How do they do it? Anyone who has ever watched an automated materials handling system in action has probably asked that question. How does the system maintain just the right spacing between cartons on a conveyor? How does the stretch wrapper know to stop at the top of the load? How do the cranes in the automated storage and retrieval system (AS/RS) line up the forks to put away a pallet in a storage location without an operator there to guide them?

The answer: Sensors.

They may not be as sexy as RFID tags and bar codes, but sensors are a key enabling technology in automated materials handling systems.

What's more, those little devices that look for cartons on a conveyor and monitor the motor vibration, just could be the next automated data collection device to cut the cord and go wireless. While wireless sensing is still in the very early stages of development, proponents say it may enable the collection of more data from more points in the supply chain than is possible when you have to run wires and cables.

When it comes to automated materials handling, sensors play a role different from other data collection technologies.

Bar codes and RFID tags collect identifying information about whatever item they're connected to. Sensors, on the other hand, capture information about the physical environment surrounding that product.

Once captured, the sensed data is converted into digital information that is fed to a programmable logic controller (PLC) or warehouse control system to drive a process.

“When a sensor senses a part coming down a conveyor, that information will be used by a control device, like a PLC, to send a signal to a robotic arm to pick up a part or for a sortation system to sort a carton,” says Cliff Whitehead, manager of strategic applications at Rockwell Automation (440-646-3779)

That information can also be used to monitor system performance. “You need a sensor to tell you what's going on so you can run a conveyor faster, automate a process and get feedback on the status of your system,” says Tony Udelhoven, sensors division director at TURCK (763-553-7300). “The alternative is for people to manually monitor those conditions.”

While industrial sensors have been around for more than 50 years, the requirement for more sensors in materials handling is increasing as more automated processes are introduced. That's because in automated processes, a sensor is the eyes and ears of the control system.

“For many years, we've tried to make simple devices that mimic the human senses to reduce the cost of production,” says Jim Pankiewicz, product marketing specialist for Omron Electronics (847-843-7852). “They may perform a simple function, like saying that something is there or not. Or, they might inspect a finished part to make sure it's perfect rather than having someone physically measure it. Whatever the case, it all comes back to a device that mimics some human sense.”

In factory automation, sensors may be used to monitor the temperature of a vessel, the vibration of a motor, or the fill level in a container. In automated materials handling, “sensors tell you how many, how big, where it is, and when did it come by,” says Wayne Meyer, product manager of industrial sensors for SICK (800-325-7425).

The sensor world is vast. Options range from basic sensors that detect the presence of an object, to analog sensors that capture complex information, like temperature, noise or vibration. In the broader supply chain, some users are experimenting with the combination of analog sensors with RFID and GPS technology to monitor the location of a truck, the temperature in a trailer and the identity of the products inside.

In factory automation and the materials handling world, the most common sensors are simple binary ones. These communicate the equivalent of a yes or no response to a question. There are three types: photoelectric, ultrasonic and inductive.

Photoelectric, or optical, sensors detect the presence or absence of light. While there are a variety of photoelectric sensors, they all work on a similar principle: The sensor emits a beam of light that detects the presence of an object when it breaks that beam, or reflects the beam back to a receiver on the sensor. An object is either present or it's not present.

Photoelectric sensors are most commonly found on conveyors, where they track cartons or products. But photoelectric sensors might also be used to determine the height of a load. Or they can be used in conjunction with other sensing devices to position the forks on an AS/RS to put away a pallet. “The key to any application is less the capability of the sensor as it is the cleverness in how the sensor is applied,” says Tom Draper, marketing manager for Balluff (800-543-8390, www.balluff.com).

Ultrasonic sensors are used to detect the presence or absence of an object in a different way. Instead of light, these sensors emit an ultrasonic wave. Ultrasonic sensors can be used in dirty environments, where dust and particles in the air might impede a photoelectric sensor. “Ultrasonic sensors are also being used to keep automatic guided vehicles (AGVs) on their path and to create invisible bumpers that detect objects in the AGV's path for collision avoidance,” says Gary Grigyes, product manager for electric sensors for Pepperl & Fuchs (330-486-0001, www.am.pepperl-fuchs.com).

Inductive sensors detect the presence of ferrous and non-ferrous metal object. Like ultrasonic sensors, they can operate in environments where photoelectric sensors can't be used. One common application is to provide information for robotic picking applications.

Also commonly used in materials handling are sensors that determine position. These include linear and rotary encoders and laser distance meters. These sensors may use a time of light measurement technology—sending out an optical pulse and then listening for the result. They could also pick up pulses from a conveyor or motor shaft as they turn. In either case, the sensor determines with a high degree of accuracy how far something has moved and at what speed.

Often, one sensor type is not enough to get the job done. In the complex world of automated materials handling “it's common to find multiple sensor technologies on a single line or a single machine to create a solution,” says SICK's Meyer.

For industries that rely on sensors, wireless could be a big boon. Wireless connectivity, after all, has contributed to productivity gains in DCs and factories. Going mobile enables workers to take bar code scanning, label printing and voice technology directly to the spot the work is getting done. RFID, meanwhile, completely automates data collection.

Until now, however, sensors have still required wires to do their job, especially in automated materials handling applications. There are two reasons for that.

1) Information latency. This phenomenon is the speed at which information travels through a wire compared to through the air. “A wireless mesh network is transmitting 100 to 200 millisecond data rates,” says Cliff Whitehead, manager of strategic applications for Rockwell Automation. “That sounds fast, but a motion control application, like a high speed conveyor and sortation system, requires faster data rates than that.”

2) The need for power. While many sensors may only need to come on for brief periods of time at spaced intervals, a photoelectric eye on a conveyor traveling at 600 feet per minute “is going to be on all the time,” says Whitehead. “If you're going to the expense of running power to the sensor, you might as well put a signal cable in as well.”

By all accounts, most sensor providers are still in the R&D stage when it comes to wireless. However, there are a few solutions already in the market.

Banner Engineering, for instance, has developed a wireless sensor solution to detect the presence of automotive doors on an overhead conveyor system in an assembly line and to monitor motor and bearing temperature in an AS/RS.

Two developments are making new applications possible.

1) A power management system. “The key is to efficiently power the sensor, or cycle it on and off, in a way that extends the battery life,” says Darvin Kaelberer, Banner's wireless business development manager.

2) The development of wireless mesh networks to communicate data. In a mesh network, intelligence is built into the sensors in way that allows them to organize a network and hand off information from one sensor to the next, like a bucket brigade, until it reaches a centralized gateway connected to a network. “The sensors figure out the best way to get that information back to the gateway,” says Robert Shear, director of market development for Dust Networks.

While the technology is marching forward, most experts still believe broad-based implementation of wireless sensors in automated materials handling will be some time off. “For now, status monitoring, especially in process manufacturing, is the most common application for wireless sensors,” says Shear. “Things happen a little slower and an information infrastructure is already in place. You're just adding more sensors.”