MPP E - long settling time after switch-over from coil to pinpointer probe, long recovery time ...

[Polymer]

I've basically finished building my waterpoof MPP E now.

When switching from coil to pinpointer probe or viceversa,
the threshold goes haywire and takes quite long to "recover" from the switchover.
The wait feels like ages when detecting.

Is there any way to make this time short(er)?

I have also noticed that the recovery time after detecting a solid signal
is also quite long - the detector is sort of "numb" for a while after this happens
and can't detect anything.

What part of the circuit do I have to look at to maybe change this?

Thanks!

[waltr]

A quick look at the Rev E schematics shows that probably adjust values of C9 & R6. These set the speed of the SAT circuit.

Take a look at the schematics and instructions for the Hammer Head detector about these values.

https://www.geotech1.com/cgi-bin/pag...head/index.dat

[Polymer]

A quick look at the Rev E schematics shows that probably adjust values of C9 & R6. These set the speed of the SAT circuit.

Take a look at the schematics and instructions for the Hammer Head detector about these values.

https://www.geotech1.com/cgi-bin/pag...head/index.dat

Thanks Waltr! I have measured with the oscilloscope at TP 1 to see what happens.

1.) No metal present - Threshold is at minus 2 volts (sound output just ticking occasionally)

2.) Pass a piece of metal by the coil that causes a strong signal - The voltage at TP 1 will jump up to plus 4 volts.

Now here comes the part I do not understand:

3.) Once the metal has passed the coil, the voltage at TP 1 will swing down to minus 4 volts.

The TP1 voltage will then slowly move back up to minus 2 volts and the detector is ready to detect again.
This process takes anywhere from 2 to 5 seconds.

I am not sure if this is purely SAT related and am interested to know
if the negative voltage swing below the adjusted threshold can be somehow avoided.

Do commerecial PI's have this "feature". I do not have one to compare.
Maybe something is wrong with my build?

[waltr]

If you can (dual channel scope) also look at TP4 (integrator output) at the same time.

The action does sound correct and is the action of the Integrator & SAT circuits. This is also called a 'motion detector' since it responses to a change of target. So yes, this is a feature of commercial PI detectors.

Target going into coil's field produces a rising Voltage. Target leaving coil's field drops Voltage so SAT goes in oppisite direction since it is AC coupled.

The 2 to 5 second to return to 'no target' Voltage is R6 (1Meg) discharging C9 (470nF). On other PI MDs, like HH, R6 is a pot + fixed R to adjust the SAT decay time. The SAT op-amp also has a Gain of 47 (R22/R21) which increases the Voltage swing of the integrator output. R22 could also be a pot to adjust SAT gain.

Also double check the cap values of C9 & C8. Could be you have either of these too large since it is easy to miss-read cap value markings. If C9 is 4.7uF instead of 0.47uF the decay would be too long. The HH I built decays in less than a 1/2 second and is perfect for me.

[Polymer]

OK, I've scoped TP1 & TP4 simultaneously .

TP4 goes from -150mV to +4.2V when swinging metal and recovers very quickly. It does not swing to negative voltage.

TP1 goes from -1.3V to +4.0V when target approaching coil and swings down to - 3.8V when target leaves coil.
This signal then "recovers" slowly back to -1.3V (back to threshold).

The odd thing is that this happens so pronounced when the target is relatively close to the coil.
I am using a 3.4g gold ring to test. At a distance of say 8 inches I get a signal and will also find it on the return stroke of the coil.

Doing the same with a distance of 2 inches from the coil I cannot detect it on the return stroke of the coil due to the long "recovery" time.

It seems worse than iron masking, because I will get no signal at all for the duration of "recovery".

This seems like a flaw to me, being inherent to the design, funny component(s) and/or mistakes on my part messing things up.

I have checked and measured the values of C8 & C9 they are correct - 1.0uF & 0.47uF respectively, both are film caps.

