new pulse induction early stages

[ant17]

hi guys i am new to the forum and i am trying to build pulse induction metal detector. Currently i have the pulse generator and igbt transistor to pulse the coil. The pulse generator is tl494 pulsewidth modulator with 0- 90% duty cycle and independent frequency adjustment which drives the gate of a igbt with the emitter grounded and the collector is connected to one end of the coil and other end to the positive in the power supply but when i put my oscilloscope on the collector which is also connected to the the bottom end of the coil and the ground clip went to the gnd of power supply but the waveforms do not change when a metal target is brought close to the coil i thought it would am i wrong if somebody could help i would be greatful

[Qiaozhi]

Have you seen this book? -> http://www.geotech1.com/forums/conte...metal-detector
This will answer most of your questions.

[golfnut]

This is normal.

These machines dont look for change in Tx waveform with target present.


The general idea is

charge L to steady state,
break the charge path quickly,
the fast decay of H field is the Tx pulse,



Rx listens for a return after all remnants of the Tx have just finished, and compares it to a base line..

Outcome 1 baseline and return samples at same level -- No target
Outcome 2 baseline and Rx level different - - target present.

Study the surf drawing and it will become clearer.

S

[Davor]

You'll be able to see some change if you wave a piece of ferrite.

[ant17]

thanks guys

thanks guys keep the info coming

[Carl-NC]

You want to look at the decay region, within ~50us of turn-off, for very slight changes. You may need a preamp to see those changes, they are that small.

[ant17]

pi metal detector

i have seen other waveforms which looks like a a very sharp pulse where as on my scope it looks just like a square wave that i can adjust the pulse width and frequency