Hello people,
I must admit I don't understand story about manual and automatic mode of operation completely.
As far as I know when process is switch to automatic control that means that system works in closed loop. There is a set point that is set by operator, controller, actuator and finally proces variable. For example set point is with some kind of electronic device converted to to 4-20 mA signal that goes into PLC. PID controller is implemented in PLC, and mA signal goest to actuator (for example it's some kind of regulating valve that is used to control process temperature). In feedback path is usually temperature transmitter that converts PV (temperature) to mA signal that is led to PLC.
If mode of operation is automatic then whole system works in a closed loop with all parts I've just stated, right?
When operator switch to manual, does that mean that only feedback path is open (closed loop is broken), and control system works in open loop:
Set point --> PID controller -->Actuator-->Process?
Is whole story about manual/automatic control tailored to some kind of software switch that break feedback path, or there's more to it.
In my modest experience, I have worked with Siemens FB41 continuous PID block and in that scheme I can see that when MAN is ON (bool) output from controller is just "Manual" variable (real) and all PID structure is simply bypassed.
There is one more thing.
In school I learned that PID form is:
CO = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt
and in Laplace domain that is:
Gpid = K+Ki/s+Kds, where Gpid is defined as ration of Laplace transforms of CO(t) and e(t)
where CO(t) represent controller output (in time) and e(t) is error (in time) which is defined as SP(t)-PV(t).
Recently, after I have read one article about PID I saw the following form:
CO = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt + CObias
where CObias is defined as controller output when error is zero and disturbance is not present and needed to maintain PV depending of SP.
It also stated that CObias must exist because when error signal is zero there must be some controller output to maintain output PV.
I can understand that but how to obtain Laplace transfer function then?
Why bias is not included in most textboox I have read?
Why many software for simulation like Matlab and Simulink doesn't include this bias value in PID block parameters?
I tried to explain this to myself:
CO(t) = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt + CObias
Since, we're interested into process dynamics and that means we want to test how system reacts to variable change that is exactly why Laplace tranform is used.
What does that means?
First of all, definition of transfer function(TF):
TF is defined as ratio of Laplace transforms of output and input respectively when all initial conditions are zero.
Hmm, this requirement zero initial conditions is somewhat unrealistic, but soon I realised that this does not means that all changes are from 0 to some final value, it can be from from some initial value IV to some final value FV, linear system will behave same just like change from 0 to some new value (FV-IV).
So in my opinion, to obtain transfer function of PID we need to introduce new variable, called deviation varibale which is defined as current value minus steady state value:
co(t) = CO(t)-CObias, so new form will be:
co(t) = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt
CO(t) has initial condition CObias, but co(t) has initial condition zero, so Laplace transform can be applied.
Simulation software such as Simulink is good for analyse of process dynamics, that's why systems are often test to step change.
Can you, please, comment this?
Oki, that was theory, what about practice?
I have worked with Siemens FB41 PID ant that PID block doesn't have CObias and that is where this story begins to confuse me.
I feel I'm pretty close to completely understand this but need your help.
I have read in that same article that most tuning software has CObias value, but in FB41 I cannot see one.
I also tried to simulate CObias in Simulink (adding constant to CO output of PID block) but system behaves like that CObias is disturbance and finaly reach steady state after some time.
So I'm not really sure if this CObias is needed, because if PI is used, integral part represent continuous sum and hence that is CObias, if P-only is used, process will most likely have offset in it's output (response to step input), so controller will have some output value different from zero.
Did you have to manually set up this CObias or not?
This thing really bothers me.
If you made up this far, I thank you for your time and patience.
I hope your answers will help me to understand and overcome this issue.
I have set up only one PID controller in my life so far, and it worked well but at that time I didn't know that CObias exists. In Matlab and in my textboox there were only Kp, Ki and Kd.
Thank you very much and I'm looking forward to your replies.
