1、 TECHNICAL REPORT ISA-TR100.00.02-2009 The Automation Engineers Guide to Wireless Technology: Part 2 A Review of Technologies for Industrial Asset Tracking Approved 2 October 2009 ISA-TR100.00.02-2009 ISA-TR100.00.02 The Automation Engineers Guide to Wireless Technology Part 2 A Review of Technologi
2、es for Industrial Asset Tracking ISBN: 978-1-936007-28-8 Copyright 2009 by ISA. All rights reserved. Not for resale. Printed in the United States of America. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanic
3、al, photocopying, recording, or otherwise), without the prior written permission of the Publisher. ISA 67 Alexander Drive P. O. Box 12277 Research Triangle Park, North Carolina 27709 USA ISA-TR100.00.02-2009 Copyright ISA, 2009. All rights reserved. - 3 -Preface This preface, as well as all footnote
4、s and annexes, is included for information purposes and is not part of ISA-TR100.00.02-2009. This document has been prepared as part of the service of ISA towards a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to pe
5、riodic review. Toward this end, the Society welcomes all comments and criticisms and asks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone (919) 549-8411; Fax (919) 549-8288; E-mail: standard
6、sisa.org. The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards. The Department is further aware of the benefits to USA
7、users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards, recommended practic
8、es, and technical reports to the greatest extent possible. Standard for Use of the International System of Units (SI): The Modern Metric System, published by the American Society for Testing in this diagram the bottom axis is time. ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights reserved. - 21
9、-Figure 8. Linear chirp waveform. The system performance and range determination are easy to see using the analogy of chirped-frequency radar. Consider the case where a chirped-frequency radar transceiver that outputs a certain frequency, F1, at time T1. Note: In a typical instance, a sawtooth wavef
10、orm that begins at a frequency F0with the “chirp” duration of some total time Tt is used. After this time Tt, which had an associated output frequency F(Tt) = F0+ aTt, the frequency is reset to F0 and the process repeats. The situation is illustrated in Figure 9. This signal proceeds down its merry
11、way a distance X, taking time T, until it encounters a surface and is backscattered toward the radar transceiver. After another time, T, the reflected signal is received by the radar transceiver. Remembering that distance = velocity times time (X = V * T) it has taken the signal a time interval 2T t
12、o travel down and back from the reflector. Since the speed of the electromagnetic signal is C, the speed of light, then it is easy to find the distance, or range, from the radar transceiver to the reflector. This is how classic radar systems work. ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights
13、 reserved. - 22 -Figure 9. Illustration of the band-limited chirped-frequency radar signal. Note that while the frequency varies, the amplitude of the signal remains constant. In the chirped frequency case, the situation is similar but back at the transceiver, instead of monitoring time, T, the freq
14、uency that the radar transceiver is currently outputting, F2at time T2, is recorded. The difference in frequency, F = F2 F1 is calculated, and then by knowing the slope of the linear frequency ramp function, a, the time difference T is determined. Knowing T allows you to determine X, the distance (r
15、ange) to the reflector. This is illustrated in Figure 10. ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights reserved. - 23 -Figure 10. Chirped-frequency-based range determination. While it may sound complicated, the reality is that using a chirped-frequency radio for location information relaxes
16、the time resolution requirements for range measurement resolution by shifting the measurement into the frequency domain. Back in the industrial field transmitter realm, there is a requirement for multiple devices to have connectivity to the asset under measurement (to provide the X, Y and potentiall
17、y Z coordinates of the asset). This may be achieved by having the asset within range of multiple gateways, or by using relative location information for other field transmitters that are within range of the asset. Note that there must be an infrastructure or network/system connection in place which
18、will transport the range information to the appropriate software application. It is worth reiterating that the asset “tag” must be within range of multiple gateways in order to achieve X, Y and potentially Z positional information. Typical location accuracy is in the cm range with an overall asset-t
19、o-asset or asset-to-gateway separation in the 1-100 meter range. While not specifically diagramed, the infrastructure support system is therefore similar to that shown in Figure 4. 6 Asset Tracking Utilizing IEEE 802.11 a/b/g Focus on Received Signal Strength Numerous techniques for RTLS are based o
20、n the strength of the signal received by an assets attached radio changing and associating that received signal strength variation with a change in the separation distance between the gateway/access point and the assets receiver. It is well understood that the further a receiver is from a transmitte
21、r, the less the received signal strength. This fundamental principle is based on the 1/r2EM field law (sometimes referred to as the inverse square-law). In terms of communication systems, this means that the received signal strength (RSS) follows, for a line of site instance, ISA-TR100.00.02-2009 Co
22、pyright 2009, ISA. All rights reserved. - 24 -where R is the receiver-transmitter separation distance.5In this scenario, the variation in the received signal strength (RSSI) is predicated on all other parameters remaining constant so that any change must be due to a change in distance between receiv
23、er and transmitter. Therefore, if the receiver knows the transmitters output level, then the distance R can be determined. This in turn leads to contour lines for the signal strength from a base station, as shown in Figure 11. It becomes an easy matter for the wireless device, which is attached to t
24、he asset, to report to the access point that it is associated with its RSSI value (along with its unique radio/tag ID). That information is then processed either locally in the access point, AP (if it is intelligent) or passed to a software application for it to determine the location of the tag wit
25、h respect to the (typically) fixed location of the AP. Figure 11. The received signal strength decreases as 1/R2with separation distance leading to contour lines for signal strength. Knowledge of the tags RSSI value for a transmission from a single AP puts the tag within a certain range of the AP (i
26、n radial coordinates, the distance is known but not the angle). To determine the X, Y, and potentially Z coordinate of the tag requires the tag to be in communications range of a number of APs. This situation is shown in Figure 12. The RSSI-based techniques have been applied using a multitude of wir
