Explaining the benefits of thin-film pressure sensors

Thin film technology now enables accurate, stable, reliable and low-cost sensors to be produced for fluid pressure measurement applications on OEM machinery and hydraulic systems, thereby 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 its parent company Sensor-Technik Wiedemann (STW) in Germany has 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 sensors based on new technologies. 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 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

Higher performance, lower cost

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.

Many of the initial applications of thin film sensors took advantage 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 Sensor-Technik's 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 350degC 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 proven 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 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 customised designs

The production techniques used to manufacture thin-film sensors have now matured, such that standard units can be manufactured very efficiently at low cost and with high standard specifications, with 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 sensors 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 exterior of the sensor is totally stainless steel, including the fitting, housing and sensor diaphragm, as well as 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 of -40 to +125degC, so there are no problems in cold weather starting or in hot machinery: maximum fluid temperature is 150degC. 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 0-80degC is +/-1 per cent FSD, with linearity and repeatability better than +/-0.5 per cent.

Experience counts

The man behind this latest development from Sensor-Technik is Morten Moller, the MD of the UK company and the man with the experience of sensors in many industrial applications. 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 signalling, 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 ample choice of standardised units. Pressure connections are also standard G1/4 male, with a sealing gasket to DIN 3869, or NPT1/4; there is even a G1/4 connection with a manometer adapter to EN 837-1. Electrical outputs are 4-20mA on two wires or 0-20mA or 0-10V on three wires, or even a ratiometric output is available with a DC supply down to 5V. Please request the datasheet for further technical information.

Potential applications

Morten sees the main areas of application for the Sensor-Technik M01-OEM sensors as being in hydraulic equipment and vehicles, such as cranes, road rollers, excavators, agricultural and construction machinery, forklift trucks, waste disposal, tunnelling equipment. "All control functions of working hydraulics are improved by accurate monitoring of the hydraulic pressures, 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 making the data and information available for all machinery manufacturers to improve the automation and control of machine processes, for forward-thinking designers to take the advantage.

* Sensor-Technik's publication Electronic Sensing and Mechanical Measurement: The Engineering Guide to Pressure, Load, Force and Rotary Measurement is available on request from Sensor-Technik by emailing [email protected] or visit www.sensor-technik.co.uk.