  As part of the Advanced LIGO+ (A+) project we have developed, produced, and
characterised sensors and electronics to interrogate new optical suspensions.
The central element is a displacement sensor with an integrated electromagnetic
actuator known as a BOSEM and its readout and drive electronics required to
integrate them into LIGO's control and data system. In this paper we report on
improvements to the sensors and testing procedures undertaken to meet enhanced
performance requirements set out by the A+ upgrade to the detectors. The best
devices reach a noise level of $4.5\times 10^{-11}{\rm m}/\sqrt{\rm Hz}$ at a
measurement frequency of 1 Hz.
