Conveyor controls keep moving along

As functions and interfaces with other equipment become more complex, control components on conveyors have to do more.

As material handling becomes more complex, the controls that regulate conveyors have to become more sophisticated.

Conveyors control the movement of products and packages within a plant. But what controls the conveyors?

Like just about everything else on a packaging line, conveyors have been called upon to do more things, with more variety. And, again like everything else on the line, they’re accomplishing that through the increasing sophistication of programmable logic controllers (PLCs), motor drives and other electronic controls.

More sophisticated controls allow conveyors to keep up with several trends, perhaps the most important of which is increased changeovers. Conveyors have to respond to changes in package sizes in three basic ways: altering their speed; changing the timing and, in some cases, the physical spacing of diverting accessories like sorters, gates and lane dividers; and interfacing with the machinery they feed into and out of.

“With the increasing demands by marketing and consumers for different products and packaging comes the need for fast changeovers,” says James Wussow, electrical engineering customer service manager for Arrowhead Systems. “Advanced controls allow for recipe-based, tunable programs that allow for one-button changeovers. The controls components themselves need to more versatile. They need to be able to communicate with other equipment and be reconfigurable for the next change.”

Distribution center

One of the biggest general trends in conveyor control, conveyor suppliers say, is a rise in distributed-drive systems. The standard arrangement was to have variable-speed drives clustered together in a control panel and hard-wire them out to the motors they control. Now, it’s becoming more common to locate the drives near the actual motors.

There are several advantages in doing so, says Joel Kasper, project manager for Nercon Engineering and Manufacturing. Easier maintenance is one. New National Fire Protection Agency regulations require maintenance people to don cumbersome protective equipment before entering a control panel; having the drives out on the conveyor line makes for easier maintenance. It also makes things easier to have all the input/output (I/O) points and wiring are near the motor. In addition, it’s quicker and cheaper to install a distributed system, since there’s no need for long wiring between the control panel and the motors.

“Those forces have conspired to pretty dramatically increase the number of distributed-drive systems,” Kasper says.

On the other hand, distributed-drive systems have some disadvantages, says Doug Imes, automation manager for Hartness International. Distributed drives have to be tested in the field, he says: “One of the things you give up is the ability to completely test and check out the system in the factory before it ships.”

Another disadvantage is that distributed drives are in an exposed position on the conveyor lines, which may compromise their durability. “At this point, we’re still discovering whether or not these conveyor-mounted drives are more reliable. The expectation is that they’re not,” Imes says. “However, we don’t have data to support that at this point.”

Regulation of sorting devices, and keeping track of flow near them, are among the most important functions of conveyor controls.

PLCs the norm

Programmable logic controllers are the most common controllers for conveyors. The usual setup is to have one PLC for an entire conveyor system.

“For the most part, a single PLC is capable of running very large conveyor systems,” says Rod Farver, controls director for Dorner Manufacturing Corp. “Multiple PLCs are sometimes deployed, but [usually only] for reasons of ease of service or deployment.”

The most common function for a conveyor PLC is interacting with the machinery at either end: getting permission to run from downstream equipment and getting a call to carry product from upstream equipment, and speed references in applications where different speeds are used. Many conveyor systems run at a single speed, which simplifies the control situation. But others use variable-speed motors, such as sorter/filer areas that use belt segments running at different speeds to ease a mass of bottles into a single file.

When the conveyor runs at a single speed, it often interfaces with machinery on either side on a simple yes/no basis. This greatly simplifies the control setup: It allows use of the most basic control, a “dry contact” switch that uses an electromagnet to pull down a contact and cut off the motor that runs the conveyor. The signal to activate the electromagnet and cut the current comes from the PLC of an adjacent machine.

However, sometimes conveyor response has to be more versatile. A conveyor might have to change speeds, or it might have to interact with multiple sensing devices such as photoelectric sensors, bar code readers or cameras. Those kinds of applications require more sophisticated communication.

Ethernet on the rise

A common communication protocol for such applications is Ethernet. It’s versatile, non-proprietary, easy to configure and widely used in control equipment.

The major disadvantage of Ethernet in these applications is that it’s not usually capable of “cascading”-that is, allowing communications among multiple controllers and devices through a single line or loop of wire. Every block of I/O ports needs to be wired separately to whatever device it needs to communicate with. That can get complicated, in an application that requires sophisticated communications.

Ethernet is “like a star [wiring] topology, where you have to have a router or switch, and then from that box you go to the different equipment,” says Sophie Tisseaux, automation and control director for Sidel. Such wiring can be complex, time-consuming and expensive, especially as the price of copper wire rises.

Other communication protocols, such as DeviceNet, allow cascading communications. This greatly simplifies the wiring topology, since one loop of wires can carry information across a string of devices and controllers. The disadvantage is that such setups demand the use of expensive proprietary software; Ethernet, as an open protocol, is much less expensive.

However, there’s a development in Ethernet-based hardware that may overcome its inherent disadvantage, says Jerry Koch, product director of software and controls for FKI Logistex. Some component manufacturers have developed integral switches that allow Ethernet networks to cascade data, allowing the use of Ethernet while avoiding the costliness and hassle of star topology.

“What you’re seeing now is, you actually end up wiring a topology that’s more of a traditional daisy chain or linear type of network layout, instead of having to run every device back to a central switch,” Koch says.

Inspection interface

Conveyors often are expected to interface with a variety of inspection/detection devices along the line, such as photoelectric sensors, light curtains, cameras and bar code readers. The interfaces, especially with yes/no type devices like photoeyes, have tended to be hard-wired discrete connections (meaning they’re either on or off). But, depending on the sophistication of the device, more complex networked communication setups may be appropriate, Farver says.

“Networked sensors offer features that hard-wired devices can’t,” he says. “But the additional cost still stands as an obstacle to deployment. For the most part, we continue to interface to simple sensors via hardwire connections. These are normally pulled to a small network I/O block and connected back to the controller via Ethernet. Smarter gear, such as bar code reading or vision equipment, will be connected by Ethernet or traditional serial communications, depending upon the application.”

Hytrol Conveyor Co. supplies an inspection accessory that allows maximum flexibility in information usage. Its EZLogic accumulation system is now in a Gen3 version. EZLogic comprises a photoeye and controller that keep track of how many cases are in an accumulation zone and signal the rest of the conveyor to start and stop accordingly. In the Gen3 version, the photoeye is separated from the controller to maximize flexibility and ease installation.

The EZLogic’s controller is usually wired to whatever will actuate a zone, whether it be a motor drive, a motor control card or a pneumatic solenoid valve. This helps it interface with downstream equipment, says Boyce Bonham, Hytrol’s manager of product development: “A palletizer may [deposit] 10 boxes in a layer, and we can tell it [whether] you’ve got 10 boxes on the conveyor or not.”

Data management

Another important issue in conveyor control is how much data management the control system is expected to do. PLCs are designed to execute commands quickly with a limited amount of data storage space. That works well for most food and beverage applications, Koch says. However, in some applications, the material handling system has to handle motion control, discrete controls for functions like routing, database management and interfaces with higher-level software. In those cases, PC-based controllers, with their larger memories, are more useful than PLCs.

“If you think of information relative to every single carton-the size of the carton, the bar code for the carton, the desired destination, the spacing before, the spacing after-you start handling a larger amount of data than a PLC can handle directly,” Koch says.

Conveyor controls are like any other electronic components: Increasingly sophisticated technology allows them to keep up with increasing demands.

“The controls components need to more versatile,” Wussow says. “They need to be able to communicate with other equipment and be reconfigurable for the next change.” F&BP