Where a wired approach can present logistical and cost-related problems, such as when accessing remote locations or crossing roads, wireless DAQ offers an alternative. With a protocol such as Zigbee, for example, each measurement node can communicate wirelessly to other nodes and, via an Ethernet gateway, to a PC.
Clearly wireless DAQ has its advantages, but there are attendant pitfalls of which users should be aware. B&B Electronics has published a white paper, 10 Commandments of Wireless Communication, which identifies the most common pitfalls. These can be summarised as follows:
11. Since RF power attenuates as a logarithmic function, the dBm scale is most useful.
12. As frequency rises, available bandwidth typically rises, but distance and ability to overcome obstacles is reduced. However, lower frequencies require larger antennae to achieve the same gain.
13. Receive sensitivity is a function of the transmission baud rate so, as baud rate goes down, the receive sensitivity goes up.
14. RF background noise establishes a noise floor, the point at which the desired signals are lost.
15. Walking away from a newly commissioned wireless installation without understanding how much fade margin exists is the most common cause of wireless woes.
16. Contrary to popular opinion, no 'black art' is required to make a reasonable prediction of the range of a given radio signal.
17. In a clear path through the air, radio signals attenuate with the square of distance. To double the range therefore requires a four-fold increase in power.
18. Antennae increase the effective power by focusing the radiated energy in the desired direction. Using the correct antenna not only focuses power into the desired area but it also reduces the amount of power broadcast into areas where it is not needed.
19. Use high-quality RF (radio frequency) cable between the antenna connector and antenna and ensure that all connectors are of high quality and carefully installed.
10. Acceptable bit error rates are many orders of magnitude higher than for wired communications. Most radios quietly handle error detection and retries automatically, but this comes at the expense of throughput and variable latencies.
Avoiding these problems will help to ensure that the wireless DAQ application is successful from the outset and does not require costly modifications to optimise its performance.
Hardware and engineering costs
Although wireless DAQ hardware is more affordable than it was, say, five years ago, the total project cost can in some cases be higher than for a wired system because of the installation expertise needed; in a wired systems the engineer can see what is connecting to what. However, Tristan Jones, National Instruments UK & Ireland technical marketing team leader, argues that skills in standards like Bluetooth, Zigbee and IEEE 802.11 Wifi used on PCs are more widespread than industrial fieldbus skills, which can make a significant difference to the investment required for a wireless DAQ project: "Programming an API to a National Instruments wireless device is exactly the same as if you were using USB or plugging in a PCI data acquisition device. We want to make it as easy as possible for an engineer to implement wired or wireless DAQ.
"When you are talking about real-time process control, sometimes the best way to do it is to embed a wireless platform near to automation equipment like PLCs (programmable logic controllers). PACs (programmable automation controllers) are often referred to as the next generation of PLCs, with more functions and intelligence to handle I/O modules to measure parameters like strain, temperature, and analogue inputs and outputs.
"Instead of taking measurements and pulling them back into a centralised PID (proportional-integral-derivative) loop to control things like temperature, conveyor belt speed or whatever, it may be better to provide local optimisation on a PAC platform like National Instruments' CompactRIO (compact reconfigurable I/O).
"This has enabling FPGA technology which is highly parallel and deterministic, in a similar way to which the firmware on televisions sets can be updated over the air. In this way you can tackle things like optimising energy efficiency, for example, by coupling the vibration of a gearbox to accelerometers to analyse its harmonics and carry out predictive maintenance."
Where no mains or DC power is available, one of the main concerns with wireless DAQ is battery life. However, because the power requirement is relatively low, wireless DAQ designers are coming up with ingenious ways of harvesting the energy that is available 'for free' in some environments. For example, microgenerators can harness energy from vibrations on machines being monitored, then use this to power the sensor and communication functions of the sensor nodes. Perpetuum, a company that manufactures a range of microgenerators, was established from the University of Southampton (UK) in 2004 to patent vibration energy harvesting designs.
