Field based process control system with auto-tuning

A smart field-mounted control unit powered over a process control loop and for controlling a process through a control output signal based on a measured process variable, comprising:

• input-output means adapted to be coupled to the process control loop and for receiving power from the process control loop;
• microprocessor means coupled to the input-output means and comprising:
• control means having a process variable input and a control output, wherein the control means generates the control output signal on the control output in response to the measured process variable received on the process variable input and based on control function parameters;
• and
• tuning means coupled to the control means for tuning the control function parameters to the process based on the measured process variable;
• and
• a memory coupled to the microprocessor means for storing the control function parameters.

A smart field-mounted control unit which is powered by a process control loop and controls a process through a control output signal as a function of a measured process variable, comprising:

• an input-output circuit configured to be coupled to the process control loop and for receiving power from the process control loop;
• a controller having a process variable input and a control output, wherein the controller generates the control output signal on the control output in response to the measured process variable received on the process variable input and based on control function parameters;
• a tuning circuit comprising:
• an excitation circuit having an excitation output coupled to the control output, which generates an excitation signal that rises and falls;
• an estimating circuit coupled to the process variable input, which estimates a model of the process based on a rising dead time L.sub.R, a falling dead time L.sub.F, a rising rate-of-change R.sub.R and a falling rate-of-change R.sub.F in the measured process variable in response to the excitation signal;
• and
• a parameter calculating circuit coupled to the estimating circuit and the control circuit, which calculates the control function parameters based on the process model and control function rules;
• and
• a memory coupled to the control and tuning circuits for storing the control function parameters and the control function rules.

In a process control apparatus having a controller with a process variable input and a control output, wherein the controller controls a process through a control output signal generated on the control output as a function of a measured process variable received on the process variable input, the improvement comprising:

• a tuning circuit comprising:
• excitation means having an excitation output coupled to the control output for generating an excitation signal which rises and falls over time;
• estimating means for estimating a process model based on a rising dead time L.sub.R, a falling dead time L.sub.F, a rising rate-of-change R.sub.R and a falling rate-of-change R.sub.F in the measured process variable in response to the excitation signal;
• and
• parameter calculating means coupled to the estimating means for determining the function of the controller based on the process model.

A method of determining tuning parameters for a process control system which controls a process through a control output signal as a function of a measured process variable and a set point, the method comprising:

• varying the control output signal over time such that the measured process variable rises and falls;
• determining a rising dead time L.sub.R in the measured process variable;
• determining a rising rate-of-change R.sub.R in the measured process variable;
• determining a falling dead time L.sub.F in the measured process variable;
• determining a falling rate-of-change R.sub.F in the measured process variable;
• and
• tuning the function of the process control system based on L.sub.R, L.sub.F, R.sub.R and R.sub.F.