As safety and reliability requirements are rising all the time, reluctance actuators are being used more and more to improve functionality in modern machinery and equipment, in cars, and even in consumer goods. The reliability of the complex overall system depends on the functionality of the individual electromagnets. Actuator systems are found in automobiles, power plants, elevators, industrial safety engineering and medical technology. External and internal variables that may cause technical parameters to deviate from their original calibration, leading to malfunctions during operation, cannot be wholly ruled out. The damage that can result from this necessitates that measures be taken to significantly improve safety and reliability. Diagnostic systems that detect damage at the earliest possible stage are an indispensable tool. The Steinbeis Transfer Center for Mechatronics in Ilmenau has developed MagHyst®, a measuring principle to determine the magnetic state of reluctance actuators upon every switch, thereby detecting other changes, too.
The system detects changes in the field coil, caused by heat or wear, that can lead to magnetic short circuits. It can thereby prevent failures by ensuring actuators can be replaced in good time. In short: it makes predictive maintenance possible. At present, the functionality of electromagnets is normally determined mechanically by measuring the force-distance curves, which generally means that the actuators have to be removed from the propulsion system. Inspecting electromagnets in this way requires a lot of effort, and the method also does not include suitable sensors that are able to take direct measurements in the worn or damaged areas. MagHyst® is an innovative measuring principle that uses the field coils of magnetic actuators as sensors, making it possible to assess both static and dynamic behavior via Ψ(i,δ) graphs. The magnetic properties of a magnetic actuator generally remain stable over its entire service life, and its magnetic behavior is only affected by changes in the mechanical or electrical subsystem. The measured curves deliver information on an electromagnet’s critical functional parameters and quality indicators such as switching time, friction in the system, energy reserves and changes in working stroke. As such, MagHyst® allows for a hitherto unrivaled standard of condition monitoring for complex, safety-critical systems. Errors can be detected at an early stage through continual monitoring and sampling, which also makes it possible to estimate a system’s predicted working life. This type of functional diagnostics has now successfully established itself in a wide variety of safety-critical applications.
The graphs to the right illustrate the potential of this new measurement technology. Figure 1 shows the typical dynamic Ψ(i,δ) curves when an electromagnet is switched. The dynamic processes during switching are clearly visible: attraction (1-2) and decline (4-5), attraction delay and decline delay. Points 1 to 5 show the progression of the switching operation of the system being measured. Deviations from these points indicate the nature of the error that has occurred; curves may deviate in either direction depending on the source of the error. Before testing an electromagnet, a reference curve like this must first be recorded. Comparing the test results to the reference curve as a whole or to individual points delivers information on the state of the magnets being tested. The magnet measured in Figure 2 (red) is displaying much higher friction than would normally be expected. Compared to the reference curve (black), a much higher current is needed to switch the magnet. This type of error reflects the condition of the mechanical subsystem. Figure 3 shows the curve differs in the case of an error in the electrical subsystem. In this case, there is a short circuit in the coil, leading to a reduction in magnetic power which can impair the functioning of the magnet.
These striking examples show that reliable, non-destructive inspection of reluctance actuators is possible – and that changes in the static and dynamic behavior of these kinds of actuators can be visualized. One major benefit is that measurements can also be taken under load, or of reluctance actuators integrated into other functional units. So changes not just in electromagnets but also in valves, clutches and brakes are easy to detect. This measurement technique can be used at any point in an actuator’s service life: to verify performance parameters during development, as part of the final inspection during manufacturing, and in condition monitoring and error analysis during operation. The latter application in particular is of major benefit, as it allows machine operators to prevent malfunctions that could lead to major damage.