Daniel Hofer and Bernhard Vetterli look at the technology behind OEM transmitters.
OEM transmitters - from Keller - are systems that can be described as 'embedded' in the best sense of the word - and in two different ways. Firstly, the sensor and the downstream electronics are embedded in the same housing and secondly, the transmitter capsules themselves are suited for embedding in application-specific systems. Depending on requirements, the output signal is standardised and temperature-compensated (ratiometric or digital).
Sensor miniaturisation?
Thanks to the chip-in-oil (CiO) technology developed at Keller, the trend toward sensor miniaturisation is now a reality. This development can offer impressive advantages: an extremely compact structural design, high resistance to electrical noise fields and high vibration resistance thanks to low mass and short conduction paths.
Beneficial functions
To put it clearly, CiO technology means that an ASIC is fitted directly next to the pressure sensor - in the same housing - to provide users with a whole range of beneficial functions
However, this does not make the pressure measurement capsule any larger: its external dimensions remain the same. This transmitter concept is available in housings 4L-9L, starting from a diameter of 11mm.
Sintered-in pressure-resistant glass lead-throughs feed the transmitter signals outwards. The internal wiring uses short, lightweight bonding wires - with the total exclusion of air in oil.
First, this approach eliminates the need to connect filigree signal processing boards with multi-wire cabling in the rest of the installation process for the pressure transducer. And second, there is no need to protect the downstream electronics against moisture and condensation.
High-grade steel housing
Together with the high-grade steel housing, the glass lead-throughs act as feed-through capacitors, forming a Faraday cage. This makes the CiO technology extremely resilient to electrical fields. Even field strengths of 250V/m at frequencies of up to 4GHz are unable to influence the measurement signal. The digital interface must be protected by the equipment manufacturer.
The ASIC is designed as a microcontroller with the corresponding peripherals, so the sensor signals can be registered with high resolution and dynamism (sampling rate: 2 kHz).
In addition to the process pressure as such, the temperature of the pressure sensor is measured and is used for mathematical temperature compensation when the signal is processed.
OEM transmitters supply two output signals: a ratiometric analogue voltage output and a digital inter-integrated circuit interface (I2C).
Summary
Keller's C-series OEM transmitters herald a new chapter in the history of high-integration pressure measurement technology. The chip-in-oil concept moves signal processing directly into the protective oil-filled pressure measurement capsule housing, made of stainless steel.
Linearisation, temperature compensation and parameterisation are handled here. For integration into higher-level systems or battery-powered devices, versions are available with a ratiometric voltage output or with a serial-digital I2C interface.
Various structural designs can be supplied depending on the specific application.
Daniel Hofer and Bernhard Vetterli are with Keller AG, Winterthur, Switzerland. www.keller-druck.com
