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221 lines
10 KiB
Plaintext
221 lines
10 KiB
Plaintext
{{
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*****************************************
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* Inductive Sensor Demo v1.0 *
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* Author: Beau Schwabe *
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* Copyright (c) 2007 Parallax *
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* See end of file for terms of use. *
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*****************************************
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Test Circuit:
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10pF 100K 1M
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FPin ───┳──┳── SDF(sigma-delta feedback)
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│ ┣──── SDI(sigma-delta input)
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L 100K
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GND GND
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Test Coils:
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Wire used was the "Radio Shack Special" GREEN (about 27 gauge)
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25T (Coke Can form) = 2.1MHz
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15T (Coke Can form) = 3.9MHz
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5T (Coke Can form) = 5.3MHz
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50T (BIC pen form) = 3.2MHz
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How does it work?
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Note: The reported resonate frequency is NOT the actual resonate LC frequency. Instead it is where the voltage produced from
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the LC circuit was clipped.
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In the example circuit below:
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C L
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A ────┳──── GND
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│
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B
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When you apply a small voltage at a specific frequency to an LC circuit (at point "A") that is at or near the resonate
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frequency of LC, it is not uncommon to measure 10's or 100's of times the amount of voltage (at point "B") that you are
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applying to the LC circuit. (at point "A")
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In the "Test Circuit" above, point "B" passes through a diode which then basically feeds a divide by 2 voltage divider:
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100K 100K
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B ───┳── GND
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│
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C
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...So in order see the sigma-delta ADC "clip" the frequency sweep result, the output from the LC circuit only needs
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to generate about 6.6 Volts above ground. (0.6V drop across the diode, and since the ADC is only sensitive to about
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3V, it works out to be about 6.6V after the voltage divider.)
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A typical magnitude plot of a frequency sweep applied to an LC circuit might look something like this:
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*
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*
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*
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*
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* *
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* *
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* *
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* *
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* *
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***** *****
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...With 'clipping' the pattern looks more like this:
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X****
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* *
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* *
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* *
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***** *****
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...The 'X' denotes the location of the reported resonate frequency. The reason this is slightly off is for
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two reasons really. 1) lazy - I didn't want to fiddle with the voltage divider combo... adjusting so that the
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"peak" was also where the ADC happened to "clip". 2) some benefit - When you apply a frequency to a tuned LC
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circuit that's resonate frequency is the same as the applied frequency, the LC acts like a dead short. A
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situation not exactly great for Propeller I/O's
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Now that we have that out of the way, what happens next? How can we use this so called "coil" as a sensor?
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If a frequency sweep is initially preformed to determine the resonate frequency clip point, then it just so
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happens that adding additional "metal" (<- Does not need to be ferrous) causes the resonate frequency to shift
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to a HIGHER frequency.
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Once you determine the "clip" frequency and you use one of the available counters to constantly feed that
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particular frequency back to the LC circuit, the resulting ADC output is proportional and somewhat linear when
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metal objects are introduced to the coil.
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Assume frequency increases from Left to Right. With a slight resonate shift to the right, the ADC reports a
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lower "de-tuned" value because the voltage magnitude no longer "clips" at the reported resonate frequency.
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Typical ranges are full scale between 65535 (no metal) and 0 (metal saturation)
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X *****
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* *
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ADC reports value here --> * *
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* *
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***** *****
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Slight shift to the right
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I also made mention that the response is somewhat linear. As the LC resonance shifts and the ADC value begins
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to lower, the slope is steepest near the "clip" point. Therefore, the slightest shift results in larger value
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changes. Since the coil is actually the least sensitive to metal the further away it is (Law of squares) and
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most sensitive to metal the closer it is, the resulting combination acts to linearize the output. I need to
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point out that some LC combinations will exhibit plateaus and other anomalies caused by varying parasitic circuit
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conditions that will affect the overall output, so a little bit of trial and error is necessary to get things
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the way you want them.
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}}
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OBJ
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Freq : "Synth"
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ADC : "ADC"
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gr : "graphics"
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Num : "Numbers"
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CON
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FPin = 0
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UpperFrequency = 6_000_000
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LowerFrequency = 2_000_000
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bitmap_base = $2000
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display_base = $5000
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VAR
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long FMax, FTemp, FValue, Frequency
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PUB demo
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'start and setup graphics
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gr.start
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gr.setup(16, 12, 128, 96, bitmap_base)
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FindResonateFrequency
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DisplayInductorValue
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PUB DisplayInductorValue | X
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Freq.Synth("A", FPin, FValue)
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repeat
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ADC.SigmaDelta(@FTemp)
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'**************************************** Graphics Option Start *********************************************
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'clear bitmap
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gr.clear
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'draw text
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gr.textmode(1,1,7,5)
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gr.colorwidth(1,0)
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gr.text(0,90,string("Inductive Propeller Sensor"))
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gr.colorwidth(1,5)
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X := (65535 - FTemp )*200/65535
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gr.plot(-100+X,15)
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gr.textmode(1,1,7,%0000)
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gr.colorwidth(1,0)
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gr.text(-100,-20,string("Resonate Frequency ="))
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gr.text(35,-20,Num.ToStr(FValue,10))
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gr.text(-100,-36,string("ADC Frequency Response ="))
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gr.text(65,-36,Num.ToStr(FTemp,10))
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'copy bitmap to display
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gr.copy(display_base)
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'**************************************** Graphics Option Finish *********************************************
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PUB FindResonateFrequency | P
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dira[FPin] := 1
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FMax := 0
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repeat Frequency from LowerFrequency to UpperFrequency step 1000
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Freq.Synth("A", FPin, Frequency)
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ADC.SigmaDelta(@FTemp)
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if FTemp > FMax
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FMax := FTemp
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FValue := Frequency
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'**************************************** Graphics Option Start *********************************************
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P := (Frequency - LowerFrequency)*100/(UpperFrequency - LowerFrequency)
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gr.colorwidth(1,5)
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gr.plot(0,0)
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gr.line(P,0)
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gr.colorwidth(3,5)
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gr.line(100,0)
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gr.colorwidth(2,0)
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gr.plot(P,(FTemp/1024)+10)
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gr.colorwidth(0,1)
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gr.plot(P+1,5)
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gr.line(P+1,50)
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gr.copy(display_base)
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'**************************************** Graphics Option Finish *********************************************
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DAT
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{{
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┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
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│ TERMS OF USE: MIT License │
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├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
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│Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation │
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│files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, │
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│modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software│
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│is furnished to do so, subject to the following conditions: │
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│ │
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│The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.│
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│ │
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│THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE │
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│WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR │
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│COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, │
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│ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. │
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└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
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}} |