Additional Information about Gain, Tracking, and Transition Counter

The QX-870 Industrial Raster Scanner is an optical device. Optical devices deal with a wide range of brightness, in a way similar to a camera. If the image is too bright, the exposure must be reduced. If the image is too dark, the exposure must be increased. The scanner will only work properly if it is within the range of brightness that can resolve light and dark changes, or "transitions", in the field of view. If the gain is too low, the entire symbol looks like a dark bar to the scanner. If the gain is too high, the entire symbol looks like a white bar to the scanner. Brightness conditions can be affected by the distance between the symbol and scanner, or by the consistency of the material on which the symbol is printed.

AGC (Leading Edge or Continuous)

AGC self-adjusts the optical-electrical gain to hold brightness to one range for all materials and distances. The Auto Calibration menu shows what AGC is doing to control brightness, as in the example shown here:

The value -35.0 indicates that brightness has been reduced by 35 dB. The symbol used in this example was very close to the scanner, using high laser power. As the laser power is lowered and/or the symbol is moved farther from the scanner, the attenuation (brightness reduction) value becomes lower. The user does nothing to adjust the gain in order to keep the scanner decoding; AGC performs the task of raising gain to accommodate the greater distance.

The actual attenuation factor is determined by the return light from the symbol and the "SETPOINT" value that the control system is attempting to hold. (SETPOINT is an abstracted number range that has to do with the feedback system's dynamic range.) A very low SETPOINT value of 0-20 would tend to keep the symbol amplitude low. A very high SETPOINT value of 150-255 would tend to keep the symbol amplitude high. The gain will change to hold the feedback value equal to the user SETPOINT setting of amplitude (Gain Level) at all times.

Disabled (Fixed Gain)

Fixed Gain is a gain mode in which there are no changes in the gain system. The system takes the total gain range and divides it into 255 steps. Then the value of the Gain Level setting is converted directly into the gain value. Nothing will change this value unless the user changes the Gain Level setting.

When To Use Fixed Gain vs. AGC

The AGC settings (Leading Edge or Continuous) are preferable for most applications, as they provide the best overall scannable area. However, there are times when AGC will have difficulty locking onto the symbol of interest, especially when extraneous non-symbol objects in the field of view appear to be symbols. AGC can only base its measurements on one object. If that object is not the actual symbol, AGC performance and consistency are adversely affected.

Leading Edge bases its calculations on the first object it detects, even if the first object is not a symbol. This is helpful if the symbol is always at the leading edge of the decode direction.

Continuous is the recommended setting for Microscan scanners. Continuous always samples throughout the entire scan but is most interested in the object with the most sharply-defined bars. This can occasionally mislead the scanner when bar-like objects or specular reflection enter the field of view.

Fixed Gain is preferable if AGC repeatedly fails to choose the correct Gain Level setting for the application. In some applications, symbols might be surrounded by a greater number of symbol-like false candidates than actual symbols. The Fixed Gain setting will never change its value, so once it is set, surrounding objects in the field of view will not distract the scanner from decoding actual symbols.

Fixed Gain is also useful in applications using symbols that have very low bar counts, or in applications using Symbol Reconstruction on symbols with tilted or skewed placement.

The AGC Minimum and AGC Maximum parameters are useful in applications with depth of field limitations that are not conducive to Fixed Gain operation.

Set AGC Sampling Mode to Disabled (Fixed Gain) and find the range of gain levels that yields successful decodes in the application (both the lowest and the highest gain levels).

Enter those values in the AGC Minimum and AGC Maximum fields. This will leave the scanner room for gain changes to increase depth of field (within the limitations of what works for the application).

Maximum gain may be too high for symbols on white backgrounds, because it may attempt to process false bars. Especially in applications using symbologies that require more processing, setting gain too high can slow decode performance substantially.

Setting a minimum gain can prevent the system from occasionally "under-gaining" because of previous false candidates that appear very bright.