Technologies of identification and positioning in real-time
To identify the objects of interest and monitor their whereabouts. It all depends on objectives and circumstances.
If the goal is recognition of subscribers for the provision of regionalized services (e.g., weather forecast), the error in a dozen kilometers of the special role will not play, but if we are talking about the positioning of the chip-on-Board for automatic Assembly, it will be about microns.
If you need to quickly find the right part, the frequency of the survey in the system can be minimal – only at the moment when the part is needed or when inventory. The rest of the time the system can spend in sleep mode. But if you want to control the route and speed of movement of trucks in the shop, you will need the sampling frequency to several times per second – real time.
The wagon on intercity route is recommended to be monitored using satellite positioning system, but once she gets into the covered unloading area or in the repair box, communication with the satellites is lost and you want something else.
And these features use a lot. Of course, there are many different types of systems of identification and positioning.
In this topic we will focus on systems of identification and positioning. But in order not to drown in a sea of information, we leave aside locating system (radio, acoustic, infrared), where the location of the object is determined by the reflected signal. Will not consider robotic Assembly systems where the position of an object is not measured by the system, and set it. Leave unattended and intelligent video surveillance systems with their methods of object recognition.
We are in Topeka will focus on positioning systems, using a label – whether it is actually label, GPS, Wi-Fi device or cell phone.
The application of systems of identification and positioning (location) of material objects – people, vehicles, moving parts and objects is an important direction of optimization of technological and business processes. Such systems are already applied in various fields. From monitoring patients, staff, medicines and equipment in hospitals to control the location of tools, Assembly units and workers on the Assembly line. From searching for victims in emergency situations – to monitor animals in the free content to identify patients.
The diversity of fields and areas of use spawned a variety of technologies.
Requirements for positioning systems
Before proceeding to the comparison of the systems, define the criteria for comparison.
As mentioned above, the system must provide:
a) identification of the controlled objects;
b) optimal positioning accuracy;
C) the optimum frequency of the survey.
In addition, important criteria are:
g) radius (maximum distance from the labels to the elements of the infrastructure);
d) immunity to interference;
e) resistance to multipath fading (the effect of reflected signals);
g) small size and weight of the label;
h) low energy consumption label (to save battery);
I) ease of deployment and use;
K) electromagnetic compatibility, the need for frequency permissions;
l) the cost of the solutions.
Types of positioning systems
For positioning are composed of several groups of technologies.
First of all, this satellite navigation system – GPS, GLONASS, Beidou, Galileo and others.
The largest group of radio frequency technology including RFID tags – RFID.
In a separate group can provide technology infrared and ultrasonic positioning.
Among the RF technologies can provide the technology, originally designed to provide communication services, or otherwise adapted for positioning (Wi-Fi, Bluetooth, cellular), and those who on the physical properties of modulation most suitable for positioning is CSS (ISO24730-5), UWB, NFER and others.
Leave radio-frequency technology for a dessert, but let's start with the global navigation.
Global positioning system
Do not dwell on the technological aspects – they are well known. Let's get right to the features. The best accuracy for today GPS provides. Positioning accuracy is already no worse than six metres. A new generation of satellites launched currently ensure the accuracy of at least 60-90 cm
A General lack of global systems – dependence on the conditions of use. It is almost impossible to locate inside buildings, in basements or tunnels, signal strength is severely compromised under the cover of trees and even clouds. On reception of GPS signals may be affected by interference from land-based sources. Because the GPS orbits have inclination of about 55 degrees, the accuracy in the high latitudes is reduced since GPS satellites are visible low on the horizon. In this respect, the GLONASS satellites have the advantage of the inclination of their orbits about 65 degrees (for the entire territory of Russia).
Positioning in cellular networks
Positioning in cellular networks appeared one of the first (long before global positioning). This is due to the widespread cellular communication and the relative simplicity of the technique Cell Of Origin – at the location of the cell, which is connected to the subscriber. The accuracy of such positioning is determined by the radius of the cell. For "picocells" is 100-150 meters, for most base stations, a kilometer or more.
