Envelope weighing apparatus with gripping compensation for variable thickness envelopes

A weighing station for an in-line scale comprising:

• (a) a capstan motor comprising a first precision servo motor;
• (b) a capstan roller operative coupled to the capstan motor;
• (c) a pinch roller linked to the capstan motor for rotation in synchrony with the capstan roller;
• (d) a tension arm motor comprising a second precision servo motor coupled to the pinch roller for controllably repositioning the pinch roller relative to the capstan roller so as to provide a gripping force for controllably gripping an article between the pinch roller and the capstan roller;
• (e) a rigid tension arm coupled to the tension arm motor and supporting the pinch roller so as to allow free rotation of the pinch roller responsive to the capstan motor, the tension arm arranged for translating rotational force of the tension arm motor into forcing the pinch roller toward the capstan roller to provide said gripping force;
• and
• (f) a motion damper coupled to the tension arm to suppress vibration of the tension arm when it closes on an article at high speed;
• and further comprising:
• a first servo amplifier electronically coupled to the capstan motor and arranged to drive the capstan motor to a selected speed of rotation while an article is gripped between the capstan roller and the pinch roller, and to acquire torque data from the capstan motor;
• and
• a second servo amplifier electronically coupled to the tension arm motor and arranged to drive the tension arm motor to apply a selected constant force to the pinch roller while an article is gripped between the capstan roller and the pinch roller, thereby minimizing slippage of the article during weighing and avoiding weighing inaccuracy resulting from variations in thickness of the article.