Link: AWID Home Page
by Douglas Cram
VHS vs. Beta; Windows vs. Macintosh; 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, and I use this term to describe any attempt to identify a particular vehicle, whether it is a car, truck or forklift, by tagging it with a radio frequency 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 transmit its encoded ID number(s) to the reader or receiver. Active tags are often referred to as transponders. Some active tags constantly transmit, and some only transmit when they detect RF from the reader/interrogator.
Active Tags: Advantages and Disadvantages
The primary advantage of active-tag AVI systems is that they can read tags while the vehicle is moving, and from a relatively long distance. This is attributable to the fact that the tag has its own power source with which to transmit its ID number, and the reader antenna is quite large in size. I have seen active tag systems that will read tags at a distance of 15 to 25 feet, passing by at 60 mph or more. Most manufacturers' specifications claim more reasonable speeds like 30 mph, and there is a system on the market that uses 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. 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.
AWID LR-911 AVI Passive Reader 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.
Additionally, every active windshield tag I have seen is attached to the windshield using Velcro. While that makes it convenient to move the tag from vehicle to vehicle, it reduces security. Most active 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. Human error figures predominantly anytime a tag in the 902 to 928 MHz frequency range can be held in the human hand, because this frequency is highly susceptible to body blocking. Tags like this are also all too easy to leave in the wrong 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. Because a dying tag experiences reduced read range, battery problems may also cause frustration among users. With most active tags, a dead battery means that the entire tag must be replaced unless it has a replaceable battery.
Passive Tags: Advantages and Disadvantages
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 comparatively limited read zone of approximately 10 feet, they avoid the problems caused by long read range mentioned above. There is much more security in the use of a passive tag that is glued to the windshield on the inside of the vehicle and can't be transferred to other vehicles. 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. While applications exist where vehicles tagged with passive tags can be read at 10 to 15 mph, most 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 much less expensive than active tag systems-usually about half the cost.
Passive windshield sticker - Intellitag by Intermec 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? (These may include customer training and retuning of readers due to possible interference.)
Match the Features to Your Application
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? Some control systems used in the gated entry and parking industry only accept RS-232.
• 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.
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. AWID is a manufacturer of RFID products used in the access control, automated vehicle ID and asset monitoring industries. Doug can be reached at 770-590-0915.
If you have any questions, please feel free to contact us: e-mail: [email protected]
Tel: (845) 369-8800 Fax: (845) 369-1195
Address: 382 Route 59, Section 292, Monsey, NY 10952
Copyright 1998-2003, Applied Wireless Identifications Group, Inc.
Comments