Each analog input should be continuously monitored to determine if there is a process threshold violation.
A threshold violation is activated if the analog value is too high or too low, which implies a high threshold and a low threshold. In addition, there are two types of threshold:
- Alarms: a warning, without consequences for the installation.
- Faults: an anomaly, immediate shutdown of the installation.
To allow the installation to start, a time delay is introduced to allow time for the machine to heat up and reach its operating point.
When one of the alarm thresholds is reached, a time delay is started, in order to prevent unwanted tripping due to sensor noise. Once the delay has elapsed, the alarm is lifted. If the measurement drops back below the permitted range, the alarm is stopped.
When one of the fault thresholds is reached, a time delay is started, in order to avoid unwanted tripping due to sensor noise. Once the time delay has elapsed, the fault is removed and the machine is stopped immediately.
To restart the installation, an acknowledgment of the fault is mandatory. After which, if the measurement is dropped below the authorized range, the restart authorization will be issued by the PLC. The analog threshold parameters and the time delays are adjustable by the operator using an HMI.
