Eddy current technology for high precision monitoring

Gap monitoring plays an important role in precision engineering. However, in machinery such as hydraulic pumps, gap calculations have proven to be difficult if not impossible. Pumps are exceptionally compact and exert considerable restrictions on the space available for the integration of sensors. They operate using a cylinder with a piston to create hydraulic pressure and a balance has to be kept between the need for maintaining adequate hydraulic pressure and ensuring the sliding properties of the moving parts.

The chief difficulty in monitoring the gaps is that they are exceptionally small - often less than 20 microns wide and periodically less than 10 microns. Eddy Current technology, which does not depend on contact, is being seen as the solution. These new displacement sensors consist of a probe connected to a driver unit, which contains the high frequency oscillator that drives the probe. It is the high frequency oscillation, which is then produced that induces eddy currents in the target material.

Micro-Epsilon high precision sensors

Micro-Epsilon Messtechnik GmbH & Co. KG the provider of high precision measurement sensors has recently developed a method to monitor gaps, based on this technology. The company’s eddyNCDT 3060 is a displacement measuring system that offers a combination of precision, speed and temperature stability. The device is used to monitor the lubrication gap and thermal expansion, as well as to determine shaft movement and concentricity of machine parts and drive components.

Its inductive position measuring system consists of a compact controller, a sensor and a cable and is factory-adjusted to ferromagnetic or non-ferromagnetic materials. The system is designed for fast, non-contact displacement measurements. It is actively temperature compensated and designed for ambient temperatures up to + 200 ° C and an ambient pressure of up to 20bar. Measurements are made with a cutoff frequency of up to 20 kHz.

Sensonics probes shipped to methanol plant

Other Eddy current based systems are also coming on to the market. UK based Sensonics, the provider of various monitoring systems using both contact and non-contact techniques, recently introduced a new stainless steel option to add to its range of API 670 compliant XPR04 Eddy Current Proximity Probes. It is designed to be used on turbines, pumps, compressors and gearboxes.

The first shipment of this improvement has now been supplied to a leading European Pump OEM for the protection of two Turbo Pumps and one Electro Pump at a Methanol Plant in the Middle East. The Sensonics range of linear variable differential transformer (LVDT) offer contact displacement monitoring measurements from a few mm's up to 600mm and are designed for extreme environments.

Zetec’s ‘first affordable’ handheld system

Meanwhile, Zetec Inc., a global leader in non-destructive testing (NDT) solutions and subsidiary of Roper Technologies, Inc., recently launched MIZ-21C, which Jesse Herrin, Eddy Current Systems Product Manager for Zetec describes as, “the first truly affordable handheld eddy current instrument with surface array capabilities.” The device can deliver fast, accurate inspections in demanding NDT applications including aerospace, oil and gas, manufacturing, and power generation. The surface array solution can reduce inspection time by up to 95% compared to traditional handheld pencil probes. The MIZ-21C is available in three models with single frequency and surface array options to meet a wide range of inspections needs and budgets.

Omni Instruments

The UK based Omni Instruments sells a range of Eddy current based proximity probes for non contact vibration and displacement measurement. Its stainless steel PRS02 is a straight mounted eddy current probe with a 5mm diameter that covers a displacement measurement range of 0-2.5 mm with a sensitivity of 8mV/µm. Its 8mm diameter PRS04 probe covers a displacement measurement range of 0-2.5 or 0-4mm and its larger 20mm diameter PRS08 probe covers a displacement measurement range of 0-8 mm. Omni also has a specially designed ‘high temperature’ probe, the 16mm diameter PRS05 that covers a displacement measurement range of 0-5 mm.

Monitran’s range

Another UK based company, Monitran the supplier of cost effective sensors, has also introduced a range of Eddy current probes. Its most common probe for monitoring high speed pumps with plain metal bearings is the MTN/EP080, which has a diameter of 5mm and is able to measure small vibration levels with gaps up to 2mm. Other Monitran Eddy Current probes include: the MTN/EP200; the MTN/EP340; and the heavy-duty MTN/EPHD80. Controlling the probes are drivers such as the MTN/ECPD/85 and the MTN/ECPD/60.

Outlook

The market for Eddy Current testing is growing at close to 10% CAGR, according to a recent report, and could be worth $1.68 billion by 2022. The factors which are driving this market include advancements in eddy current testing technologies, safety regulations by governments and increasing infrastructure development in emerging economies.