Experimental results of using a linear step motor as a programmable spring

It is demonstrated experimentally that a variable-reluctance linear step motor can be controlled as a programmable spring with a spring constant of dynamic range 2000 to 1 for robot applications. This large dynamic range is mainly attributed to the direct-drive use of this variable-reluctance linear step motor. Using direct drive provides a high ratio of output force to friction because of the lack of commutation brushes and a reduction gear. The large dynamic range of achievable spring constants also demonstrates that a very simple electronic commutation-logic feedback can simplify a statically and dynamically complex variable-reluctance step motor to the point that it can be algebraically linearized. For a large operating range in force, velocity, and frequency, this linearized model can be approximated by a second-order ordinary differential equation; i.e. the force on a mass given by Newton´s second law with the addition of a small friction force