Active RFID Tags
Benefits of Wireless Identification

Active RFID Tags have many benefits when you want to identify items wirelessly. You can read a truck-load of tagged items in seconds. Private tag information can be secured by password or cleared on command.

They operate over distances up to 100 metres, have the intelligence to initiate conditional transmissions to a reader.

They can store a number that makes a tagged item ‘one-of-a-kind’ and even store instructions about how to manufacture it, use or look after it.

Usually these features are benefits, but not always...

It depends on the application.

The features of active RFID tags

One of the main differences between active tags and passive tags is that the active tag has a battery.

You’d expect an RFID tag manufacturer to have good reasons to integrate a battery into an RFID tag, because...

A battery will limit some applications

• The tag has to be bigger to house the battery. Fitting a large tag to a small item can be a challenge!
• It significantly increases the RFID tag cost and therefore the price.
• Unless the battery in the tag can be replaced, the life of the battery may limit the life of the tag.
  

But there are significant benefits.

A manufacturer of RFID tags knows that the RFID tag benefits mostly outweigh the disadvantages, because...

Having power available from a battery supports a number of features that make active RFID tags superior.

Some tag manufacturers claim that their tags have a battery life of ‘up to’ five years. How long depends mostly on how often and how long the tag transmits, so the words ‘up to’ perhaps indicates a battery life of five years, if your tag doesn’t spend a lot of time transmitting and spends most of its time asleep.

Check the specified battery life (for a given duty cycle) in the manufacturer’s data sheet.

Here are some important things about active RFID tags...

• Active RFID tags have greater operating range
  
• You can read more tags in a shorter time
  
• You can make private information secure
  
• The identifier data format may be standard or customized
  
• They are usually bigger
  
• Active tags can do much more
  
• They cost much more than passive RFID tags
  
• Tags are often easy to protect against harsh environments
  

Let’s look at each of these in more detail...

The Operating Range of Active RFID Tags

Depends on...

Transmit power and receive sensitivity

Active RFID tags transmit RF energy.

This energy carries the ID and other information to RFID tag readers. The higher the transmit power the greater the range.

The battery in an active tag provides enough energy to allow the tag to transmit up to the legal maximum power, enabling it to operate over a greater distance than would otherwise be possible.

Active RFID tags also receive RF energy, from a reader.

This RF energy carries the information from the reader to the tag. A tag with higher receive sensitivity will be able to receive weaker RF signals than a tag with lower sensitivity. It will be able to ‘hear’ better. The battery powers the receiver circuitry.

The antenna and its orientation

An active RFID tag antenna is used to transmit and receive, though not at the same time.

The RF signal is strongest when a tag antenna and reader antenna are ‘facing’ each other.

However in many applications you can’t guarantee they will be facing each other, so it’s better to engineer an RFID system based on the shortest range, arising from worst case tag orientation.

Whether a tag is transmitting to, or receiving a signal from a reader...

the RF signal strength is weakest when the RFID tag antenna is edge-on to the reader’s antenna.

Weaker RF signal strength means shorter operating range.

In many practical applications the tags will be randomly arranged and so you’d expect some tags to be positioned edge-on to the reader with the RF at its weakest and the range at its shortest.

Frequency

In general, the higher frequency active RFID tags have greater operating range.

So tags operating in the higher UHF or microwave frequencies operate over greater distances than tags that operate at lower frequencies. However, the higher frequencies have some limitations of their own...

Surroundings

The RF, to and from tags operating at UHF or microwave frequencies, is likely to be partially absorbed by water and blocked, or reflected, by metal objects. This reduces the range they can operate over.

Tags operating at lower frequencies aren’t affected to the same extent but may have other disadvantages.

You can get active RFID tags that are specifically designed to operate while attached to metal items You have to mount the tags in a specific way, and by design they’re spaced off from the surface of the metal by an exact amount. The metal surface then becomes a reflector and effectively part of the RFID tag antenna.

You can read more tags in a shorter time

You can read active RFID tags much faster than passive tags.

Typically you can reliably read a hundred or more tags every second. Important if you’re trying to identify tagged items in a truck passing RFID tag readers, and only have a few seconds to do it.

Here’s why they’re so fast because...

• They’re always ready for action, with battery power instantly available.
  
• They can process information fast, enabled by sophisticated on-board circuitry.
  
• They send information faster by using higher frequency RF energy to carry it.
  

Some types can put themselves into a sleep mode, when they don’t need to be read, to minimize the current drain from the battery. When a reader wants the tags to respond, it can wake them all simultaneously and read each tag individually.

But it’s not an ideal world and anything that degrades the quality of the RF interaction between tags and reader will slow the rate at which the tags can be read.

Security and Privacy

Long range RFID tags might be of benefit to one application but a disadvantage to another.

