US 2003/0197066 A1 · Slope predictive control and digital PID control

Abstract

The present invention provides an airflow control loop that uses averaged airflow measurements without the problems that are normally encountered with averaging measurements, such as the delay introduced into the airflow control loop. This is accomplished, in the present invention, through a predictive control scheme. The predictive control scheme of the airflow control loop calculates the damper sensitivity, calculates the damper runtime needed to achieve setpoint, and then runs the damper for the determined period of time. In addition to the unique airflow control loop used to maintain the constant airflow into the room, the present invention also implements a digital form of proportional, integral, and derivative (PID) control to maintain the room temperature. The digital form of PID control uses non-linear gains which vary according to how far the measured temperature is from setpoint.

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Drawings

Claims38

Dependent claims

A method for controlling a Variable Air Volume (VAV) box in a pressure independent VAV temperature control system comprising:

receiving an indication of an airflow setpoint;
measuring an airflow in the VAV box and calculating an error between the airflow setpoint and the measured airflow;
and
until a new airflow setpoint indication is received, repeating the steps comprising:
based on the error, predicting a damper runtime to move a damper in the VAV box to achieve the airflow setpoint,
if the predicted damper runtime is less than a minimum runtime, not moving the damper and measuring the airflow to calculate an average airflow over a period of time,
if the predicted damper runtime is not less than the minimum runtime, measuring the airflow in the VAV box while moving the damper for the predicted damper runtime or until the airflow is measured to have crossed the airflow setpoint,
in response to moving the damper, measuring the airflow to calculate the average airflow, and
based on the average airflow, recalculating the error.

6 more elements

A pressure independent Variable Air Volume (VAV) temperature control system comprising:

a VAV box having an airflow control damper for controlling an airflow delivered to a room and an airflow sensor for measuring the airflow;
a temperature sensor within the room for measuring a temperature of the room;
and
a micro-controller configured for executing computer-executable instructions for:
receiving a room temperature measurement from the temperature sensor and using the temperature measurement to calculate an airflow setpoint,
receiving an airflow measurment from the airflow sensor and calculating an error between the airflow setpoint and the airflow measurment, and
until a new room temperature measurement is received from the temperature sensor and a new airflow setpoint is calculated, repeating the steps comprising:
based on the error, predicting a damper runtime to move the damper in the VAV box to achieve the airflow setpoint,
if the predicted damper runtime is less than a minimum runtime, not generating a signal for moving the damper and receiving airflow measurements from the airflow sensor to calculate an average airflow over a period of time,
if the predicted damper runtime is not less than the minimum runtime, receiving airflow measurements from the airflow sensor while generating the signal for moving the damper for the predicted damper runtime or until one of the airflow measurements is determined to have crossed the airflow setpoint,
in response to movement of the damper, receiving airflow measurements from the airflow sensor to calculate the average airflow, and
based on the average airflow, recalculating the error.

9 more elements

A pressure independent Variable Air Volume (VAV) temperature control system comprising:

a VAV box having an airflow control damper for controlling an airflow delivered to a room and an airflow sensor for measuring the airflow;
a temperature sensor within the room for measuring a temperature of the room;
and
a micro-controller configured for executing computer-executable instructions for performing a PID control having a proportional term, an integral term and a derivative term, the computer-executable instructions comprising:
in response to receiving a room temperature measurement, calculating a temperature error between the room temperature and a temperature setpoint;
in response to determining that the temperature error is greater than a temperature threshold, calculating an airflow setpoint using a significantly increased PID output and accumulating the integral term;
after the expiration of a time increment, receiving a new room temperature measurement and using the new room temperature measurement to recalculate the temperature error;
in response to determining that the temperature error is less than the temperature threshold, determining whether the temperature error is within a deadband of the temperature setpoint;
in response to determining that the temperature error is within the deadband of the temperature setpoint, recalculating the PID output by updating the proportional term but not updating the integral term and calculating the airflow setpoint using the recalculated PID output;
and
in response to determining that the temperature error is not within the deadband of the temperature setpoint, recalculating the PID output by updating the proportional term and the integral term and calculating the airflow setpoint using the recalculated PID output.

8 more elements

Dependencies are inferred from the claim text.

Description6,299 words

Summary1,656 words

#The present invention relates generally to control systems. More particularly, the present invention relates to Variable Air Volume (VAV) temperature control systems.

