Anti-windup and noise protection that can be used with any general digital controller. The anti-windup technique allows the implementation of noise filtering techniques to decrease or eliminate chatter of the control output at the limit. In a higher order controller that uses a feedforward signal, the anti-windup technique prevents an unexpected change in the control output when a step change occurs in the feedforward signal. The technique can be used to provide anti-windup protection in both single layer and multi-layer control schemes.

A system and method of controlling air quality within a room. The method includes determining a first discharge air temperature setpoint based upon a room temperature setpoint and a first value indicative of an air temperature within the room, and determining a first air flow control signal based upon the first discharge air temperature setpoint and a second value indicative of an air temperature of discharge air being provided into the room. The method further includes controlling an air flow device based upon the first air flow control signal, and maintaining a heating device employed to influence the air temperature of the discharge air at a maximum heating level. The method additionally includes monitoring a level of carbon dioxide within the room to determine whether the level is below a predetermined threshold.

A PID control system and method which maintains continuity of the control output in spite of changes in controller parameters. The controller parameters include a proportional gain, an integral gain, a derivative gain, and a manual/automatic mode parameter. The control output at the time of a parameter change is set equal to the previous controller output, and the integrated error is set to a value consistent with the previous controller output according to a PID controller equation. This value for the integrated error serves to ensure the sustained continuity of the control output after the controller parameter change. Alternatively, the control output at the time of a parameter change is set equal to a first value continuous with respect to one or more previous control output values, and the integrated error is set to a value consistent with the first value according to the PID controller equation. Furthermore, the system and method employs integrator anti-windup. If the control output saturates, i.e. falls outside a control region defined by a lower control bound and an upper control bound, the system and method of the present invention prescribe (a) setting the control output to the value of the control bound which is exceeded, and (b) setting the integrated error equal to a value consistent with the exceeded control bound according to the PD controller equation.

A control apparatus is disclosed that comprises a primary proportional, integral, differential ("PID") controller capable of receiving a first setpoint and a first process variable and generating therefrom a second setpoint; and a secondary controller capable of receiving the second setpoint and a second process variable and generating therefrom an output control signal, wherein the primary PID controller is capable of receiving from the secondary controller a feedback signal 1) that indicates that a previous value of the second setpoint exceeds a limit associated with an output control signal of the secondary controller, and 2) that transfers a value of a signal from the secondary controller. The primary PID controller is then capable of limiting the contribution of the integral calculation component in a PID calculation that generates a new current value of the second setpoint. The integral calculation component may be excluded, included, or partially included in the PID calculation in order efficiently minimize the effect of undesirable erratic output signals.

A system and method for automatically tuning a PID controller resident within a PID control loop. The PID control loop includes a PID controller and a process. The process supplies a process variable which is compared to the loop input. The result of the comparison is supplied to the PID controller, and the PID controller drives the process. A relay is applied to the loop input. The relay compares a set point value to the process variable. If the set point value is greater than the process variable, the relay drives the loop input with a first amplitude value. If the set point value is less than the process variable, the relay drives the loop input with a second amplitude value. In response to the set point relay, the process variable develops a sustained oscillation. The period and amplitude of the sustained oscillation are measured. A new set of PID controller parameters are calculated from the period and amplitude of sustained oscillation. In particular, the oscillation period and amplitude are used to calculated a time constant and dead time for a standard process model. The time constant and dead time are used to calculate the new PID controller parameters either (a) directly through the formulae associated with the Ziegler-Nichols reaction curve method, or (b) through the intermediate step of calculating an ultimate period and frequency from the time constant and deadtime.