How does Motor Torque change along the travel of the seat ?

A Ford SUV Third Row Seats have Power Folding Seats.  With one-touch of a button seat travels from seated to folded position. Or from folded to seated position.  This requires some sort of obstacle detection.  While traveling if seat hits a suitcase, it will reverse.

Brush DC Motor drives the seat.

Please try to explain following sentence from a paper:

"With the considerable weight of a seat that is involved the motor torque changes along the travel of the seat, sometimes even reaching negative values in those positions where obstacles would most likely need to be detected."

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naseeamAsked:
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CompProbSolvCommented:
I can make an educated guess about most of that statement.

When the seat is being lifted from a folded position, the greatest torque is needed at first.  The key is to look at the center of gravity of the back of the seat (generally located in the center of the back) and how it is affected by gravity.  If the seat back were horizontal, the entire weight has to be lifted against gravity.  As the seat back gets more vertical, there is a mechanical advantage between how much the seat axis rotates and the amount that the center of mass is lifted, so less torque is needed.

When the seat back goes beyond vertical, it falls back.  The motor restricts this fall, which is translated as negative torque.  This assumes that the lifting torque was defined as positive; it could have been defined opposite to this.

When the seat back is falling back (when negative torque is required) the last phrase implies it is the most likely time for obstacles to be detected.  That makes some sense as the further back it reclines, the more likely it is to hit something behind it.
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Tiras25Commented:
From what I understand it sounds like it works like a garage door openers or a window motor.  The motor doing less work depending on where on the axis it is moving.  While the motor has to lift the majority of the weight of the seat, the torque output will be highest, when the seat is on the downward part of the axis, the motor has to use less force, and possibly go into negative values(reverse) to keep the seat from falling, essentially setting the seat down.  
Therefore when the seat starts going down, and it is supposed to be coasting, and all of a sudden their is a spike in the voltage required, the internals can infer there may be an object in the way and reverse itself.
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