Saturation and Mechanical Force, conceptual

[robtheg]

Been working on a solenoid actuator design and the simulation software is indicating that, as I increase amp-turns beyond what is reasonable, the force output of the piston keeps increasing. This is in contrast to what I expected. My understanding was that as the piston reaches magnetic saturation, the output force would level off. I've been unable to increase my understanding enough to confirm whether the simulations are correct or if I've set up the problem incorrectly.

Here's some definitions I'm using.
H = applied magnetic field (ie. from the coil)
J = induced magnetic field (extra magnetic field from the realignment of magnetic domains in the armature)
B = total magnetic field (H+J)

And my conceptual example:
Blue = Body
Orange = Coil
Red = Piston
Spring pulls the piston upwards

Let's assume the body has a low reluctance compared to the armature, the piston is a magnetic material (soft iron or whatever) and that the coil can handle as many amps as we want.

Conceptual issues I'm trying to understand and would appreciate some direction:
1) I understand the downward magnetic force on the piston depends on the air gap. Is this force also proportional to the J field? Or is it a function of B field? I understand that magnetized materials also generate and internal demagnetization field. Does mechanical force arise from the interaction of this field with the B or H field?
2) What are the high-level concepts for calculating magnetic force? I'm essentially looking for a control volume understanding: Does a field line entering or exiting a material generate a force in a particular direction, etc?

Thanks for any help

 

[MagBen]

Your design sounds solid to me. The saturation can occur to both the piston and armature. When the piston is saturated,there is not contribution to the force. However, there is still contribution from the armature and the coil even when the armature is saturated since B = J (from the soft material)+ H (from the coil), J has limit, but H is unlimited and will increase as amphere turns increase.
No need to consider demagnetized field in htis case.
Could not fully understand your second question.

 

[israelkk]

When the iron parts are saturated the magnetic circle behaves as though as the iron parts are made or air.

 

[robtheg]

Hey thanks for your responses.