Constant speed strap drive control unit

1. A method for operating a strap drive control apparatus used to position a ventilation control member in an animal facility, the method comprising:

inputting an initial spool diameter of a spool driven by a stepper motor;

inputting a strap thickness for a strap attached to the spool;

inputting a strap configuration of the strap on the spool;

determining an effective spool diameter based on an initial position of the strap relative the spool and the strap configuration;

attaching one or more positionable control members to the strap;

determining an initial stepper motor speed required to move the control member at a desired control member speed;

repositioning the one or more control members at the control member speed by operating the stepper motor at the initial stepper motor speed to rotate the spool;

receiving an input from an encoder about a new position of the strap;

calculating a new effective diameter of the spool using the input from the encoder for the position of the strap and the strap thickness;

determining a modified stepper motor speed based on the new effective diameter of the spool that would cause the stepper motor to move the one or more control members such that the one or more control members moves uniformly at the desired control member speed; and

repositioning the one or more control members at the desired control member speed by operating the stepper motor at the modified stepper motor speed.

2. A ventilation control system for an animal facility, the ventilation control system having a movable ventilation control member mounted in an opening of the animal facility, the ventilation control member being movable relative the opening to control the amount of air that is able to pass through the opening, the ventilation control system also having a positioning strap attached to the ventilation control member, the strap having a strap thickness, and a strap drive control apparatus configured to control the positioning of the ventilation control members, wherein the strap drive control apparatus comprises:

a rotating spool having an initial spool diameter;

an electrically driven stepper motor to move the positioning strap by winding or unwinding the strap on the rotating spool;

an encoder; and

a controller configured to operate the stepper motor, the controller configured to:

determine an effective spool diameter based on an initial position of the strap relative the spool;

determine an initial stepper motor speed required to move the control member at a desired control member speed;

reposition the control members at the control member speed by operating the stepper motor at the initial stepper motor speed to rotate the spool;

receive an input from the encoder about a new position of the strap;

calculate a new effective diameter of the spool using the input from the encoder for the position of the strap and the strap thickness;

determine a modified stepper motor speed based on the new effective diameter of the spool that would cause the stepper motor to move the control member such that the control member moves uniformly at the desired control member speed; and

reposition the control members at the desired control member speed by operating the stepper motor at the modified stepper motor speed.

3. The ventilation control system of claim 2 wherein the controller receives the initial spool diameter and the strap thickness.

4. The ventilation control system of claim 2 wherein the controller receives a strap configuration of the strap on the spool and determines the effective spool diameter based on an initial position of the strap relative the spool and the strap configuration.