Microcontroller of the power supply of a fast field cycling relaxometer

This paper describes aspects of the implementation of a digital control system to be used in a Fast Field Cycling (FFC) Nuclear Magnetic Resonance relaxometer (NMR). This technique is very demanding from the point of view of experimental equipment requiring a modulation of the magnetic flux density, and that, the magnetic flux density cycles between homogeneous, steady-state regimes, with switching times within 1–3 ms for most experiments with soft materials. The main purpose of this paper is the development of a digital control system capable of modulating the Zeeman field of a FFC-NMR relaxometer. The developed embedded system is replacing the analog controller currently used in the main power supply of FFC equipment. This is accomplished with the use of a Microchip dsPIC microcontroller and implementing some additional filters and driving on-chip peripherals to interface with sensors and power electronic devices. The software and hardware were designed to allow fast transitions as required in a FFC-NMR measurement. The experimental results show that transitions between the high polarization field and a low relaxation field are successfully realized fulfilling the FFC-NMR specifications.