Behaviour of Controller on loss of Input Signal
Behaviour of Controller on loss of Input Signal
(OP)
Hello All,
I have this challenge with a controller I am trying to program. I need to understand what is actually happening and that is the reason why I have created this forum. I plan to contact the manhufacturers of the controller for help in addition but I do not want to be lazy. I want to do my research first.
Basically my controller reads a PV (pressure) and varies the output (actuator). When I break the input or remote setpoint signal, I get a controller eror, which is normal. On restoration of the input or PV signal, the controller error clears, which is also fine. It is what happens to the output which is of concern for me.
The actuator output does not 'freeze'when I break the input signal but goes down to zero. On restoration of the signal, the actuator output falls to zero, stays there for some time (maybe "integrating")and then rises to the value it was before the signal break.
I would like a bumpless transition from loss of signal to restoration, where the controller just continues acting from where it was without having to ramp the output first to zero, 'soak' for som time and then rise again to the value pre-signal break.
Has anyone successfully accomplished this bumpless transfer on any controller?
Regards.
I have this challenge with a controller I am trying to program. I need to understand what is actually happening and that is the reason why I have created this forum. I plan to contact the manhufacturers of the controller for help in addition but I do not want to be lazy. I want to do my research first.
Basically my controller reads a PV (pressure) and varies the output (actuator). When I break the input or remote setpoint signal, I get a controller eror, which is normal. On restoration of the input or PV signal, the controller error clears, which is also fine. It is what happens to the output which is of concern for me.
The actuator output does not 'freeze'when I break the input signal but goes down to zero. On restoration of the signal, the actuator output falls to zero, stays there for some time (maybe "integrating")and then rises to the value it was before the signal break.
I would like a bumpless transition from loss of signal to restoration, where the controller just continues acting from where it was without having to ramp the output first to zero, 'soak' for som time and then rise again to the value pre-signal break.
Has anyone successfully accomplished this bumpless transfer on any controller?
Regards.





RE: Behaviour of Controller on loss of Input Signal
TTFN
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RE: Behaviour of Controller on loss of Input Signal
What I am aiming to achieve is to prevent the valves from closing (being driven fully shut) when there is a sensor break. It is an undesirable state for the process and so I need to program this anomaly out, if you see what I mean.
I have an alarm that will warn me that there has been a controller fault and then I can send an engineer to the local panel to investigate. So, yes, I have a means of knowing that the controller 'messed up'
RE: Behaviour of Controller on loss of Input Signal
When the device returns from its error condition switch back over after a suitable time delay to allow for settling. Or, neater, compare the difference between actual and saved input values and have a ramp routine to slowly adjust the value used by controller from the saved value back to the actual value. That might not be necessary if you have a sluggish controller response anyway.
RE: Behaviour of Controller on loss of Input Signal
If configured as ‘Hold’ the output of the loop will stay at its last good value.
On exit from sensor break the transfer is should be bumpless – the power output will step from its held value to the control value.
That's the first thing I have come up with.
RE: Behaviour of Controller on loss of Input Signal
Personally I would manage the controller input as in my post above since that is the source of the issue but, as is usual in PLC control, there's more than one way to skin a cat.
RE: Behaviour of Controller on loss of Input Signal
Thanks for your recommendation. I will get work on it and will update you on any progress over the next couple of days.
Regards.
RE: Behaviour of Controller on loss of Input Signal
TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Behaviour of Controller on loss of Input Signal
I think I have come up with a modified program and I hope to test this tomorrow and feed you guys back with what I did and the outcome
Regards.
RE: Behaviour of Controller on loss of Input Signal
RE: Behaviour of Controller on loss of Input Signal
The system is expected to be in auto during normal operation. Operation in manual mode will initiate an indication in the control room. We do not want to distract the operator too much.
I get your point though and that sounds like a robust design.
RE: Behaviour of Controller on loss of Input Signal
RE: Behaviour of Controller on loss of Input Signal
Keith Cress
kcress - http://www.flaminsystems.com
RE: Behaviour of Controller on loss of Input Signal
RE: Behaviour of Controller on loss of Input Signal
Keith Cress
kcress - http://www.flaminsystems.com