Measurement / Test & Inspection
Measurement

Wireless Measurement & Data Acquisition

Increased quality control requires measurement of increasingly more parameters in a cost-efficient way.

November 1, 2013
KEYWORDS bluetooth / WLAN
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Professional measurement systems such as laser distance meters, laser scanners, level-instruments, machine-control and monitoring systems are often mobile or temporarily installed and are thus ideal applications for wireless communication. The main advantage for going wireless in these applications is connectivity—without constraining cables—to existing networks, the cloud, desktops, laptops, smart phones and tablets.

Wireless Scenarios in Measurement & Data Acquisition

Many measurement and data acquisition devices use traditional serial interfaces (UART, RS232, RS422 or RS485) or Ethernet interfaces as the means to connect to the data acquisition / human interface device. Either the connection is a serial point-to-point connection to a PC or a serial / Ethernet connection to the existing infrastructure requiring a wireless solution based on Bluetooth or WLAN (Wireless LAN) which is also commonly referred to as Wi-Fi.

TECH TIPS

In order to get disturbance-free communication it has to be secured that WiFi is not disturbed. With frequency planning, it is possible to allow multiple technologies to operate in the same band. It is also possible to beforehand choose channels that are not to be used in order to avoid interference with other wireless systems used in the same environment.

An alternative to the above described solutions is to design-in a wireless chipset solution. This approach often makes sense only for large production volumes due to the cost involved in the design and regulatory approvals.

The serial cable replacement solution would either use Bluetooth or WLAN, both of which are standard technologies in most computers. For an ad-hoc connection, Bluetooth is the most suitable technology while WLAN often is best suited for connections through an Ethernet infrastructure. While Classic Bluetooth today has the larger installed base, for lower bandwidth applications, Bluetooth low energy is an alternative. Bluetooth low energy is particularly suited if low power or a very fast connection / re-connection are required where required data rates are relatively low.

Mobile devices are widely used in everyday life. By installing a tailored “app,” the everyday mobile device becomes a powerful and cost efficient tool for the measurement device. The app can be designed to gather certain data, to perform specific tasks such as to act as a Human Machine Interface (HMI) panel, a remote control or a gateway.

Wireless Sensors and Actuators

Increased quality control requires measurement of increasingly more parameters in a cost-efficient way. Especially when it comes to new installations it is of interest to use wireless communication systems including “wireless power” which is typically achieved through batteries. For many smaller devices a coin cell battery would be enough for several years of operation. But as an alternative to a battery there are also several energy harvesting alternatives. Bluetooth low energy is an efficient technology for these types of applications.

Internet of Things

With the rapid deployment of infrastructure and access, mobile devices can be accessed over the Internet. This evolution is called internet of things, web of things, or embedded web, and will be a common way of collecting data from devices and systems. A cellular connection is a good alternative for many applications but requires a substantial power source and is relatively expensive both to purchase and to operate in relation to the cost of smaller devices or sensors. In these cases a preferred implementation is a local low power wireless technology that covers the distance between the device and a gateway connected to the existing infrastructure or the cellular network.

Wireless Technologies Options

Devices are installed in tough industrial settings, mines, tunnels and vehicles. These applications require a secure and reliable transfer of measurement and control data, putting special demands on the wireless solution. No one wireless technology offers all of the features and strengths that fit the requirements of various application scenarios. The main requirements can be high data throughput, robustness or low power (the latter especially for battery operated devices). WLAN is often used in data acquisition where high data throughput or infrastructure connectivity is needed. Bluetooth is often used for HMI, programming, service / maintenance as well as real-time control tasks. During the last few years, other technologies like IEEE 802.15.4 (ZigBee, Wireless Hart etc.) and Bluetooth low energy have become increasingly used for sensors, actuators, HMI and configuration.

Wireless Coexistence

When more than one wireless technology is used in parallel, there could potentially be disturbances so it is important to optimize coexistence of various wireless technologies in order to get a disturbance-free operation.

WLAN uses 13 overlapping 20 MHz wide channels, two to three non-overlapping channels and operates with Direct-Sequence Spread Spectrum (DSSS).

In Classic Bluetooth, the 2.4 GHz radio band is divided into 79 hopping channels of 1MHz each where a new channel is chosen every 625 µS. Bluetooth low energy uses frequency hopping on 40 2MHz channels at the same hopping rate. Both Bluetooth versions apply the Adaptive Frequency Hopping (AFH) feature which detects potential channel interference; for instance, a WLAN 802.11 b, g, n device transmitting in close proximity. If such interference is found the channel is automatically blacklisted. These blacklisted channels are later re-tried in order to handle temporary interference.

IEEE 802.15.4 has 16 channels with a bandwidth of 5 MHz and is using Direct-Sequence Spread Spectrum (DSSS).

Enhanced WLAN Coexistence Possibilities

As you see above the 2.4 GHz frequency band is very crowded. This is especially true for WLAN which is well-established throughout offices on to the production planning. In order to get disturbance-free communication it has to be secured that WLAN is not disturbed. Another solution is to use the 5 GHz band (IEEE 802.11a) for the WLAN communication links. The 5 GHz band is increasing in popularity in industrial applications.

With frequency planning, it is possible to allow multiple technologies to operate in the same band. If one wants to use WLAN and IEEE 802.15.4 in parallel there is room for a smaller number of IEEE 802.15.4 channels in-between the three non-overlapping WLAN channels in order to get a disturbance-free configuration and there is also enough room enough for a high number of Bluetooth channels. It is also possible to beforehand choose channels that are not to be used (black listing, as mentioned above) in order to avoid interference with other wireless systems used in the same environment.

The Bluetooth Specification coexistence mechanisms cover most potential interference issues; however, measurement and data acquisition applications need assurance that Bluetooth will not disturb the WLAN communication during Classic Bluetooth service discovery and connection setup when AFH is inactive. To solve these potential interference issues, the Bluetooth Low Emission Mode optimally configures how often and how long a Classic Bluetooth product is to conduct inquiry and scanning, paging and page scanning respectively. This mode minimizes the impact on other wireless systems without losing stability on the Bluetooth link setup. Contrary to Classic Bluetooth, during the scan no radio transmission is sent, which means whereby Bluetooth low energy does not interfere with the radio environment. 

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