Seat suspension mechanism

A seat suspension mechanism comprising:

• an upper frame and a lower frame that are supported so as to be capable of a separating and approaching operation relative to each other, with a link mechanism therebetween;
• and a spring mechanism which elastically biases the upper frame,
• wherein the spring mechanism includes:
• a first spring part which biases the upper frame in a direction in which the upper frame separates from the lower frame;
• and
• a second spring part which, when the upper frame is in a predetermined up-down movement range including a balanced point, exhibits a spring characteristic of biasing the upper frame in a direction in which the upper frame approaches the lower frame, to create a constant load spring characteristic by being combined with the first spring part,
• the seat suspension mechanism further comprising a body weight adjustment mechanism which adjusts a position of the upper frame to the balanced point in a state in which a person is supported,
• wherein the first spring part includes a lower frame-side torsion bar which is connected to the lower frame and is twisted by movement of the link mechanism and an upper frame-side torsion bar which is connected to the upper frame and is twisted by the movement of the link mechanism,
• wherein the body weight adjustment mechanism includes:
• an elastic force adjusting unit which is connected to the upper frame-side torsion bar and adjusts the position of the upper frame by twisting the upper frame-side torsion bar;
• a torsion angle detection sensor which detects a torsion angle of the lower frame-side torsion bar;
• and
• a control unit which compares the torsion angle of the lower frame-side torsion bar detected by the torsion angle detection sensor with a balanced point torsion angle that the lower frame-side torsion bar has when the upper frame is at the balanced point, and outputs, to the elastic force adjusting unit, a control signal instructing to twist the upper frame-side torsion bar by a predetermined amount to make the torsion angle of the lower frame-side torsion bar agree with the balanced point torsion angle, and
• wherein a damper is suspended between the lower frame and the upper frame, and a movement zone of a piston in a cylinder of the damper corresponding to the predetermined up-down movement range including the balanced point when the upper frame moves up and down relative to the lower frame is free running zone where a damping force in the free running zone is lower compared with a damping force in a zone other than the free running zone.