Sensors for High-Tech Detection

Technologies once requiring a highlevel of expertise are now simple to use and affordable.

The demand for consistently high-quality products drives most industries to improve their manufacturing processes, but few face the pressure that medical equipment and device manufacturers do. They must rely on a variety of sensors to inspect products throughout the production process, to ensure the highest product quality and to comply with the guidelines of good manufacturing practices. Not only do sensors help detect defective products before they ship, they also make it possible to identify and correct problems early in the manufacturing process, reducing waste and improving yield.

Photoelectric Sensors: Old Technology with a New Look

Photoelectric sensors --sometimes called electric eyes or photo eyes -- have been around for more than half a century. Their inception was revolutionary, allowing many inspections that humans performed to be automated, making the inspections more reliable and less costly.

The theory is simple: an emitter sends a beam of light, and a receiver detects it. The beam can be within the visible spectrum or be infrared. If an object breaks or alters the beam of light between the emitter and receiver, the receiver triggers some kind of reactionthe garage door doesn't close, or a machine stops, or a defective product is rejected from the line.

Advances in photoelectric technology have kept pace with advances in electronics. Today's photoelectric sensors are smaller, cheaper, faster, more versatile, and more reliable than their predecessors. State-of-the-art optics, microprocessors, and packaging techniques are now used to address diverse applications. Sensing ranges now are as close as a few millimeters to as far away as 250 meters. Overmolded housings protect sensor components from vapor and moisture. Some photoelectric sensors can distinguish between colors, while others provide analog output. Some have the emitter and receiver in the same housing, so only one side of a line needs wiring.

A major advance has been miniaturization; novel optical design and electronic components make it possible to build sensors that fit on a dime, without compromising performance. Another advance is the use of lasers in high-end sensors, because the tight, far-traveling beam accurately detects small objects, even over long distances. Durable fiber optics cable can now be attached to photoelectric sensors to pipe the sensing beam into extreme environments that the sensor itself can't tolerate, and for other remote sensing applications.

With so much choice, prices of course vary, but many sensors cost less than $75. The following are just a few applications to which photoelectric sensors are suited:

• Detecting the presence of colored caps on bottles of children's cold tablets.
• Counting tablets for solid-dosage medication packaging
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• Verifying the position of glass vials before the vials are filled.
• Detecting the level of liquid in a clear bottle, without touching the liquid
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Despite their remarkable versatility, photoelectric sensors have limitations. They have trouble detecting some clear objects and don't perform well in environments where grease, dirt, or dust interferes with the beam. And they only sense a point, not an area. Fortunately, two other types of sensors address these problems: ultrasonic sensors detect clear objects and penetrate dust, and vision sensors inspect a plane made up of thousands or millions of points, rather than just one.

Ultrasonic Sensors: It's a Dirty Game

Ultrasonic sensors use sound waves, rather than light, to detect objects. For that reason, they are the first choice for dirty environments, where the lens of a photoelectric sensor could become coated with grime or dust in the air and interfere with a photoelectric beam. They also can detect glass objects through which a photoelectric beam passes unaltered. Ultrasonics are often used to sense levels of liquids in tanks, to gauge the changing diameter of a roll of material, and to control the speed of a web operation by monitoring the tension of the material between two rollers. They work best with dense targets that reflect sound waves; sound-absorbing materials such as cloth and foam rubber make poor targets.

Vision Sensors: The Bigger Picture

Unlike a photoelectric sensor, which uses single photoelectric cell, a vision sensor has many photoelectric cells that capture an entire scene. The amount of detail the image shows -- the resolution, expressed as the pixel count -- depends on how many photoelectric cells the sensor has. For example, some visions sensors from Banner Engineering can capture 1.3 million pixels. As a result, the sensor "sees" with great clarity, whether centimeters or meters away from the object.

After capturing the image, the device that processes the image -- either the vision sensor itself or a separate PC -- analyzes the image by comparing it to the reference image stored in its memory. So, for example, if the operator sets up the device to recognize a blister pack with eight undamaged tablets, the device knows to reject a pack that has only seven tables or a broken tablet. Furthermore, it can make that determination regardless of where the packet is located within the camera's field of view. And it can inspect asymmetrical objects no matter how they are rotated within a full 360° range. Vision sensors with a remote TEACH feature make it possible to change inspections remotely.

Vision Systems versus Vision Sensors

Vision technology has been around for about 40 years, but until recently was available only in complex, expensive, multi-component systems. Vision systems require one or more cameras, custom software, and a PC to perform the image processing. They usually necessitate hiring an outside vision consultant to design and integrate the system. In addition, because vision systems are application-specific, they can't easily be modified for other inspections. Finally, they are difficult to support, usually requiring additional outside help. PC-based vision systems start at about $10,000 but can cost several times that amount. Vision systems have their place, but the advent of less costly, less complicated vision sensors is shrinking the number of applications that warrant a vision systems. Vision sensors -- sometimes called smart cameras -- do not require a PC to run an inspection (although some can be connected to one for Ethernet or bus networks).

Vision sensors are smaller and less expensive than vision systems, starting at about $1000 for a basic one and about $2500 for a full-featured model. Consequently, manufactures can use vision sensors -- and more of them -- in applications for which a vision system can't be justified. As a result, vision sensors can solve more inspection challenges.

Compared to photoelectric sensors, vision sensors are more powerful and give machine designers more flexibility. A designer can use one vision sensor to inspect for multiple features in a large area, rather than multiple photoelectric sensors that require time-consuming, costly, and precise fixturing. And vision sensors are far less likely to be thrown off target by the

Vision sensors also are more versatile than photoelectrics. If the operator changes the target object -- from, say, over-the-counter personal medications to bulk generic drugs -- there's no need to adjust or swap out the sensor. The operator simply teaches the vision sensor what the new object should look like -- which the operator can do remotely. In addition, because a good, basic vision sensor now costs about as much as ha ndful of photoelectric sensors, it is an economically viable solution for many applications.

Discovering More Vision Applications

The lower cost and improved performance of vision sensors have led machine designers and process engineers from a wide range of industries to incorporate vision sensors in applications that once relied on humans or multiple photoelectric sensors -- or were not inspected at all. Industrial applications of vision sensors include verification, gauging and measuring, orientation, flaw detection, and sorting. Here are just a few examples:

• On a filling line in a tablet manufacturing plant, it ensures that a complete desiccant package drops into the bottle before it was capped.
• It verifies the dimensions of a medical component.
• Inspects the date/lot code and data matrix bar code on bottles of a prescription medication.
• On a packaging line, the sensor ensures the correct label is attached and is in the correct location.
• Inspects cannulas at high speeds to verify that each cannula is straight and the correct amount of epoxy attaches it to the syringe barrel.

Looking Forward

It's an exciting time for automated machine sensing. Technologies that once required a high level of expertise is now simple to use and affordable. Ongoing development will produce sensors that are more sophisticated but still cost-effective, for more uses than before. The challenge now is to make machine designers aware of the innovations in photoelectric sensors, advances in ultrasonic sensors, and migration to vision sensors, so they can benefit from these booming technologies.

To learn more about sensor theory and applications, visit www.bannerengineering.com/training/ TMD