The moving coil hangs from the weighing cell via a variable-length suspension. An integrated electrostatic motor varies the length of the suspension and allows the coil to move (see image below). Force measurements are carried out using a 10 kg Sartorius weighing cell with 10 µg resolution.
The electrostatic motor is composed of three parallel plate electrodes (see image below). The central electrode is grounded and fixed. The upper and lower electrodes are mobile. By independently modulating the voltages on the two mobile electrodes, a net electrostatic force is obtained and the electrodes move. The movement of the mobile electrodes is transmitted to the lower part of the suspension via three levers separated by 120°.
The BIPM watt balance