Strapping machine having primary and secondary tensioning units and a control system therefor

An apparatus for bundling one or more objects with flexible strap material, the apparatus comprising:

• a frame;
• a dispenser of the strap material retained by the frame;
• a strap material accumulating compartment fixed to the frame and having an inlet that receives a free end of the strap material from the dispenser, and an outlet;
• a track retained by the frame and adjacent to the group of objects, the track releasably receiving a portion of the strap material about but spaced apart from the group of objects;
• a feed and tension unit retained by the housing at the outlet of the strap material accumulating compartment and having a first servo motor, a drive roller rotatably received by the first servo motor, a motor encoder coupled to the drive roller and providing drive roller signals indicating a position of the drive roller, and a position sensor and a pinch roller biased against the drive roller so that the pinch and drive rollers feed the free end of the strap material about the track under control of the first servo motor, and the position sensor being coupled to the pinch roller and providing pinch roller signals indicating a position of the pinch roller;
• a cutting and sealing unit retained by the frame and adjacent to the track and feed and tension unit, the sealing unit having a second servo motor that actuates first and second hand gripper arms, the first gripper arm retaining the free end of the strap material while the pinch and drive rollers retract a first upstream portion of the strap material under control of the first servo motor to provide a primary tensioning of the strap material about the objects, the second gripper arm retaining a second upstream portion of the strap material following primary tensioning, and the sealing unit having heating and cutting members, actuatable under control of the second servo motor, that seal the free end and second upstream portion of the strap material together and sever the second upstream portion from a remainder of the strap material, respectively;
• a secondary tension unit retained by the frame and having a biased roller and a gripper unit adjacent to the first upstream portion of the strap material, the gripper unit capable of gripping the second upstream portion of the strap material while the biased roller forces a downstream portion of the strap material in a direction transverse to a direction of the strap material to provide a secondary tensioning of the strap material about the objects prior to sealing and cutting by the heating and cutting members, the secondary tension unit having an electrically operable disabling unit that selectively disables the secondary tension unit in response to a disabling signal;
• and
• a control unit electrically coupled to the first servo motor, motor encoder, position sensor, second servo motor and disabling unit, the control unit providing forward signals to the first servo motor to control feeding of the strap material about the track and providing reversing signals to the first servo motor to control primary tensioning of the strap material about the objects, the control unit monitoring the drive and pinch roller signals and halting the first servo motor when the drive and pinch roller signals have a predetermined relationship to a first selected value, and the control unit providing the disabling signal to the disabling unit to selectively disengage the secondary tension unit.