experiments [4], which is based on the system
originally designed for the rocket experiment [2]. While
developing a compact and portable ADR, we faced a
problem limiting the temperature controlled time in the
cooling cycle. Because heat capacity of the refrigerant
material in a ‘‘salt pill’’ is small for compact ADRs,
difficulties arises in keeping constant temperature for a
long time. Particularly, we found small residual temperature
difference between the aimed and measured temperatures,
which gradually increased in time when we
controlled the temperature of our ADR with the standard
PID (Proportional, Integral, and Derivative control)
method. The problem originates in the principle of the
standard PID which is naively applied to ADRs, making it
critical to keep stable temperature with a small refrigerator.
Bernstein et al. [8] have demonstrated quite steady control
of the ADR temperature by rejecting thermometry readout
noise and optimizing varying parameters which determine
dI=dt, although their method assumes the magnet current,
I, is sufficiently high. We have succeeded in solving the
problem with an improved PID method by adding a new
term in the standard PID considering the physical properties
of the paramagnetic salt, and demonstrated it with our
portable ADR system. In this article, we introduce the
outline of the improved PID method, and the experimental
results with our ADR system are presented.