Not only that, but the more cables there are, the more complex the wiring. This in turn means a far greater potential for system hazards and errors. However, sensors that require no power or signal cables can avoid all of these problems.
Research carried out by the BRE and Satchwell Control Systems has examined this 'cordless' sensor technology. The result is the development of a compact, inexpensive sensor that complies with all regulations, and will operate maintenance-free for ten years, with data being transmitted every one to five minutes.
Variants of such a sensor could potentially be used to monitor such variables as illuminance, humidity, pressure and occupancy.
Cordless technologies
The two key technological hurdles to be overcome are how to power a cordless sensor and what means of communication should be used. How the sensor system should be organised is a further consideration.
The trends that make cordless sensors possible are much lower power consumption by electronics, higher performance batteries and more efficient methods of communications.
Several related battery-powered devices suggest the potential for cordless sensors:
Power sources
Possible power sources for cordless sensors fall into three categories: stored, environmental and beamed.
Modern lithium cells will function for up to 15 years at low load. One existing temperature sensor will run for two years using such a battery, and future sensors may well run for longer.
However, some sensor applications require more power than this and a longer maintenance-free period would be of value.
Ambient energy sources may be used to charge batteries to power a sensor. Small photoelectric cells generate enough electricity under typical light levels to power a sensor.
Other possible sources of energy are temperature differences, harnessed through the Peltier effect, magnetic fields from the mains or vibrations and noise converted using a piezoelectric device.
Power may also be beamed to a sensor in the form of radio waves, light, infrared radiation or ultrasound.
Radio power is limited by regulations. One possible way to obtain a higher power density is to use a phased array transmitter which can beam radiation in a chosen direction. This could be used to efficiently beam power and communicate with sensors at greater range.
Means of communication
Cordless sensors need a means of communicating data to an outstation. Autonomous sensors have a permanently active clock and program which consumes extra power and causes problems with synchronisation of different sensors. Other sensors require a trigger signal from their controlling outstation to instigate action.
When an outstation communicates with several sensors, a multiplexing mechanism is required. One way to do this is to use a different radio frequency for each sensor. This is efficient, but needs expensive hardware.
Another method – using a single radio frequency – is to encode each sensor with digital address information, and transmit the address information in order to interrogate a specific sensor.
Alternatively, each sensor can be programmed with a distinct delay time for response, and all of the outstations' sensors can be interrogated by broadcasting a 'wake up' signal to all of them.
Cordless systems do not impose any major limitation on the type of information being communicated. Outstations can translate measured data from sensors to standard formats.
Remotely-powered sensors
Remotely or environmentally-powered sensors derive their power entirely from external environmental or transmitted sources, and do not need a battery. Every cordless sensor that uses more power over its lifetime than it can store has common features.
A generator charges an electrical storage device which powers the sensor. The sensing element produces an analogue signal, which is encoded and used along with an address to modulate a transmitted signal.
A remotely-energised radio sensor uses a 'wake up' circuit based on a simple crystal set radio using a germanium diode. Having received a signal, the sensor decodes it and responds according to its program. This could be to delay a fixed time and transmit its measured data.
In general, buildings are broken down into zones which cover a relatively small area within range of communication of a single outstation. This may be one room or one floor of a building depending on the type of system.
The outstations may be connected in a conventional manner, or through similar cordless mechanisms.
Systems using several cordless sensors need to be consistent, and to remain so when new sensors are added. This involves a central record of identification information for sensors, including addressing and timing information.
Regulations and health issues
British regulations governing the use of the radio spectrum are quite strict in order to prevent radio interference and health risks. The limited frequencies available restrict the options for radio cordless systems, and discourage the use of broadband techniques.
The power restrictions limit the range of cordless systems, while interference with other electronic devices and between adjacent cordless systems is a problem which needs to be carefully addressed.
For the purposes of communication between cordless devices in a zone of a building, powerful transmitters are not needed. Moreover, the transmitters are only used intermittently, reducing mean power densities considerably.
There is more concern over the use of radio frequency energy to power devices, as the mean power densities needed are much higher. If unbeamed radiation is used to power devices at a distance, the transmitter must be quite powerful to generate adequate electrical power at the receiver.
Cordless sensors do have some disadvantages. The hardware is more expensive, but the costs associated with wiring offset this to some extent. They are more bulky, particularly when using radio communications, but a conventional sensor has the added bulk of its wiring.
Cordless sensors also have limited data transmission rates (though probably high enough rates for the applications being considered here).
The communication range of cordless sensors can be limited by attenuation of the signal. However, advanced radio communication techniques currently being investigated may considerably increase the range of low power radio communications.
Most cordless sensors need battery power. The need to frequently replace these batteries is the biggest obstacle to their application. The next stage in development will be to try and eliminate batteries by exploiting developments in lower power electronics and signal processing.
Source
Building Sustainable Design
