Written By: Jürgen Reiser, Product Manager, Electronic Pressure Measurement, WIKA Alexander Wiegand GmbH & Co. KG, Germany and Martin Armson, Director of Electronic Pressure Measurement, WIKA Instrument Corporation, USA
In today’s world, safe working conditions are increasingly mandated by legislation and voluntary codes of practice. Automated load monitoring systems using pressure transmitters are becoming instrumental in creating safe working environments. They play an important part in making certain that machine control systems used in utility vehicles including cranes, lifting platforms and fork-lift trucks perform with precision, accuracy, and most importantly reliability. ‘Failsafe’ is the keyword that drives system designers to be ever more creative. System redundancy is a design concept that helps meet these regulatory requirements. Pressure transmitter based systems must guarantee that systematic failures within hydraulic controls are eliminated and random error functions are identified so that all moving parts are shut down if a dangerous condition exists.
In typical load and load-moment monitoring applications, pressure is measured 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 may lead to a critical condition of the system. A malfunction may be caused by extreme environmental conditions (EMC, humidity, shock and vibration, or physical damage). It can also be caused by abnormal operating conditions including an overload of the 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 and/or system output signal.
Machine designers take many preventative steps to provide a continuously safe operating condition. 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 that an additional pressure port with separate sealing is required, as are additional electrical connectors with mating connectors. A larger number of electrical components are required to use two individual pressure sensors. From a safety standpoint, this increased complexity presents additional opportunities for failure.
The WIKA Answer: ‘2 in 1‘ Pressure Sensor MHS-1
WIKA’s MHS-1 pressure transmitter offers the first integrated dual pressure sensors, manufactured specifically for safety- critical and safety-related applications in mobile hydraulics. Dual (2 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. Note: the pressure sensor with the smaller measurement range is selected for the maximum possible system pressure as both pressure sensors have only a single mechanical process connection. The use of identical pressure ranges or a reverse scaling of the second signal to 20-4 mA is also possible.
A random hardware or software error of the safety-critical load 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 if a hazardous condition is detected. Therefore, the probability of a dangerous failure is substantially reduced. This solution increases the process security and is cost effective since the mechanical and electrical installation costs are clearly 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 and independent test laboratory. The requirements of the new European Machinery Directive (2006/42/EC), which becomes mandatory in Europe at the end of 2009, have already been met.
Safety-related characteristics available from WIKA for the pressure sensor include, “Mean Time to Failure” – (MTTF values). A complete safety-related control 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 environmental conditions
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.
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.