Wireless motion detectors have low power consumption

Motion detectors have become ubiquitous in homes and workplaces as a form of protection, and as a means of triggering light sources indoors and out. However, these detectors need wires to function, making their installation laborious as well as costly. The good news is that this is all about to change. ABB, in cooperation with one of its partners, has developed a wireless motion detector.

Figure 1: The wireless motion detector.

Using embedded system technologies, not only have the wires disappeared but engineers have produced a flexible, reliable, compact and inexpensive device that can operate for at least five years using just standard alkaline batteries.

Low power consumption was one of the fundamental demands of the detectors and it had to be provided without compromising reliability.

This was met using low power-consuming components and embedded system technologies.

Electromagnetic susceptibility, which increases with a rise in system impedance, also had to be addressed. To avoid this, all sensitive signal lines were kept short.

So, not only have the cables disappeared making installation easier, but the end result is a reliable and inexpensive motion sensor that consumes, on average, less than 20 µA.

In addition, it allows for integration into the existing Busch-Jaeger product portfolio in terms of cost, design and performance.

To carry out the different functions of the detector, three interconnected modules have been developed - the sensor, radio and controller modules.

The sensor module is the most delicate part of the system as it contains the passive pyroelectric infrared sensor (PIR) that captures the monitored movements and transforms them into infinitesimal electrical signals.

Because small signals are particularly prone to electromagnetic interferences, they need to be amplified as close to their source as possible.

Figure 2: On the sensor module, the amplifier and the PIR sensor have been mounted in a way that avoids electromagnetic interferences. a) front fitted with PIR sensor; b) back with electronics and amplifier.

Therefore, an ultra-low power amplifier has been mounted directly beside the PIR sensor connections.

The sensor has been taken from the Busch Watchdog Professional product series and adapted to the power consumption requirements of the new device.

The radio module ensures communication between the wireless motion detector and one or several actuators (ie, radio-controlled light switches).

To fulfil the low power consumption requirements in this module, only one-way communication from the motion detector to the actuator is used to avoid permanent reception readiness.

A typical transmission takes place in the ISM band (industrial, scientific and medical) at 868 MHz (KNX-RF protocol) and a range of up to 300 m is possible in the open.

The radio and sensor modules are mounted on the controller module that also contains a power supply and provides the means for background illumination sensing and parametrisation.

Its core component is a mixed-signal microcontroller that processes all sensor signals and system parameters, finally generating the binary presence signal that is transmitted to the actuator by the radio module.

Figure 3: Controller module fitted with three sensor modules and one radio module.

The microcontroller supports several power down modes that allow the temporary shutdown of individual processor functions, and their subsequent recovery, within a few microseconds.

By operating only the necessary circuitry needed to perform a particular function, additional - in fact substantial - power savings are achieved.

Ultimately, the success of a wireless appliance depends as much on its design as it does on the choice and availability of an appropriate power supply. Users demand energy sources that are cheap and compact, and which function for a very long time.

Therefore, a variety of power supply solutions were investigated as to their suitability in a wireless motion detector.

Some of the criteria that had to be met included:

• A potential source should function uninterrupted for at least 10 years;
• It must be able to deliver an average current of 20 µA and sustain current peaks of 25 mA. These typically occur during radio communications;
• The temperature range should extend from at least -20 to +60°C.

Six possible candidates are listed in the box.

Power supply possibilities for the wireless motion detector.

Batteries constitute the most obvious solution. Four 1.5 V alkaline cells ensure a lifespan of five to seven years, just a little short of the 10-year goal.

Moreover, this option is very cheap and batteries can be acquired virtually anywhere.

The downside, however, is their limited temperature range and self-discharge rate, which happens to be quite significant.

To overcome these disadvantages, the more expensive but longer lasting lithium ion disulfide cells (Li-FeS2) could be used instead.

A power supply based on solar cells is ideal for the wireless motion sensor.

Solar cells constitute an ecological alternative to a battery supply. They require no maintenance or replacement and are particularly suitable for self-sufficient working.

In fact, a solar cell-based power supply prototype for the wireless motion detector has already been designed by ABB engineers.

Photovoltaic cells produce most of their energy during daylight, and therefore some energy storage is required for night operation.

Figure 4: Principle of the solar cell supply. Solar cells produce power during daylight and additional energy storage for night operation is provided by C1 and C5.

The electric double layer capacitor (EDLC) stores the energy generated by the solar cell during the day.

These capacitors, also known as Gold Caps, deliver energy densities that are 300 times that of conventional capacitors.

They can be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred, or at most a thousand, recharge cycles.

However, loading the EDLC can take several hours. Therefore, another smaller capacitor is normally connected in parallel to reduce the initial start-up time of the powered device.

The solar cells consist of amorphous silicon. This type is much cheaper than crystalline cells and its effectiveness does not depend on even illumination.

This is important because motion sensors are often operated in partially shaded locations. With the above solution, a cell of size 57 x 50 mm suffices to reliably power the wireless motion detector.

Thermoelectric generators use the Seebeck Effect to create power from temperature differentials. The effect is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference exists between those points.

This effect is usually very small, but recent generators have achieved as much as 20 µW on a single chip with a temperature difference of 5°C.

To apply the thermoelectric principle to the wireless motion detector, a sufficient temperature gradient must be attained inside the device.

While solar energy could be used to this end, thermoelectric generators are not yet suitable for indoor applications.

A fuel cell is an electrochemical device similar to a battery, but it is designed to continuously replenish the reactants consumed.

In other words, while a battery has limited internal energy storage capacity, the fuel cell produces electricity from an external fuel supply of hydrogen and oxygen.

Research has shown that the development of fuel cells for portable computers and mobile phones is possible, but high costs and a limited lifespan currently prevent their use in wireless motion detectors.

With piezoelectricity, certain crystals generate a voltage in response to an applied mechanical stress, for example one generated by wind or by any other form of thermal airflow with the aid of the von Karman Effect.

This principle, which notably describes the flapping of a flag, states that flow around a bluff body will generate vortices on alternate sides of body.

The effectiveness of this method, however, is insufficient for powering the wireless detector.

Wireless energy transfer works by transmitting electromagnetic energy from an external power source to the receiver.

This would require the mounting of an expensive and space-consuming emitter coil in the actuator.

In any case, customer acceptance of such technology is still low.

The wireless detector combines reliability with ultra-low power consumption. It can be placed anywhere and is easy to install.

Normal off-the-shelf alkaline batteries ensure a lifespan of at least five years and this is extended to more than 10 years when lithium ion disulfide cells are used. The detector complies with all current EMC regulations.