Current Measurement

As part of the »Performance Monitoring System – LEISTMON« project, a Hall-Sensor was integrated into a specially developed chip together with other analog and digital components. Highly integrated Hall-Sensors are capable of carrying out contact-free current measurement. A total of eight of these ASICs are arranged equidistantly in a ring around the electric conductor to be monitored with the aid of suitable packaging. Thanks to this symmetrical layout and use of the minimum number of Hall-Sensors, which is determined by means of simulation, unavoidable changes in conductor position during installation have only a minimal effect on measuring accuracy.

The measurement principle itself is – similar to the Rogowski coil – based on Ampère’s circuital law. Accordingly, the Hall-Sensors measure the strength of the magnetic field generated by the current-carrying conductor which is around the conductor, and the measuring system’s microcontroller calculates an approach of the discrete circulation integral via the eight measurement points and hence the actual current.

Using a special kind of the phase-locked loop – with patent application – a comparator can be dispensed with. The synchronization of the measurement to the mains frequency is already implemented in the firmware. By precisely synchronizing of the measuring rate to the mains frequency and the constant system clock of the microcontroller, the mains frequency can be accurately determined. All remaining values such as active power and apparent power are calculated from the existing measurements.

Technical Design

The starting point was a special 3D-Hall-Sensor cell, which was manufactured in a standard CMOS process. Additional components on the chip carry out the linearization, balancing, and compensation of disturbance variables. A simple serial interface allows several ASICs to be daisy-chained together and connected to a microcontroller. The chip’s dimensions were optimized such that the flexible conductor can have as narrow a bending radius as possible following assembly of the sensor chips. This permitted optimization of the overall terminal size.