I've found good literature for offline proccesses, and I've gotten good results. However, when it comes to online applications, I've found nothing good. Applying any filter without a linear phase response will give me too much distortion once I get to displacement. However, any filter I've used with linear phase response gives me way too much "time lag" (apart that it's very high order). Below is the proccess most people apply:
Raw Acc --> [High Pass Filter (HPF)] --> [Integrate (Int)] --> [HPF] --> [Int] --> [HPF] --> Displacement Signal
As you can imagine, this procces is trivial if you use non-causal filters... But I've not gotten a good solution for the online proccess.
My current conclusion is that I need a "low order, linear phase, causal filter." There is active research in these types of filters, but they are somewhat complicated to apply. So before I start making leeway, I decided to ask this in a forum to see if anybody could give some input or fresh ideas.