Gas station without pumps

2012 July 12

Capacitive sensing with op amps

Filed under: Circuits course — gasstationwithoutpumps @ 09:54
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I’m not very happy with the lab exercise in Capacitive sensing.  The basic design questions are good:

  • What capacitance do we add by touching something?
  • What frequency should the oscillator run at without the touch?
  • What % change in frequency can we detect reliably? How will we do that detection?

but I’m not happy with my choice of an LM555 chip for the oscillator.  It is a fine choice for many purposes, and a good chip for students and hobbyists to learn about, but the Fairchild Semiconductor LM 555 data sheet does not do a good job of explaining how the chip works nor exactly what the logic function is for triggering the flip-flop.  As a result, students will probably end up just copying the astable circuit without really understanding it.

I think I would prefer to have them use an op-amp multi-vibrator circuit.  It can’t quite be modeled with only linear op amps, as it relies on clipping of the output to get nearly a square wave.

Op amp multivibrator. The first op amp is just to set up a virtual ground halfway between the power rails. The second op amp does the oscillating (at about 8.12 kHz for these components). Circuit drawn with CircuitLab, which is not capable of simulating it.

The op-amp multivibrator is a simple design.  When the output is high, C1 is charged until V_minus is higher than the positive input of the op amp (about 34.7% of the way from Vbias to Vout=Vdd).  Then the output goes low (taking about 6µsec, limited by the slew rate of the op amp), and the capacitor discharges until it is below the positive input (about 65.3% of the way from Vbias to Vout=0 with these values for R4 and R5). So Vminus swings from (1-0.6526) Vbias = 0.1737 Vdd to Vbias+0.6526(Vdd-Vbias)=0.8263Vdd.  The time it takes to charge or discharge should be about \ln (V_{hi}/V_{lo} \tau,  where the time constant τ=R3 C1.  For the values of R4 and R5 here, the charge time should be 1.560 τ, and the period 3.119 τ. With the given values of R3 and C1, this should be a period of 102.9 µsec, or a frequency of 9.7 kHz.   I’m measuring 8.1 kHz, not 9.7 kHz, but I think that the discrepancy is due to the slew rate limitations (which add about 12 µsec to the period) and possibly stray capacitance.  With C1=47 nF, the frequency is 203.6 Hz, when the calculation says it should be 206.7 Hz, well within the accuracy of the components.

Connecting the foil-and-plastic-wrap sensor to Vminus adds a little stray capacitance, dropping the frequency to 8.01 kHz. Touching the sensor drops the frequency to 7.5 kHz–7.7 kHz, adding about 5 µsec–8.5 µsec to the period, consistent with adding about 50pF – 83 pF to C1.  This is a little smaller than the 163 pF that I estimated using the parallel-plate model in Capacitive sensing, but consistent with my measurements using the LM555 circuits.

I can get a much stronger effect if I remove C1 from the circuit, and use only stray capacitance.  The period drops to about 6.2 µsec (roughly 160 kHz), but (because of the slew rate limitations of the op amp) is not full scale and is essentially a triangle wave.  Touching the sensor reduces the frequency to about 35 kHz – 45 kHZ, an increase of 16 µsec – 22 µsec in period, consistent with an additional capacitance of  155 pF – 220 pF, which is about what was predicted by the parallel plate model. I’m a little reluctant to put this signal into an Arduino board, though, as the no-touch signal spends most of its time in the non-digital region between high and low, and only barely crosses the thresholds that would allow the ATMega328 chip to detect the signal.

One interesting idea would be to take advantage of the change in amplitude of the oscillation to make an all-analog touch detector.  I’m afraid that would not be very robust, though, as an increase in the period of only 12 µsec is enough to make the oscillation full scale.

2012 July 9

Capacitive sensing, part 2

The obvious next step with the capacitive sensor is to make the sensor actually control something.  This is not essential for the circuits class—for that class it is better to have the students viewing the waveform on the oscilloscope and seeing the frequency of the output vary  as the touch varies.  But I was curious about whether the various grounds (oscilloscope, USB cable, wall wart power supply) were making a difference to the capacitive sensing, so I wanted to redo the capacitive sensor to be a fully battery-based application, with no external ground connections.

To display whether the sensor had been touched or not, I turned on or off the LED on pin 13. I used the same software detection mechanism as in the previous post, counting the number of pulses it takes to get a total time for the signal being low. Touching my laptop (to reduce the resistance to ground) no longer makes a difference in the sensitivity of the switch, now that the battery ground is separated from the laptop ground.

// Capacitive sensor switch
// Mon Jul  9 06:05:54 PDT 2012 Kevin Karplus

// To use, connect the output of the LM555 oscillator to
// pin CAP_PIN on the Arduino.
// The code turns on the LED on pin 13 when a touch is sensed.

