My experiments were done with the sensor on a table or suspended over it. When it became clear that it would not meet my needs, I put it aside and looked at inductance. That was also problematic and had very limited range with the coil size I could accommodate. Ultrasound (500 KHz) worked, but was very sensitive to the distance and beam spread. I ended up with reflected IR. You may want to consider ultrasound also. Good luck. John