control parameters for PID

E

Thread Starter

engineer

hi all,

i am using one of those single loop pid controllers and it has got a parameter "manual reset value" along with usual pid. the unit is %.
range is 0% to 100% or -100% to +100% for heat cool.

tried consulting the manual and it reads "this value is applied as a bias to the control output. manual reset is applied with integral action both on & off".

couldn't make out much.
 
Congratulations on finding another confusing PID term :p

Your single loop controler output is normaly the output of its internal PID algorithm (which in an old pneumatic single loop controller may well be a mechanical "calculation"). However, the "Manual Reset Value" can be added to alter the actual output of the controller. So....

Controller Output = PID Algorithm Output + Manual Reset Value.

If the PID Algorithm inside the controller calculates an actuator position of 50% and your Manual Reset Value is 25% then your single loop controller output is (50+25) = 75%

No, I have no idea why its called "manual reset value". In practice, this value can be used to connect an external bias from another PID controller acting in "Feed Forward" mode.

Rob
www[.]lymac.co.nz
 
Manual reset is a term from the days of pneumatic controllers when proportional only control was applied. If say you were controlling a process at a set point of 50% you could adjust the manual reset to bias the output until the process variable was equal to set point (zero offset). If then a load change occurred in the process such that an offset (difference between set point and process variable) developed then the operator could re-adjust the manual reset until the process variable was again equal to the set point. This has always been the problem with proportional only control that offsets will occur when process load changes occur.
 
For you history buffs...

The "I" term in PID is called alternatively Integral or Reset. The PID algorithm in pneumatics, electronics, and digital is really an automatic reset term that drives the reset term to zero over time by use of integration. The integral time constant is a dimensionless number that is often expressed as "repeats per minute." A repeat is roughly the number of times in a minute that the algorithm calculates a reset term.

The simple controller here sounds like a P-only controller, not PID.

Dick Caro

Richard H. Caro, CEO, CMC Associates
Certified Automation Professional (ISA)
Buy my books at the ISA Bookstore:
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i used to believe the same as you said R. Caro.
but the parameters i had to fill included the following along with others, pb for heat cool o/p, integral action time, derivative action time, manual reset value.
 
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