Loop dynamics is the study of how the system process variable (PV) changes,as a function of time,in response to a sudden input change or upset. The upset may be caused by process disturbance,a load change,or an adjustment of the setpoint.
What is Process Time Lag?
In an ideal loop,there are no delays.That is when an input occurs,there is an instantaneous output response and the error between the new SP and the the PV is corrected immediately.
In practical systems,output lags behind inputs. Any delay in response to an input signal is called a Process Time Lag.
The sharp change in input signal signal to the system is called a step change. The transition from one input value to another input value occurs in virtually zero time. However the output does not rise to its new value with the same rate of change as the input. A certain measurable amount of time is needed for the output to rise to its new final value.
What are the causes for Process Time Lag?
Process Time lags are caused by the following three properties of a process.
1.Capacitance. 2.Resistance. 3.Dead time.
Delays caused by capacitance and resistance often are called transfer lags.
Capacitance is the change in the amount of material or energy required to make a unit change in potential.
Example - Thermal Capacitance ,Liquid Capacitance,Electrical Capacitance.
Thermal Capacitance is the amount of heat energy that must be added to an object to raise its temperature.
Liquid Capacitance is the volume of liquid that must be added to a tank to increase the pressure at the tank bottom by one unit.
Electrical Capacitance is the amount of electrical charge that must be stored in an electrical capacitor to raise its voltage.
Resistance is the opposition to the flow of material or energy. It may be expressed as the amount of potential required to produce a unit change in the quantity of material or energy moved in a unit of time.
Examples - Thermal Resistance, Liquid Flow Resistance, Electrical Resistance.
Dead time or Transportation lag involves the movement of mass of mass from one point to another. Dead time is the fixed interval of time that elapses between the moment when a component receives an input signal and the moment when a response to that input is first detected.