When a piezo-actuator (whether amplified or non-amplified) is actuated against a spring load, it converts electrical energy into both motion and force. This force will vary according to the amount of expansion/contraction achieved by the piezo-actuator under the applied electrical field. When activated with an applied electric field, the piezo-actuator moves against the spring load until it reaches a force balance condition. If at this point the spring load were to be removed, the stored potential energy within the piezo-actuator would be converted completely into additional motion. Therefore, when working against a spring load, the amount of displacement that the piezo-actuator can produce in the spring load is less than the piezo-actuator’s free zero-load displacement. The amount of displacement that the spring can be compressed or stretched is a function of the spring stiffness and the piezo-actuator stiffness.