Robust Control of a Two-Terminal Cryogenic Current Comparator

A digital $H\infty$ controller for a two-terminal cryogenic current comparator is designed. To this end, a set of mathematical models covering the actual system is proposed. Simulation results compare the open- and closed-loop systems based on the proposed controller and the traditional integral control. According to these results, the new controller can significantly reduce the noise in the superconducting quantum interference device sensor.

I. Introduction

The cryogenic current comparator (CCC) bridge is widely used in high-accuracy metrology. It is the chosen system for many national metrology institutes (NMIs) to calibrate standard resistors against the quantum Hall resistance. In this system, a superconducting quantum interference device (SQUID) sensor is used to fix the current ratio. A SQUID can detect extremely small changes of magnetic field, and it is a nonlinear device. This combination of characteristics can produce rectification, flux jumps, or saturation of the SQUID controller. These undesired effects can increase the standard deviation, produce systematic errors [1], or even impede the measurement. Since its origin, the SQUID stabilization and a low null error were obtained using an analog integral controller. Recently, more complex control strategies have been applied. For example, some NMIs have included digital filters in the current sources [2] or have used feedforward techniques [3] to decrease the effect of the time constants difference between the arms of a four-terminal CCC. In addition, an NMI has presented a dynamic model of the CCC and a control strategy [4]. In a more recent publication, a high-frequency control loop has been applied to decrease the effect of noise in the SQUID [5]. Aside from these works, little has been published on the dynamic model of the CCC and its control.