Can you help me understand this parameter?
As these are series elastic actuators with a spring element in them, there is some hysteresis that comes along with the torque calibration. When you apply a load in one direction on the actuators and return it back to 0 deflection you won't necessarily return to an absolute reading of 0 Nm.
We do our best to calibrate and model this hysteresis in each actuator, but that is your standard source of error in the "effort"/torque readings you will see.
Here are some example calibration plots to show you the hysteresis found in our Series Elastic actuators.