Watt balance: redefining the kilogram

The prototype kilogram – the standard for today's kilogram – is getting a little long in the tooth. There are plans to redefine the unit on the basis of a natural constant. At the Swiss Federal Office of Metrology (METAS), the project is being realized by means of electrical units and with the aid of a watt balance.

METAS doesn't forecast tomorrow's weather; instead it realizes units of measurement such as the meter or the kilogram. The kilogram is at the focus of a new project. The goal is to redefine the unit which is still based on a physical standard dating back to 1889 instead of on a natural constant. The so-called prototype kilogram consists of a 39 mm x 39 mm cylinder made from a platinum-iridium alloy. It is kept in Paris. As a material object, this artifact changes over the years through deposits of dirt or through wear. Forty copies of the prototype kilograms were originally distributed around the world. One of them is kept in a vault at METAS. When the prototypes were compared with one another over the years, differences in the microgram range were found.

This uncertainty has led to the decision to define the kilogram, like other units, by means of a natural constant. The project will be realized by means of electrical units and involves the use of a watt balance. A watt balance is a highly complicated measuring device that needs to record several quantities with extreme accuracy. In cooperation with maxon motor, METAS in currently developing such a balance. A complex experiment achieves a precise comparison of mechanical and electrical power.

Experiment in high vacuum
One important function of the watt balance is performed by motors from maxon motor. The experiments for redefining the kilogram take place under conditions of high vacuum, which means that high demands are placed on the drives. Therefore, vacuum-compatible maxon brushless heavy duty motors (EC22HD, 100 W) equipped with MR encoders and the maxon EPOS2 24/5 positioning controller are used in this application.
To prevent the formation of particles, only brushless maxon motors are used. The system used is a 3-axis robot which performs various movements in order to carry out precise measurements during the experiment. Among other things, the maxon drives are responsible for the precise movement of a turntable and for the horizontal and vertical movements of various components of the robot.

 © maxon motor ag

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