"Induction Balance" coil tuning

[Thomas M]

We have a circuit that is a 50kHz 'induction balance' metal detector. Two
transmit coils that canel each other out and a receiver coil that picks up
any disturbance in the canceled field. Winding the coils is difficult to
get them exactly tuned and canceled. Is there an electrical
component/circuit that can be added that will let us 'dial in' the fine
tuning?

 

[John Larkin]

On Mon, 17 Jan 2005 08:49:03 -0500, "Thomas M" <nospam@yahoo.com>
wrote:

We have a circuit that is a 50kHz 'induction balance' metal detector. Two
transmit coils that canel each other out and a receiver coil that picks up
any disturbance in the canceled field. Winding the coils is difficult to
get them exactly tuned and canceled. Is there an electrical
component/circuit that can be added that will let us 'dial in' the fine
tuning?

Yes. An RC network that has adjustable amplitude and phase shift would
take a bit of the transmit signal and inject it into the receiver; you
could then adjust for null.

John

 

[Jamie]

Thomas M wrote:

We have a circuit that is a 50kHz 'induction balance' metal detector. Two
transmit coils that canel each other out and a receiver coil that picks up
any disturbance in the canceled field. Winding the coils is difficult to
get them exactly tuned and canceled. Is there an electrical
component/circuit that can be added that will let us 'dial in' the fine
tuning?



well yes.

a simple Pot between the 2 of them supplying the single to the coils.
use the center tap as the feed from the OSC supply.
the outer will connect to the coils.!
1 to each 1 leg of each coil.
the remaining legs are tied together to the common.

i don't know the amount of current you are using but
wire wound pots come in a variety of sizes.
just use a low R Value...
depending on how far off your coils are going to be will
determine how wide of a range you need.
you could also have 2 AMps driving the coils with a balance control.
etc.