// The Arduino measures the number and width of LOW pulses on pin 2 until
//    the sum of the pulse width exceeds TOTAL_PULSE_USEC.
// The LED is turned on if the number of pulses is less than NO_TOUCH_NUM

#define CAP_PIN (2)

#define TOTAL_PULSE_USEC (100)	// total of pulse widths (in  microseconds) before reporting
#define NO_TOUCH_NUM (50)

void setup(void)
{
    pinMode(CAP_PIN,INPUT);
    pinMode(13, OUTPUT);
}

void loop(void)
{
    uint32_t pulse_sum=0;
    uint16_t num_pulses=0;
    while (pulse_sum<TOTAL_PULSE_USEC)
    {   pulse_sum += pulseIn(CAP_PIN,LOW);
        num_pulses++;
    }
    digitalWrite(13, (num_pulses<NO_TOUCH_NUM? HIGH:LOW));
}

I had to set the threshold a bit higher than I expected, indicating that the frequency shift was not as large without all the connections to ground, but the sensor still works with a moderately firm touch. With a light touch, the LED flickers on and off, indicating that the frequency shift is near the threshold. Raising NO_TOUCH_NUM too high has the LED always on. Even raising it from 50 to 55 makes the switch too sensitive—the LED turns on when my finger is still 5mm away from contact. Perhaps the ideal setting is 52, as that detects a light touch fairly reliably, but doesn’t fire until my finger is within about 1mm of the surface.

Note: it might make more sense to leave NO_TOUCH_NUM at 50 and adjust TOTAL_PULSE_USEC down to 96, since the larger number can be adjusted more finely.

If I wanted to avoid the flicker, I would debounce the detection.  I can’t use the Arduino Bounce library for this, as the signal that needs debouncing is in software, not on a pin.  It is a trivial matter to add a delay whenever the output changes, though:

// Capacitive sensor switch
// Mon Jul  9 06:44:33 PDT 2012 Kevin Karplus

// To use, connect the output of the LM555 oscillator to
// pin CAP_PIN on the Arduino.
// The code turns on the LED on pin 13 when a touch is sensed.

// The Arduino measures the number and width of LOW pulses on pin 2 until
//    the sum of the pulse width exceeds TOTAL_PULSE_USEC.
// The LED is turned on if the number of pulses is less than NO_TOUCH_NUM.

// pin that LM555 output connected to
#define CAP_PIN (2)

// total of pulse widths (in  microseconds) before reporting
#define TOTAL_PULSE_USEC (96)

// minimum number of pulses to indicate that sensor is not touched
#define NO_TOUCH_NUM (50)

// time (in milliseconds) to wait after a change in state before
// testing input again
#define DEBOUNCE_MSEC (100)

static uint8_t was_on;

void setup(void)
{
    was_on=0;
    pinMode(CAP_PIN,INPUT);
    pinMode(13, OUTPUT);
}

void loop(void)
{
    uint32_t pulse_sum=0;
    uint16_t num_pulses=0;
    while (pulse_sum<TOTAL_PULSE_USEC)
    {   pulse_sum += pulseIn(CAP_PIN,LOW);
        num_pulses++;
    }
    uint8_t on = num_pulses<NO_TOUCH_NUM? 1:0;
    digitalWrite(13, on);
    if (on!=was_on)
    {   delay(DEBOUNCE_MSEC);
        was_on=on;
    }
}

Debouncing does not completely eliminate multiple flashes. If I hold my finger very steady just touching the sensor or my whole hand flat about 1mm away, the LED will flash on and off about 5 times a second (the speed of the flashing is determined mainly by DEBOUNCE_MSEC).

Really good code would not require fixed NO_TOUCH_NUM and TOTAL_PULSE_USEC, but would learn what values are usual for off and on states and set the thresholds to separate them reliably.  I’ve not figured out a good way to do this as unsupervised learning. It would not be very difficult to have initial settings, gather averages for on and off states, then dynamically adjust the threshold, but I don’t know how stable such threshold setting would be. I suspect that it would end up rather unreliable, and highly dependent on good initial settings.

2012 July 8

Capacitive sensing

I decided that it was time to try a new circuits lab today.  Although I have the INA-126 instrumentation amplifier working (with a 220Ω gain resistor for a gain of about 370), and I did get some EKG stick-on electrodes, I don’t feel like trying to debug the EKG lab today.  I could do a little more with the instrumentation amplifier and the new function generator, adding the function generator output to the microphone output to see try to separate the differential gain from the common-mode gain, but I’m getting a bit tired of op-amp and instrumentation amp circuits.  I’ll try to come back to them later.

Other sensor ideas from What sensors for circuits class? that I have the parts for now include phototransistors, LEDs and IR emitters, and reflectance light sensors, but I don’t feel like playing with phototransistors today either.  That leaves one rather do-it-yourself sensor: a capacitance touch switch.  I was thinking of implementing this with LM555 timer chip as an oscillator, detecting capacitance increase as a change in the frequency of the oscillator.

