Integrated Sensors
One way of doing this is by identifying a selection of
sensors that are capable of satisfying a substantial portion of the market, then
integrating them along with the wireless sensor network transceiver. If, as is
frequently the case, the associated "actuator" in the application is just an
electrical switch, or information to be stored in RAM or passed on to a host
computer, the node may be completely integrated. However, this approach has some
difficulties:
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The sensors may be difficult to
integrate. Ideally, one would like sensors that are integratable on the same
die as the wireless sensor network transceiver, for a completely integrated
solution; however, even if this is technically feasible, it may drive the cost
of the fabrication process up to the point that it is not practical. An
alternative solution is to place the sensor(s) in separate dice but in the same
package as the transceiver — a method that attains much of the usefulness of
total integration, if not all the cost benefits. In any event, many sensors
still cannot be integrated at all.
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The system is inflexible. From the
user's standpoint, a purchased network node is application-specific; it cannot
be used for any other application. From the manufacturer's standpoint, if a
different sensor is needed, even one of the same type but differing in
specification (accuracy, response time, etc.), nothing can be done short of
designing and manufacturing a new integrated circuit, a time-consuming and
expensive task. Because the actuators in
this example are either simple switches or, in the degenerate case, information
itself, it is relatively easy to reprogram the nodes to perform multiple
functions based on the information received from the network (e.g., control the
switches in a different manner). Even here, however, significant limitations
exist. For example, the voltage and current capabilities of the switches in a
completely integrated design will be limited to those compatible with a modern
digital integrated circuit fabrication process. This means that, unless
extraordinary measures are taken, the current will be limited to a few tenths of
an ampere and the voltage to 3.6 V or so. This precludes the node from directly
controlling household appliances, for example, although a logical output from
the node can certainly control a semiconductor switch on a second die (made in a
high-current, high-voltage process) that, in turn, can control a household
appliance.
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Legacy sensors, perhaps those associated
with a previously installed wired sensor network, cannot be reused. In some
applications, for example, sensor networks in nuclear reactors, where access to
placed sensors is difficult, this is a significant handicap.
Despite this (rather foreboding) list, some integratable sensors
may find wide application. One example is a temperature sensor. Making a
moderately precise and accurate temperature sensor of limited range in an
integrated circuit is relatively easy; many possible designs are available, most
of which revolve around the temperature dependence of the voltage drop across a
forward-biased silicon diode, typically −2
mV/°C.[2] Such a sensor
may have wide application in areas as diverse as heating, ventilation, and air
conditioning (HVAC) as well as industrial control, where the actuator can be a
switch controlling a heater in a thermostat, for example, and in health
monitoring, where the actuator may be a display, or simply data stored in a
computer.
In most cases, however, it is not possible to integrate both
sensor and actuator in all wireless sensor network nodes used in an application.
This forces the node to have a transducer interface to the outside world. The
question then becomes how to design the interface to achieve the maximum
possible market penetration.
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