Perpetuum's magnetic resonator can be used with narrow-band vibration sources, such as electric motors. This device includes a thermal compensation mechanism to maintain the correct resonator frequency over the full industrial temperature range. For applications where the vibration source is wide-band, such as rail vehicles, the microgenerators have a broader response from which to optimise performance.
Air quality monitoring at process plant
Smart wireless technology has also enabled a cement plant in Colton, California, to comply with air quality emissions requirements by monitoring nitrogen oxide (NOx) emissions inside a rotating cement kiln. Emerson Process Management supplied Calportland Company with a system comprising four Rosemount wireless temperature transmitters, one wireless differential pressure transmitter and a smart wireless gateway. Calportland chose to use wireless data acquisition because it was seen as the best option for meeting the requirements for its challenging application. Steve Tyrrell, the company's senior electrical supervisor, explains: "The rotation, extreme temperature and the location of the kiln 6-12m above ground made using a wired solution impractical. With a rotary kiln, continually adding process variable instrumentation to optimise control can become overwhelming.
"Wireless allows the movement of process indicators to various positions on the kiln. The minimal maintenance requirement of the wireless option also ensures reliability of the process signals for greater process control. We thought this was a perfect opportunity to apply wireless."
The kiln is 165metres long, four metres in diameter, rotates almost twice per minute and operates at temperatures as high as 1500 ° C. Within this Calportland uses a selective non-catalytic reduction (SNCR) process of spraying ammonia into the kiln to control NOx emissions. To optimise the process requires the temperature of the ammonia and process gases to be monitored, as well as the kiln's slight vacuum. Calportland had tried using a slip ring around the kiln to check these parameters, but the ring failed due to wear and thermal expansion of the kiln that damaged the insulators isolating the process signal.
Tyrrell is very pleased with the wireless DAQ system, commenting: "By installing this wireless network, we were able to monitor and treat the NOx in the kiln successfully when there was no other alternative. Wireless was a brilliant option for our project. This has allowed us to comply with the NOx emissions regulations and improve control over the process."
Installation of the four devices was completed in one day. The pressure transmitter was installed on the injection shroud to measure the extremely low vacuum inside the kiln, while the temperature transmitters were installed at different locations around the kiln.
The self-organising network transmits signals reliably to a smart wireless gateway despite the devices being installed at opposite sides of the kiln. At times the line-of-sight transmission path between devices and the gateway is blocked but, despite this, no data is lost. The gateway is integrated with the facility's existing PLC control system.
Patient monitoring with ultra low power wireless
In complete contrast to the industrial process control applications for wireless DAQ, medical applications can also benefit (see panel). An ultra-low-power wireless technology from Toumaz technology is an example of this, as co-founder Keith Errey explains: "One simple and well-known way is to use ambient light energy. For example, our product can run on silicon solar cells. Very-low-power wireless is undoubtedly leading to significant new opportunities for the development of revolutionary devices."
Whether the application is medical, industrial or for use in almost any other environment, creating networks for sharing data across different communications technology standards can be costly and time-consuming. There are vast numbers of wireless sensors and other embedded systems on the market today, many of which have their own operating systems, programming languages and hardware peculiarities. Network developers have traditionally had to write customised software to enable data to be shared between these devices.
But now the Hydra project, which is co-funded by the European Commission, is developing middleware for networked embedded systems. Hydra middleware enables developers to incorporate heterogeneous physical devices into applications by means of easy-to-use web service interfaces for controlling any type of physical device irrespective of its network technology - such as Bluetooth, RF, Zigbee, RFID (radio-frequency identification) and Wifi.
ICT Results showcases EU-funded ICT research and activities such as Hydra. Markus Eisenhauer, project co-ordinator at the Fraunhofer Institute for Applied Information Technology, is reported as saying: "Hydra aims to reduce the complexity by developing a service-oriented middleware. It will help manufacturers, software developers and systems integrators to build devices that can be networked easily and flexibly via web services to create cost-effective, high-performance solutions."