Design questions include

  • What capacitance do we add by touching something?
  • What frequency should the oscillator run at without the touch?
  • What % change in frequency can we detect reliably? How will we do that detection?

Capacitance

Let’s do a first-principles calculation of capacitance of a touch, using a simple parallel-plate model.  Let’s say that the area of the touch, pressing lightly with a finger, is about 1cm^2.  We need to know the dielectric constant of the insulator and its thickness.  Let’s say we use a cheap plastic  wrap from the grocery store.  According to Wikipedia, common plastic wraps are about 0.5 mils (12.5 µm) thick and are now generally a low-density polyethylene (LDPE).  According to Bert Hickman (referring to sites that no longer exist), the relative dielectric constant for LDPE is 2.2, with a breakdown voltage of 3000v/mil (or 1500v for 0.5mil).  Of course, he also claims that the common wrap material is a mix of PVC and PVDC, which seems to be out of date information. The A to Z of Materials site for LDPE claims

Dielectric Strength (MV/m) 27
Dissipation Factor 1kHz 0.0003
Dielectric Constant  1kHz 2.3

That dielectric strength would be 337V at 12.5µm, which seems more reasonable than Bert Hickman’s numbers.

I also have 3M contractor’s plastic for masking large areas when painting. It claims to be 8.9 µm thick, but I can’t figure out what the material is (they just say “high density plastic”).  If it is HDPE (a possibility), the A to Z of Materials site for HDPE claims

Dielectric Strength (MV/m) 22
Dissipation Factor 1kHz 0.0005
Dielectric Constant  1kHz 2.3

which gives the same dielectric constant as LDPE, but the breakdown voltage would be about 195V at 8.9µm—still plenty for a touch sensor.

If I have 1 cm^2 (1E-4 m^2) area, 5 mil thickness (12.5E-6m), relative dielectric constant of 2.3, and ε0=8.8542E-12 F/m, I get a capacitance of 163pF for the touch.

Frequency

The LM555 can be set up as an astable oscillator:

LM555 astable oscillator circuit

LM555 astable oscillator circuit from the Fairchild Semiconductor LM 555 data sheet.

The data sheet gives the period of the oscillator as 0.693(R_A+2R_B)C_1 and the frequency as \frac{1.44}{(R_A+2R_B)C_1}. If I want a slow oscillator, I’ll need to use a large resistance.  The largest resistance they show in the data sheet is RA+2RB=10MΩ, and I happen to have some 3.3MΩ resistors, so 9.9MΩ seems reasonable.  If I had no stray capacitance, 163pF and 9.9MΩ would give a period of 1.6msec (620Hz).  But, of course there will be substantial stray capacitance—probably much larger than the touch capacitance.  In any event, the increment to the period of the oscillator by adding the touch capacitance should be about 1.6msec, so we need to be able to detect that small a change in the period.  The percentage change will depend mainly on the stray capacitance.

One thing that is not clear from the LM555 data sheet is what value should be used for C2.  I suspect that it is just a bypass capacitor to reduce noise in the reference voltages used for the threshold, so a 4.7µF ceramic capacitor (of which I have several) should do fine.  Design notes I’ve seen on the web suggest that 0.1µF is plenty, though a bypass on the Vcc input of that amount is also recommended.

Experiment

I set up the circuit on the breadboard, with RA=RB=3.3MΩ, C1 being just the stray capacitance in the circuit, and C2 being a 4.7µF ceramic bypass capacitor. I also put a 4.7µF bypass capacitor between pins 1 and 8, as well as a 470µF electrolytic capacitor, to make sure that the power supply was clean.

The resulting oscillator has a period of about 0.2msec (4.4kHz), but it fluctuated a lot, so that I could not get a clean trace on my oscilloscope. Touching the insulted sensor pad (made from Al foil and Glad Cling Wrap) caused the period to increase enormously, but to a pulse train of 3 pulses that repeated at 60Hz.  I believe that what I was mainly doing was coupling in a 60Hz noise signal, rather than detecting a change in the capacitance to ground.

I could decrease the sensitivity of the circuit to noise by reducing the resistances and adding some more capacitance to C1. Adding 1nF cleans up the signal, but doesn’t eliminate the problem.  I get 107±1Hz without a touch, and 60Hz with a touch.  I’m still mainly coupling in 60Hz noise!  I’d have to reduce the impedance to reduce the 60Hz coupling, reducing RA and RB.

Leaving in the 1nF capacitor and reducing RA and RB to 100kΩ results in a non-touch frequency of 4.05kHz and a touch frequency of 3.6kHz.  This is about a 30µsec difference in period, consistent with an 150pF change in capacitance. But this frequency change seems to be almost independent of the area of contact, which is not consistent with a parallel plate model.  The lower frequency still seems to be modulated by a 60Hz hum.

