Automated assembly systems can be classified into two major categories: synchronous and asynchronous.
Synchronous systems are ideal for high-volume assembly of small products with long life cycles. Think spark plugs or syringes. Synchronous systems are driven by a central camshaft that rotates at a constant speed. Each assembly station is associated with one or more cams on the shaft. As the cams rotate, they trigger the stations to perform their tasks.
The problem with synchronous systems is that they’re not that flexible. Adding an assembly station can be difficult, because it may disrupt the carefully controlled timing of the system. Another problem is timing. Each process on a synchronous line needs to take about the same amount of time. So, processes that take longer than a second or two—leak testing, for example—are difficult to integrate.
Asynchronous systems don’t have these issues. In an asynchronous system, conveyors transfer parts on pallets from one station to the next. Assembly operations and pallet transfers are triggered by sensors or switches, instead of cams, and the overall system is governed by one or more computers or PLCs. Stations can easily by added or repositioned. These systems are ideal for assembling products with varying features.
Asynchronous systems are typically slower than synchronous ones in terms of assemblies per minute. However, asynchronous systems are generally more efficient. That’s because asynchronous systems let engineers create buffers between stations, so downtime at one station does not directly affect the output of the following station. With a synchronous system, a fault at one station immediately starves every station down the line.
Assemblers have a couple of options for transferring pallets from station to station in an asynchronous system.
The most common option is a conveyor that carries pallets on twin strands of belt or chain, such as the TS family of conveyors from Bosch Rexroth. Each strand rides in a track within an extruded aluminum profile. The profiles are separated by space, and only the edges of the pallet are supported. The strands provide just enough friction to move the pallet. But, if the pallet encounters a gate or another pallet, it stops and the strands slide freely beneath it.
The strands can be belts, roller chain or flat-top chain. Belts are made from static-dissipative rubber, and generate the most friction for moving pallets. They are used when pallets must be positioned precisely. With thick belts made from low-durometer rubber, the product may be able to ride directly on the conveyor, without a pallet. One drawback of belts is that they are more difficult to repair or resize than roller chain or flat-top chain.
As its name implies, roller chain consists of steel and plastic rollers linked together in a chain. Roller chain can carry heavy loads with minimal chain tension and power consumption. With roller chain, starts and stops have a low impact. The chain moves forward, but the rollers can spin. So, pallets accelerate slowly after stopping—a good feature if the conveyor is moving a sensitive product or a heavy product.
Flat-top chain is a roller chain topped with flat caps of low-friction plastic or steel. Because it can flex from side to side, flat-top chain is used for carousel and serpentine layouts. An advantage of both roller chain and flat-top chain is that individual links can be removed for easy repair or resizing.
The TS family of conveyors are available for handling products as small as cosmetics cases or as large as automotive transmissions. The conveyors can be arranged in many ways: side-by-side, rectangular, carousel, serpentine or even one atop the other.
Many options are available for moving pallets from one conveyor to another. Transfer modules lift and shift pallets from one conveyor directly across to another. They can be used at the end of a line or in the middle of one. Elevator modules are used in over-and-under conveyor setups, or to transfer pallets to another level of the factory. Curved modules or turntables transfer pallets between parallel conveyor sections.
There are also many options for stopping pallets. Cushioned stops bring pallets to a gentle halt. Lift-and-tilt and lift-and-rotate modules facilitate manual assembly operations. Lift-and-locate units raise pallets off the conveyor for automated processes. Dowel pins in the stops engage holes in the pallet for precise positioning. If the process exerts downward force on the pallet, such as a pressing operation, cams can be used to lift the pallets instead of pneumatic cylinders.
Hold-down rails can be attached to the sides of the tracks to prevent pallets from tipping. These rails are used at stations where screwdriving or other operations might tilt the pallet. Another way to provide additional locating accuracy is with guide rails. Located below the conveyor track, the rails match corresponding slots underneath the pallet. The rails are tall enough to slide into the slots and stabilize the pallet, but not so tall that they lift the pallet off the strands.
The newest option for transferring pallets from station to station are linear motor transfer systems, such as the ActiveMover system from Bosch Rexroth. These systems work on the principle of a “smart” track—consisting of multiple linear servomotors—and a “dumb” mover outfitted with powerful magnets. Coordinating the activation of electromagnetic coils in the motors propels the movers around the track.
This arrangement provides multiple advantages for automated assembly. Topping that list is speed. The ActiveMover can move pallets at a maximum speed of 150 meters per minute and a maximum acceleration of 4 g for a 1-kilogram payload or 1 g for a 10-kilogram payload. Fast transfer times have the obvious advantage that automation spends less time moving parts between stations, creating more time to perform operations.
Precise control of acceleration and deceleration of the pallets is another benefit. Movers can be programmed to move with the optimal speed, acceleration and deceleration so assemblies can be transferred as quickly as possible without dislodging parts. Such precise control over pallet motion also optimizes processes such as vision inspection, printing, labeling and curing adhesives.
A third benefit of linear motor transfer systems is their ability to provide independent control over individual pallets. Typically, conveyors move pallets at a fixed speed, and, obviously, all pallets must move in the same direction.
Linear motor conveyors break this paradigm, giving each pallet its own independent speed, acceleration and trajectory. This lets assemblers move unloaded pallets at top speed and acceleration, which helps reduce cycle time. It also enables flexible pitching, in which different stations can work on parts one at a time, two at a time—or any multiple—depending on the process. Pallets can even backtrack to a previous station.
With the ActiveMover system, pallets are 165 millimeters wide and can carry a fixture as wide as 500 millimeters. Maximum payload per pallet is 10 kilograms.
An integrated measuring system ensures precise indexing of the pallets, eliminating the need for lift-and-locate units. Stop positions, set via software, can be anywhere around the system, even curves. Positional repeatability is ±0.01 millimeter. The conveyor communicates via ProfiNet or Ethernet I/P with any process control system.