Ankle Calibrations
From DIDEAS Wiki
Nonlinearities
There are a few nonlinearities that must be considered when using the ankles:
- Transmission ratio dependence on ankle angle
- Parallel spring stiffness dependence on ankle angle
- Series spring stiffness dependence on ankle angle
- Series spring softening curve
Transmission Ratio
Due to geometry, the transmission ratio (from motor angle to ankle angle) varies as ankle angle changes. This function can be computed by measuring the moment arms from the ankle joint center of rotation to each of the ballscrew attachment points (motor housing and series spring clevis) along with the length between these two attachment for a given ankle angle. However, it may be more accurate to determine this function experimentally in one of the following ways:
- Manual motion of ankle joint:
- command zero motor current
- rotate the ankle joint throughout its range of motion manually while recording motor angle and ankle joint angle
- Motor-drive motion of ankle joint:
- command a motor trajectory so to move the ankle joint through its range of motion
Using the first method, the following curve was obtained:
