SIEMENS

The automatic wristwatch has been showing us how it’s done for decades. Our environment is full of energy sources that can be used to drive equipment. In the case of time pieces, a sophisticated mechanism uses the wearer’s body movements to wind the spring. The watch runs entirely without a battery, but only if it’s moved often enough.
A similar idea is behind self-powered radio sensors, which are currently making headway in industry. They take all kinds of measurements, transmit them via radio to a control room, and obtain their energy by tapping directly into environmental sources, such as light, vibrations, temperature gradients, and air flows. All of this is referred to as energy harvesting because special converters harvest ambient energy, generate electricity, and store it in a capacitor until enough of it is accumulated to power the sensor.
Self-powered sensors offer many advantages. They are easy to install because they eliminate the wires required for transmission of signals and for the supply of power. And because there is no need to replace batteries, they are practically maintenance-free in operation, while demanding nothing of the environment.
That saves real money, particularly in complex industrial applications. "The use of wired sensors is expensive, especially if a large number of devices have to be connected, which is the case in large plants such as steel or paper mills," says Dr. Leif Wiebking, a sensors specialist at Siemens Corporate Technology (CT).
"Installation often requires more than an hour for each individual sensor, compared to only between five and ten minutes for radio sensors," he adds. In many cases, use of radio sensors can obviate the installation of hundreds of meters of wire — wire that in an industrial environment can be damaged, leading to additional costs over the service life of the sensor.
The trend is therefore away from wire and toward wireless data transfer — a trend that will become increasingly important in the years ahead. Market researchers at ON World expect radio sensors to experience average annual growth of around 77 % and predict that their worldwide market volume will total $3 billion by 2011. They consider industrial automation, building automation and control, and the status monitoring of machines to be attractive areas of application. "And because radio sensors make the most sense when both the power cord and the battery can be eliminated, self-powered devices will carve out a big piece of this pie for themselves," predicts Wiebking.
Wiring would be extremely expensive if it were used to modernize existing systems that are characterized by limited accessibility. That’s why sensors are the preferred method for modernizing such systems. It was largely due to such scenarios that Siemens was the first company to develop self-powered radio sensors that are tough enough for industrial use and require no wires. With these sensors, old systems can be monitored and utilized more effectively, and the cost of retrofitting is not so high as to nullify its advantages.
There are also many potential industrial applications. "Self-powered radio sensors are particularly attractive wherever something is moving or rotating, such as the industrial robots in the automobile industry," explains Prof. Leonhard Reindl of the Institute for Microsystem Technology at the University of Freiburg. "Wiring for such applications is very costly and time-consuming to install, and rotary motions require expensive sliding contacts," he adds. Another interesting potential field of application is in extreme environmental conditions, such as the presence of high voltages or the monitoring of areas that have to be protected against potential explosions.
System condition monitoring is another area that is tailor made for the use of radio sensors. "It makes a lot of sense to monitor the condition of machines for possible wear," says Wiebking. Damaged bearings are not a rare occurrence with electric motors. But if a motor fails, production comes to a halt. "If the drive system for the rollers in a steel mill or a roll in a paper machine fails, for example, damages of up to €100,000 per hour are not unusual," says Wiebking.
The condition of the bearing could be continuously monitored with the help of a sensor, allowing the wear to be recognized in time so that the component can be replaced while the machine is shut down for some other reason. This can be accomplished by bolting an electronic unit the size of a matchbox onto the motor. The unit analyzes the motor’s vibration spectrum roughly once a minute and sends the data via radio to a control unit. In the event of irregularities — if the machine is not running smoothly, for instance — the control unit issues an alarm so that a technician can check and replace the bearing either immediately are during the next scheduled outage. Such services have another major benefit: The radio sensor draws its energy from a machine’s vibrations. A piezoelectric converter continuously generates electrical energy from the vibrations and stores it in a capacitor during the period between measurements.