27、eless technologies. This also means that the frequencies used by these differing technologies also vary, which implies, following Table 1, that the actual RSSI value relative to the actual transmitted signal level (which is different for different wireless technologies) within a single facility will
28、 be different for the different technologies. While this may appear to be somewhat convoluted, the general notion is depicted in Figure 12 for a typical RTLS system. For ease in the discussion, Figure 10 and the accompanying descriptions are based on RSSI and an IEEE 802.11 (WiFi) infrastructure. _
29、5In general situations, the received signal strength decreases as 1/Rn. ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights reserved. - 25 -Figure 12. RTLS positional information for the tag using RSSI requires that the tag be within range of multiple APs. For improved location determination, the s
30、ize of the APs RF footprints should be reduced (less transmitted power) with a higher density of APs being deployed. A measurement based solely on intensity may be problematic, for any change in the received signal strength may be interpreted as a change in separation distance between the AP and the
31、 tag, while in reality the variation may be caused by a number of reasons (e.g., decrease in the transmit power, decrease in the receiver sensitivity, introduction of a “new” object that attenuates the signal, etc.). Although a change in received signal strength may degrade asset location accuracy a
32、nd performance, asset location is still provided with a measure of variation based on the change in the network. However, deployed location performance will be restored at the time that network performance has returned to normal operating status. There are also other issues associated with RSSI-base
33、d asset tracking. For example, RSSI is not measured in specific units. Instead, each wireless device vendor uses an arbitrary set of numerical units. It is incorrect therefore to attempt to match a given RSSI value with, say, a power ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights reserved. - 2
34、6 -unit such as mW which leads to serious problems if devices from multiple vendors are supposed to interoperate. While not strictly for RTLS, RSSI is of great interest to any WiFi device since much of the perceived performance of a wireless network is based upon inferences made via the use of RSSI.
35、 Examples are: the higher the RSSI, the higher the transmit data rate (up to a maximum); client devices tend to monitor the RSSI on a channel (frequency); when this value drops below a certain threshold, the device assumes that the channel is clear to send and transmits data; the association and dis
36、association of client roaming between multiple APs is almost entirely determined by RSSI. Therefore, while RSSI may not provide the optimum asset tracking capability, it is used for a wide range of client-host operations. See Figure 13. Infrastructure requirements for a WiFi RSSI-based asset trackin
37、g system are not dissimilar from that of any typical data or voice-deployed wireless network. WiFi tags are managed as any other wireless client, with the exception that voice and data solutions are given network priority to maintain quality of service and production application availability. WiFi T
38、ags are maintained on separate VLANs to maintain separation from production wireless applications. There are solutions that offer state-of-the art deployment tools for verifying that infrastructure requirements are met, and in cases where the requirements are not met, problem areas are indicated wit
39、h resolution options provided. It should be noted that a typical VoIP wireless network provides excellent location granularity and at least one solution provides software clients for tracking VoIP phones. Figure 13. Spatial resolution is provided in multiple axes only if the tag (target in this figu
40、re) is in communications with multiple APs. ISA-TR100.00.02-2009 Copyright 2009, ISA. All rights reserved. - 27 -While the RF footprint is obviously dependent on the transmit power and antenna gain (directionality), the typical indoor specified WiFi-compliant range is on the order of 50 m. For a lar
41、ge facility this value gets mapped into the geometry of the actual location, resulting in a coverage map along the lines of that shown in Figure 14. While vendors differ in the RF channel assignments for the WiFi APs, the need for consistency in terms of over-the-air security (WEP, WPA, WPA2, etc.)
42、and ease for the tags to associate with the APs as they roam through the facility is obvious. RF signal coverage and density are essential for IEEE 802.11 RTLS solutions. In particular, received signal strength and the number of audible access points per location throughout the facility are two of t
43、he most critical determining factors that will affect the level of tracking accuracy. However, no matter the density and RSSI values, all IEEE 802.11 systems can provide asset location to varying degrees either X,Y, map, zone or presence functionality. Asset location granularity is dependent not onl
44、y on the wireless environment, but also on the requirements that each facility has for those critical pieces of equipment. Figure 14. Predicted WiFi coverage for an office setting. Note that this figure does not differentiate between RF signals from different APs. ISA-TR100.00.02-2009 Copyright 2009
45、, ISA. All rights reserved. - 28 -One advantage of the RSSI approaches is that the RF coverage for RTLS can be easily determined through utilization of the “location coverage” visualization provided through specific site survey products. This, in conjunction with the “network requirements” visualiza
46、tions, makes it easy to visualize and report on areas of strong coverage and also to identify areas where the network might be improved for RTLS location accuracy, if asset location requirements dictate increased accuracy. In essence, some site survey products allow users to manage a wireless networ
47、k, while reporting and planning location performance for asset tracking. Vendors comment frequently that the prospect of potentially thousands of WiFi-based tags communicating over a companys wireless network is sure to draw concern from an IT department. In most cases, specifically for battery-oper
48、ated lifetime reasons, the tags communicate only sporadically through the network. In certain cases, tags have been integrated with some method of motion sensor which, coupled together, means that the tags may be programmed to only “announce” themselves when the tagged object is actually moved. Once
49、 an object is at rest, the location is recorded and the tag essentially remains in “sleep” mode until movement is detected. In an attempt to make the use of WiFi-based tags as easy as possible and AP vendor neutral the tags are to operate as IEEE 802.11b/g compliant with an IEEE 802.11b radio (2.4GHz) and, at least, WEP security. Note: there are a number of IEEE 802.11 “new” standards that are under consideration, such as IEEE 802.11n. At the time of this writing the editors of this document felt that a candidate WiFi-based asset tracking syst
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