With Hydra, devices such as electricity meters, televisions, refrigerators and stereos, as well as heating and lighting systems, can be networked without having to know what goes on inside. "We have some prototypes and demonstrators running where we have used an ordinary Playstation 3 as a home control centre," comments Eisenhauer.
Agriculture can also benefit from Hydra. In one trial, pigs were fitted with RFID tags so their movements could be tracked. Eisenhauer explains the reasoning behind this: "We can locate each pig in the shed or outside and we can use this to control the heating and ventilation system. If the shed gets too crowded the temperature rises and then the heating system responds to that."
In Hungary a Hydra trial group suffering from chronic diabetes is able to test their blood glucose content with a sensor connected by Bluetooth to a mobile telephone. An application on the telephone automatically forwards the data to middleware on a central server. If the glucose levels are poor, the middleware can send advice to the patient, notify doctors and nurses, or - where there is extreme danger - call an ambulance.
Given these developments in middleware, user-friendly programming for wireless sensor networks and cost-effective energy harvesting devices, it is no wonder that designers of consumer products, medical devices, plant and machinery are all starting to take advantage of wireless DAQ. In many cases a wireless system is the only feasible option, while in others wireless is becoming a competitive or even lower-cost alternative to traditional wired systems.
Wireless DAQ benefits non-intrusive medicine
Imperial College in London is to start clinical trials of an ultra-low-power, ultra-small 'digital plaster' that monitors vital signs wirelessly. Volunteers and patients will be provided with a wearable Toumaz Sensium 'digital plaster' that can continuously monitor temperature, heart rate, respiration and other vital signs. Based on a non-intrusive, real-time, low-power wireless monitoring system supplied by Toumaz, the focus of the trial will be on verifying that the physiological data acquired by the digital plaster system within a clinical setting is equivalent to that acquired using current gold-standard monitors in hospitals - equipment that is often expensive, bulky and fixed, such that patient mobility is impaired. In contrast, the digital plaster is a disposable device powered by thin batteries that give a working lifetime of several days.
Dr Stephen Brett, a consultant in intensive care medicine at Imperial College Healthcare NHS Trust, an honorary senior lecturer at Imperial College London and the person leading the trial, explains: "This technology has the potential to improve the capturing of patient's vital signs within all areas of the hospital - enabling key physiological data to be acquired at an increased frequency, with the minimum of inconvenience to patients, and without the requirement to connect patients to immobile pieces of equipment.
"This raises the possibility of technology improving hospital safety systems, enhancing the efficiency of adding vital sign data to patient records, and potentially freeing valuable nursing staff time for other patient care responsibilities."
Professor Chris Toumazou FRS, founder and chief executive of Toumaz Holdings, and director of the Institute of Biomedical Engineering at Imperial College London, adds: "Once proven in a hospital setting then the digital plaster can be deployed in the home in the same way."
As well as the digital plaster described above, very small implants can also be equipped with wireless technologies. Since the first Zarlink Pillcam video capsule was approved in the USA by the FDA in 2001, more than 500,000 patients have swallowed Pillcam video capsules, with capsule endoscopy reported as being more accurate than traditional invasive methods for diagnosing disorders of the small intestine.
More recently, in November 2009, Zarlink Semiconductor announced that its custom RF (radio frequency) integrated circuit is being used in Given Imaging's new Pillcam Colon 2 camera capsule for wireless examination of the colon. Launched at the Gastro 2009 meeting in London, the Pillcam Colon 2 is due to be marketed in Europe from 2010.
Kevin Rubey, chief operations officer at Given Imaging, says: "Colon 2 is a patient-friendly capsule endoscope that may complement traditional procedures to visualise the colon in patients who are unwilling or unable to undergo colonoscopy."
The Pillcam Colon 2 capsule, which can be easily swallowed by the patient, contains a tiny camera at each end, a light source and Zarlink's ultra-low-power RF transceiver. Images are transmitted from the Pillcam Colon 2 to an external data recorder as the capsule passes naturally through the patient's colon