $15,000 to $50,000.
Safety is the other reason. It is everybody's first priority. AGV cart manufacturers like Jervis B. Webb (Framington Hills, Mich.) and FMC Technologies, Inc. ( Chalfont, Pa.) design their vehicles for safety. Webb's SmartCart has ultrasonic sensors and a bumper that senses objects and stops the cart when an impending collision is detected. FMC's Self-Guided carts have a similar system that uses laser bumpers for safe obstacle detection.
Facility throughput is a key factor determining the type of material handling equipment used. Conveyors are still best for high throughput operations. "If you don't have a high throughput and don't want a permanent obstacle like a conveyor blocking an area of a plant, then a cart or an AGV is a great solution," says Mark Longacre, marketing manager, FMC Technologies.
AGV carts, however, are not well suited for complex hub-and-spoke layouts where there are 20 or 50 pick-up and drop-off points and a random mix of movement. Nor are they suitable for heavy-duty manufacturing. But they are ideal for moving parts or boxes to the assembly point or receiving area. The FMC Self-Guided carts are flatbacked and have no lifting capabilities. They have a 1,000- to 1,500-lbs. capacity and can tow 5,000 lbs. Jervis B. Webb's SmartCarts are available in three models. Their capacity ranges from 800 to 3,600 lbs. Tow capacities are between 1,000 to 4,000 lbs.
Carts in Action
Not surprisingly, AGV carts are a good fit with automobile manufactures' lean, just-in-time and sequenced manufacturing processes. Carts are a safer and more efficient way to deliver parts to the assembly line. "Instead of conveyors, the carts make more sense if you can exploit their advantages, their size, and how you engineer your manufacturing system," says Nidamuluri Nagesh, technical integration engineer, GM (Warren, Mich.).
GM's new Delta Township assembly plant near Lansing, Mich., has a fleet of about 40 SmartCarts it is using for a variety of applications including carrying seats and instrument panels from the dock to the assembly point. Robotic assist arms lift parts on and off of the carts. This GM plant is assembling Solstice and Saturn Sky.
Lift trucks are not used in the new plants, Nagesh says. "For safety reasons and ergonomics, we prefer the Smartcart." Tuggers are also being used in the plant, he adds. To his knowledge, there have been no injuries caused by the carts in the Lansing facility.
The DaimlerChrysler Belvedere Assembly Plant (Belvedere, Ill.) uses FMC's Self-Guided carts to link receiving docks with the assembly line. Parts are received in racks, which are taken by cart to assembly lines for models like the Dodge Caliper. DaimlerChrysler has 10 carts in its fleet of 31 AGVs.
Honda, Toyota, John Deere, and Harley-Davison all use AGV carts at their manufacturing facilities because they solve cost and safety concerns and issues. Jervis B. Webb's V. P. and COO, Brian Stewart, says carts are "low-cost, and have high reliability and high quality. They can be implemented in hours or days." At Harley-Davison, he explains, SmartCarts were delivered on a Friday, and installed and running Saturday afternoon. "Harley made six changes to the line on Sunday, and the carts were running production on Monday."
Battery charging
AGV carts know when their batteries are low and need recharging. The carts usually carry 24-volt batteries. Many of the carts opportunity charge by pull up to the nearest battery charger embedded in floors or mounted on walls. The plates carry a DC current, which won't injure people when they walk on them. Some companies use battery rooms for recharging their carts.
At GM's Lansing plant, Nagesh says he makes sure there are enough carts to keep the product-movement and battery-recharging loop moving constantly. The charging stations are placed in locations where carts tend to wait, which is a productive way to give batteries a two-minute recharge.
Guidance Systems
Self-guided carts are guided by magnetic tape epoxied to floors, reflectors, lasers or a wire embedded in the floor.
FMC carts have a laser guidance system that uses 10- x 2-in. reflectors to calculate triangulation equations that guides navigation. About 40 calculations are done per minute. The company chose this type of guidance so its carts can be used with full-size AGVs in the same facility. The practice of using AGV carts and lift trucks lets managers decide if they want to use more or less complex systems for an application. When all of the vehicles use the same guidance system, one host computer can be used to track them. This leads to better vehicle traffic control and prevent logjams at intersections on the floor.
Webb's SmartCarts are guided by magnetic tape epoxied to warehouse floors. It is a flexible system; the tape can be easily changed to create new paths. The tape is impervious to wear and tear. For example, Stewart says, "Harley-Davison has one area where there are hundreds of lift trucks driving over it every day as well as workers walking over it and it has not been damaged at all."
Software
AGV carts know what their tasks are thanks to onboard software that controls where they go and what they do. Wireless networks are used to communicate with the carts.
SmartCarts are controlled with SmarTools software. It guides the cart based on choices made at decision points that are programmed in it. The software communicates through RF back to a main computer that updates it and tells it where to go. Workers on the floor can program the plug-andplay software in five minutes. "GM has embraced this technology and it has save a lot of money doing that," Nagesh says. Traffic control and scheduling are run from a PC server.
SmartTools can be integrated into other warehouse software. However, GM only uses the software to monitor the carts. "In our culture," GM's Nagesh says, "they monitor everything. In the dispatch room they know where all of the vehicles are and what they are doing. They get failure messages when something goes wrong."
FMS Technologies carts use Layout Wizard software. Using the carts' computer display, users can define the road system or paths in its facility. The path is then downloaded to the carts, and they proceed on their paths when the host computer tells them to. The carts use RF to communication with host computer. The chatter between the systems is frequent, but does not impede other signals being transmitted because not a lot of data is exchanged with each communication.
Lessons Learned
AGV carts are a good fit for lean manufacturing and lean material handling, says GM's Nagesh. Using carts are "great when you are reengineering and leaning out your assembly processes. They are very flexible." Carts can even be used for parallel processing. When using carts, Nagesh says, "Keep it as simple as possible. The simpler the better. If you have a dedicated task, two or three carts can do the same thing. If you want to get into the system level, and use a lot of carts, then you need the traffic control software. The carts can carry all sorts of dunnage." It all depends on how cleverly the mechanical framework of the cart is designed, he says. And pay attention to the simple details. "How the part is put on the cart, and how it is taken off. Managers need to pay attention to these details."
