Inventory management systems now rely on passive RFID technology for real-time automatic identification of products. For many applications, the return on investment using RFID is acceptable. These systems must be able to capture all of the on-site inventory information in real time, which requires the RFID system to be able to read 100% of all tagged items. The readability of an RFID system is a function of many variables, including: label size, orientation, placement, and Finder Antenna (IA) design. Unfortunately, there are "black holes" that are not readable for all single-antenna designs. By analyzing and validating these black holes, the industry has developed a way to achieve 100% readability with the diversity of ISO 15693/ISO 18000-3 (13.56-MHz) item-level systems.
High frequency (HF) RFID systems such as smart trucks/containers play a role in this area, and many manufacturers and solution providers offer such products. These inexpensive systems use passive RFID tags (less than 25 cents in mass production), a technology that has great potential for tracking clinical high-value items, some of which have a certain shelf life. For example, more than 250 stents may be stored in lockers commonly found in hospital heart catheterization laboratories, with an estimated total value of $375,000. Depending on the size of the hospital, it is possible to use four such lockers, the items in which are consumed every four months, equivalent to the value of such a locker "hands-on" for up to $1.125 million per year. Implantable cardiac defibrillators (ICDs) are also high value items in hospitals. They are small (approx. 3 & TImes; 4 & TImes; 6-inch packaging), but worth between $10,000 and $20,000. They are usually stored in a secure space such as a locker. In such applications, the use of RFID can reduce the cost of inventory due to insufficient or excessive stocking of certain items, and can better control the whereabouts of these valuables.
A basic RFID system consists of a host system and multiple RF components (Figure 1). The RF component includes an RF querier (reader and antenna) and a tag. The purpose of the querier is to communicate with the field tag. For passive systems, the querier also supplies power to the tag via the transmitted RF signal. The querier is responsible for protocol processing, powering tags, reading tag information, writing information to tags, and ensuring that information is effectively passed to the host system.
The ISO 15693 standard states that passive tags are only activated when "placed" in the RF field. To activate a passive tag, the voltage induced by the RF field (VTag) must be high enough to reach the minimum voltage level required for the RFID chip embedded in the tag to operate. The VTag value is a function of the size/direction of the label and the magnitude of the magnetic field strength. For an ideal loop, VTag can be expressed as:
VTag = 2Ï€f0NQB(Scosa) (1)
among them:
N=the number of windings of the label coil,
Q = the quality factor of the label,
B = magnetic field strength,
S = area of ​​the label coil,
a = the pointing angle of the label
Figure 1: A basic RFID system consists of a host system and multiple RF components
The magnetic field strength (B) is generated by a circular finder antenna (IA) and can be expressed by Equation 2:
B = (μ0INa2)/2r3 (2)
among them:
I = IA coil current,
N = number of IA coil windings,
a = IA coil radius,
00 = magnetic permeability without interference,
r = distance to IA.
From these equations, we can deduce the relationship between the size and orientation of the label and the field strength induced along the IA axis. When the tag and the querier are close, although the coupling relationship is established by the complex reactive near-field relationship between the two, it can only be expressed by the above equation, especially when r "a and deviates from the axis of the querier. When it is coupled, the coupling relationship is difficult to predict accurately. In practical object-level applications, the tags are usually close to the finder antenna, so for this reason, the choices do not depend entirely on these predictions.
Figure 2: The smallest RFID tag used in this study, only the size of the coin
This mechanism is critical to understanding RF black holes, which are both related to the design of the IA and the tag, as well as the interaction between the two. HF tags come in a variety of designs and sizes and are generally divided into two broad categories: planar and three-dimensional (3D) designs. Flat labels are the more common type of tissue carrier, while three-dimensional labels contain ferrite and are much smaller. The labels used in this study were all flat. Since performance is a function of tags and IA, the functions of three different sizes of commonly used tags are discussed here. The tag size shown in Figure 2 is the smallest, and Figure 3 shows two different sizes of IAs with different designs. The response to the reader was recorded in the case of only one tag and multiple tags in the sensing field. This is quite representative of the practical application of many products in the induction field. The purpose of these measurements is to sketch out a three-dimensional space that represents the actual system and locate any RF black holes that exist.
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