SCANNING TECHNOLOGY

All laser scanners include two optical systems. One system, which is known as the “scanning optics”, includes the laser, lenses to focus the laser beam, and a scan mirror that moves the laser beam rapidly across the bar code. The other system, which is known as the “collection optics”, has two functions. One is to collect laser light that reflects off the bar code, and to concentrate that light on to a photodetector.The other is to prevent light from other sources, such as sunlight, from reaching the photo-detector. The ability of the collection system to perform both of these functions well has a huge impact on the performance of the scanner.

There are two different kinds of collection optics used in laser scanners today. One is a “static” system, such as that used in MEMS (microelectromechanical systems) based scanners, while the other is a “scanned” system.

Both kinds of systems can do a good job of collecting and concentrating laser light, but scanned collection optics are better for rejecting bright external light sources, which can interfere with scanner performance. Scanned optics, however, require the use of a relatively large scan mirror, which means that MEMS scanners, with their inherently small scan mirrors, are unable to use scanned collection optics. Instead, MEMS scanners are forced to make do with static optics.

STATIC v SCANNED

Figure 1 illustrates how a scanner with static collection optics sees the bar code. The static collection system must be designed to be able to collect reflected light along the entire length of the scanned laser line so that a bar code anywhere along the line can be seen and decoded. The area from which light can be collected is called the “field of view” (FOV), and is outlined by the dotted oval.

When scanning in a dark room, the only light striking the bar code within the oval is laser light, so the scanner does not see any other light which could interfere with scanning performance. When operating under bright lighting conditions, however, external light sources will also illuminate the bar code and surrounding label within the field of view.

Some of this light will be reflected towards the scanner and will be collected by the collection optics, and concentrated on to the photo-detector. This external light tends to mask the laser light, making it more difficult to decode the bar code. If the external light is bright enough, it can completely hide the laser light and the scanner will not function.

Figure 2 shows what a scanner with scanned collection optics sees. Notice that the large oval field of view of the static system has been replaced with a small circular field of view centered around the moving laser spot.

When scanned collection optics are used, the field of view can be much smaller than with static optics, because the scanner does not need to see the entire scan line at once. Instead, the field of view is scanned along with the laser beam, so the scanner is constantly looking only at the part of the bar code where the laser is striking at any moment.

The amount of undesired ambient light collected by a scanner is proportional to the area of the field of view, and as can be seen, the use of scanned optics allows the area of the field of view to be dramatically reduced. Scanned collection optics can generally be designed with a field of view that is as small – at around five percent of the field of view of a static system, giving the scanner a 20-to-1 advantage when operating in bright lighting conditions.

In a scanner with static optics, various electronic techniques can be employed that help the scanner distinguish the laser from bright external light sources. These techniques can sometimes prevent total failure of the scanner in bright sunlight. This isn’t a perfect solution to the problem, however, because the added circuitry inherently introduces some electrical noise into the system. This noise will be present even when the scanner is operating in dimly lit environments, degrading performance even when bright light is not present.

In other words, these techniques can be used to avoid total failure in bright sunlight, but at the expense of reduced performance when sunlight isn’t present. No such additional circuitry is needed in a scanner with scanned collection optics, so no additional noise is added, and the scanner can achieve maximum performance in any kind of lighting conditions.

Scanned collection optics require the use of a large scan mirror, because the larger the mirror, the more light can be collected and the greater the working range of the scanner. LP scan elements can carry large mirrors, but MEMS devices cannot. As a result, MEMS scanners must use stationary collection optics, so their working range is reduced and their performance can be more severely degraded in some lighting conditions.

SPEED v PRODUCTIVITY

The oft-claimed advantage of a MEMS based scanner is that it produces a large number of scans per second. This, however, should not be confused with higher scanning productivity. In other words, a higher number of scans per second won’t necessarily translate to an ability to scan more bar codes per hour.

As scan speed increases, the signals that the scanner must process increase in frequency, making it more difficult to distinguish them from noise. As a result, the working range of the scanner will be decreased. It is very difficult to design reliable MEMS devices that run at less than a few hundred scans per second, which is too fast for optimum signal quality in a small scan engine.

A Liquid Polymer (technology developed by Motorola) scan element, on the other hand, can be designed to operate at whatever speed is best for the scanner in which it is to be used. In the LP-based SE950 and SE955 scan engines, for example, the scan element is designed to run at 100 scans per second, which has been determined as being fast enough to make the scanner extremely responsive, but not so fast as to ruin the signal quality.

