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The project

Anticipating that we would use our wireless expertise, our client asked ASH to design and demonstrate the technology feasibility of safely powering and remotely communicating with a microprocessor to be housed in a hazardous explosive environment which changes temperature through a 130-degree range and which, once fitted, is then inaccessible.  

Light, through a fibre, was the only energy source.

  • Safe data communication with a remote microprocessor to
    be installed in an inaccessible container also containing a potentially explosive atmosphere.
  • Operational Life: 30 years
  • Operational Temperature range: -50 to +85 °C
  • ATEX: <200 µJ stored energy,
    <150mW power transmission (even under fault conditions)
  • ESD: no conductive paths.
  • Minimal Bill of Materials cost.
  • Bi-directional use of multimode optical fibre
    and non-laser LEDs
  • Selection of optimal fibre and wavelengths
    for data and energy transmission
  • Power-up management to limit current consumption
  • Optimal fibre path geometry to intercept light energy separately from data
  • Met environmental constraints with 2Mb/s read
    200kb/s write data transmission speeds


Design Challenges

Key Design choices: Most commonly, remote sensing applications are served with a combination of low power radio, powered by batteries or possibly energy scavenging with backup battery. Because of the stored energy limit, any form of battery was inappropriate. The energy transfer/ESD limits meant that wired power was ruled out. This left scavenged energy, either by mechanical movement, light or inductive pick up. The unpredictability of the mechanical and radio frequency aspects of the operating environment meant that optical power transfer was the logical choice. 

Power over fibre: The cost requirements of the project (both in terms of bill of materials and manufacturing cost) and the operational temperature ranges meant that use of lasers was less desirable. This meant that the data requirements, limits in sensor transmitted power, and range necessitated careful selection of the optical fibre and LEDs used.   

Data communications both ways on a single fibre: The combination of non-coherent LED light sources, and the need for the transmission of power one way, and data both ways meant that multiple transmitters and receivers needed to be aligned optically into a single fibre Restricted energy: The need for circuitry to respond to power and receive data, make measurements, and to reply within a very limited energy budget makes for some very challenging design criteria in the sequence and thresholds at which different parts of the circuits are activated.

Other applications that might benefit from this approach

The requirements for this project were driven by safety critical operation within an explosive environment, but other applications may drive similar requirements, including:

Very High Voltage environments – 400kV: Applications such as sensing on electrical distribution primary transformers mean that use of conductors to connect to equipment at these potentials can be problematic.  These transformers are taken out of operation for service very rarely, and thus the continuous operational life (and elevated operating temperatures) may prohibit use of batteries.  Other circumstances may prevent use of photovoltaics.

Very high pressure: Operating in high pressure environments mean that equipment may need to be oil-filled, and composite components with electrochemical volume changes (i.e. rechargeable batteries) may be a problem.
 
Medium blocking rf signals: Operation under water, for example, often blocks use of radio transmission of both power and data.  If, for other reasons, wired solutions are not possible or desirable, then optical connection offers new possibilities.

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