If you like I'd be interested to see if you get similar results measuring TP 1 & TP4 (or equivalent on your HH design) with the target moved far and close.

There must a better way to do this ... or not ? (other than errors which are of my making)

[waltr]

On my HH the integrator output is quit fast. The Voltage changes within milliseconds of target passing into and out of coil's field.

The SAT output is slower, < 1second.

Did you check the 1 Meg Ohm resistor?
Did you try decreasing the value of the 1Meg resistor?
Try putting a 100k across the 1M and see if the recovery time decreases.

[Qiaozhi]

OK, I've scoped TP1 & TP4 simultaneously .

TP4 goes from -150mV to +4.2V when swinging metal and recovers very quickly. It does not swing to negative voltage.

TP1 goes from -1.3V to +4.0V when target approaching coil and swings down to - 3.8V when target leaves coil.
This signal then "recovers" slowly back to -1.3V (back to threshold).

The odd thing is that this happens so pronounced when the target is relatively close to the coil.
I am using a 3.4g gold ring to test. At a distance of say 8 inches I get a signal and will also find it on the return stroke of the coil.

Doing the same with a distance of 2 inches from the coil I cannot detect it on the return stroke of the coil due to the long "recovery" time.

It seems worse than iron masking, because I will get no signal at all for the duration of "recovery".

This seems like a flaw to me, being inherent to the design, funny component(s) and/or mistakes on my part messing things up.

I have checked and measured the values of C8 & C9 they are correct - 1.0uF & 0.47uF respectively, both are film caps.

If you like I'd be interested to see if you get similar results measuring TP 1 & TP4 (or equivalent on your HH design) with the target moved far and close.

There must a better way to do this ... or not ? (other than errors which are of my making)

I don't have an MPP REV-E on the bench at the moment, but the recovery time you're experiencing seems to be incorrect. The SAT time should be almost the same as Hammerhead. The MPP uses a 470nF cap and a 1M fixed resistor, whereas the HH uses a 470nF cap and a 1M pot in series with a 100k resistor. If you want to be able to adjust the SAT, then replace the 1M fixed resistor with the same 1M pot + 100k resistor combination used in the HH.

However, I'm certain that you have a problem with your MPP build. I have just tested a Crossbow detector, which has a similar SAT configuration, and in Dynamic mode on the slowest setting (470nF + 1M + 100k) the recovery is still very fast. Even if I hold a coin close to the coil until the audio threshold disappears, moving the coil away and then immediately across the coil produces a response.

[green]

OK, I've scoped TP1 & TP4 simultaneously .

TP4 goes from -150mV to +4.2V when swinging metal and recovers very quickly. It does not swing to negative voltage.

TP1 goes from -1.3V to +4.0V when target approaching coil and swings down to - 3.8V when target leaves coil.
This signal then "recovers" slowly back to -1.3V (back to threshold).

The odd thing is that this happens so pronounced when the target is relatively close to the coil.
I am using a 3.4g gold ring to test. At a distance of say 8 inches I get a signal and will also find it on the return stroke of the coil.

Doing the same with a distance of 2 inches from the coil I cannot detect it on the return stroke of the coil due to the long "recovery" time.

It seems worse than iron masking, because I will get no signal at all for the duration of "recovery".

This seems like a flaw to me, being inherent to the design, funny component(s) and/or mistakes on my part messing things up.

I have checked and measured the values of C8 & C9 they are correct - 1.0uF & 0.47uF respectively, both are film caps.

If you like I'd be interested to see if you get similar results measuring TP 1 & TP4 (or equivalent on your HH design) with the target moved far and close.

There must a better way to do this ... or not ? (other than errors which are of my making)

Trying to simulate integrator with spice. Doing something wrong. Could you attach a scope picture of (TP3 and TP4) and (TP4 and TP1) while swinging the target across the coil.