I must admit I don't understand story about manual and automatic mode of operation completely.
As far as I know when process is switch to automatic control that means that system works in closed loop. There is a set point that is set by operator, controller, actuator and finally proces variable. For example set point is with some kind of electronic device converted to to 4-20 mA signal that goes into PLC. PID controller is implemented in PLC, and mA signal goest to actuator (for example it's some kind of regulating valve that is used to control process temperature). In feedback path is usually temperature transmitter that converts PV (temperature) to mA signal that is led to PLC.
If mode of operation is automatic then whole system works in a closed loop with all parts I've just stated, right?
When operator switch to manual, does that mean that only feedback path is open (closed loop is broken), and control system works in open loop:
Set point --> PID controller -->Actuator-->Process?
Is whole story about manual/automatic control tailored to some kind of software switch that break feedback path, or there's more to it.
In my modest experience, I have worked with Siemens FB41 continuous PID block and in that scheme I can see that when MAN is ON (bool) output from controller is just "Manual" variable (real) and all PID structure is simply bypassed.
There is one more thing.
In school I learned that PID form is:
CO = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt
and in Laplace domain that is:
Gpid = K+Ki/s+Kds, where Gpid is defined as ration of Laplace transforms of CO(t) and e(t)
where CO(t) represent controller output (in time) and e(t) is error (in time) which is defined as SP(t)-PV(t).
Recently, after I have read one article about PID I saw the following form:
CO = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt + CObias
where CObias is defined as controller output when error is zero and disturbance is not present and needed to maintain PV depending of SP.
It also stated that CObias must exist because when error signal is zero there must be some controller output to maintain output PV.
I can understand that but how to obtain Laplace transfer function then?
Why bias is not included in most textboox I have read?
Why many software for simulation like Matlab and Simulink doesn't include this bias value in PID block parameters?
I tried to explain this to myself:
CO(t) = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt + CObias
Since, we're interested into process dynamics and that means we want to test how system reacts to variable change that is exactly why Laplace tranform is used.
What does that means?
First of all, definition of transfer function(TF):
TF is defined as ratio of Laplace transforms of output and input respectively when all initial conditions are zero.
Hmm, this requirement zero initial conditions is somewhat unrealistic, but soon I realised that this does not means that all changes are from 0 to some final value, it can be from from some initial value IV to some final value FV, linear system will behave same just like change from 0 to some new value (FV-IV).
So in my opinion, to obtain transfer function of PID we need to introduce new variable, called deviation varibale which is defined as current value minus steady state value:
co(t) = CO(t)-CObias, so new form will be:
co(t) = K*e(t)+Ki*integral(e(t))+Kd*de(t)/dt
CO(t) has initial condition CObias, but co(t) has initial condition zero, so Laplace transform can be applied.
Simulation software such as Simulink is good for analyse of process dynamics, that's why systems are often test to step change.
Can you, please, comment this?
Oki, that was theory, what about practice?
I have worked with Siemens FB41 PID ant that PID block doesn't have CObias and that is where this story begins to confuse me.
I feel I'm pretty close to completely understand this but need your help.
I have read in that same article that most tuning software has CObias value, but in FB41 I cannot see one.
I also tried to simulate CObias in Simulink (adding constant to CO output of PID block) but system behaves like that CObias is disturbance and finaly reach steady state after some time.
So I'm not really sure if this CObias is needed, because if PI is used, integral part represent continuous sum and hence that is CObias, if P-only is used, process will most likely have offset in it's output (response to step input), so controller will have some output value different from zero.
Did you have to manually set up this CObias or not?
This thing really bothers me.
If you made up this far, I thank you for your time and patience.
I hope your answers will help me to understand and overcome this issue.
I have set up only one PID controller in my life so far, and it worked well but at that time I didn't know that CObias exists. In Matlab and in my textboox there were only Kp, Ki and Kd.
Thank you very much and I'm looking forward to your replies.