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Automatic Vehicle ID: Passive vs. Active Tags

Douglas Cram and Mike Bigbee

VHS vs. Beta, Windows vs. Apple, Thai vs. Chinese food. It seems as though many of the choices we face in life are elusive. Conversely, the choice between passive tags and active tags in automated vehicle identification applications is usually much better defined. It's a matter of matching the features and benefits of each technology to your particular application. The purpose of this writing is to provide you with the tools to do exactly that. First, let's clarify some terminology:
AVI is an acronym for automated vehicle identification. The term is used to describe any attempt to identify a particular vehicle, whether it is a car, truck or forklift, by tagging it with a radio frequency (RF) identification tag.
A passive tag is an RFID tag that has no battery. It operates by absorbing RF from a reader and reflecting its encoded ID number(s) back to that reader, thus drawing its power from the reader.
An active tag contains a battery and uses the power from that battery to assist in transmitting its encoded ID number(s) to the reader or receiver. Some active tags constantly transmit, and some only transmit when they detect RF from the reader/interrogator.
Active tags
The primary advantage of active-tag AVI systems is that they can read tags while the vehicle is moving from a relatively long distance. This is attributable to the fact that the tag has its own power source to assist with the transmission of its ID number. There are active tag systems that will read tags at a distance of 15 to 25 feet passing by at 60 mph or more. Most manufacturers' specify speeds like 30 mph, and there are systems on the market that use active transponder tags and a receiver unit that claims ranges in excess of 25 feet. Most (but not all) automated toll collection systems use active tags.
Let's examine some of the disadvantages of active-tag AVI systems, beginning with operational issues. Having 20 feet of read range can backfire when you're using AVI for gated entry applications. For instance, if two cars are sitting in line at a controlled gate, the reader may read and approve access for the tag on the second car instead of the first, essentially allowing the first car in on the second car's credential.
Then, if the access system is programmed for anti-pass back, the gate will refuse to open for the second car because the reader believes it has already granted the second car access. Multiple lanes, side by side, can present a similar challenge when the reader in lane 1 actually reads the tag in lane 2. Another oft-mentioned drawback is the confusion and technical difficulty created when the entry reader in a garage or lot reads the tag of an exiting vehicle.
This problem is solved if the system has the ability to control the size of the read zone. (Most do.)
Additionally, most windshield tags are attached to windshields using Velcro. While that makes it convenient to move the tag from vehicle to vehicle, it reduces security. Most tag manufacturers offer a tag that can be attached to the vehicle using screws to partially alleviate this security concern, even though these tags are mounted outside the vehicle.
And then there is the battery issue. We all understand that batteries will eventually drain and need replacing. The most commonly quoted battery life is three to six years, depending upon operating parameters and environmental influences.
It should be noted that a dying tag does not reduce read range. It works up to the range specified, and then stops. A dead battery means that the entire tag must be replaced unless it has a replaceable battery.
Passive tags
Now, let's look at the advantages and drawbacks of the passive-tag AVI systems. The most obvious advantage of a passive tag is that there is no battery to expire, so the tag life is virtually unlimited. Because passive tags have a well-defined and limited read zone of approximately 15 feet, they avoid the problems caused by long read range mentioned above. Passive tags are also available in form factors that allow them to be mounted on the outside of a vehicle or other object.
The drawbacks of a passive tag system are primarily the limited read range -- 15 feet or less -- and that the speed of the object being tagged must be limited in the read zone. Vehicles tagged with passive tags can be read at 10 to 15 mph; some manufacturers recommend having the vehicle come almost to a stop in the read zone.
Cost considerations
It should be obvious that with multiple manufacturers selling both active and passive tag systems, any discussion of exact costs would be inaccurate at best. It is accurate, however, to state that passive tag systems tend to be less expensive than active tag systems -- usually about half the cost. You should also consider the issue of tag life. Will the customer be happy having to replace dead tags or batteries in a few years? What, if any, post-installation costs are involved?
Matching the features
In light of this information, it should be relatively easy to decide which type of system is appropriate for your particular application. Do you need to read vehicles or other objects at speed? Are the vehicles approaching a gate or other barrier that requires them to come to a stop regardless? Do you need 20 feet of read range, or is 10 feet sufficient? Budgetary issues can often help you decide which technology to consider, assuming both types meet your application requirements. In addition to these performance and cost issues, pay attention to other issues such as:
* Does the access controller with which you are integrating accept a standard Wiegand output?
* Does the manufacturer have a good reputation for support of their products after the sale?
* What are your customers' expectations for performance?
* Does the manufacturer offer the capabilities of altering the timing and duration of signal output?
* Does the manufacturer have an easily accessible training program to assist you before and during the installation?
In addition, don't forget that electro-magnetic and radio frequency interference can wreak havoc with even the best-designed system. Frequency hopping (spread spectrum) technology can help avoid a significant amount of fixed frequency interference. It should be pointed out, however, that the FCC controls the frequency on which these tags operate for the specific reason of eliminating as much as possible RF interference.
As always, buyer beware. Do your homework and best wishes for a trouble-free and profitable installation!

Douglas Cram is the Vice President of Sales and Marketing for AWID Inc. He can be reached at (770) 590-0915. Mike Bigbee is Area Vice President, Transportation Mobility Solutions, Sirit Technologies. He can be reached at (972) 243 7208, ext. 2116.

Article Abstract from August, 2003




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