[Polymer]

On my HH the integrator output is quit fast. The Voltage changes within milliseconds of target passing into and out of coil's field.

The SAT output is slower, < 1second.

Did you check the 1 Meg Ohm resistor?
Did you try decreasing the value of the 1Meg resistor?
Try putting a 100k across the 1M and see if the recovery time decreases.

The integrator output is fast - instantaneous to my eye. So all good there.

I have connected a 100k resistor in parallel with the 1M.
The recovery time is very snappy now - estimated a few 10ths of a second - , albeit with a small sensitivity loss.

Will be checking the 1M resistor for quirkies.

[Polymer]

Qiaozhi, thanks for the confirmation of something being off.

Got out my second MPP E board to compare, made with differing component manufacturers eg. the integrator/sat capacitors etc.

It has the same behavior This is independent of supply voltage (tested 9.6 to 12 volts)

The recovery time after hitting negative rail after metal moving away to begin of detecting is 2-4 seconds.

Waltr's suggestion of placing a 100k in parallel to the 1M resistor is a good temporary "workaround".

I am none the wiser though - am travelling tomorrow and will be taking the MPP E along.
A shame I didn't notice this problem earlier. Might find some time for a fix in a few hours. Have to unpack
my measuring gear - its already packed. Hoping to maybe get this solved and knowing what was wrong before I leave.

[waltr]

I just did an LTSpice model of just the SAT stage. The integrator stage is not easy to model in spice. Spice must be run for many cycles for the integrator stage to settle.

With the component values in the RevE MMP the recovery time of the SAT stage after Negative saturation is almost 2 seconds.

With R6 changed from 1Meg to 100k the recovery time is about 340msec.

There is the possibility that if U4a (TL071) is not real good (excessive input current) or there is leakage on the PCB to its input pins then the recovery time will be shorter. Could this have happened in the prototypes?

I based my design off the HH since it had many adjustments and options. I did end up setting the SAT discharge R (MMP R6) to 100k for a recovery time I liked.

[green]

Trying to simulate integrator with spice. Doing something wrong. Could you attach a scope picture of (TP3 and TP4) and (TP4 and TP1) while swinging the target across the coil.

See what I was doing wrong. Scope pictures not needed. Thanks

[green]

See what I was doing wrong. Scope pictures not needed. Thanks

My spice simulation. Wasn't thinking earlier, needed to jumper R7 C3 for simulation. Changed R9 to 100k like waltr suggested. Increasing R5(1meg higher than needed?) seems to help with simulation. Changed C5 also. Not sure simulation is same as detector.

[Polymer]

I just did an LTSpice model of just the SAT stage. The integrator stage is not easy to model in spice. Spice must be run for many cycles for the integrator stage to settle.

With the component values in the RevE MMP the recovery time of the SAT stage after Negative saturation is almost 2 seconds.

With R6 changed from 1Meg to 100k the recovery time is about 340msec.

There is the possibility that if U4a (TL071) is not real good (excessive input current) or there is leakage on the PCB to its input pins then the recovery time will be shorter. Could this have happened in the prototypes?

I based my design off the HH since it had many adjustments and options. I did end up setting the SAT discharge R (MMP R6) to 100k for a recovery time I liked.

Waltr, you saved the bacon! I have put in a 100k resistor. I am happy as a lark and my detector is now packed and ready to go.

Your reasoning makes sense to me and appears to be a distinct possibility.

Come to think of it, if I recall correctly the MPP E when I originally built it was quite funky reaction-wise.
After cleaning the board (IPA, ammonia, water) I believe it appeared sort of sluggish.
I thought it would need more time to dry properly. Got sidetracked and didn't think about it anymore.

Anyway, I'll be intensively using the MPP E the next 5 weeks and hope to have some interesting insights to write-up
other than whining about this-that or the weather ...

Thank you again waltr.

[Polymer]

See what I was doing wrong. Scope pictures not needed. Thanks

I am glad for that, no time right now