A rotary series elastic actuator (SEA) comprising: a gear transmission assembly ; a motor coupled to the gear transmission assembly , the motor to deliver torque or power to the gear transmission; a spring assembly coupled to the gear transmission, the spring assembly including: a first torsional spring including a first set of elastically deformable spirals ; and a second torsional spring including a second set of elastically deformable spirals , the first torsional spring coupled to the second torsional spring via a spacer; the rotary series elastic actuator being characterised by an internal mechanism residing between the first torsional spring and the second torsional spring , the internal mechanism configured to detect a rotational deflection of the spring assembly and provide the rotational deflection as a measured torque input value to a controller of the motor , wherein the internal mechanism includes: a base arm ; a sensor ; a swing arm movably coupled to the base arm and shaped as a circular arc, the swing arm including a first arm and a second arm; a first pulley coupled to a first end of the base arm ; a second pulley coupled to a second end of the base arm , the second end opposite the first end; and a string coupled the first arm of the swing arm and the second arm of the swing arm and tensions around to the first pulley, the second pulley, and a rotational shaft of the sensor. a gear transmission assembly ; a motor coupled to the gear transmission assembly , the motor to deliver torque or power to the gear transmission; a spring assembly coupled to the gear transmission, the spring assembly including: a first torsional spring including a first set of elastically deformable spirals ; and a second torsional spring including a second set of elastically deformable spirals , the first torsional spring coupled to the second torsional spring via a spacer; the rotary series elastic actuator being characterised by an internal mechanism residing between the first torsional spring and the second torsional spring , the internal mechanism configured to detect a rotational deflection of the spring assembly and provide the rotational deflection as a measured torque input value to a controller of the motor , wherein a first torsional spring including a first set of elastically deformable spirals ; and a second torsional spring including a second set of elastically deformable spirals , the first torsional spring coupled to the second torsional spring via a spacer; the rotary series elastic actuator being characterised by an internal mechanism residing between the first torsional spring and the second torsional spring , the internal mechanism configured to detect a rotational deflection of the spring assembly and provide the rotational deflection as a measured torque input value to a controller of the motor , wherein the internal mechanism includes: a base arm ; a sensor ; a swing arm movably coupled to the base arm and shaped as a circular arc, the swing arm including a first arm and a second arm; a first pulley coupled to a first end of the base arm ; a second pulley coupled to a second end of the base arm , the second end opposite the first end; and a string coupled the first arm of the swing arm and the second arm of the swing arm and tensions around to the first pulley, the second pulley, and a rotational shaft of the sensor. a base arm ; a sensor ; a swing arm movably coupled to the base arm and shaped as a circular arc, the swing arm including a first arm and a second arm; a first pulley coupled to a first end of the base arm ; a second pulley coupled to a second end of the base arm , the second end opposite the first end; and <EOS>