Servo error detection of bi-directional reel-to-reel tape drives using fine line tachometers

An apparatus for controlling the motion of a web of material in a web drive, the apparatus including:

• a first reel assembly for rotating in a first direction to supply the web of material and for rotating in a second direction to take up the web of material;
• a second reel assembly for rotating in the first direction to take up the web supplied by the first reel assembly and for rotating in the second direction to supply the web of material to the first reel assembly;
• a first fine-line tachometer coupled to the first reel assembly;
• a second fine-line tachometer coupled to the second reel assembly;
• a web tension sensing device producing a direct current output for sensing the tension in the web between the first reel assembly and the second reel assembly;
• a control mechanism for controlling the rotational speed of the first and second reel assemblies said control mechanism being continually adaptive to dynamic changes of the first and second fine-line tachometers and the directly coupled output of the web tension sensing device as the web is moved from the first reel assembly to the second assembly and continually adaptive to dynamic changes as the web is moved from the second reel assembly to the first reel assembly;
• said control mechanism includes motion control logic means responsive to said first and second fine-line tachometer and the direct current output from said web tension sensing device for calculating the appropriate rotation for each reel to drive the web between the reels with a predetermined profile with negligible web tension disturbances.

A method for controlling motion and tension of a web of material as it is being transferred from one reel to a second reel, comprising the steps of:

• sensing the output of a fine-line tachometer from both the first and second reels;
• comparing the output from both fine-line tachometers to a fixed frequency clock to obtain a velocity count from both the first and second reels;
• converting the velocity count from both the first and second reels to a velocity of the web at the reels;
• counting the fixed frequency clock pulses between the fine-line tachometer outputs of both the first and second reels to obtain the angular velocity of the web at both the first and second reels;
• obtaining the radius ratio between both the first and second reels;
• obtaining the linear velocity of the web at the first reel from the angular velocity of the web at the first reel from the angular velocity of the web from the first reel and the radius ratio;
• obtaining the linear velocity of the web at the second reel from the angular velocity of the web at the second reel from the angular velocity of the web from the second reel and the radius ratio;
• obtaining the directly coupled output from a tension transducer;
• monitoring the differences of the linear velocities from the first and second reel and the directly coupled output from the tension transducer;
• and
• indicating any error in the differences of the linear velocities between the first and second reels and any difference between the direct current output of the tension transducer and a reference current.

An article of manufacture for use in an apparatus for controlling the motion of a web of material in a web drive, the apparatus including a first reel assembly for rotating in a first direction to supply a web of material and for rotating in a second direction to take up the web of material, a second reel assembly for rotating in the first direction to take up the web of material supplied by the first reel assembly and for rotating in the second direction to supply the web of material to the first reel assembly, a first fine-line tachometer coupled to the first reel assembly, a second fine-line tachometer coupled to the second reel assembly, a web tension sensing device producing a directly coupled output for sensing the tension in the web between the first reel assembly and the second reel assembly, a control mechanism which is continually adaptive to dynamic changes as the web is moved from the first reel assembly to the second assembly and continually adaptive to dynamic changes as the web is moved from the second reel assembly to the first reel assembly, said control mechanism including motion control logic means responsive to said measuring means for calculating from said web position error and said inertia values the appropriate torque for each reel to drive said position error to zero along with a predetermined profile with negligible tape tension disturbances;

• said article of manufacture in said control mechanism comprising a computer-readable storage medium tangibly embodying a program of executable computer instructions which may cause said apparatus to control the motion of the web of material, said article of manufacture to:
• sense the output of a fine-line tachometer from both the first and second reels;
• compare the output from both fine-line tachometers to a fixed frequency clock to obtain a velocity count from both the first and second reels;
• convert the velocity count from both the first and second reels to a velocity of the web at the reels;
• count the fixed frequency clock pulses between the fine-line tachometer outputs of both the first and second reels to obtain the angular velocity of the web at both the first and second reels;
• obtain the radius ratio between both the first and second reels;
• obtain the linear velocity of the web at the first reel from the angular velocity of the web at the first reel from the angular velocity of the web from the first reel and the radius ratio;
• obtain the directly coupled output from a tension transducer;
• monitor the differences of the linear velocities from the first and second reel and the direct current output from the tension transducer;
• and
• indicate any error in the differences of the linear velocities between the first and second reels and any difference between the directly coupled output of the tension transducer and a reference current.