I’ve recently completed an Honours Degree in Software Development and as part of this I spent a few months off and on working on a proof of concept tracking system utilising Bluetooth Low Energy (BLE) in order to track small asset tags. I believe this system could quite easily be scaled up to an industrial scale with further work but don’t personally plan to follow this up in the near future due to working on other projects. As such I figured I’d release a lot of what I have and hopefully it can help somebody else.
There will be a series of posts over the next few days/weeks releasing various materials of the project, they are mostly released as is and may contain minor errors.
If you do something cool with this then please get in touch, otherwise do with it all as you wish.
All posts in this series may be found by selecting the tag below.
Literature Review
Introduction
There has been a 129% year-on-year growth in e-commerce orders in recent years (Columbus, 2020), and this growth is expected to continue in the future. This means that the efficient processing of packages in a variety of warehouse settings must be addressed and any potential improvements identified if companies are to keep up with the growing demand.
This report will focus on identifying key issues where a real time tracking system based on a Bluetooth Low Energy (BLE) tag and Wi-Fi gateway infrastructure may apply and what limitations or benefits such a system may bring when compared to existing RFID-based solutions, this will be done by investigating existing Radio Frequency Identification (RFID), optical and BLE systems to identify their benefits and disadvantages when compared to a BLE-Wi-Fi system.
Relevant Issues
There are several common issues that point towards areas where a potential solution may be able to improve on; overstocking, which is having more inventory than required; inventory shrinkage, which is misreporting inventory numbers and picking time, which is the process of gathering inventory at point of sale or when inventory must be moved.
Overstocking
Overstocking is effectively when the value of inventory exceeds sales, in a perfect situation a business will have the item they need when they need it which means that warehouses are only storing what needs to be stored, reducing storage and processing costs. However, most U.S-based retailers have $1.31 worth of inventory for every $1 in sales as of December 2020 (United States Census Bureau, 2020) which shows an overstocking percentage of just under 33%.
One primary cause of this issue is companies over-correcting due to the prospect of being out of stock when a customer requires an item which may lead to worse issues than holding too much inventory such as the lost opportunity cost of the sales revenue, customer frustration, brand reputation loss and a higher-than-normal cost to rush delivery of replacements.
A partial mitigation to this problem would be to be able to accurately track inventory accurately in real time which would allow companies to purchase stock with the knowledge that they have reliable statistics of inventory.
Inventory Shrinkage
Another common issue for businesses is inventory count errors, otherwise known as inventory shrinkage, which is when the physical count and value of stock does not reconcile with the reported value. The average inventory shrinkage percentage was 1.62% in 2019, with 27.3% of companies surveyed reporting over 2% inventory shrinkage (National Retail Federation, 2020). This represents a major cause of financial loss for companies.
A common way to resolve this issue is to perform manual stock takes, which are time-consuming, error-prone, and expensive. Stock takes are often performed on a yearly basis at minimum to ensure correct asset reporting at the end of the financial year, but some organisation, especially those working in the fast-moving consumer goods group may have to perform stock takes multiple times per year. One way to reduce inventory shrinkage would be to be able to accurately track inventory in real-time which will reduce the chance of further errors in tracking or logging of any data to a back-end inventory system, whereby an automated solution can do both at the same time.
Picking Time
Finally, picking time is the process of workers moving about the warehouse environment and retrieving required stock for sale or movement. Picking time is estimated to constitute up to 55% of total warehouse operating expense (Koster, et al., 2007) and so should be a major focus of thought and improvement.
A common functionality in today’s warehouse management system is the ability to direct staff via the most efficient route to pick up items, this does still require having trustable data on the exact location of inventory which is where a real time location system could improve the process dramatically.
Inventory Tracking Technologies
There are a variety of existing inventory tracking technologies, including both active and passive systems. By far the most common in recent years has been radio frequency identification (RFID) systems which use radio frequency waves to identify, track and locate inventory.
RFID Systems
RFID systems come in two main forms, commonly known as active and passive.
Active RFID tags are battery powered and continually broadcast a signal up to 100 meters depending on the frequency of the tags, with most active tags operating in the ultra-high frequency band. Active RFID is generally reserved for high-value assets due to their high cost of individual tags with the cost ranging from roughly $5-$100 per tag depending on the functionality, range, and purchase volume according to several websites online that were offering tag solutions.
Active RFID systems do have much lower installation costs though due to often not requiring specialised hardware that often requires expensive professional to install and maintain.
Passive RFID systems do not contain a battery at all and respond to requests by using the electromagnetic energy transmitted from an RFID reader, passive RFID typically only has a range up to 5-6 meters although can reach 30+ meters in ideal conditions (atlasRFIDstore, 2019). Although passive RFID systems are used by many organisations due the seemingly low purchase costs of the individual tags, this is often a false economy due to the high cost of readers etc which will be covered more thoroughly in the next section. Typical prices for passive RFID tags are around 29p (AliExpress, 2021) per tag in low quantities, but can go significantly cheaper in bulk.
A typical passive RFID implementation requires purchase and setup of antennas, RFID tags and RF readers all of which can be costly.
