Tim Stokes, an auto ident specialist with Sick (UK), discusses the alternative technologies that are available for automatic part identification, and how to select the right one for a given application.
Psychologist Abraham Maslow once said "I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail." These words, dating from 1966, have been adopted by industry as a warning against over-reliance on a familiar tool or technology.
Maslow's fascinating observation of human behaviour is particularly apt in the field of auto-identification technologies. For more than 15 years, users in factory and warehouse automation have been confused by some claims that one auto-ident technology suits all applications: RFID, camera or laser.
Clearly, Maslow's observation, sometimes called the 'golden hammer', underlines the fact that there is no 'silver bullet.' Put another way, a single technology is not necessarily the answer; an intelligent choice of technologies is what truly makes the difference.
It is natural to stick with a technology that is familiar and comfortable, not least because it eliminates the risk that data communications between devices will be disrupted. This can be a false security.
Achieving effective and consistent auto-ident results is about using the best technology for the task at hand; the optimum solution will not be the same each time.
Aside from the auto-ident device chosen, it is equally important to consider how the device will perform on many platforms and interface to multiple devices, such as PLCs and fieldbus systems.
Weighing up the benefits
Given that there are a range of auto-ident technologies for different tasks, let us examine the pros and cons. The three major auto-ident technologies are:
- Radio Frequency Identification (RFID);
- Vision cameras covering matrix and line scan technologies;
- Laser readers.
Cameras and laser readers, which encompass 1D and 2D code scanning, are the basis for a large proportion of auto-ident operations.
Centralised or decentralised?
Is a centralised system needed with all data and production history held on central PCs and servers? Or is the production history held on the label of each product as it progresses through the process? If a decentralised system is in operation, then RFID could be the only real choice.
RFID offers unique benefits:
- Tags can be read from any direction, so locating the reader is not as critical as optical systems and line-of sight is unnecessary. The tag to reader range can be very large compared to the other technologies.
- For high-speed throughput, reading is very rapid and many tags can be read at once.
- Tags can carry large volumes of complex data, which can be rewritten, amended or updated easily.
- RFID performs well in harsh factory environments where there is dirt, dust or high moisture levels, for example.
But RFID can be expensive. Reliable transponder tags are about ten times the cost of a printed label and many tags may be needed in the supply chain as well as in the factory; the effect on ROI (return on investment) can be significant.
RFID systems also need careful set up in areas where radio signals could be masked or reflected - for example, where multiple metal surfaces are present.
Reading codes - choices and options
If a centralised system is acceptable, then either a barcode or 2D code is likely to be the answer. Generally the 2D code is becoming widely used because it can be read in any direction and stores a larger amount of data in a smaller space than a 1D code.
However, 2D codes need a camera to read them, which is more expensive but worthwhile if large amounts of on-pack data are necessary. A camera is also better at reading low-contrast coding in 1D and 2D.
A simple laser reader will be the answer if all that is needed is to read simple SSCC (standard industry) or in-house barcodes on the side of a pallet, say, to check goods in or out. With wide scanning height and width, it can sweep the entire side of a pallet, with cell sizes down to 3mm.
Cameras can read 1D, 2D and plain text coding. They can provide live and stored images for analysis and storage. Reading is omnidirectional with just one device, and poor code qualities can be read consistently.
Matrix scanner cameras offer the benefit of reading in any direction or orientation. They can cope with low contrast and poor aspect-ratio codes, and can compensate for non-existent items on the code itself such as missing quiet zones and unprinted pixels.
Matrix cameras are very similar to those used in smartphones, with a lens that focuses the image onto a receiver comprising many pixels. For consistent reading, the smallest bar of the barcode should cover a minimum of two or three pixels in the receiver. So if the width - the X dimension - of the smallest bar in the barcode is too small, the code will need to be read closer to the camera than if the X dimension is larger.
As the barcode gets further away from the camera, the light needed on the code increases according to an inverse square law. A lot more light is needed at larger distances - but if it is too bright it could be disturbing to production operatives or disrupt other sensors.
Matrix cameras capture multiple images of the code as it passes by. The 'frame rate' needs to be balanced with the speed: too fast and the camera processor will have insufficient time to decode the data before the next barcode arrives; too few frames read and the scanner will not 'see' the code properly; the 'just right' zone is usually between one and three frames per label.
Decoding time can also be affected by the print quality and contrast ratio parameters set. For a typical black-on-white code, the contrast and print quality would be set at 'good' or 50 per cent as a starter; below this level of contrast, print marks would be ignored and not decoded, reducing the amount of time taken to decode the label.
Line scan cameras
Instead of focusing an image onto a matrix of pixels, in a line scanner, the configuration of lens and pixels results in a 1mm line scanning across the light-sensitive read area. A much higher frame rate is possible with the image data building up a picture as it is scanned. The reading width, length and range of line scan cameras are generally greater than for laser scanners.
With its sensitivity and the ability to cope with a range of light levels, a line scan camera is at the top of the range in terms of performance and cost, with a price differential of four times or more over other devices. The range of applications can include telecodes and optical character recognition (OCR) as well as 2D and 1D code reading. Even 'no read' occurrences are also recorded as an image for print and maintenance checking.
Laser scanners use a laser beam directed onto a rotating polygon and reflected onto the code where it is read. Some devices use time-of-flight technology to measure the distance between the laser and the code, to achieve precise focus.
Although eye-safe, the laser spot is very bright and the polygon scanning action over the code is a rapid 1200Hz. The high power of the laser and its tightly focused spot enables reading over long ranges without disturbing operatives.
To achieve omnidirectional code reading, two or more laser scanning heads are required. They offer a wider and better depth of field than matrix cameras of the equivalent price, but perform less well where there is a very low contrast, poor aspect ratios or badly-printed codes.
For basic 1D code reading functions, laser cameras provide a low-cost option with an excellent depth of field and large reading field width. They are not sensitive to external light pollution and do not need additional illumination. Codes can be read easily whether the object is still or moving.
Having identified the right auto-ident technologies, achieving optimum communication between devices is the next step. To begin with, choosing common interface hardware such as ethernet, fieldbus or serial is advisable and will save commissioning time. Using a common configuration software tool - like SOPAS from Sick - allows quick and consistent commissioning of all types of auto-ident devices.
To achieve faster communication and eliminate errors in data transfer between devices, the ID-Pro platform from Sick is the last piece in the jigsaw. The ID-Pro platform achieves seamless connectivity between laser barcode scanners, 2D/3D vision cameras and RFID interrogators. It can ensure that all product identification equipment in a manufacturing or logistics environment works smoothly together without interruption.
If I had a hammer...
As good as a hammer is, you need all the tools in a toolbox to achieve the right solution to a given problem. No one device suits all applications. With the most up-to-date software, it is now much easier to integrate a wide range of auto-ident technologies in a single network.
The most impartial advice is likely to come from a vendor offering all three technologies, especially where they can become part of a seamless communications network.
Follow the link for more information about Sick auto-ident products and systems.