7.1.10 Loop Break

A break in the control loop could be due to a number of factors such as a disconnection or open circuit of the control device (contactor, thyristor, etc). The loss of control could be catastrophic to the process. An early warning of the condition may be initiated by the controller as a Loop Break Alarm (L.br section 4.3.10.).

 

The loop is considered to be broken by the controller if the PV does not respond to the output in a given time.

 

Since the time of response will vary from process to process a Loop Break Time can be set manually or overridden by ‘P’ code parameter P34. This parameter can also be set automatically by the Autotune algorithm.

 

It is assumed that, so long as the requested output power is within the output power limits, set by op.Lo and op.Hi in Level 2, the loop is operating in linear control and is therefore not in a loop break condition.

 

However, if the output is in saturation and the PV has not moved by >0.5*Pb in the loop break time and in the right direction, a loop break condition is considered to have occurred.

 

If an Auto Tune is performed the loop break time is automatically set to Ti*2 for a PI or PID loop or to 12*Td for a PD loop. For an On/Off controller loop break detection is also based on loop break time as 0.1*SPAN where SPAN = Range High – Range Low. Therefore, if the output is in saturation and the PV has not moved by 0.1*SPAN in the loop break time a loop break will occur.

 

If the loop break time is OFF the loop break time is not set.