Live adjusting Z with stepper and sensor

Hello all

Im making project for laser engraver. I managed to get running height adjustment of neje master laser with servo motors (one on each side). Now Im exploring possibility to add some kind of sensor (ultrasound, laser,...?) for keeping same height of laser head on uneven material and adjusting this height on the fly with servos.

Can you point me in right direction what kind of sensor should I use (i would mount it very near laser beam so it could measure distance correctly) of course stepper speed should be fast to adjust to material shape.

Thanks

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{cough} "its current location" {cough}

Well spotted!
It's a standard reply, which I didn't write. I'll edit it.