Development of an intelligent digital integrator for long-pulse operation in a tokamak

A new digital integrator with high input voltage and a long-time low drift speed has been developed in JT-60. This integrator uses an integrating method with a pair of a voltage-to-frequency converter (VFC) and an up and down counter (UDC) (VFC-UDC unit). However, this method has a saturation of the VFC for large input, leading to integration error. To improve it, the new digital integrator is composed of three sets of VFC-UDC units in parallel and a digital signal processor (DSP). Three VFC-UDC units with different input ranges integrate an identical input signal respectively, and the DSP selects a suitable integrated signal among three integrated outputs at a sampling frequency of 10 kHz and makes a chain of integrated signals. The performance of the integrator has been tested using a disruptive discharge in JT-60. A good integration result has been obtained though large signal is input. Also, fundamental characteristics of the VFC, linearity, thermal drift, dead band, which are causes of integration errors, are described.