Optical detection of gaseous carbon dioxide, water vapor (humidity), and oxygen is desired in Portable Life Support Systems (PLSS) incorporating state-of-the-art CO2 scrubbing architectures. Earlier broadband detectors are nearing their end of life, and recent advances in laser diode technology make replacement of earlier technology compelling. The function of the infrared gas transducer used during extravehicular activity (EVA) in the current spacesuit is to measure and report the concentration of CO2 in the ventilation loop. The next-generation PLSS requires next-generation CO2 sensing technology with performance beyond that presently in use on the Shuttle/International Space Station extravehicular mobility unit (EMU). Accommodation within spacesuits demands that optical sensors meet stringent size, weight, and power requirements. A sensor is required that is compact, low power, low mass, has rapid sampling capability, can operate over a wide pressure range, and can recover from condensing conditions.
The first version of a laser diode (LD) CO2 sensor developed for this purpose was based on wavelength modulation spectroscopy (WMS), and incorporated oxygen and humidity measurements. This work reports on a new version of the sensor (Ver. 2.0). New features include significant redesign of the sensor digital electronic firmware and software, and an innovative implementation of the sensor calibration over a wide pressure range, which results in a simpler calibration procedure and shorter response time.
To simplify the sensor calibration procedure and eliminate external computational software, the following innovative approaches and changes to the hardware, programmable logic firmware, and microcontroller software have been implemented in the new version of the PLSS CO2 sensor (Ver. 2.0). First, pressure-optimized waveform parameters are used to decrease the pressure-induced variance in the WMS optical absorption signal. This is implemented on the sensor’s programmable logic device (field programmable gate array, FPGA), which contains the optimized parameters tabulated as a function of the total pressure. The waveform parameters are adjusted in real time so that the pressure dependence of the optical absorption signal is effectively compensated, which eliminates the need to implement the complicated pressure calibration.
Second, a new digital communication channel from the microcontroller to the FPGA is created. It is used to transmit the pressure data acquired from the PLSS pressure transducer to the FPGA, which in turn uses it to generate the pressure-optimized waveform. Third, an autonomous digital pressure sensor is implemented on the FPGA, which may in principle be used as a backup to the main pressure sensor of the system, providing pressure measurement redundancy. Fourth, the microcontroller software was restructured to manage onboard calibration, using a cyclic redundancy check (CRC) for communication error detection, ensuring faster response time. The implemented innovations resulted in a four-fold increase in the sensor response time and removal of external processing software.