The requirements are high on pressure sensors, which are used in pneumatic or hydraulic braking systems. This article shows that the use of pressure sensors “as a modular system” is expedient even due to these extended requirements.

The technological leader for tractors – AGCO GmbH with its premium brand Fendt – also uses pressure sensors from the M01 modular system by Sensor-Technik Wiedemann GmbH (STW) in the braking systems of their large-scale tractors.

Which prerequisites must pressure sensors in braking systems fulfil, and what benefits result from the use of a modular structure pressure sensor program?

Why pressure measurement in braking systems?

Complex braking systems are not only found in cars, but also in commercial vehicles, mobile machines and stationary systems for transporting passengers. Fairground rides in the leisure industry are also dependent on brakes, which are absolutely reliable and can be accurately controlled.

In the case of pneumatically-activated brakes, the pressure is measured in the reservoir, but also the current brake pressures in the individual circuits have to be recorded in order to sensitively and evenly decelerate moving masses. As a result of this pressure measurement, additional convenience and safety functions are facilitated such as anti-blocking systems, start-up assistance, traction control, handbrake assistance, emergency braking systems, track and lane guidance systems etc. The more accurately the pressure can be measured, the more energy-efficient the pressure generation; the smoother the braking action and the more accurate the statements which can be made regarding the technical condition of the brakes. Therefore, up to six pressure sensors can be found in one single braking system with different pressure ranges, pressure connections, output signals and connectors.

Failures regarding pressure measurements lead to very different consequences: From the illumination of a warning lamp, via an increased energy requirement in the pressure generation or the deactivation of individual convenience functions, right up to a blockage of the vehicle or system.

The technical requirements

In particular in braking systems, a loss of accuracy, functional failure of the pressure measurement or even pressure sensor breakage are unacceptable. High demands exist from a functional safety point of view due to the areas of application. These also include the characteristic ability of self-diagnosis which is expressed in the so-called “diagnostic coverage”: Which proportion of possible errors is recognised by the sensor itself and therefore does not lead to faulty measurement values? The M01 sensor used in the Fendt tractor achieves the remarkable value of more than 70% in the conditions prevailing during application, and an average time of more than 250 years until a dangerous failure (MTTFd) occurs.

The following rule applies: no measurement value is preferred to an incorrect measurement value.

A further complicating factor is that the pressure sensors are installed on the pressure lines of the braking system, and therefore are often permanently and directly exposed to contamination, salt water, extreme temperature fluctuations and severe vibration. The pressure sensors have to be extremely resistant to all types of environmental influence. Simultaneously, the measurement of the pressure must be carried out with an accuracy of +/-1 % or better.

Reliability and availability – also under extreme conditions – are vital and distinguish both the pressure sensor component as well as the overall product: The tractor.

Are the requirements fulfilled?

The effort to qualify a sensor for these tough requirements is not insignificant. In order to verify the defined mechanical robustness, amongst other things, the transmitters have been subjected to a particularly tough vibration test. The standards ISO 16750-3 and DIN EN 60068-2-80 specify a test over a total period of 282 hours: Vibrations of 18 times the force of gravity, in all three axes, simultaneously with 70 temperature cycles between -40°C and +125°C – anything which functions during and after this tough test may be designated as “extremely robust”. The large amount of effort required to conduct this test alone makes it necessary to ensure these results are transferable to as many components as possible.

Likewise, EMC/ESD properties must be qualified: Tests acc. 2009/64/EC, DIN EN 55025, ISO 7637, ISO 16750-2, ISO 10 605 and UN/ECE R.10 are standard, complex and therefore arduous.

Undesirable variation diversity is the result

As a result of the use in various tractor models - with pneumatic and hydraulic braking systems – different pressure ranges, pressure connections, output connectors and signals are necessary. Changes to the configuration are complex and create further variations. From further developments, for example “brake by wire”, it may become necessary to change from an analog output signal to a digital CAN-bus output. Use in further safety-critical applications, such as in the load-lifting equipment for commercial vehicles, would require further development and certification effort.

Despite the enormous range available on the market, sensors which fulfil all of the stated criteria are not always immediately found.

The advantage of a modular system

The causes and the price for the resulting variation diversity are therefore known: Every variation must be searched for, selected, negotiated, procured, stored and documented. High costs for development and qualification, and more complex logistics, both in production and also spare parts supply, are the result.

Therefore, both customers and suppliers have an understandable interest in reducing variation diversity.

If the advantages of identical parts cannot be exploited, due for example to imperative technical reasons, then the use of a modular system is the next and best alternative.

In order to minimise the abovementioned effort as much as possible, it was economically and technically crucial to use a series of pressure sensors which fulfilled all requirements: The M01 with its millions of possible variations provides the required characteristics:

• Safety requirement acc. PL b (acc. ISO 13849)
• Reliability with regard to tightness thanks to welded stainless steel measurement cells
• 14 pressure ranges between 0.1 and 2000 bar, relative and absolute reference
• 20 pressure connections
• 9 output signals, analog and digital
• 10 connection plugs made from metal and plastic
• Economical production in an ISO/TS16949-certified company in Germany thanks to the modular structure. 

As a result of the exact definition of the modules in the modular system, a sensor variation which verifiably fulfils the high requirements can be selected at the beginning of the development of the end product.

Summary

The use of modular structure sensor technology has significant advantages also, or especially, regarding safety-relevant applications. Based on hard technical and economic facts, AGCO Fendt GmbH has consciously made use of the M01 modular system by STW GmbH.

Of course, aspects which are valid for safety-relevant systems such as braking systems can also be transferred onto other challenging applications.