From Contraction Sensing with Smart Braid McKibben Muscles (2015):  "We tested the actuator in a custom-made test apparatus in which we applied different pressures and loads while measuring both inductance and the ground truth actuator contraction. Pressure control was achieved with a custom feedback loop driven by a proportional valve (Enfield LS-V05s) and a pressure transducer (WIKA A-10). Contraction of the actuator was measured from the motion of a sliding carriage affixed to the actuator end. A string potentiometer was used to measure the position of the carriage (UniMeasure LX-PA, 10" range)."