RFID readers transmit and receive radio waves to allow for communication with the RFID tags, antennas are connected directly to the readers and convert signals from the reader into radio waves which are received by the tags. Based on figures released by AirFinder in 2020, who manage systems for Amazon, Verizon and other large organisations, passive RFID readers may reach costs of $3000 per device when taking in to account additional related costs (AirFinder, 2020) which can become very expensive when readers are often needed at every entrance and exit at minimum, with a large number required for any form of close to real-time tracking of assets.
Nowadays RFID tags are generally reusable with 500,000 read/write cycles in some cases (Infineon technologies, 2004) which can lead to cost savings due to not having to replace RFID tags after single uses. Although this does not aid reducing the initial installation costs, it can save a significant amount of money over the lifetime of a tracking system. Companies would need to have processes in place to handle reuse in a smart manner though.
There are also installation costs to consider, passive RFID systems in particular use specialised equipment such as the antennas which need to be properly directed, network infrastructure configured, and tests perform.
Where both solutions do tend to converge somewhat is ongoing maintenance costs where the cost of both systems is roughly comparable. However, the cost of either is seen as the dominant barrier to adoption by companies (Huber, et al., 2007).
The primary issues that a BLE-WiFi solution will have to solve or at least improve on would be the high initial cost of implementation due to specialised equipment and/or high costs of tags, the requirement for specialists to install and maintain the system and the overall high maintenance costs.
Near Field Communication (NFC) Systems
NFC is a very similar technology to RFID in that it operates within the same band and uses electromagnetic induction to transmit information. Where RFID requires very expensive hardware to read and write data however, an NFC tag can be reprogrammed or read by using an off the shelf device such as an Android phone, like Bluetooth Low Energy tags.
NFC is limited to a read range of around four to ten centimetres (Shobha, 2016) which does not make it ideal for real-time location tracking, but it could be of use in an approximate location system.
Optical Systems
Optical systems are one of the newer solutions on the market that attempts to provide real time location services, it appears that there has been little published research on the topic and there are only several providers that could be found at the time of writing.
Currently there are only a few Optical-based tracking systems on offer, one of which is Vero Solutions’ TotalTrax, Inc. system (Anon., 2016) which works on the basis of a grid of markers normally mounted on the ceiling which allow forklifts equipped with special hardware to track where they are in each space, this is all fed back to a central server generally using a WiFi network. This is in addition to RFID readers placed on the forklifts and RFID tags placed on pallets to identify what inventory is moving.
Initially optical systems appear to be a derivative of standard RFID systems, however, there are various advantages to this over a standard RFID implementation when installed in a large warehouse where inventory is primarily moved via forklifts rather than personnel. All inventory is tracked in real time, a live view of all equipment can also be recorded, the overall cost should be lower than typical RFID systems as cheap passive RFID tags can be used along with static RFID readers rather than the typically expensive mobile RFID readers plus antennas. Also, as each forklift or truck is reported to a server in real-time, it should be possible to see significant cost savings due to reduced picking times, as the nearest asset can be redirected as required.
Alternative Tracking Systems
Tile (Tile, 2021) is a well-known tracking system that utilises Bluetooth Low Energy like the proposed solution. The solutions’ primary purpose is for personal use in tracking items such as phones, laptops, and the likes, with a range of up to 40 feet in normal conditions. A single Tile Sticker costs £17.50 according to the manufacturers site, which would be prohibitive in a workplace environment where you may require thousands potentially, whereas an off shelf BLE transmitter with the addition of a replaceable battery would cost £3.25 (AliExpress, 2021), this is the single-unit cost however and may be reduced considerably by removing the replaceable battery and/or buy buying in bulk.
Where Tile fails as a real time tracking system is that it works by connecting to a device (normally a mobile phone) and then uses an app in tandem with that devices GPS to attempt to track the Tile, tracking the position of the tag only works whilst the Tile is connected to that device unless using the ‘Tile Community’ feature, which I’ll discuss below, and with the short range of the Tiles this makes it difficult to track tiles accurately, also as the system only connects to a single device, it is not possible to get an accurate location or even direction, simply an estimated range from the devices location.
The Tile has a feature whereby if the tile is marked as lost, any device running the Tile app can track it regardless of the owner. When a Tile comes within range of a device, the device transmits its location to the Tile Community servers which then pings the owners device anonymously. There is a potential privacy issue here in that Tile does not state whether it stores or processes this location data in any manner.
A BLE-Wi-Fi system would work around this issue by having each tag reporting to several Bluetooth receivers connected to Wireless access points which would allow for accurate trilateration in real time.
Bluetooth Low Energy / Wi-Fi Summary
Bluetooth Low Energy (BLE) is a newer technology, first released in 2011 as Bluetooth 4.0. It was primarily designed to be an extremely low power consumption technology whilst still being compatible with previous versions of Bluetooth and common modern-day devices and has seen high levels of adoption in the IoT space as a result.
BLE is like RFID systems in that it broadcasts a small quantity of data often every 5-60 seconds and as such many of the typical advantages of an RFID system apply, such as their extremely long battery life which often extends to between 2 – 5 years depending on how often they are set to broadcast.
