Lords of the Rings

Introduction

Berran Industrial Group designed a packaging machine that attaches small tags to rings and then seals each ring in a small plastic bag. The jewelry company wanted an automated way to verify that each bag contained a ring, but for several reasons the sensors we use for other applications could not reliably perform that detection. Our solution, after much trial and error, was to use an OMNI-BEAM accoupled sensor with fiber-optic cables.

The Problem

The patented packaging machine that Berran built for the jewelry manufacturer attaches small price tags to rings and then seals each ring in a small, clear, plastic bag. The process, which takes just a few seconds, has several steps. First, the feeder moves the leading end of a continuous, perforated roll of poly bags into position. Then an air jet emits a puff of air that opens the 3-inch × 2-inch pocket in the roll, and a ring drops down the shoot and into the bag. When the sensor detects the ring as it falls into the bag, the sealer bar moves into position and quickly heat-seals the bag, which is then cut from the roll. Puff, drop, seal, cut—at a rate of about 40 cycles a minute.

The value of the rings necessitates tracking them throughout the process, so the jewelry manufacturer specified that a sensor inspect each bag before it is sealed to make sure it contains a ring. This requirement proved problematic for several reasons:

• Because the sealer bar closes the bag right after the ring drops into it, the presence of the ring has to be detected during the fraction of a second that is actually falling.

• Wrinkles in the surface of the bag reflect light, which could trick the sensor into thinking that the ring has dropped before it actually drops, triggering the sealing bar too early. As a result, the bar could crush an expensive ring or, the ring could fall to the floor and be damaged or lost.

• The sensor has to be versatile enough to accommodate the differences in sizes of rings.

• The path the ring follows while falling into the bag is inconsistent; it can fall anywhere within the 2-inch width of the bag.

• The sensor has to fit into a confined area.

Adding to the challenge, the customer required a detection rate of 100 percent.

We tried photoelectric sensors, several fiber-optic sensors, and a capacitive ring sensor. It was a massive problem. Every sensor supplier with whom we work looked at the problem and said they had the solution. Some of the setups we tried worked some of the time—if the ring dropped just the right way—but none worked with the reliability that the customer needed.

We'd evaluated and rejected about a dozen sensors when I called in Ken Prahlow, one of the owners of MC² Inc., in Cleveland, OH, a representative for Minneapolis-based Banner Engineering.

The Solution

Ken borrowed a solution widely used in the automotive and metal stamping industries, recommending that we try an OMNI-BEAM™ sensor in opposed mode, coupled with an ac amplifier. In addition, he suggested using a pair of fiber-optic cables with 40-mm slotted ends as the "eyes" of the sensor—one cable connects to the emitter in the OMNI-BEAM, the other to the receiver—and placing an end on either side of the roll of plastic bags just below the shoot.

Here's why Ken suggested this setup:

• Ac-coupled amplifiers detect only quick changes in the light level. They are too sensitive for many applications because they might respond to unwanted conditions, such as vibration. But for this application, it provides the sensitivity needed to detect a ring—no mater how delicate—at the instant it drops into the bag.

• The fiber optic array senses the entire width of the bag, not just a point, so the path of the dropping ring doesn't matter.

• Because the sensor uses an infrared LED, it only detects infrared light, which is outside the visible spectrum. Ambient light on the production floor is irrelevant.

• With cables on either side of the bag, the sensor operates in opposed mode— it looks through the bag—so the wrinkles no longer cause a false trigger. As the ring drops into the bag, the receiver on one side senses the quick decrease in the amount of light it's picking up from the emitter on the other side.

• The fiber-optic cables are small enough to fit into the machine.

The sensor works, 100 percent of the time. The beauty of the solution is not that it used a dazzling new technology; in fact, the OMNI-BEAM has been around for a long time. The beauty is that the solution applies a proven technology to a dilemma that the industry's usual solutions couldn't reliably handle.