Inventory management systems today heavily rely on passive RFID technology to enable real-time, automatic product identification. For many applications, the return on investment from RFID is not only acceptable but also highly beneficial. These systems must be capable of capturing all on-site inventory data in real time, which means the RFID system must be able to read 100% of all tagged items. The readability of an RFID system depends on various factors such as label size, orientation, placement, and the design of the Finder Antenna (IA). Unfortunately, single-antenna designs often have "black holes"—areas where tags cannot be read. By identifying and addressing these black holes, the industry has developed methods to achieve full readability using the diversity of ISO 15693/ISO 18000-3 (13.56 MHz) item-level systems.
High-frequency (HF) RFID systems, such as those used in smart trucks or containers, are playing a significant role in this domain. Many manufacturers and solution providers offer cost-effective systems that use passive RFID tags, which can cost less than 25 cents when produced in bulk. This technology holds great potential for tracking high-value clinical items, some of which have limited shelf lives. For instance, more than 250 stents might be stored in lockers commonly found in hospital catheterization labs, with an estimated total value of $375,000. Depending on the size of the hospital, four such lockers could be in use, with items being consumed every four months—equating to a yearly value of up to $1.125 million. Similarly, implantable cardiac defibrillators (ICDs), which are small (about 3x4x6 inches) but valued between $10,000 and $20,000, are typically kept in secure locations like lockers. In such scenarios, RFID can help reduce inventory costs by preventing overstocking or understocking, while also improving control over these valuable assets.
A basic RFID system consists of a host system and multiple RF components, including an RF querier (which includes a reader and antenna) and a tag. The primary function of the querier is to communicate with the tag in the field. In passive systems, the querier also supplies power to the tag via the transmitted RF signal. The querier handles protocol processing, powers the tags, reads and writes information to them, and ensures that the data is effectively transferred to the host system.
According to the ISO 15693 standard, passive tags are only activated when they are placed within the RF field. To activate a passive tag, the voltage induced by the RF field (VTag) must be sufficient to reach the minimum operating voltage required by the RFID chip embedded in the tag. VTag depends on the size and orientation of the label, as well as the strength of the magnetic field. For an ideal loop, VTag can be expressed as:
$$ V_{\text{Tag}} = 2\pi f_0 N Q B (\cos \alpha) $$
Where:
- $ N $ = number of windings in the label coil,
- $ Q $ = quality factor of the label,
- $ B $ = magnetic field strength,
- $ S $ = area of the label coil,
- $ \alpha $ = angle of the label's orientation.
The magnetic field strength $ B $ is generated by a circular finder antenna (IA) and can be calculated using the following equation:
$$ B = \frac{\mu_0 I N a^2}{2r^3} $$
Where:
- $ I $ = current in the IA coil,
- $ N $ = number of windings in the IA coil,
- $ a $ = radius of the IA coil,
- $ \mu_0 $ = magnetic permeability of free space,
- $ r $ = distance from the IA.
From these equations, we can understand how the size and orientation of the label affect the field strength along the IA axis. When the tag and querier are close, the coupling relationship is established through the reactive near-field interaction, which is complex and difficult to predict accurately. However, in practical object-level applications, tags are usually located near the finder antenna, making these predictions less critical in real-world implementations.
Figure 1: A basic RFID system consists of a host system and multiple RF components.
The smallest RFID tag used in this study is about the size of a coin, as shown in Figure 2.
Figure 2: The smallest RFID tag used in this study, only the size of a coin.
This mechanism is essential for understanding RF black holes, which depend on both the design of the IA and the tag, as well as their interaction. HF tags come in various designs and sizes, typically categorized into planar and three-dimensional (3D) types. Planar labels are the most common, while 3D labels often include ferrite and are much smaller. The tags used in this study were all planar. Since performance is influenced by both the tag and the IA, this discussion covers the functions of three different sizes of commonly used tags. The tag shown in Figure 2 is the smallest, while Figure 3 illustrates two different IA sizes with distinct designs. The response from the reader was recorded in both cases where only one tag and multiple tags were present in the sensing field. This setup closely mirrors real-world applications involving numerous products in the induction field. The goal of these measurements is to map out a three-dimensional space that reflects the actual system and identify any existing RF black holes.
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