May
19
Measuring distances in test rigs
Every type of test rig today represents a complex interaction of mechatronic disciplines. These consist of the mechanical design and the complete software for the control of the test rig and measurement technology or sensors. The measurement technology itself always contributes significantly to the expertise of a test rig. In other words, the performance of the test rig depends on the precision of the sensors. This is why eddy current sensors are frequently used on test rigs. These sensors often originate from Ortenburg, the head office of measurement technology company Micro-Epsilon.
Eddy current displacement sensors are used in many different applications, yet the requirements are often very similar. Typically, requirements are for nanometre resolution with the sensor as compact as possible and which is resistant to external influences. In addition, the sensor often needs to be easily adapted and integrated with other components or system parts, as the target is usually found inside the test rig. The measurement task itself could be gap measurement, distance or displacement measurement.
The requirements outlined above are satisfied very well by eddy current sensors. Compared to the test rig, the sensor itself is a very small component; currently the smallest sensor in the world has an external diameter of just 2mm. Nevertheless, the component has an extremely important task. So how does an eddy current sensor work?
Measuring via eddy current is based on the extraction of energy from an oscillating circuit. This energy is required for the induction of eddy currents in electrically conductive materials. Here, a coil is supplied with alternating current whereby a magnetic field forms around the coil. If an electrically conducting object is present in this magnetic field, eddy currents are produced in it. According to the Lenz Rule, the field of these eddy currents is opposed to the field of the coil, which causes a change in the coil impedance. This distance-dependent impedance change can be captured at the controller as a measurable factor using the change in the amplitude of the sensor coil.
The principle can be used for all electrically conductive materials. As eddy current penetrates insulators, even metal behind an insulating layer can be used as a measuring object. A special coil winding enables very compact sensor designs, which can even be used in high temperatures up to 320°C. All eddy current sensors are insensitive to dirt, dust, moisture, oil and pressure. This means they are ideally suited to harsh industrial environments.
The eddy current sensors of the eddyNCDT series cover measuring ranges between 0.4mm and 80mm whereby the sensor geometry is directly dependent on the measuring range. A measurement channel can achieve a maximum resolution of 0.09nm.
Controlled welding
A fully automatic test rig has been set up at the Technical University of Brunswick, which establishes the quality of the seam during the welding process when the seam flanks are continuously moving. The test rig simulates dual-axis stressing of the weld sample. Special attention is paid to the acquisition of the weld edge movement of the sample plate, because the reliable acquisition of this controlled variable is critical to the success of the welding. The fact that displacement measurement is not influenced by external factors such as dirt, smoke and electromagnetic fields, was therefore key to selecting a suitable measurement system. Due to its very high robustness, the displacement measurement system from Micro-Epsilon is used. The eddyNCDT series sensor has a measurement range of 4mm and provides micrometre accuracy for a controlled sequence of the weld edge movement. The non-contact technology also ensures a longer, maintenance-free operational period in the test rig. The stability of Micro-Epsilon’s eddy current sensors with respect to high temperatures was a decisive advantage in this particular application.
Test rig for tribology
Eddy current sensors from Micro-Epsilon are also being used in a new high-performance test rig at the Institute for Tribology and Energy Conversion Machines at the Clausthal Technical University. With this new test rig, which is currently under construction, the tribological, flow mechanics and rotor dynamics of highly loaded plain bearings at maximum circumferential speeds of the shaft will be examined. Based on many positive experiences already from using Micro-Epsilon eddy current sensors in existing test rigs for the examination of hydrodynamic radial plain bearings or plain bearings with particularly high loads, sensors from the eddyNCDT series will also be used in this new test rig. In doing so, the shaft reaches circumferential speeds of up to 200m/s; typical test rigs achieve a maximum of 120m/s.
The position of the test bearing housing and the relative movement between the rotor and the test bearing is detected in the test rig using eddy current displacement sensors. The bearing gap of the plain bearing between bearing surface and rotor is also measured using eddy current sensors. There are 22 measurement channels in the test rig. The eddyNCDT miniature sensors with a measuring range of 0.5mm are integrated directly into the design in order to enable the bearing gap to be measured with maximum precision.
Engine test stations
Eddy current sensors from Micro-Epsilon are also being used in automotive test stations. For example, the secondary movement of pistons during the different strokes is can be measured. To do this, several sensors are integrated directly with the pistons, forming a flat surface on the piston wall. The cables are routed along the connecting rod and drive shaft to the outside, via a swing arm. During operation, particularly when the engine is producing torque under load, it can be established for example, whether the piston in the cylinder has too much ‘play’ and whether this would adversely affect the service life. If, for example, the sensors are moved to another location in the crankcase, the “breathing” of the cylinder head gasket during the stroke can be tested. There are space constraints for all applications on an engine. The engine housing normally has to incorporate multiple coolant channels. Sensor integration and cable routing are therefore a challenge, since no channel can be modified or damaged. As well as being difficult to install, the sensor also has to withstand a harsh environment for sustained periods: temperatures of up to 200°C, pressures of up to 2,000bar and contact with fuels, oils or air-fuel mixtures.
The applications outlined above are examples of how sensors are being used in today’s test rigs. The integration of sensors in test rigs will continue to gain importance as, last but not least, test rigs must provide high quality measurements, which is taken for granted these days. This requirement can only be achieved with corresponding mechatronics competence for the complete system, which is often a decisive factor in this market.
