Thin-film pressure sensor technology matures

Morten Moller reviews the development of thin-film pressure sensor technology, and advises: "The time is right to offer standard thin film sensors in a simple package at a low price"

Morten Moller reviews the development of thin-film pressure sensor technology, and advises: "The time is right to offer standard thin film sensors in a simple package at a low price, to enable accurate monitoring on hydraulic systems and fluid machinery".

Thin film technology now offers accurate, stable, reliable and low-cost sensors for fluid pressure measurement applications on OEM machinery and hydraulic systems, improving overall performance and control.

The best modern designs of industrial pressure and load sensor are based on thin film technology - a technology that has developed alongside and benefited from the advances in micro-electronics and ASICs.

Sensor-Technik and their parent company Sensor-Technik Wiedemann (STW) in Germany have been active in this field for over 20 years.

The founders of the company, Katharina and Wolfgang Wiedemann, have been driven by a passion for sensor solutions based on new technology.

As a result they have developed and patented many aspects of the manufacturing technology now used for producing thin film strain gauges for pressure and force measurement sensor applications.

STW thin film strain gauges have many advantages when used as the active element of a fluid pressure sensor.

* The Titanium Nitride (TiN) thin film coating technology used makes the sensor element particularly small and compact, and has impressive specifications in terms of precision and long-term stability.

* The thin films used in the strain gauge can be deposited on, and bonded to, various styles of substrate - including ceramic materials, but most commonly to high grade special steels - to create an IP69 sealed transmitter, filled with a dry inert gas.

* The sealed capsules can be directly welded into the pressure ports of small industrial housings, with standard process connections.

* By integrating temperature compensation for the sensor element directly into the thin film itself, the response is faster and exact, and the need for external circuitry is reduced.

Excellent stability, accuracy and reliability have resulted from the STW development of Titanium Nitride thin film manufacturing techniques.

Add to this the very low unit costs available when sensors are produced in quantity, and these sensors offer everything desirable for an industrial measurement transducer - given the right overall housing.

INITIAL PRODUCTION APPLICATIONS.

Many of the initial applications of thin film sensors made a feature out of the small sensor size and the designer's ability to provide a unique housing to suit each specific application.

Engineering and perfecting devices for special applications established the Sensor-Technik reputation as a supplier of accurate and reliable thin-film sensors.

Typical examples were those developed for monitoring transient pressures in internal combustion engines at 350C, for automotive combustion development engineers, and for a wide range of other in-vehicle monitoring duties and measurements in hydraulic machinery.

Sensor-Technik sensors have been proved in the tough environmental conditions found in heavy construction machinery and vehicles, for many applications in engines, cooling and hydraulic systems, in terms of temperature, shock, vibration and EMC resilience.

Units are now supplied for standard production to tractor manufacturers such as Massey Ferguson, and for hydraulic systems on cranes and lorries, for example units are supplied to the Liebherr Group for construction equipment.

Further application examples, and guidance on the choice and specification of such sensors is given in the Sensor-Technik "Engineering Guide to Pressure, Load, Force and Rotary Measurement", available on request.

STANDARD AND OEM MODELS.

The production techniques used to manufacture thin film sensors have now matured, such that standard units can be manufactured very efficiently, at low cost, with high standard specifications, using automatic test and calibration adjustment.

Sensor-Technik has taken the next step forward, and is now offering the advantages of their small size, high specification thin film sensor in a standard industrial unit, offering a good cost/performance ratio.

The new Sensor-Technik M01-OEM standard units now in production are targeted at the higher pressure applications normally associated with hydraulic systems, construction machinery or vehicles, having models covering pressure ranges from 0-60bar up to 0-800bar.

Models with pressure ranges extending down to 0-5bar will be added later.

These standard models are chosen to offer the best specifications available.

The sensor material is totally externally stainless steel, in terms of fitting, housing and sensor diaphragm, including all parts in contact with the fluid monitored, to give a high media compatibility.

There are no O-Rings, seals, ceramics, or bonding materials in contact with the fluids measured, and the volume of process fluid needed within the sensor is minute.

The M01-OEM unit functions over a wide ambient temperature range, -40 to +125C, so there are no problems in cold weather starting or in hot machinery: maximum fluid temperature is 150C.

The standard sensor fitting threads and small size make the sensor easy to install, and experience in machinery applications leads to a high 200V/m EMC tolerance, producing a rugged sensor.

Absolute accuracy over 0-80C is +/-1% FSD, with linearity and repeatability better than +/-0.5%.

EXPERIENCE IN APPLICATION.

The man behind this latest development from Sensor-Technik is Morten Moller, the MD of their UK company, and the man with the experience of sensors in many industrial applications.

For many years Morten was the regular sensors columnist in the Industrial Technolgy magazine, reporting on hydraulics and pneumatics sensor technology.

The design specification of the new standard Sensor-Technik sensor is based on his experience of the industry.

Morten comments: "Thin film sensors are at the heart of most of the advanced high specification pressure transmitters available across industry now, so the time is right to offer these sensors in a simple package at a low price, to encourage wider use of accurate monitoring on hydraulic systems".

Having worked in many industries with many OEM customers, Sensor-Technik has chosen the external signaling, connection and environmental requirements to suit most typical machinery applications.

With four types of electrical connection, from moulded cable, to a panel connector (AMP Super-Seal 1.5), DIN bayonet (to DIN 72585 with IP69K rating) and M12 circular connectors (IP67), there is no lack of choice from the standardised units.

Pressure connections are standard G1/4 male, with a sealing gasket to DIN3869, or NPT1/4: there is even a G1/4 connection with a manometer adapter to EN837-1.

Electrical outputs are 4-20mA on 2 wires or 0-20mA or 0-10V on 3 wires, or even a ratiometric output is available with a DC supply down to 5V.

Please request the datasheet for further technical information.

THE FUTURE.

Morten sees the main areas of application for his Sensor-Technik M01-OEM sensors as in hydraulic equipment and vehicles, such as cranes, steam rollers, excavators, agricultural and construction machinery, forklift trucks, waste disposal, tunneling equipment.

"All control functions of working hydraulics are improved by accurate monitoring of the hydraulic pressures," he adds, "and the experience of Sensor-Technik in providing sensors for this industry is unique".

With the production lines rolling, committed to the manufacture of large volumes of standard sensors, Sensor-Technik is offering the opportunity for all machinery manufacturers to improve the monitoring and control of fluid pressures in their automation systems and machine processes, for forward thinking designers to take the advantage.

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