Automatic vehicle identification and vehicle access control applications very often rely on systems that consist of readers and tags or transponders. A variety of AVI systems is available to be used in a wide range of applications. Over the past few years a new type of technology is getting increasingly popular: Automatic Number Plate Recognition (ANPR or LPR). Usually this technology physically consists of a camera with built in software for OCR (Optical Character Recognition).
A very basic description of the functional process is:
- The camera takes a picture when it identifies a license plate (using infrared technology)
- The camera isolates the plate, adjusts the brightness and contrast and segments it into characters
- The pattern of each character is analyzed to convert the picture into text
- The output is sent over IP or over a serial connection to an access control or vehicle management system
A wide variety of license plate camera is available. The technology was developed during the late seventies and it has found its way into numerous applications. The increased accuracy has made it suitable for vehicle access control systems as well.
Benefits of ANPR
The big advantage of using ANPR for AVI applications is that it makes use of something that almost every car already has: a license plate. It takes away the need to distribute and mount transponders or tags. And tags that are not issued, do not need to be revoked as well. This makes ANPR very suitable for those situations where it is unachievable or undesirable to issue and administer tags. Typical applications that come to mind are:
- Vehicle access control applications with one off or short term authorizations: visitors at industrial estates, suppliers in city centers, short stays at exclusive parking sites, etc.
- Vehicle management applications dealing with car populations that are rapidly changing but are managed accurately: rental cars, rental trucks, temporary vehicles in the supply chain
- Applications with a need for sudden need to identify a vehicle and keep track of it: advanced parking management applications where the license plate is recorded for extra security or to create fail safe systems
- Situations where specific RFID bandwidths cannot be used because of interference or regulations
ANPR is not only a good alternative to transponder based systems in some situations, but it can also be a good addition to installations where traditional vehicle identification technologies are used. Nedap has been using ANPR successfully for city access where suppliers are allowed access using ANPR and buses, taxis and ambulances are allowed access through a highly reliable transponder based system.
ANPR Cameras come in different forms and shapes. The Nedap ANPR for example deliberately moved away from the traditional tube design to make it easier applicable in typical access control situations. The camera is easily mounted on walls and poles and can withstand harsh weather conditions.
Probably more important than the physical aspects are the aspects related to software. Most ANPR cameras were never built to act as a reader for vehicle access control and will not support the common standards used to connect readers to access control and management systems (like Wiegand, RS422, RS485, IP). It is therefore important to select a reader that will support your application. Many access control systems will not be able to deal with full license plate identifier strings sent over IP. They are only capable to deal with numbers (in Wiegand format). Nedap has added a Wiegand Interface Module to its ANPR portfolio to be able to convert license plate information to a Wiegand number and thus support all common access control systems.
Another thing to consider is that the OCR software in an ANPR camera will rely on libraries that are supporting license plates from specific countries. Each country is using its own characters, colors and designs. So it is important to verify whether adequate OCR software is available to support you region. In many applications the OCR library should be able to support a range of countries.
It is important to realize that ANPR cameras use a combination of traditional camera technology and therefore need to be installed and configured properly. Most cameras have a specific or configurable focal point, will need to be mounted on a specific height to support the right viewing angle and will support a specific limited driving speed. ANPR needs a visual connection between license plate and camera, so one can imagine that ANPR accuracy can easily be influenced by light shining into the camera lens or dirt on the license plates. These are typical circumstances where transponder based systems do not have problems. ANPR performance has increased over the years when it comes to accuracy and reliability. When installed properly an approximate 98% accuracy typically can be achieved, which in practice means that about two of every hundred cars are not read properly, which might lead to an administrator intervening manually.
A last thing to consider is that in some vehicle access control situations, ANPR will require a bit more administrative work when compared to using transponder based systems: some transponders can be moved from car to car with the authorized driver: a license plate always is fixed to the car, so every car change will cause an administrative effort to be made.
ANPR for vehicle access control: conclusions
In situations where transponders cannot be distributed, using ANPR has proven to be a very viable option. ANPR cameras have rightfully earned their place in the variety of technologies that can be used for automatic vehicle identification. In situations where an accuracy of near a 100% is needed, transponder based systems should be in favor. High end AVI systems can also be used in combination with ANPR to support a mixed population with short term and long term vehicle access authorizations.