Condition monitoring

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The concept of Condition Monitoring (German condition monitoring is based on a periodic or a permanent detection of the) machine state by measurement and analysis of physical quantities, for. B. Vibrations , temperatures , position / approximation.

aims

Condition monitoring pursues two goals: safety and machine efficiency. It is comparable to structural health monitoring for static components.

Based on the sensor data that may be analyzed in real time, a reliable and very fast reacting safety system (emergency shutdown, "trip") can be implemented. In comparison to this, previous systems (e.g. simple vibration sensors ("Earthquake" switches)) are generally less precise and then do not make any contribution to clarifying the causes of the damage. Online condition monitoring (continuous condition monitoring) enables an emergency shutdown based on the analyzed and stored data - and thus a subsequent analysis of the disruptive factor.

The monitoring of the machine condition is an essential requirement for condition-based maintenance. This strategy replaces the reactive or preventive maintenance that was usual up to now. In the case of the latter, the relevant machine was shut down and components checked or replaced at regular intervals. This type of machine maintenance often led to intact components being replaced and the remaining service life being wasted.

But not only maintenance should be optimized by means of such machine monitoring. Thanks to modern CM components such as the vBox from Fraunhofer IPT , conclusions can be drawn about the necessary deployment of machine operators or service employees . Such systems are therefore particularly suitable for ramp-up optimizations in a new series production.

Modern CM systems place the highest demands on sensors , measurement data acquisition and forwarding and automatic measurement data processing (analysis, diagnosis) as well as system-specific knowledge. However, it also offers the greatest potential for cost savings, since the service life of critical machine elements can be practically fully utilized and at the same time necessary repair measures can be scheduled in coordination with the production plan.

Condition-based maintenance as a cross-sectional subject from the fields of mechanics , acoustics , systems theory , electronics and computer science is in development. However, it can already be very accurate today, especially when monitoring individual components. In the case of complex systems, however, it becomes increasingly blurred, since the greater the system complexity, an increasing number of signals from the most varied of sources overlap. Here, pure expert systems prove to be the only professional solution for monitoring critical machines. These systems, specially developed for one type of machine, offer - depending on the expansion stage - maximum protection for people, the environment and the machine, as well as maximum use of component life cycles.

Another shortcoming up to now has often been the lack of suitable sensors to be able to record signals directly in the wear or damage zones. In the future, microsystem technology may be able to provide a remedy, e.g. B. by sensors in thin-film technology , which can be attached directly to the structure to be monitored.

The challenges of this strategy can be seen in:

  • the search for suitable measuring points and sensors,
  • Finding meaningful parameters (state variables) for the damage to the components of interest,
  • the targeted application of signal analysis and pattern recognition,
  • as well as the enormous flood of data.

Or to sum it up in one sentence: "What has to be monitored when, where, how and with what?"

What condition monitoring cannot do is detect and avoid spontaneous component failures, such as B. the force rupture of a wave.

Material fatigue, on the other hand, can be the subject of condition monitoring. By measuring the loads (e.g. forces or torque), the load cycles can be counted which certain components are exposed to during operation. Damage accumulation hypotheses put these borne loads (e.g. in the form of load collectives) in relation to the bearable load on the component. The result is a statistical estimate of the remaining service life of the monitored components, from which optimal maintenance cycles can be derived. The advantage is that components can be exchanged before a technical crack and subsequent fatigue fracture occurs, and the usable supply of the components can still be used as fully as possible. An alternative method of monitoring material fatigue is acoustic emission monitoring. This allows plastic deformations, crack propagation or changes in the contact between metallic surfaces to be detected. The aim is to detect signs of material fatigue before a component fails spontaneously.

An alternative to monitoring the remaining service life or material fatigue would be to subject the components in the cross-sections at risk of cracking to a crack test (e.g. using a dye penetrant method). If the components are accessible at all, crack tests are time-consuming and naturally only represent an intermittent evaluation method.

