DD Coil Design (Help required)

[EEE]

Hi everyone,

I am trying to design Induction-balance coil using DD configuration, similar to Tinkerer's one for his IB-PI project. I dont want to build a full metal detector to test outdoor. I will have Tx stage (Power MOSFET driven by logic and powered by power supplies) and Rx stage (diff preamp).. I will connect the output of preamp to an oscilloscope and observe its response. I am interested in the response of objects of same size but made of different materials and same material of different size objects.

I want to build a coil with good sensitivity and depth range I am thinking of 30 cm OD (Outer Diameter) coil. I have looked at Emery's and bbsailor's design notes for coils.. I am going to explain the procedure that I am gonna follow:

1) I have 22 AWG and 24 AWG enamelled copper wires.
2) On a piece of wood I will fix nails in a D shape (20 cm radius)
3) I will wind 2 coils 23 turns of 22 AWG copper wire to create Tx and Rx coils.
4) I will use true-aluminum duct tape to wrap each coil.
5) on a piece of a cardboard, I will place both coils and the overlap distance will be 10 cm then glue them.
6) will place another cardboard on the top of the coils.

I would like to hear your comments about my procedure. I also several questions:

1) How can I test the balance of the coils, before fixing them to the cardboard or connect them to the TX and RX circuits? Shall I connect TX coil to a sinewave generator and also connect a cap in parallel with it, make the coil resonate and then move the RX coil above the Tx coil until null is acheived (or very close to 0V)?

2) Will this search coil configuration will give a good response? I would like high current to flow thru the coils 1A up to 3A.. Tx pulse will be between 100us to 400us.

3) Is it important to have a space between the coil windings? I know space in between will lower the capacitance but does that matter in terms of sensitivity?

3) Do I need to paint the cardboard with graphite for shielding? Is it necessary?

4) Is the aluminum duct tape good for shielding? I got that from HH project paper.

5) I have made before a coil of 24 turns 16 AWG (30 cm OD) it had inductance of 370 uH and I wrapped the coil with PVC. I didn’t test it yet thu as the circuit is not built yet. Is HH1 circuit a good start? I looked at Tinkerer’s IB-PI circuit but it is more complex.

Thank you all

[mikebg]

LABORATORY EQUIPMENT

EEE, if you need equipment for measuring impulse response or frequency response of different targets, then the sensor should be differential type, because there are strong interferences in the laboratory. I use an old "oOo" loop configuration:
http://www.geotech1.com/forums/showp...2&postcount=18
No need of shielding if you use preamp with input symmetry

[EEE]

Mikebg, thank you for the response. Do you suggest any inductance values for Tx and Rx1/2? What about the sizes of the coil with respect to eact other? Lets say the OD of Tx coil is 30 cm. The Rx coils have to be identical and also when placed on the top of Tx coil they both have to pick up the Tx signal with the same amplitude for this config to work properly.

What I understand from the circuit diagram is that the two Rx coils will pick up the same back emf from Tx. If there is no metal target, the preamp output is zero as the responses of both coils will cancel out.

I know what I saying might be very obvious but I like to discuss things to details and I also I am in the learning phase.

thanks,

[mikebg]

EEE, we do not need a back EMF induced in RX coil because it reduces the modulation index of target signal, loads stray capacitance in RX input, requires opamp having high CMRR and saturates preamp. So each RX coil in OOO configuration should be placed in induction balance with TX coil. Please read patents US3, 002,262 and US3, 823,365 for this loop configuration. If you have time, try to find errors in the following thread:
http://www.geotech1.com/forums/showthread.php?t=17289

[Carl-NC]

EEE, since you are just bench-testing target responses, there is no need to shield the coils. You can set the balance looking at the PI response, it will be obvious when the coils are balanced. You'll probably find that the 'on' balance and the flyback balance don't occur at quite the same point. I would use the 22awg for TX and the 24awg for RX. You could also experiment with 32 turns on the RX.

[EEE]

EEE, we do not need a back EMF induced in RX coil because it reduces the modulation index of target signal, loads stray capacitance in RX input, requires opamp having high CMRR and saturates preamp. So each RX coil in OOO configuration should be placed in induction balance with TX coil. Please read patents US3, 002,262 and US3, 823,365 for this loop configuration. If you have time, try to find errors in the following thread:
http://www.geotech1.com/forums/showthread.php?t=17289

EEE, since you are just bench-testing target responses, there is no need to shield the coils. You can set the balance looking at the PI response, it will be obvious when the coils are balanced. You'll probably find that the 'on' balance and the flyback balance don't occur at quite the same point. I would use the 22awg for TX and the 24awg for RX. You could also experiment with 32 turns on the RX.

Thank you. I will try both DD and oOo Configrations. oOo config is similar to Figure 8 config as both Rx coils are in balance with the Tx coil, am I correct?

[mikebg]

Thank you. I will try both DD and oOo Configrations. oOo config is similar to Figure 8 config as both Rx coils are in balance with the Tx coil, am I correct?

EEE, in the attached image, both RX windings in left side drawing forms figure of 8 config, which is in induction balance (IB) with the TX coil. However each RX winding receives large AIR signal because is not in IB. This makes difficult damping of RX tank circuit and IB of RX for all operating frequencies. An improvement can be made by adding second TX winding in opposite direction as shown in the right side drawing.

[Jose]

Hello Mikebg
Please tell me what would be the optimal relationship between the diameter of TX and RX, in the left figure.
On the right is according to understand the principles of Dave Emery.
Thanks
Jose

[mikebg]

Hello Mikebg
Please tell me what would be the optimal relationship between the diameter of TX and RX, in the left figure.
On the right is according to understand the principles of Dave Emery.

Jose, I think that diameter of RX loops and distance between them shoud be increased to achieve maximal depth as shown below.

[mikebg]

Jose, there is an idea how operator to adjust induction balance in the field. Inside the red knob is placed a core made of metal or ferrite. When operator rotates the knob, the core moves and delivers more signal in one of RX coils.

[Jose]

Thanks for answering Mikebg
Very good artwork
A greeting from Argentina
Jose

[EEE]

EEE, if you need equipment for measuring impulse response or frequency response of different targets, then the sensor should be differential type, because there are strong interferences in the laboratory. I use an old "oOo" loop configuration:

http://www.geotech1.com/forums/showpost.php?p=128142&postcount=18
No need of shielding if you use preamp with input symmetry

Mikebg, thank you for your contribution. I have a question in regards to "oOo" configuration. Basically, using this type will remove the background noise as if the target is not present, the output of the receiver coils will be null. However, I think this will also remove some of the target response.

For example, if the metal target is closer to receiver coil A but still have effect on receiver coil B. Some of the target response will be eliminated. I think 1 Rx coil is more adequate to capture the full target response. What do you think?