Wireless: Bluetooth vs. Bluetooth
25 April 2008
Most industrial applications require more than a standard Bluetooth wireless solution concerning robustness, optimised performance and enhanced reliability. Further, the implementation should be flexible enough to configure for a specific application by optimising throughput, latency, power consumption, range, or a combination.
Bluetooth radios are available as components ready to mount in motion control platform enclosures. Source: ConnectBlue
To even be considered for industrial applications, a wireless technology has to operate well in a noisy environment. Features such as adaptive frequency hopping (AFH) minimise the impact of potential interference and forward error correction (FEC) enables a receiver to detect and correct errors in a transmission. In these ways, Bluetooth technology becomes a highly reliable and robust wireless technology. However, more is needed to optimise performance for industrial applications.
Looking at traditional Bluetooth technology versus industrial Bluetooth technology can illustrate necessities for tough industrial applications.
Most Bluetooth solutions adapt well to a changing radio environment by enabling or disabling FEC and thereby increasing or decreasing the data packet size. However, for many demanding industrial applications, this automatic adjustment does not provide enough performance. Instead, industrial applications have better use for Bluetooth modules that provide possibilities to control the use of FEC and limit the data packet size.
Most Bluetooth solutions poll for data every 40 slots by default. Since each slot is 625 µs wide, this means an additional transmission delay up to 25 ms, which is too long for many industrial applications. Fast packet handling can, however, be achieved in a low cost CPU by decreasing the default polling time and by using a more efficient Bluetooth stack. With this implementation, configuration can be set for the shortest possible poll time and packet types can be limited to the smallest data packages (DM1), whereby a transmission time of about 5 ms can be achieved.
Bluetooth and WLAN co-existence
Since Bluetooth and WLAN radios use the same frequency band, there is always a risk of interference. Normally, degraded performance can only be seen on the WLAN traffic. Thanks to AFH, Bluetooth technology will automatically remove frequencies that are blocked. Typically, this means that frequencies used by the WLAN network will not be used by Bluetooth. However, since the AFH algorithm takes a few seconds to get active, the WLAN network may be affected a few seconds after a Bluetooth connection is set up. Frequencies previously removed are again tested after 30 seconds. Hence, if there is no traffic on the WLAN network for 30 seconds, the removed frequencies are added again, which means that the WLAN network may again be affected. Further, since AFH is only used on an active Bluetooth connection; it is not used for inquiries and connection attempts. These attempts may affect the WLAN network.
To avoid the WLAN network disturbances, there are some configuration options available. For instance, it is possible to configure a channel map that will allow the users to manually exclude frequencies that are used by the Bluetooth module. Hence, if the frequencies of the WLAN network are known, which is normally the case, these frequencies can be excluded.
Though the channel map replaces the AFH algorithm, none of the frequencies are used for inquiries and connection attempts. To solve this issue during connection attempts, ConnectBlue modules have a unique feature of decreasing the page timeout, which is the time a module continues to setup a connection before giving up.
For the remaining problem with inquires causing disturbances, it is possible to decrease the maximum output power of the embedded module so that it will not disturb the WLAN network.
With the addition of Enhanced Data Rate (EDR), Bluetooth technology can support bit rates of up to 3 Mbps over air providing maximum rates of 2,178 kbps in one direction (asymmetrical link). In other words, Bluetooth with EDR supports three times higher throughput than Bluetooth without EDR. To take full advantage of Bluetooth with EDR, a powerful CPU with generous memory capacity is needed. For Bluetooth devices used in industrial applications, this is typically not the case, which is why most of these solutions have to use Bluetooth without EDR. Furthermore, the Bluetooth EDR packets do not support Forward Error Correction (FEC), which in return increases the risk for more re-transmissions and longer delays.
ConnectBlue has developed a unique solution for its products that makes it possible to control the asymmetrical communication to achieve maximum throughput. This means that the embedded one-chip solution modules can achieve file transfers of almost 650 kb/s at a rate of 921 kb/s.
Long range possibilities in wireless solutions are typically important in industrial applications, not just to get a long range but also to overcome physical barriers and get enough 'spare capacity' to over-come changing circumstances because of mowing machines/machine parts, etc. To get the needed range, optimisation is needed on output power, antenna type, radio design, radio environment and antenna position. On top of that, the actual Bluetooth configuration may need modifications as well.
When the range increases, radio interference also increases. As mentioned earlier, the solution of enabling the error control tool FEC also causes the radio to toggle between enabling/disabling FEC and toggle between different data package types, which in turn may decrease performance. With industrially configured embedded modules, FEC and DM1 packets can be forced to provide the best possible performance also at long ranges.
Another improvement tools is the enabling of Fast Connect (interlaced page scan), which provides for shorter connection times and gives more robust connection attempts at long ranges.
Industrial Development Centre in Olofström, a Swedish R&D centre, has developed a powerful Bluetooth based data logger, BlueCenter DL141E, for use in industrial applications. The wireless communication is critical because the Bluetooth communication rate is variable. Each sample has a timestamp and can be matched with other samples asynchronously. To verify the communication, a great number of tests have been conducted in a special laboratory for wireless radio units at the Industrial Development Centre in Olofström. Different types of radio frequency signals have been used to interfere with the Bluetooth radio and the data communication. The logged data in the wireless data logger has been compared with real data logged continuously over more than one year. Parameters such as the resending of data, non-correct data, sample rate, asynchronous time, etc. were logged. “The conclusion from the tests is that the connectBlue Bluetooth radio module showed a consistent and robust behaviour in the continuous tests conducted over a full year,” said Carl-Axel Ohlsson, manager automation & technical quality control at Industriellt UtvecklingsCentrum in Olofström AB (IUC).
The Bluetooth based wireless input/output (I/O) systems from Phoenix Contact provide fast I/O update. Via the patented ID key, they are also very easy to install. The wireless I/O products from Phoenix Contact are installed in machine control, automotive robot applications, warehouse transportation systems, crane control, process control as well as merry-go-round installations eliminating the need for vulnerable and high-maintenance conductor lines, drag chains and slip rings.
Schneider Electric in the UK has developed a Bluetooth Management System to simplify for operators to manage pole mounted RTUs used in electrical energy distribution. Instead of shutting down the power line before climbing up the pole and connect a configuration PC, the operators connect a PC supporting Bluetooth to the RTU. The operator can easily upgrade the software, reconfigure the RTUs, and diagnose the electrical energy distribution on site using their PC from a distance of up to 100 meters. Operation of an 11 kV electrical distribution network is potentially lethal to the utility personnel. To minimise risks, the switch control cabinets are placed above the no climb guard and wireless control of the RTUs is deployed. The Bluetooth solution provides galvanic isolation between the PC and the RTU, which further increases the personal safety for the operator.
In Italy, ABB has developed the world’s first Bluetooth enabled Circuit Breaker, Emax, which protects low voltage distribution systems and equipment. From the front panel, the operator configures current limits, over current timers, diagnostics and event logging. All these parameters are accessible via Bluetooth through a handheld computer (PDA). Thereby, the operator can conveniently reach the circuit breaker even when installed in harsh environments where computers and cables would not be functional.
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