Kalman filters in process control systems

A control system for use in controlling a process, the control system comprising:

• a control unit including a process variable input and a control routine unit communicatively coupled to the process variable input, wherein the control routine unit generates a control signal for use in controlling the process based on a process variable value received at the process variable input;
• a Kalman filter unit coupled to the control unit, the Kalman filter unit operating once during each of a number of execution cycles to produce a process variable estimate, the Kalman filter unit including;
• a control signal input coupled to receive the control signal produced by the control routine unit,
• an interface including a process variable feedback input that receives a process variable measurement signal less frequently than once per the execution cycle time of the Kalman filter unit,
• a process model coupled to receive the control signal at the control signal input to produce an initial process variable estimate,
• a correction unit coupled to use the process variable measurement signal received via the process variable feedback input to produce a correction signal from a residual, wherein the correction unit includes a first combiner, a switch unit and a gain unit, and
• a second combiner coupled to the process model and to the correction unit to combine the initial process variable estimate with the correction signal to produce a further process variable estimate;
• wherein during the execution cycle of the Kalman filter unit at which a new value of the process variable measurement signal is available and during a predetermined number of execution cycles after the execution cycle at which a new value of the process variable measurement signal is available, the switch unit operates to provide a new value of the residual to the gain unit to produce the correction signal, wherein the new value of the residual is determined by combining the initial process variable estimate with a value of the process variable measurement signal at the first combiner;
• wherein during execution cycles of the Kalman filter unit after the predetermined number of execution cycles after the execution cycle at which a new value of the process variable measurement signal is available, the switch unit operates to provide a stored value of the residual to the gain unit to produce the correction signal, wherein the stored value of the residual is determined during the one of the execution cycles of the predetermined number of execution cycles;
• and
• wherein the process variable input of the control unit is coupled to receive the process variable estimate based on the further process variable estimate.