Siemens SM2 monitoring system manual Accuracy of the SM2 data Monitoring method inaccuracies

Accuracy of the SM2 data Notes on SM2 operation

Monitoring method inaccuracies

Different inaccuracies can occur depending on the monitoring method used:

1.Event-driven monitoring method

This method supplies very precise data at the cost of increased system workload. Problems can occur only when the duration of events is monitored.

Monitoring cycle Monitoring cycle

. . . .

Duration

As shown above, the duration of an event (and, if applicable, an activity count) is assigned to the second monitoring cycle even though part of it should be assigned to the first cycle. The relative magnitude of the monitoring error decreases in inverse proportion to the length of the monitoring cycle.

2.Sample-driven monitoring method

The accuracy of this monitoring method is subject to the laws of statistics.

A requirement for the validity of the monitored data is that the samples are independent of the monitored events. SM2 uses the system timers to control sampling by having itself activated at regular intervals.

Hardware interrupts are used for this purpose. However, such an interrupt is not permitted whenever the CPU is in a non-interruptible state. This results in a sampling delay and thus in a certain dependency on system events.

Certain unavoidable system activities cause further delays between interrupt accep- tance and sampling by SM2. If statistical independence of the samples is assumed, the accuracy of the monitored data depends on the number of samples.

An assessment of the accuracy can be obtained by using confidence intervals (e.g. deviation of not more than 1% in 99% of all cases).

It should be noted that a high sampling rate increases the system workload. Therefore, a long monitoring cycle is preferable to excessively frequent sampling.

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