So, make pads have two projector upgrade slots, this way we can either choose have a huge field (two field expander) or a decent field with "force active" module.
That's exactly why they don't have two slots. I see the projector module as a single circuit (which is the circuit included in the recipe to make the pad), so altering it in two separate ways would overload it; there's only so much the circuitry of a pad can take (or should take), for balance reasons.
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Also, it could be a Field upgrade anyway, as it keeps the field on all the time.
Projector Modules affect "how" particles are projected from a pad, once activated; not "when." Do they split up? By how much? Are they spread over a larger area?
Field Modules affect the actual "particles" that are created by the physical projector. Making particles harmful (damage module), or inverting the particles so they negate existing power (drain module) or "delayed energizing" the particles so they don't "seek" electric items until they're a certain range from the pad (armour priority) are all field effects.
Physically activating a pad is caused by the pressure plate (in the recipe) which then allows power-flow to the projector. There is a valve effect which the field expansion modules "hold open," detecting constant flow with the detector cables in their recipes (which is why the pads shut off when everything that can be charged has been charged or people move beyond the range of the field).
As you can see: neither of the module slots are involved in the initial "activation" of the pad, itself, and it's not a functionality I want to add to them.
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Permanently active fields with either damage or drain field upgrades would be good as they can be used for traps (mob/player).
I don't disagree, at all... I just don't see a feasible way of implementing it via projector/field upgrade modules, as they simply don't fit the bill, and general upgrade slots (that alter the "effectiveness" of a pad) definitely don't, either.
That said, I've taken note of the feature request and will think on it... but no promises: if I can't think of a balanced way to implement it, I'm afraid it won't happen.