For a more accurate determination of coordinates using the data from several base stations. There are several methods.
Angle of arrival – the direction of the subscriber. The method is based on the fact that the base station has three to six antenna arrays, each serving its own sector (at its frequency), the Location is determined at the intersection of sectors of several stations. The more sectors in the cell, the narrower each sector and the smaller the area of intersection of the sectors. So, higher the accuracy. Usually accurate to approximately 100-200 meters.
Time of arrival – arrival time. In this method, a measured time of arrival of the signal from the subscriber for a minimum of three base stations. To achieve the required accuracy of synchronization of base stations using atomic clocks or signals from the satellite. The accuracy of the method is about one hundred meters.
Hybrid method is to equip the mobile phone GPS receiver.
In addition to these there are a number of proprietary technologies:
Mobile Positioning System (Ericsson) – accuracy 100 m;
RadioCameraTM – accuracy 50 m;
SnapTrackTM (Wireless Assistant GPS) – accuracy up to 15 m;
CursorTM (CPS) – accuracy 50 m;
Finder (CellPoint) is a precision of 75 m.
Price of the solution is higher, the more accurate the positioning.
Object identification in cellular networks is possible, but normally such task are not put.
Wi-Fi positioning
When you consider that the number equipped with WiFi devices in 2011 reached 1.2 billion, including 513 million smartphones and 230 million computers, the rapid spread of systems Wi-Fi positioning is quite natural.
The simplest way of positioning in WiFi networks, like the cellular – base station is connected to the subscriber. Method used to provide various services, depending on the type of connected device and its location. The range of your WiFi access point is usually 30-200 meters. This determines the positioning accuracy.
To improve the positioning accuracy to measure the radio signal strength, time of its distribution from the subscriber to the access point the direction to the source signal.
As in the case of cellular networks, wifi networks identification of the object is possible, but normally such task are not put.
Local navigation
Local positioning systems include optical (usually infrared) and ultrasonic system. Their range is small – 3-10 meters.
Their advantage is that since light and sound do not pass through walls and doors, they provide "room level accuracy" – the fact of being the controlled object in a particular room. This is important, for example, in medicine.
Infrared positioning
Mobile label in the positioning infrared system emits infrared pulses, which are received by the receivers of the system that have fixed coordinates. The location of the label is calculated by the Time-of-flight (ToF) – time of signal propagation from source to receiver. The drawback of this method is the sensitivity to interference from sunlight. The use of the IR laser increases the range, accuracy, but unfortunately the cost. Positioning accuracy of this method is 10-30 inches.
Ultrasonic positioning
In the ultrasonic positioning system used frequencies between 40-130 kHz. To determine the coordinates of the label is usually measured ToF up to four receivers.
The main drawback – sensitivity to losses of the signal in the presence (appearance) even "easy" obstacles, false echoes, and to interference from sources of ultrasound, for example, ultrasonic flaw detectors, ultrasonic cleaning machines in manufacturing, an ultrasound at the hospital. To exclude these disadvantages, you need to carefully plan the system.
The advantage of ultrasonic systems – high positioning accuracy, reaching three inches.
Local positioning systems are used rarely and their use is declining with the development of radio frequency technologies.
positioning System using passive radio frequency identifiers (RFID)
The main purpose of the systems with passive RFID labels – identification. They are used in systems traditionally used bar codes or magnetic cards – in systems of recognition of goods and cargo identify people, control systems and access control (ACS), etc.
The system includes RFID tags with unique codes and readers, and operates as follows. The reader continuously generates radio waves of a given frequency. CHIP label, getting into the zone of action of the reader, uses this radiation as a power source and transmits the reader ID code. The range of the reader is about a meter.