The longer range of an active tag may seem like one of the more important RFID tag benefits, but if security is your main priority, you may want short-range operation, to make it difficult for anyone to ‘listen in’ on the RF signal.

If security is important, you might prefer to define your own identification format, instead of using a standard format, such as the Electronic Product Code (EPC), so that an unauthorized reader couldn’t interpret the data even if it could access it.

Lock your information - With some standards, including EPC, you can ‘lock’ the data stored in the memory of active RFID tags.

Kill the tag - To prevent unauthorized access to private information stored on active RFID tags, some standards, including EPC, have a password-enabled ‘kill’ command that allows you to permanently disable a tag when you no longer need it.

The format of the identifier

A typical standard identifier formatting code may have this type of structure...

• Information about the version of the formatting code itself
• Data that identifies who the manufacturer is
• Data that identifies what broad class the items belong to, eg. TVs
• The serial number of an individual item within this class

A custom identifier format may provide a higher level of security, if you need it. However, this usually isn’t necessary and a standard identifier format is often adequate.

Imagine a world where every RFID system used the same identifier format, where the numbers were always the same length and came out in the same order according to what they meant. Then all tags and readers throughout the world could speak the same language and understand each other.

However...

We don’t have such a universal RFID data format and in some applications, such as security, this could be a bad thing rather than an advantage as it would make it easier for unauthorized readers.

Identify individual items - Whatever the format, you may want enough numbers available to let you allocate one unique number to each individual item that you want to tag. If you want to individually identify each of millions of products coming out of your factory each year then you’ll need to have tens of millions of unique numbers available. So you would need to use an identifier that would support this.

Standards such as the Electronic Product Code (EPC) will let you do this and may allow different organizations in different countries to read active RFID tags that have been written to by others.

Active RFID tags are usually bigger

They are usually bigger because they contain a battery.

Lower frequency tags may have a large area to support the larger RFID tag antenna needed for low frequency (LF) operation, but can be made extremely thin. Some LF passive tags are like a stick-on label. But active tags are usually thicker because of the battery.

In general, all other things being equal, the bigger the tag, the bigger the battery and the longer the battery will last.

Active RFID tags can do much more

They have sophisticated electronics that enables them to carry out sophisticated functions.

Depending on the degree of sophistication, tags may be able to...

• Act alone without any prompting from RFID tag readers, making decisions for themselves. They may ‘sleep until woken’. Or they might wake themselves, using their own on-board alarm clock, and routinely report their presence to a reader. If the tag failed to check in, the reader could raise an alarm.


• Store instructions programmed into their memories by a reader. The instructions may tell the tag how often to wake up and transmit. Or may tell it not to wake up until woken by a reader. The memory might be programmed with a list of instructions that could be read as the tag moved past a particular operation on a factory assembly line.


• Detect when it’s moving, if it’s fitted with a simple motion sensor. It could then be programmed to waken and report to the reader when it senses it’s moving.


• Detect if the attached item had been overheated, if it’s fitted with a temperature sensor. An over-temperature alarm could be reported.


• Detect tampering.
  
  
• Sense a low battery - when the internal battery is near the end of its life.

They cost much more than passive RFID tags

RFID tag cost depends on...

• Complexity – the more functions, the more circuitry and more cost
• Cost to manufacture – manufacturing process is more expensive
• Volume – economies of scale not as great, used in smaller quantities
• Packaging – robust, sealed environmental packaging costs more
  

In general, passive RFID tags cost cents, while active RFID tags cost dollars.

But don’t consider RFID tag cost in isolation, as it’s the total cost of the complete system and process, over its life, you need to consider.

If you do this, you may discover that the difference in cost between active and passive systems isn’t as great as the difference in cost between active and passive tags might suggest. It depends on your application.

Tag inexpensive items with low cost passive tags. Otherwise the tag might become a significant proportion of the total cost of the item.

Tag expensive items, such as vehicles, with higher cost active RFID tags

As a tag user, you may need to recover the cost of your tags. In a high volume operation, where profit margins are small and competition fierce, you may not be able to pass all, or any of the RFID tag cost onto your customers.

In this case you would need to cover the cost of an RFID operation through efficiency gains by reducing handling and other labor-intensive tasks.

What environment will your active RFID tags need to work in?

• Will your tags be indoors or outdoors?
• Will they get wet?
• How hot or cold will they get?
• Will they be subjected to vibration?
• Will they come in contact with corrosive substances?

Check the tag’s datasheet to ensure that the tags and other components of your RFID system will operate during the worst conditions you're ever likely to use them in.

The maximum storage temperature of active RFID tags is typically higher than the operating temperature. Some tags can survive at temperatures as high as 200 degrees C, but their maximum operating temperature may typically be 70 degrees C.

An RFID system using active RFID tags can bring huge benefits to many applications... and these applications are only limited by your imagination.

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