Quaternions given relative to NED but I want to orientate my sensor relative to Down only

Hello all, i am using the Bosch BNO055 IMU to do gait analysis. The quaternions outlutted from the sensor are relative to the NED (or ENU, im not sure) global refernece frame. I would like to keep the DOWN part of this so the sensor is aligned with gravity, but the X and Y axis stay in the orginal body frame.

the quaternion is in w,x,y,z form where the z quaternion aligns the sensor relative to north (and therefor east). If i reset this to 0 then will it not longer aligned the sensor with North and East, but still down? Also does anyone know what i will have to do to the W term if i do this?

Thank you for your help and suggestions

[James Tursa]

I don't understand your request. If you have a DOWN vector and two BODY vectors they will not form an orthogonal set of axes. What is your purpose in trying to do this?