Getting the best from Wireless Instrumentation
Flexibility, ease of installation and operation without the need of external power has enabled wireless sensor technology to broaden the reach of instrumentation.
Wireless is suitable for practically any sensor application, its portable, requires minimum wiring, can be placed on moving parts and located in areas difficult to access. Low power functionality has moved the battery life of this technology from weeks to years of operation. This article looks at the real-world use of this type of wireless technology and how to get the best results.
Mantracourt T24-WSS Wireless Wind speed sensor
Ensuring Wireless Coverage
With low power wireless systems the range will vary depending on a number of factors. You can configure some battery powered wireless systems to have a range of several kilometers. The impact of this is a significant reduction in the battery life of the device.
As with any RF signal, concrete and brick walls can impede signal strength and antenna type can also have a big influence. A system with an on board antenna will be robust and compact but will have a shorter range than say, a device with an external antenna. Environments with metal cladding or ironwork may, or may not, impact the range of the wireless sensor. In some instances the reflection caused by the metal cladding will help propagate the radio signal, in other instances the reflections might cause a negative cancellation effect.
To achieve the best performance from a wireless sensor installation, the first step, where possible, is to undertake a site survey. By first locating a sensor and ensuring it is transmitting, it is possible to explore the site using a receiver. The sensor vendor may well be able to supply a suitable USB enabled receiver and software, enabling the recording of signal strength and data capture using a laptop. Any dead spots can soon be identified and avoided when planning the overall layout. The best range will be almost certainly be achieved when the sensors and receiver are located well above ground level as a large proportion of the signal can be absorbed by the ground; it is worth keeping this in mind when choosing the location of both the transmitter and receiver. When additional coverage is required, then it is possible to use a 'repeater', which will help extend the range and deal with obstacles and transmitting around corners.
Some wireless systems employ mesh or star networking technology. These networks can offer a higher level of sophistication, greater range and bigger self-healing networks than their non-mesh counterparts, but this is at the expense of being power hungry, lower resolution, and far more complex to configure.
Avoiding Channel Conflict and Interference
Surprisingly, low power devices that use transmission formats such as 2.4 GHz are remarkably tolerant to the usual sources of local radio interference such as WIFI, DEC, Zigbee, and Bluetooth. The main challenge to the transmission of the data comes from multiple sensors transmitting using the same wavelength, particularly if they are on the same channel. At 2.4GHz, wireless systems are able to transmit on multiple channels, so channel conflict is rarely a problem. Systems with multiple sensors and repeaters may experience some mutual interference, but the data protocol will ensure that there is no loss of information, even if there is a temporary interruption in data delivery. As well as ensuring a clear channel is used, the rate at which data is sent can be easily configured to reduce the competition for bandwidth between transmitting sensors. A final thought on this topic is that in rare cases, external signals might cause a 'jamming' effect, however this can easily be detected by the system and an alarm raised.
Maximising Battery Life
A great advantage of wireless devices is that they do not need to be located near a power source such as a 13amp socket. Indeed, wireless sensors can even be located on rotating machinery or mobile lifting equipment. These sensors will mostly be powered by a small internal battery. A long battery life is achieved by the fact that a sensor will spend the greater proportion of its time 'asleep', periodically waking up to record a measurement and then transmitting the data. In many cases, it will be possible to configure the sample rate and transmission rates of the sensor to ensure that the necessary data is collected whilst minimising the demand on the battery. A battery life of several years is realistically achievable. An alternative approach can be the use of a solar panel or energy harvesting system located close to the sensor or repeater.
Enclosure Design and the RF Window
In some instances the sensor and transmitter circuitry will need to be protected in a metal enclosure, in order to protect it in an industrial environment. Sometimes, the electronics may need to be enclosed in an IP rated box to avoid water or chemical ingress when in a process control environment or if exposed to the elements. Either way, it is important to be aware that the radio signal requires an aperture through which it can escape. One approach is to create an 'RF transmitter window', made using a small panel of fiberglass. The other alternative is to fit an external antenna to the enclosure. It is also important to remember to tighten up any cable glands when connecting cables to sensors, transmitter and repeaters. A common cause of failure is moisture ingress (capillary action) due to this oversight.
Wireless sensors are capable of generating a lot of data. It is therefore important to ensure that the data collected can be stored and used in a useful format, one that can be suitably analysed or used for a specific purpose. Some wireless sensor vendors will provide a suitable software package. This software may provide the raw data in Modbus or ASCII formats. They may also provide a data logging functionality that will enable the presentation of the data in Excel, with CSV files and charts.
Alternatively, there is equipment that will collect the data on-site using a DAQ (Data Acquisition device) or the data can be periodically sent over the GSM network either as an e-mail or text message, enabling the data to be collected remotely (anywhere in the world) or even on a mobile phone.
Wireless sensors from Mantracourt are used during chocolate production
Overall Systems Design
It is now possible to design an entire process control system using wireless sensor and wireless actuators. There are now wireless sensors that can measure linear movement, wind speed, temperature, loads and even torque.
Wireless sensors may cost a little more than their wired counter-parts, but often there are major savings in the cost of installation and benefits in the ease of re-use and flexibility that they can afford.
This article was written Matt Nicholas, Software Engineer at Mantracourt Electronics
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