Advanced Floating Point PID Controller For Cryo-Cooler Temperature Control in Infra-Red Payloads

Infra-Red (IR) payloads are used on-board for measuring the thermal/spectral properties of objects. To reduce background and enhance Noise Equivalent Temperature Difference (NeTd), it is required to cool the detector (Focal Plane Assembly-FPA) and associated optics. Linear Sterling Cryo-Coolers are typically used for cooling of FPA. A diode temperature sensor is used to sense the cold tip (FPA) temperature. Long/Medium Wave Infra-Red sensors(LWIR/MIR) require tighter control of FPA temperature (Better than 15 mK (mili-Kelvin)). Since Diode sensor has a small output dynamic range of ∼500 mV (0.6 V to 1.1 V), therefore achieving stringent control is dependent on precise performance of design blocks (low noise current bias, high resolution temperature processing and PID Controller). Design of Proportional-Integral-Derivative (PID) controller plays a very crucial role in overall performance. Overshoot, settling time and stability are affected by PID design. Cooler Drive Electronics (CDE) is used to implement these functions. Truncation losses in a typical implementation of FPGA (Field Programmable Gate Array) based PID Controller leads to higher ripple. A Micro-controller based floating point PID controller is proposed to overcome these issues, thus enhancing the functionality to provide tighter control of Cooler temperature. In this paper we have discussed design and development of a Floating Point PID controller. A prototype (Enhanced CDE) was developed and tested in close loop with the peripheral sub-systems (Cryo-Cooler, Front End Camera Electronics(FECE), and Payload Power Supply(PPS» to demonstrate performance in actual payload environment. Short Term temperature stability of ~10mK RMS (Root Mean Square) and Long Term temperature stability of ~75mK (Peak to Peak) for Cryo-Cooler is achieved against the requirements of <15mK and <100mK respectively.