Where a BLE architecture starts to shine is that the entire system may be built using off-the shelf components which keeps cost low, an indicative cost per BLE tag with the addition of a replaceable battery is £3/tag (AliExpress, 2021) which is considerably cheaper than active RFID tags, whilst keeping all the primary benefits. There is also no requirement for a costly hardware infrastructure when compared to RFID, a BLE system can communicate with a BLE-Wi-Fi gateway device, which cost around £25-200 (Fanstel, 2021)per device depending on the specification.
Both Bluetooth and Wi-Fi are well known technologies that IT professionals are often exposed to and so this should reduce the cost of installation and maintenance of BLE-Wi-Fi systems when compared to RFID-based systems which often requires hiring expensive specialists. BLE is designed to communicate with consumer devices, as such another major benefit is that BLE tags may be read directly using low-cost mobile devices which can allow for efficient picking times by aiding the process over the last few meters.
Inventory Management Software
Although all tracking systems rely on some form of hardware to report data, another important consideration is what software system the hardware reports to, ideally any developed system will be able to be used by a variety of back-end systems in a standardised manner.
There are many offerings, more than can be realistically covered in this report, as such the functionality of a popular offering will be reviewed to see what methods are available for data input and what format they require. This will inform the final design of the RTLS.
Redpoint Positioning is a company that specialises in real-time location services, they offer a product called SiteWise and SitePlan which together produce real-time visualizations using floorplans and location data (RedPoint, 2021).
The RedPoint products provide two primary methods of reporting data from 3rd party systems, the first is the representational state transfer (REST) API and the second is a Web Sockets API, both methods have their own advantages and disadvantages when it comes to reporting real-time location data.
Location Data Reporting
When reporting real-time location data, an important consideration is performance of the system. There could potentially be tens of thousands of individual tags reporting data to central servers, as such it is important to outline the primary methods of reporting this data.
REST API
REST API’s utilise HTTP connections to report or receive data, REST has been one of the standard ways to allow communication over the internet for many years and works well for applications where secure connections are required intermittently to download images, documents etc.
REST works by having a client request data from a server, this then starts a conversation where the client requests a ‘handshake’ which is effectively an authorisation request, the server then responds by opening a connection if authorised, data is exchanged and then finally either the client or the server severs the connection. This works well for one off or intermittent requests but can generate a lot of overhead when dealing with continuous data streams such as location data due to the constant need to re-open connections.
Web Sockets
Web Sockets were the answer to the overhead issue. When a client connects to a web socket the connection is kept open which means that the client can continue to send or request data and the server can also push data to the client as it see’s fit. To put this in perspective, testing has found that with small payload sizes, where HTTP-based requests were used they may make up to 10 roundtrips per second, Web Sockets are capable of performing up to 75 times as many roundtrips in the same time (Laine & Saila, 2012).
This has real world implications as it means that there is less time required to process each change and therefore it is possible for more data sources to be reporting data with less overall processing power required. By extension it means that a Web Socket system may require fewer data gateways which should have the effect of reducing hardware costs of a Web Socket-based reporting infrastructure.
Summary
In summary, there are a variety of existing tracking systems which could allow an organisation to better organise their assets in a warehouse setting, each with their own unique benefits and disadvantages compared to the others.
Typical RFID-systems benefit from very low cost of individual tags and a lack of batteries for passive tags which can reduce ongoing maintenance costs, they do suffer from potentially large initial installation costs due to requiring specialised skills and may also have increased maintenance costs due to high cost of readers and antennas. Active-RFID tags allow for Realtime tracking unlike passive, but the costs per tag are prohibitive for low-cost assets.
BLE-systems will benefit from lower installation and maintenance costs due to using off the shelf components that are generally better understood such as mobile devices, routers, and easily reprogrammable tags. However, the individual cost of the tags is likely to be higher than in RFID-based systems.
Tile and other commercial systems have proven that BLE is a technology that is ready to be adopted and as costs continue to decrease, bringing BLE to the asset/inventory management area appears to make sense. The overall capability of a BLE-system is at least comparable to RFID in every area other than cost per tag and this can be remedied to some degree through economy of scale.
References
AirFinder, 2020. A Breakdown of 7 RFID Costs, From Hardware to Implementation. [Online]
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[Accessed 11 January 2021].
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[Accessed 22 February 2021].
AliExpress, 2021. NRF51822 2V-3.3V Bluetooth 4.0 Wireless Module For iBeacon Base Station Intelligent Control System Beacon BLE Module 4MA W/ Case. [Online]
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[Accessed 15 February 2021].
Anon., 2016. Optical Tracking – Vero Solutions. [Online]
Available at: https://vero.solutions/technology/optical/
[Accessed 15 February 2021].
atlasRFIDstore, 2019. Active RFID vs. Passive RFID: What’s the Difference?. [Online]
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Koster, R. d., Le-Duc, T. & Roodbergen, K. J., 2007. Design and control of warehouse order picking: A literature review. European Journal of Operational Research, 182(2), pp. 481-501.
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[Accessed 15 February 2021].
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