Wireless sensor technology is suitable for practically any sensor application and offers numerous benefits. Matt Nicholas, software engineer at Mantracourt Electronics, takes a look at the real-world uses for the technology and how users can get the best results
There are many benefits to wireless sensor technology. Not only is it portable, but it requires minimum wiring, can be placed on moving parts, and can be located in areas that are difficult to access. Furthermore, low power functionality means the battery life for the technology now runs into years.
The range of low power wireless systems will, however, vary depending on a number of factors. Although it is possible to configure some battery powered wireless systems so they have a range of several kilometers, this may significantly reduce the battery life of the device. In addition, as with any RF signal, concrete and brick walls can impede signal strength. 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 a device with an external antenna, for example.
Environments with metal cladding or ironwork could also impact the range of the wireless sensor. In some instances the reflection caused by the metal cladding will help propagate the radio signal, but in other instances the reflections might cause a negative cancellation effect.
Achieving best performance
The first step to achieve the best performance from a wireless sensor installation is to undertake a site survey, if possible. By first locating a sensor and ensuring it is transmitting, it is possible to explore the site using a receiver. The sensor vendor may be able to supply a suitable USB enabled receiver and software, enabling the recording of signal strength and data capture using a laptop. This will enable dead spots to be identified and avoided when planning the overall layout.
The best range will 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, 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. While these networks can offer a higher level of sophistication, greater range and bigger self-healing networks than their non-mesh counterparts, this is at the expense of them being power hungry, lower resolution, and far more complex to configure.
Low power devices that use transmission formats such as 2.4GHz 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. In rare cases, external signals might cause a ‘jamming’ effect, however this can easily be detected by the system and an alarm raised.
Wireless devices do not need to be located near a power source, and it is even possible to locate wireless sensors on rotating machinery or mobile lifting equipment. Such sensors will mostly be powered by a small internal battery and, as the sensor spends most of its time ‘asleep’, periodically waking up to take a measurement then transmit the data, a long battery life is achieved. 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, meaning a battery life of several years is achievable. Alternatively, solar panels or an energy harvesting system located close to the sensor or repeater could be used.
On some occasions, the sensor and transmitter circuitry will be need to be protected by a metal enclosure. Sometimes an IP rated box may be needed to protect the electronics if the device is to avoid water or chemical ingress when in a process control environment or if exposed to the elements.
Either way, however, 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 fibreglass; the other 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 software package which 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.
Equipment is also available that will collect the data on-site using a DAQ, 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 – from anywhere in the world, or even on a mobile phone.
It is now possible to design an entire process control system using wireless sensors and actuators.
Wireless sensors available today can measure linear movement, wind speed, temperature, loads and even torque. Whatever the challenge, however, it is worth contacting a suitable vendor and discussing the project with their design team.
Despite the fact that wireless sensors may cost a little more than their wired counterparts, there are major savings in the cost of installation and benefits in the ease of re-use and flexibility that they can afford.
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