Implementation of a wireless data transmission path
Through wide-area use of smart sensors and actuators, manufacturing processes may be optimised and aspects such as security (access control, safety at work), energy efficiency (lighting and temperature control) and even connection to the smart grid may be controlled. Various wireless standards, optimised for certain applications, are available for the different tasks. To simplify data exchange between wireless networks and the central controller, Internet protocol based protocols find applications in the wireless world.
Progressive miniaturisation has produced very tiny sensors. They may be deployed, among other scenarios, for monitoring and controlling machines with much enhanced accuracy. One example of this would be using Hall sensors (e.g. the TLE499x line of Infineon) for constant monitoring of all moving machine tool shafts. These may be used to monitor speed and play of the shaft. The power consumption may also be calculated based on the revolutions. Smart grid linking can help to reduce the power consumption by adapting speeds as necessary, even for individual process steps. By measuring shaft play, wear may be diagnosed earlier and equipment servicing scheduled to suit production.
Automatic temperature monitoring in the food industry is another potential application. An infrared thermometer may be fitted at the cutting table to document the integrity of the cooling chain.
Both examples demonstrate new possibilities for collecting information using small sensors, allowing more accurate control and monitoring. For data transmission between sensor network and controller, data must be translated to the relevant protocols when the transmission system changes. The gateway links the two systems. Since data must be transmitted bidirectionally, the translation turns out somewhat more complicated. Equipment available on the market is therefore generally expensive and not intuitive to use.
A gateway for data exchange between the server in the manufacturing facility and the sensor must therefore be able to integrate data into the relevant protocols bidirectionally. The wireless transmission standards are all based on the IEEE 802.15.4 protocol. Data transmission via the Internet is based on conventional Internet protocols. When the Internet was born, addresses were defined using 32 bits. Available addresses have in the meantime run out, however, since the number of Internet capable devices has increased enormously. To solve this problem, Internet protocol Version 6 (IPv6) uses a 128 bit address. This suffices to (proverbially) “allocate an IP address to each grain of sand”.
The gateway’s task of translating data between Internet and IEEE 802.15.4 protocols presupposes a powerful processor and a plethora of data processing routines. Addressing poses another challenge in a large network of sensors and actuators. The server sends data to the gateway’s IP address. The gateway extracts the address from the data stream and integrates the data into the relevant wireless protocol for transmission.
A gateway must work with several wireless standards, e.g. ZigBee for transmission of data from many intelligent sensors and WiFi for data intensive sensors such as cameras.
To simplify data exchange between the various wireless protocols and the Internet, 6LoWPAN [IPv6 over low power wireless personal area network] has been developed. The protocol allows the 128 bit IPv6 address to be compressed via an IEEE 802.15.4 compliant protocol for data transmission. The data are furthermore integrated at the sensor already into one of the most common Internet transmission protocols, TCP or UDP. Like ZigBee, 6LoWPAN is capable of building a network of several paths between sensors.
The ZigBee Alliance has integrated 6LoWPAN into ZigBee IP. Figure 1 shows the protocol layers.
Figure 1. ZigBee IP protocol levels (Source: ZigBee IP Specification, Rev. 32)
The IPv6 based wireless data transmission methodology is evident in the ZigBee IP structure. After the link layer, which transmits the data via radio waves, IPv6 compresses/decompresses the data using 6LoWPAN. The data transmission destination is determined in the network layer. The type of data transmission is specified in the transport layer – via TCP (reliable, but slow) or UDP (fast, but prone to data loss). All other layers offer additional functions such as AES data encryption.
Bluetooth, in Version 4.1 with the “logical link control and application” (L2CAP) function, also creates a basis for IPv6. This allows data transmission via separate channels.
But even with an IP-based standard like this, capable of devolving protocol administration for wireless and Internet-based data transmission to hardware, many issues remain to be addressed before a finished product can be launched into the market.
A wireless data transmission device must be certified under the CE standard in Europe or FCC in the US. Amendments to the certification regulations may furthermore force changes to the entire system. The effort of certifying an individual device is not worthwhile for small quantities. For maximum flexibility, also in the system internally, it is worthwhile using a complete wireless module. Such a module will handle the entire protocol and will usually be FCC- and CE-certified already. The module manufacturer’s design also complies with regulations for maximum transmitted power and immunity from interference.
The market offers modules of various complexities. The simplest and cheapest modules have only a transceiver chip, such as the Texas Instruments CC3000 for WiFi, as a cost-effective example (Figure 2). This chip handles wireless data transmission whilst a host processor handles data processing and embeds these in protocols. More complex modules handle embedding of the data as well, or allow the module to run user programs, thereby obviating the need for PCB developments.
Figure 2. The reference design for the CC3000 WiFi chip has been certified by the FCC / IC / CE / TELEC
Basic modules are available from about €10. Adding up the cost of developing and certifying a unique solution leads to the conclusion that such a development is not normally worthwhile until annual volumes reach comparatively high level, of the order of 10,000 per year or greater.
Another development aims at integrating multiple protocols in a module. The Texas Instruments WiLink 8 modules are an example of this. These modules offer the option of using WiFi and Bluetooth in a single module, creating solutions for an IPv6-based gateway.
To design the sensors to be smaller and more energy efficient, the microcontroller and radio front-end may be integrated into one chip. The Texas Instruments CC2538 family (Figure 3) is a good example of this. These combine an ARM Cortex M3 with a radio front-end. This and other comparable circuits enable the use of high performance sensors and controller applications where very little space is available. The Cortex M3 is capable enough to take over protocol handling and additional tasks.
Figure 3. The 6LoWPAN and ZigBee-enabled CC2538 reference design can be connected via USB to a computer. The design is already very compact, which is important for mobile sensor and actuator applications.
The number of modules and ICs on offer is constantly increasing, which has generated a specialised area of product support from manufacturers and distributors, assisting with all wireless-related issues.
About the author:
Steven Weyrich is an Application Engineer at the Arrow Engineering Solutions Centre. He specialises in Texas Instruments analogue products and has, for his Master’s thesis, built a ZigBee Internet gateway for the remote control of mobile robots.
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