Given that in many environments bar codes are not located in a position that is convenient to scan, such as low down near the floor or up on a high shelf, a scanner with more working range – like those using LP scan engines – will be much easier to use, minimizing the need for the operator to bend, reach or climb.

As already mentioned above, MEMS scanners must use static collection optics, which results in performance degradation under some lighting conditions. This, of course, throws away any potential advantage of the increased number of scans per second, for users who work in those lighting conditions.

The first scan engines built by Motorola over a decade ago actually used static collection optics. While these engines seemed to work well in most indoor lighting conditions (but not very well outdoors), we found that users who scan bar codes on high shelves sometimes had problems, because when the scanner was aimed upwards at a bar code positioned over the operator’s head, a light fixture on the ceiling was sometimes within the field of view of the static optics, drowning out the signal. This experience showed that static collection optics are not suitable for scan engines that must work well in every environment.

Even in environments where none of the issues just described are present, a MEMS scanner may still fail to live up to the promise of improved speed suggested by the high number of scans per second because each individual decode attempt may be slowed down by the inability of the MEMS device to accelerate up to full scan angle quickly.

This is visible to the user as a scan line that starts out short and grows longer at a visible rate. Decode cannot occur until the scan line becomes long enough to entirely cover the bar code, which might not happen for several scans after the scanner is activated. This slow start-up behavior can be particularly detrimental when scanning long bar codes which can’t be entirely covered by the scan line until full line length is achieved.

The MEMS device will coast for a second or two after decoding a bar code, so a second activation immediately following a first one might feel faster, because the device is already moving. In real-world applications, however, there are usually at least several seconds between attempts to scan one bar code and the next, giving the MEMS device time to coast down and nearly stop between scan attempts. In this common situation, the user will have to wait for the scan line to grow enough to cover the bar code every time a new scan is attempted, unless the scan element is permanently activated, which can waste power in battery powered applications.

The slow start-up is the result of the electrostatic drive used with MEMS devices. An alternating electrostatic field is used to attract the moving mirror first in one direction and then in the other, causing it to rotate back and forth. The electrostatic field can be quite weak, requiring several scans before it can accelerate the moving mirror up to the full scan angle.

The LP scan element, on the other hand, is driven magnetically. This magnetic drive can apply much more torque to the mirror, bringing it up to full angle in a minimum number of scans, even when a large mirror is used. As a result, startup time for the scan element does not negatively impact scanning productivity.

DAMAGED BAR CODES

One potential advantage of high scan speed is that it can give a scanner more attempts to read a damaged bar code, increasing the chances that it will read quickly. Unfortunately, increasing the scan speed also degrades signal quality, which can actually make it more difficult to read some bar codes of marginal quality. This loss of signal quality partially cancels any advantage that might be expected from the higher scan speed.

The SE950 and SE955 use a different approach, which also increases the chances of a rapid decode, but doesn’t degrade signal quality. Two individual signal processing circuits are implemented, each optimized to read different kinds of bar codes. One is designed to read damaged or poorly printed bar codes while the other excels at reading high density bar codes.

The outputs from both of these processing circuits are available to the decoder on every scan, so even though the scanner is running at 100 scans per second, the decoder gets 200 chances to decode the bar code every second.

The two signal processors are designed to complement each other, so one, the other or both will succeed in decoding the vast majority of bar codes. In the event that a bar code is extremely disfigured, however, the scanner can automatically adjust both circuits to further improve the chances of obtaining a rapid decode.

For example, if the bar code is badly scratched, the resolution of the signal processing circuits can be reduced enabling the scanner to ignore the defects. And when scanning bar codes with very narrow bars or spaces down to around 0.1 mm, maximum resolution is employed to assure accurate detection of every bar and space in the bar code.

Notice that this ability to process the bar code two ways every scan, and to vary resolution as necessary, is a far smarter way to assure quick response on bad bar codes than simply running at higher speed, such as MEMS scanners do. If a higher speed scanner does not vary its resolution as might be necessary to read a damaged bar code, it won’t matter how fast it goes because each scan will fail.

SUPERIOR SCANNING

A laser scanner is a very complex system in which the device that scans the laser is only one of several things that influence scanning performance. In every meaningful way a MEMS scanner is just like any other laser scanner. All laser scanners have always used a moving mirror to scan the laser, and MEMS is simply another way to do so.

Numerous mechanisms have been used to move the mirrors in laser scanners including stepper motors, galvanometers and brushless DC motors, to name a few. There is no reason a scanner using MEMS should be considered to be anything but a laser scanner, and delivering superior scanning performance requires technology up and beyond simply replacing the scanning mechanism.