Condition monitoring for fatigue monitoring is state of the art in many industrial applications. One example is the monitoring of the main drives of rolling mills, which, due to the process, often cannot be designed to be durable. The torque of the investment-intensive drive shafts is continuously monitored and condition monitoring allows condition-based maintenance.

In connection with spontaneous failures (violent or fatigue failure), it must be pointed out that rapid shutdown systems help to avoid costly consequential damage from spontaneous failures. This means that the machine is shut down within a few milliseconds after the damage. Experience shows that the consequences of continuing to drive defective machines are usually more extensive than the actual initial defect. In some cases, however, it does not make sense to carry out rapid shutdowns, as the systems or technical processes that are shut down as a result harbor high risks from subsequent processes. Here it is rather urgent to report all data from the monitoring systems prepared to the responsible operating personnel. According to this, targeted and, if necessary, coordinated measures for a controlled shutdown are to be initiated in order to exclude major damage to the subsequent processes. Corresponding emergency plans or operating instructions for such cases must be available.

Sub-steps of condition monitoring

The condition monitoring consists of several sub-steps:

1. Condition assessment

Condition recording is the measurement and documentation of machine parameters that reflect the current condition of the means of production (or the machining process).

2. State comparison

The status comparison represents the comparison of the actual status with a predetermined reference value. This reference value can be both a setpoint value to be maintained and a limit value that is not to be exceeded . Depending on the parameters examined, the setpoint is either determined during machine acceptance or determined by specified values. Limit values ​​are usually determined empirically by the manufacturer or user of the machine.

Condition detection and condition comparison essentially correspond to the inspection according to DIN 31051.

3. Diagnosis

It is the task of diagnosis to use the results of the status comparison to localize any faults as early as possible and to determine their cause (s) in order to be able to plan necessary maintenance measures in good time.

Inspection sequence

Condition monitoring systems can be classified according to the inspection sequence. The inspections can be either intermittent or continuous.

Intermittent monitoring can take place at regular or variable time intervals. This means that status information can naturally only be recorded at the time of inspection. Long-term developments can thus be determined, but short-term or transient events cannot be detected. The inspection intervals are either specified by the manufacturer or have to be determined based on your own tests / experience. One advantage of the intermittent monitoring is the possibility of using mobile measuring devices, which of course brings savings compared to the complete measuring instrumentation of all machines to be monitored.

Continuous (permanent) monitoring systems continuously record the machine parameters in real time. This means that long-term trends as well as sudden or transient changes in status are recorded and fully documented. The effort for such systems is - especially in terms of measurement data management - much higher than with intermittent systems. This additional effort is only justified if the highest demands are placed on the reliability of the monitored system, e.g. B. in turbines and generators in power plants. When monitoring machining processes, such as Continuous systems are often unavoidable, for example for tool breakage monitoring.

However, the subdivision into intermittent and continuous does not say anything about the diagnostic capabilities of a monitoring system.

Process monitoring and machine monitoring

With condition monitoring, a distinction must be made between process monitoring and machine monitoring. The process monitoring aims at the quality of the machining process; the most important application example is tool monitoring during machining ; whereas machine monitoring aims to protect the machine and its components.

Basically, the strategies and tools in both monitoring fields are similar. Often the same sensors and signal processing mechanisms can also be used. The significant difference, however, is that with machine monitoring, the behavior of the machine structure is the signal source - with process monitoring as a transmission path between the process signal and sensor, however, this represents a fault. If there are changes in properties in the machine structure, e.g. B. through wear of components, this represents an event to be detected for the machine monitoring. For the process monitoring, however, this change in the transmission path leads to a signal change that does not come from the machining process.

Furthermore, the signals of the process-related excitation of the machine-tool system are recorded during process monitoring. For machine monitoring, however, the process signals are to be regarded as a disturbance due to their dominance, so that the machine structure must be stimulated for system identification outside of the machining process.

literature

Individual evidence

  1. ↑ The Fraunhofer IPT vBox. Retrieved March 13, 2018 .
  2. ↑ Ulrich Klein, Vibration Diagnostic Assessment of Machines and Systems, 2003 Stahleisen.