Cost-Effective Condition Monitoring Systems in Safety-Critical Equipment

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Using 2-in-1 Pressure Transmitters for Safe, Cost-Effective Condition Monitoring

Pressure transmitters are instrumental in the development of automated load-monitoring systems, which help create safe working environments that are increasingly mandated by legislation and voluntary codes of practice. Pressure transmitters play an important role in ensuring that machine-control systems in utility vehicles including cranes, lifting platforms, and fork-lift trucks are precise, accurate, and reliable. The challenge is to create a cost-effective failsafe condition monitoring system.

System redundancy is a design concept used to help meet regulatory requirements. Pressure transmitter-based systems must eliminate systematic failures within hydraulic controls and identify random error functions so that all moving parts are shut down if a dangerous condition exists.

Typical load- and load-moment-monitoring applications measure pressure at the hydraulic cylinder of the boom, the hydraulic pump of the traction drive, or at the pressure reservoir of the brake system. Any malfunction of a pressure transmitter can produce critical dangers. A malfunction may be caused by extreme environmental conditions (EMC, humidity, shock and vibration, or physical damage) or by abnormal operating conditions including an overload of the differential pressure sensor caused by pressure spikes. Erratic software failures in the subsystem program or a defective electronic component may also be a source of an incorrect pressure transmitter or system output signal.

Equipment designers take many preventative steps to provide a continuously safe operating environment. One easy, practical, and common solution is to use two pressure sensors at the same measurement point. A redundancy check of the two output signals is made in the machine logic-control circuit to detect any unexpected deviation in pressure transmitter readings.

The disadvantages of this approach are twofold: it requires an additional pressure port with separate sealing, and it needs additional electrical connectors with mating connectors. This requires a larger number of electrical components to use two individual pressure sensors. From a safety standpoint, this increased complexity presents additional opportunities for failure.

The Answer: "2-in-1" Pressure Sensor MHS-1

WIKA’s MHS-1 pressure transmitter offers an alternative: it’s the first integrated dual-pressure sensor manufactured specifically for safety-critical and safety-related applications such as a vibration monitoring system in mobile hydraulics. Dual (two-channel) pressure measurement, separate electrical signal processing, and two independent 4-20mA output signals enable control-side redundant monitoring for pressure measurement points.

It is now possible for Channel 1 to be a pressure sensor with measuring range 0-3000 psi with a 4-20mA proportional electrical output signal, and for Channel 2 to be 0-5000PSI for 4-20mA.

A random hardware or software error in the safety-critical condition monitoring system can be detected by monitoring the dual pressure sensors in the control system, and the actuator can be set to a safe condition when a hazardous condition is detected. Therefore, the probability of a dangerous failure is substantially reduced. This solution increases process security and is cost-effective, since the mechanical and electrical installation costs are significantly lower than installing two individual pressure sensors.

TÜV Certified Safety

The MHS-1 is designed to meet both the ISO 13849-1 (Performance Level - PL) and IEC 61508 (Safety Integrity Level - SIL) safety standards. It is certified by TÜV, a recognized independent testing laboratory. The requirements of the new European Machinery Directive (2006/42/EC) have also been met.

Safety-related characteristics available from WIKA for the pressure sensor include "mean time to failure" (MTTF values). A complete safety-related condition-monitoring system can be evaluated by the user quickly and easily. The MHS-1 achieves the Safety Integrity Level SIL 2 in accordance with IEC 61508 and Safety Category 3 Performance Level D in accordance with ISO 13849-1.

Robust Design for Demanding Environments

WIKA's hermetically sealed, welded, dry thin-film measuring cell with its sputtered Wheatstone bridge offers long-term stability in applications with high dynamic load changes. Thin-film sensors feature excellent resistance to pressure spikes. In addition, an arc-eroded pressure port and cavitation damping system integral to the process connection minimizes the possibility of pressure sensor failure. The electronics are designed for harsh EMC conditions up to 100 V/m. The robust stainless steel housing and the IP 69K high pressure steam washdown-rated electrical connection are designed and extensively tested for the extreme conditions often encountered in mobile hydraulics. 

Customer-Specific Designs

WIKA has various safety-evaluated designs available for a variety of applications both with and without sensory overload detection. WIKA’s flexible instrument assembly and modern production system supports custom designs.

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