Abstract: A series of parallel pre-stressed multi-limb six-axis force sensor with double-layer architecture is proposed, the structure stability and the work validity of the sensor are analyzed. Since the proposed sensor uses sphere pair with unilateral constraint, the limbs of the sensor must be under pressure in the measurement process, so prior to measurement, a certain preload must be applied to the sensor. The viable limbs’ number of the pre-stressed six-axis force senor is determined by using the theorem of convex analysis. By solving the static equilibrium equation of sensor, the reacting forces on limbs are divided into two parts: the particular solution is generated by the external force, while the homogeneous solution is only concerned with the pre-tightening force. The number of parameters to be determined in homogeneous solution is decreased by linear transform, and the method for determining the pre-tightening force is derived to make all the limbs compressed when it is subjected to the expected range of external force. A numerical example to determine the pre-tightening force of the force sensor with seven limbs and eight limbs is performed. The sensor prototype with seven limbs is manufactured, and the experiment result is consistent well with the numerical example, which demonstrates the correctness of the theoretical analysis.

Key words: Double-layer, Linear transform, Pre-stressed, Six-axis force sensor, Unilateral constraint