The cost of systems with passive RFID labels above the cost of the system with barcodes or magnetic cards, but the use of passive RFID significantly reduces the burdens of operators.
positioning System using active RFID
Active RFID tags are used when you want tracking of objects at relatively large distances (e.g. on-site sorting). Operating frequency of the active RFID 455МГц, 2.4 GHz, or 5.8 GHz, and the radius of action up to one hundred meters. Eat active label from the internal battery.
Active tags there are two types of beacons and transponders. The transponders are activated by receiving a signal of the reader. They are used as fare payment, at a checkpoint, entrance portals and other similar systems.
Beacons are used in positioning systems real-time. The beacon sends packets with a unique identification code on command or at specified intervals. The packets are received at least three receivers located on the perimeter of the controlled area. The distance from the beacon to the receivers with fixed coordinates is determined by the angle of the direction on the beacon Angle of arrival (AoA), time of signal arrival Time of arrival (ToA) or time of signal propagation from the beacon to the receiver Time-of-flight (ToF).
The term "active RFID" covers a vast class of various products. Most of the radio-frequency positioning systems use to identify and position objects active RFID. Therefore, characteristics of active RFID tags, including the positional accuracy, and cost vary greatly, depending on the specific manufacturer.
Positioning technology "near-field"
Technology of distance measurement in the near electromagnetic field (Near-field electromagnetic ranging – NFER) uses the labels transmitters and one or more receivers. In systems NFER receiver for determining the distance measures the phase difference between the electric and magnetic components of the electromagnetic field emitted by the label. Since this difference varies from 90° about the radiating antenna to zero at a distance of half-wave, half-wave length determines the range of the system. At 1 MHz the wavelength is 300 m, and the radius of -150 m at a frequency of 10 MHz – 30 and 15 m, respectively.
Positioning accuracy in real conditions is about a meter distance up to 30 meters.
Relatively low frequency radio waves facilitates their passage into complex production environments. Radio waves bend around obstacles, not reflected. Therefore NFER technology has advantages in complex configuration spaces with lots of obstacles.
The lack of NFER systems associated with low performance antenna. For effective operation the antenna should be comparable with the wavelength. In fact it is hundreds of times less, which requires an increase in transmitter power, and consequently the dimensions and weight of the labels.
Ultra Wideband (UWB) positioning
Technology UWB (ultra wideband) uses short pulses with a maximum bandwidth with minimum centre frequency. Most of the manufacturers, Central frequency is several GHz and the relative bandwidth is 25-100%. Technology is used in communications, radar, measurement of distances and positioning.
This is ensured by the transmission of short pulses, broadband in nature. A perfect impulse (wave of finite amplitude and infinitesimal duration), as shown by the Fourier analysis provides infinite bandwidth. UWB signal is not modulated like a sine wave, and resembles a series of pulses.
Manufacturers offer different variants of UWB technology. Vary the pulse shape. In some cases, the use of relatively powerful single pulses in the other – hundreds of millions of low-power pulses per second. Applies to both coherent (coherent) signal processing and non-coherent. All this leads to a significant difference in the characteristics of UWB systems from different manufacturers.
Technology advantages: reliable operation, high precision, resistance to multipath fading.
Restrictions: the complexity of a transmitter of significant power (typical power is 50 µw, the most powerful – 10 mW).
In addition, there are restrictions on frequency control (system as a rule, it is necessary to use in areas where low-power signal is almost not detected in the background noise).
The infrastructure system is based on wired network and requires synchronization.
positioning with usage of CSS and SDS-TWR
Details about positioning using CSS and SDS-TWR I wrote in the topic (1) and (2).
This system provides positioning accuracy of three meters and a radius of 50 meters, has a high noise immunity and resistance to multipath fading, has a low power consumption of labels, does not require synchronization.
But the deployment of infrastructure is complicated by the necessity of building a wired network data transmission to the base station.
same, but using the ZigBee network and MEMS accelerometers
About this positioning system I also wrote in the topic (3).
More can be read here and here.
I note only that these improvements have simplified the infrastructure and improved the accuracy up to one meter.
Compare technology
Comparative characteristics of the described technologies is given in the table:
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