A measurement system for the currents of a 3-phase motor has been developed. In order to transfer the information through optic fibres, a sigma-delta modulator loop has been designed. The measurement point compares the mean value of a pulse train from the system controller and sends the difference information back to the controller via another pulse train. This reduces the analog part of the current measure to a minimum.
The pulse train of the system controller is realized by a high-order sigma-delta modulator. The study has dealt with the selection of a robust structure, the calculation of coefficients for a close to linear phase signal transfer function, the analysis of the signal and error transfer functions and the implementation of the modulator in programmable circuits.
A comprehensive design environment has been developed in Python - http://www.scipy.org. The VHDL code is automatically generated and a testbench allows to veryfy the proper functioning of the system. The modulator has been inserted in the current measurement loop and has shown to function as required.