#Variable Air Volume (VAV) temperature control systems control the temperature within a room by modulating the amount of cool (or warm) air that is blown into the room by the heating and air conditioning system. A pressure dependent VAV temperature control system uses a temperature control loop to accomplish a temperature control algorithm. The temperature control loop attempts to maintain a room temperature setpoint by using readings from a room temperature sensor to control a damper in a VAV box. Opening and closing the damper in the VAV box varies the airflow through the VAV box into the room.

#One drawback to pressure dependent VAV temperature control systems is that a central Air Handling Unit supplies air to many VAV boxes and when the damper of one VAV box is opened or closed to adjust the temperature in its associated room, the airflow into the other rooms will be affected. This can cause the temperature control loops to “fight” each other, as a temperature change in one room will likely cause a flow change in the other rooms. As a result of the flow changes in the other rooms, the temperatures in those rooms will also change and as the VAV boxes in those rooms react to the temperature changes, they will cause more flow changes and the cycle is repeated. Accordingly, pressure dependent VAV control systems can be inefficient for controlling a heating, ventilation and air conditioning (HVAC) system within a sufficiently large building having multiple rooms.

Brief description of the drawings3 figures

#FIG. 1 is a pictorial diagram of the Pressure Independent VAV Control System of the present invention.

#FIG. 2 is a flow diagram illustrating the Slope Predictive Control Method of the present invention.

#FIG. 3 is a flow diagram illustrating the Digital PID Control Method of the present invention.

Detailed description4,594 words

#In one embodiment, the present invention provides flow and temperature control loops to accomplish a Variable Air Volume (VAV) temperature control algorithm in a manner that addresses the problems encountered in the control of prior art pressure independent VAV control systems.

#To address the problem of noisy instantaneous signals representing flow measurements, the present invention averages the instantaneous flow measurements over a period of time. The averaging technique serves to settle and eliminate a significant amount of the noise in the instantaneous flow measurement signals, and provides a better measurement of the average airflow into the room. Normally, when flow measurements are averaged over time, delay is introduced into the control loop. This delay causes the flow controls to overshoot the setpoint because by the time the averaging of the measurements is complete, the airflow control damper has moved past the desired position. The present invention avoids the overshoot problem by using instantaneous flow readings in situations where a fast response is needed, then using average flow readings to get an accurate reading of the flow after the damper is moved and by using a predictive algorithm (rather than feedback control) for the next damper movement.

#The following description will hereinafter refer to the drawings, in which like numerals indicate like elements throughout the several figures. FIG. 1 is a pictorial diagram of an illustrative system 100 for implementing the Variable Air Volume (VAV) temperature control algorithm of an exemplary embodiment of the present invention. The system 100 comprises a VAV box 102, an airflow control damper 104, an airflow sensor 106, a micro-controller 108, and a room temperature sensor 110. The flow control loop measures the airflow into a room, using measurements from the airflow sensor 106. Airflow measurements are used to calculate the slope of the airflow damper response curve (change in flow per unit of time) every time the airflow control damper 104 moves for a measured period of time. For example, the flow control loop may be configured to calculate the change in airflow when the control damper 106 moves for at least one second. A calculated change in airflow can then be used to predict how long the airflow control damper 104 must move in order to deliver the appropriate flow rate needed for maintaining the desired room temperature setpoint.

Facts

Application

10/425,136Filed 28 Apr 2003

Priority

5 May 2000

Published

23 Oct 2003

Term ends

Not estimatedThe source records don’t support an estimate.

Classification8 codes

G05D 23/1904
Characterised by the use of a variable reference value: variable in time
F24F 11/30
Control or safety arrangements: for purposes related to the operation of the system, e.g. for safety or monitoring
F24F 11/62
Control or safety arrangements: characterised by the type of control or by internal processing, e.g. using fuzzy logic, adaptive control or…
F24F 11/72
Control systems characterised by their outputs; Constructional details thereof: for controlling the supply of treated air, e.g. its pressure

Family

Application60/203,229Us provisional application · filed 5 May 2000

Application09/850,422Continuation · filed 7 May 2001

Application10/425,136Continuation · filed 28 Apr 2003

US 6,554,198 B1Related patent

US 6,715,690 B2Related patent

US 2003/0197066 A1This patent

From the US filing history. Foreign family members aren’t listed.

Slope predictive control and digital PID control

Life of this patent

Slope predictive control and digital PID control

Life of this patent

Life of this patent

Abstract

Drawings

Drawings

Claims38

Description6,299 words

Drawings

Classification8 codes

Life of this patent

Description6,299 words