Removing the 1nF capacitor, but leaving the 100kΩ resistors, I get a no-touch frequency of 140kHz–156kHz (depending on how close my arm comes to the sensor) and a touch frequency around 45kHz–50kHz, a 15µsec difference in the period. A firm press with all my fingers depresses the frequency further, down to about 20kHz. Touching my laptop (reducing my resistance to ground) decreases the frequency further—pressing with my palm and touching the laptop gets the frequency own to about 13kHz.

Here is the complete layout for the capacitive touch sensor, showing (clockwise) the aluminum foil wrapped in Glad Cling Wrap (except for the folded over part where the alligator clip is attached, the breadboard with the LM555 chip, and the Arduino board.

I wouldn’t want to use the interrupt-driven timer code to measure this frequency, but a simple busy-wait loop that times the period could work, or I could add a counter to divide the frequency down (and average out many clock cycles).  But this is supposed to be a circuits class, not a digital design class, so adding a counter is probably not appropriate for the lab.  Perhaps a better approach is to measure the duration of the low part of the pulse, which should be about 1/3 of the total period, using the Arduino pulseIn function.  Looking on the scope, the low part of the pulse seems to be closer on 1/6th of the total period than to 1/3 at the high frequency, though the ratios are more reasonable at the low frequency.  I suspect that there are stray capacitance within the LM555 that account for some of the pulse shape error.

By averaging over 500 pulses, I see a switch from 1.15±0.03 µsec for the low pulse in the no-touch configuration to 5.4±0.3µsec for a firm touch and 2.8±0.2µsec for a light touch (and up to 220µsec for a full-palm touch while touching my laptop). Single pulse measurements may be a bit unreliable at those speeds, since pulseIn only has 1µsec resolution, but adding 500 pulses doesn’t take much time, unless the capacitance is very high. Putting the 1nF capacitor back in the circuit makes the low time look more like 1/3 of the period.  The pulse widths are about 78.4±0.05µsec for no-touch and 80µsec–85µsec for a touch.  Those are long enough to be reliably detected with even a single pulseIn call, but the difference is rather small, so setting a fixed threshold would be a bit risky.

An alternative approach to averaging 100s of pulse widths is to collect pulses until the total pulse width exceeds 200µsec and report the number of pulses collected.  That mechanism results in an easily detected change in counts (from over 150 to under 60) and a response time of about 2–5msec (most of which is probably in the serial communication).  A detection threshold would have to be set for each different sensor plate the circuit is connected to, but that is pretty straightforward.

Here is the code I used for detecting sensor touches with summed pulse widths (click to expand):

// Capacitive sensor timer
// Sun Jul  8 17:59:49 PDT 2012 Kevin Karplus
//
// License: CC-BY-NC http://creativecommons.org/licenses/by-nc/3.0/
//
// To use, connect the output of the LM555 oscillator to
// digital pin 2 on the Arduino.
// On initialization, the Arduino will print "Arduino ready." to serial.
// The Arduino measures the number and width of LOW pulses on pin 2 until
//    the sum of the pulse width exceeds TOTAL_PULSE_TIME.
// It then reports 3 numbers to the serial line:
// The first is the time since the start, in microseconds.
// The second is the number of pulses measured.
// The third is the average duration of the pulses.
// Serial baudrate is 115200.
// The start time may be reset by sending the character 'R'.

#define TOTAL_PULSE_TIME (200)	// total of pulse widths (in  microseconds) before reporting

unsigned long first_time;   // what was the time for the first tick since reset?
// All times are in microseconds.

// do_reset() empties the queue and sents the count to 0
void do_reset(void)
{
    noInterrupts();     // don't take interrupts in the middle of resetting
    first_time=micros();
    // let the user know that the Arduino is ready
    Serial.print(F("\nArduino ready.\n"));
    interrupts();
}

// interrupt handler for data becoming available on the serial input
// Reset if an R is received
void serialEvent(void)
{
    if (Serial.read() == 'R')
    {
        do_reset();
    }
}

void setup(void)
{
    Serial.begin(115200);       // use the fastest serial connection available
    do_reset();
    pinMode(2,INPUT);
}

void loop(void)
{   // keep trying to print out what is in the queue.
    uint32_t pulse_sum=0;
    uint16_t num_pulses=0;
    while (pulse_sum<TOTAL_PULSE_TIME)
    {    pulse_sum += pulseIn(2,LOW);
        num_pulses++;
    }
    uint32_t datum=micros();
    Serial.print(datum - first_time); // time since start (usec)
    Serial.print("\t");
    Serial.print(num_pulses); // number of pulses
    Serial.print("\t");
    Serial.println(pulse_sum*(1.0/num_pulses), 2); // avg pulse duration
}

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