Gas monitoring station
Based on the PoF approach illustrated in Fig. 1, an optically powered gas monitoring station has been designed to measure CH4, CO2, CO and O2, as well as ambient temperature and barometric pressure in one device. The block diagram and assembly of the gas monitoring station are shown in Fig. 3.
In Fig. 3a, the sensor signal conditioning module incorporates four gas sensor cells and a single sensor for temperature and pressure. CH4 (MIPEX-02-1-II-1.1A) and CO2 (MIPEX-02-3-II-1.1A) sensors are infrared gas sensors cells while O2 (O2-A2) and CO (CO-A4) sensor cells are electrochemical. BMP280 digital pressure sensor is used to measure both ambient temperature and barometric pressure.
The conditioned sensing signals are then processed by an ultra-low-power microprocessor ATSAML21J18B which orchestrates all other actions from LCD display, touchpad, intrinsically safe LED strobe, power supply module, etc., into a Manchester encoded digital signal. The optical transducer receiving 1% of the incoming light converts the digital signal passively into an optical signal to be transmitted back to the RTU. All the electronics is powered by the power supply module and its circuit is designed in compliance with intrinsic safety standards (IEC 60079-0 2017; IEC 60079-11 2011).
The power supply module consists of a photovoltaic converter (KPC8-T) for 1300–1600 nm laser light as the main power source, a backup battery and its charge pump. 99% of the incoming light goes into the photovoltaic converter and is converted into a typical maximum voltage of 3.6 V at a power conversion efficiency around 30% to power the whole device through a voltage regulator. A 3.6 V battery (LS14500) is used as a backup power for temporary optical power failure and to power the LED strobe when/if triggered. When the supply voltage from the optical power is lower than 2.5 V, the battery will switch in and power the device via the charge pump circuit (instead of relying on optical power). Every minute the power supply condition is monitored, and as soon as the optical power is restored, it will replace the battery to power the device.
There are four modes of operation: 1) Warm-up mode, (2) Normal mode, (3) Alarm mode, and (4) Calibration mode.
When the gas monitoring station is turned on, it will start with a 60 s Warm-up mode. During the Warm-up mode, the powering of each sensor cell is sequenced to “level” the start-up power before the station enters Normal mode. The gas concentration thresholds can be set for ‘low’ and ‘high’ alarms for each gas sensor. When any alarm is triggered, the station will go into Alarm mode and the LED strobe goes off. In-situ calibration is supported allowing user to apply Zero and Span gas during which the device enters Calibration mode to avoid any false alarm caused by calibration gases. Information under each operation mode can be accessed both locally and remotely as light carrying the information back to the RTU enables real-time remote monitoring.
Based on this design, we fabricated two PCBs for each station. One dealing mainly with the optoelectronics while the second plays a more traditional role. The two are connected and housed in an IP54 enclosure accommodating the internal optical fibre layout and all user-facing external components as shown in Fig. 3b. The total power consumption of each station is less than 40 mW electrical power provided by 120 mW optical power.
Remote terminal unit (RTU)
The RTU is the heart of the whole system: it normally sits in the air-conditioned room, connects and sends light to each gas monitoring station while collecting all the information of interests to the end users. It is intentionally structured, with light source module sending light to each station, photodetector module converting the received optical signal back to electrical signal, and data acquisition (DAQ) module decoding the electrical signal into presentable format for webpage display and data storage (see Fig. 4). All modules share a 12 V DC supply and the whole RTU consumes less than 18 W. For a system with two gas monitoring stations, two light source modules are installed each dedicated to one station.
Light source module
We choose 1550 nm wavelength light and SMF for the system as they are commonly used in telecommunication with corresponding mature and inexpensive optical components. And as many industries already have optical communication networks in place, an SMF system offers a high degree of compatibility.
The light source is used both for powering and signal transmission. To power each station, at least 110 mW optical power is required; and for signal transmission, we need broad-band light with low noise at relatively low frequency (under 10 kHz) for sensing purpose. In addition, a protection circuit for overpower protection is required for intrinsic safety. To the best of our knowledge, no light source on the market can serve all these purposes, so we manufactured the light source, which was designed to be compact, reliable, powerful and, importantly intrinsically safe.
The photodetector module converts the optical signal into an electrical signal, then cleans and amplifies it. It also provides display and controls for user to both monitor and adjust the output voltage. The module is designed to measure up to 4 optical channels. They are powered by 12 V DC and draw around 170 mA of current. The intensity of the light from each optical input port is measured by a dedicated InGaAs photodiode and converted into a current. The DC and AC components of this current are separated and converted into voltages. The DC voltage represents the background light reflected back from the optical transducer and the AC voltage represents the signal carrying the digital information of interest. The AC voltage of each channel is coupled to a driving amplifier and the gain can be adjusted on the front panel to a level suitable for DAQ module.
The DAQ module is designed to decode and process the signal from the photodetector module. It is also powered 12 V DC and draws around 200 mA current. It supports an Ethernet interface and data logging. The information each sensor—in the form of 8 Manchester packets—are read and logged to volatile SDRAM memory every second and given a timestamp. The real-time data of each sensor includes: ID of gas monitoring stations, gas concentrations of CH4, CO, CO2 and O2, Ambient temperature, ambient barometric pressure, operation modes, optical power level, and battery level.
These data with timestamps can be shown graphically on a standard web browser on a PC or mobile device via an Ethernet interface. Every minute, the information of each station is logged in the µSD card on the DAQ board with timestamps. Data logs can be downloaded as.csv files from the home webpage.
Daisy chain configuration
Two topologies are supported in this optical network. Each station can connect to the RTU separately (star topology) or in a daisy-chained connection with other stations. For the daisy-chained network, N-core optical cables are needed for a system of N gas monitoring stations. As shown in Fig. 3b, each station has two optical ports—‘Light in’ and ‘Light out’. Light comes in from ‘Light in’ port and drops one optical core dedicated to one station while the other cores go on to ‘Light out’ port to be connected to the next station’s ‘Light in’ port. The daisy chain network saves cabling from the RTU and increases distance coverage. In our two-station system, the daisy chain configuration illustrated in Fig. 4 a shows that with the same lengths of cables, the chained device extends the coverage while still enjoys the dedicated SMF for power and data transmission. The complete system ready for field trial with two gas monitoring stations daisy-chained by two 2-core 100-m long ruggedised optical cables is shown in Fig. 5.