Error-proof data entry: embedding bar code readers to improve the automated process.
Bar code readers are commonly embedded inside self-contained instruments and fully automated systems, which require limited or no human intervention to perform tasks. Typical examples range from blood gas and DNA analyzers, to single tube conveying systems, to pipetters and rack and plate handlers. With an embedded reader, an analyzer can identify each sample, determine what test needs to be performed, and identify the necessary reagent(s). The entire test is bar code verified and the information automatically uploaded to the central laboratory's database. Without the bar code reader, the sample identification, test information, and reagent identification would need to be manually keyed in by a technician, reducing process efficiency and increasing the opportunity for error.
Laser scanning is the most common method for decoding (or reading) linear bar codes. As the laser spot (or other light source) sweeps over the bar code, a photo detector in the scanner head measures the amount of light reflected back from each dark and light elements. After conversion from analog to digital, the signal is then decoded based on the algorithms for the particular bar code. Once decoded, the data can be directly outputted to the host system, or the barcode reader can perform logical tasks on the data.
Since only the widths of the bars and spaces are important in decoding a linear bar code, single-row bar codes are considered one-dimensional codes. Other types of codes (or symbologies) include stacked and 2D barcodes. These symbologies often appear to resemble a 'checkerboard' pattern and have various advantages and disadvantages over traditional one-dimensional, linear bar codes.
Scan and mechanical envelopes
The scan envelope is the total dimensional space required by the reader to decode a bar code at a specified distance, especially important in space-constrained applications. The challenge in reading a bar code at close distances is ensuring there is sufficient width for the laser to scan the entire code.
Scanner depth, scan angle, and the distance from the reader to the barcode all comprise the scan envelope and directly affect how much space must be provided within the instrument for a specific scanner to read the bar code. The scan envelope varies widely from scanner to scanner, necessitating careful planning and calculation. The depth of the bar code reader is determined easily by measuring the actual physical case of the reader. Scan width and read distance can typically be found on the specification or data sheet of the bar code reader.
Other factors that determine readability are the density of the bar code and the speed the bar code is moving through the laser beam. Typically, the higher the density of the bar code (narrower the bars), the shorter the read range. For example, if a bar code reader needs to read both a 10 mil and a denser 6 mil bar code at the same distance, the selected reader will need to have a scan envelope that accommodates the read range specifications for the 6 mil bar code traveling at the highest speed.
When evaluating different mounting solutions, first take time to note the characteristics of the bar codes and the material to which they are applied. If the bar codes are on a highly reflective surface, the bar code reader may need to be mounted with an angled bracket to avoid specular (or direct) reflections from the symbols which can 'blind' the reader. Additional factors to consider are the mounting screw locations, lighting environment, and the level of contrast between the dark and light spaces of the bar code.
A scanner must always be oriented with the line of scan perpendicular to the bars of the bar code. Bar codes on a moving line are referred to as either 'picket fence' or 'ladder' orientation (horizontal or vertical relative to the scanner). Picket fence orientation generally provides the scanner more time to read the entire bar code as it passes through the scan width. According to application engineers at manufacturer Microscan Systems Inc., a minimum of five scans per bar code provides the highest level of data accuracy. The following formula can be used to determine the number of scans per bar code for a given application.
(Scan Width - Label Length/Label Speed x Scan Rate) - 3 = number of complete scans
The reader can be flush mounted or pivoted on a bracket to accomplish the optimal orientation and to negotiate corners or other geometric challenges. Right angle readers are recommended for very tight spaces because they require less physical space and offer additional mounting versatility. By using an extended right angle mirror, the focal length can be dramatically reduced by 50% or more, depending on the application.
Some bar code readers have the communication cable positioned in a corner of the device to provide engineers with the greatest number of cable routing options. Corner positioning also conserves space to create the tightest fit possible. Bend radii should be as large as possible and extreme dynamic flexing should be avoided.
Power, triggers and software
Many bar code readers are designed with minimal power requirements in order to reduce the drain on the host instrument. Communications connectivity options range from USB and Ethernet to standard serial RS 232.
It is always recommended to use a trigger so that the system can 'know' whether there is a bar code intended to be read. Triggers can be easily divided into two different categories: serial and discrete (external). Serial triggers receive a programmed cue from a device such as a PLC or host PC, telling the scanner to look for a bar code. Discrete triggers are mounted separate from a scanner to which they are wired directly, and require less programming than serial triggers.
Additional factors to consider include what the bar code reader will need to do with the decoded data. A bar code reader can often be employed to perform basic external electrical functions such as: communicate with an auxiliary device; drive a relay that trips a gate or a light stack; or sound an alarm on a no-read.
Bar code readers today are capable of more than just scanning a bar code and outputting the data. Scanners now function like independent computers, reducing the amount of programming required on the host to process the data. Once the bar code reader scans the bar code, it can output the data in the exact format required by the instrument for a specific process. Some readers can output only the last four digits of the bar code, or re-construct a data string in any format required. Depending on the application, it can also determine if the bar code is positioned incorrectly. Interface software should be designed to initialize the reader, check for status, and create a robust real time communication protocol between reader and host.
Embedding a bar code reader inside a refrigerated compartment or next to a heater can affect performance. In addition to temperature, extreme vibration, ambient lighting, electrical noise, and dust or water exposure all should be considered. To avoid read rate issues, make sure the housing of the bar code reader meets the necessary industrial rating, or else build an enclosure to protect the reader from the environment. Many bar code readers are capable of scanning bar codes through windows built into refrigerated compartments.
Besides the reader's performance, the environment can also affect the quality of the bar code itself. For example, changes in environmental temperatures can cause condensation to form on the bar codes, distorting their appearance and readability. In closed loop systems, where there is some control over the quality of the bar codes, spend time selecting the right media for the application. Label adhesives that are not suitable for the application create a liability that can be easily avoided with a little up-front time investment.
Successful integration of the bar code reader is critical to an instrument's success. By planning for the bar code reader earlier in the process and incorporating process specifications into the design, engineers will dramatically increase the performance and accuracy of their data capture process while significantly lowering the total cost of ownership of the entire instrument.
Circle 166--Microscan Systems Inc., or connect directly at www.rsleads.com/508df-166
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|Date:||Aug 1, 2005|
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