Constant tension device

A tensioning device, comprising:

• a primary spring attached to a carrier so as to apply a primary spring force directed along an axis, the carrier being constrained to only move along the axis, the primary spring force applied to the carrier changing in accordance with a primary spring rate function as the carrier moves relative to the primary spring along the axis;
• a wire or string attached to the carrier and extending along the axis so that a net axial force applied to the carrier is applied to the wire or string;
• and
• a secondary spring having a first end attached to the carrier so as to apply a secondary spring force to the carrier, the secondary spring force directed across the axis and having an axial component that is applied to the carrier in a direction along the axis and a normal component that is applied to the carrier in a direction normal to the axis, the secondary spring force configured so that the axial component of the secondary spring force varies in accordance with a secondary spring rate function as the carrier moves relative to the primary spring along the axis;
• wherein the net axial force applied to the carrier comprises the sum of the primary spring force and the axial component of the secondary spring force;
• and
• wherein the secondary spring rate function approximates and opposes the primary spring rate function as the carrier moves axially within an operating range so that the net axial force stays within a desired range about a preferred tension as the carrier moves axially within the operating range.