Condition monitoring units provide sensing solution

Paul Boughton
Martin Cuthbert reviews a recently launched series of intelligent, industrial, hydraulic sensors. These user-friendly units combine sensing technology for flow and temperature and a built-in display as well as analogue signal output and optional switches.

Hydraulic sensors are sourced by industrial and mobile machinery OEMs, power-pack builders and service and maintenance companies. They are used in high-pressure oil hydraulic systems and in high-flow lubrication systems to provide local information to operators. They can also provide high-speed feedback to a PLC in complex systems, and often to trigger alarms or shut-off other functions if certain high/low conditions are met.

Hydraulic sensors typically measure and transmit an analogue signal for subsequent display or use by a PLC. Increasingly, as systems are becoming more complex, not only are designers seeking reliability and energy efficiency but are wanting a local display, and built-in comparators that will switch other functions on or off automatically depending on pre-set limits. Webtec's response has been to design a complete unit that acts as a central hydraulic feedback 'HUB' for flow and temperature measurement. The new series of condition monitoring units cover the range 1-360lpm at up to 420bar and 90°C.

A major focus of the design effort was to provide a highly versatile feature-rich unit that could be configured easily for different applications. Often OEMs need to install sensor units in difficult locations on a machine and cannot afford for the performance to be impaired. Unlike other flow sensors, the inlet and outlet pipe configuration has no effect on the precision or repeatability of the unit. The unit is available with BSP and SAE threads, can be mounted in any orientation and requires just one cable that carries power, analogue output and two switched signals.

The FlowHUB series has an interface consisting of three buttons. In normal operation, the display shows actual flow rate in pre-selected engineering units (lpm or US gpm), and by pressing one button the unit also displays temperature. By entering a password, the operator can navigate a simple menu tree enabling preset units to be selected, flow range over which the analogue signal is scaled to be chosen and the setup parameters for two switches to enable an operating window of flow to be easily monitored.

When designing a versatile sensor that is sufficiently rugged to operate in heavy-duty applications, such as mining, construction and steel mills, the flow sensing element has to be unaffected by fluid pulsation and mechanical vibration, while having minimal affect on the overall operating pressure of the system. The FlowHUB unit uses a unique variable orifice design which has minimal internal movement, and hence very low wear, that is sensed by a non-intrusive sensitive magnetic device (Patent pending). This combination of parts provides excellent sensitivity to changes in flow while minimising energy losses due to pressure drop.

There were three key considerations when designing the electronics to enhance the performance; fast response time, wide operating temperature range and custom fluid calibration. The 'brain' of the unit has been designed to provide high-speed feedback of flow rate to PLC systems which use the information for closed loop control. This has been made possible with a response time of better than 50ms over a wide operating range, with some models offering a 45:1 turndown ratio. The on-board electronics have been heavily insulated from the fluid they are monitoring to deliberately extend the operating fluid temperature range to 5-90°C. The FlowHUB is designed for use on hydraulic and lubrication systems which could be running at pressures of up to 420bar. The main difference between the two types of system is fluid viscosity which could vary from 5cSt (hydraulic) to 600cSt (lubrication).

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Martin Cuthbert is with Webtec Products Ltd, Cambridgeshire, UK.