Sensors & Measurement · Study deck

Piezo Touch and Biomimetic Sensing

Picture a robot hand lifting a glass jar.

Physics Phoebe is your guide for this deck.

sensorbiomimeticpiezo
Physics Phoebe, the module guide, in a scene from this chapter.
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After studying this chapter

Learning objectives

You will be able to:

  • Explain why piezoelectric sensors behave like rapidly adapting mechanoreceptors.
  • Convert a dynamic force into charge and charge-amplifier output voltage.
  • Choose feedback capacitance and resistor values from signal amplitude, saturation, and low-frequency cutoff constraints.
  • Pair dynamic piezo channels with static-capable pressure sensors when a design must measure both events and held force.
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Major section

Start With the Measurement Story

The control team must decide whether that fading means lost contact or normal sensor behavior.

  • The fast channel must notice a tap, slip, or vibration within the allowed time.
  • A second channel must show that steady grip remains.
  • This simple split does not size the sensing parts or predict every material effect.

Why it matters

If the hand must prevent a dropped jar, keep the urgent safe response near the hand.

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Major section

Start With the Measurement Story (continued)

If the hand must prevent a dropped jar, keep the urgent safe response near the hand.

  • A distant service may store trends, but it must not be the only path to releasing or tightening the grip.
  • Under the Hood follows force through charge, the amplifier, leakage, and the limits of the model.
  • The useful starting point is the contact story: force, vibration, mounting, threshold, noise, and the proof that a real touch was detected.
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Major section

Phoebe's Field Notes: The Source Itself Only Fires On dF/dt

If that force rises in 5 ms, the crystal sources 40.0 nA on average; a 1 nF charge amplifier gives a 0.200 V ideal peak.

  • Its 100 MΩ feedback path makes $\\tau=100$ ms, so a linear 5 ms tap retains about 97.5% of that peak while a slow hold leaks away.

Numbers to remember

0.200 Va 1 nF charge amplifier gives a 0.200 V ideal peak.
100 MIts 100 MΩ feedback path makes $\\tau=100$ ms
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Major section

Follow a Tap From Ceramic to ADC Code

At the piezo touch panel, the panel bends, the crystal makes charge, and the readout produces a short voltage pulse; a held finger soon becomes quiet even though the force remains.

  • That physical decay is why piezo touch imitates rapid skin response better than steady pressure.

Numbers to remember

3.3 VA 3.3 V, 12-bit ADC has a 0.806 mV code width
0.806 mV12-bit ADC has a 0.806 mV code width
Touch sensation pathway showing external load, skin mechanics, stress and strain at a mechanoreceptor, and electrochemical response.
Touch sensation pathway showing external load, skin mechanics, stress and strain at a mechanoreceptor, and electrochemical response.
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Major section

Follow a Tap From Ceramic to ADC Code (continued)

A 40 mV threshold leaves room above small vibrations while keeping the 0.200 V tap visible.

  • Along the piezo sensing path, the feedback resistor lets that voltage fall, and the ADC plus threshold labels the pulse as a tap.
  • Each labelled block can change sensitivity or timing, so “piezo detected touch” is not enough to locate a missed event.
  • Mounting changes the mechanical input before the piezo equation begins.
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Major section

Follow a Tap From Ceramic to ADC Code (continued)

A 3.3 V, 12-bit ADC has a 0.806 mV code width, so the ideal pulse spans about 200/0.806=248 codes.

  • A thick steel face may spread a fingertip force across the disc, while a loose mount can ring after impact.
  • Threshold and hold-off time therefore need tests on the final panel, not only on a bare bench element.
  • The feedback path removes the charge, so the circuit mainly reports the change at press and release.
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Deck summary

Key takeaways

The control team must decide whether that fading means lost contact or normal sensor behavior.

  • If the hand must prevent a dropped jar, keep the urgent safe response near the hand.
  • If that force rises in 5 ms, the crystal sources 40.0 nA on average; a 1 nF charge amplifier gives a 0.200 V ideal peak.
  • At the piezo touch panel, the panel bends, the crystal makes charge, and the readout produces a short voltage pulse; a held finger soon becomes quiet even though the force remains.
  • A 40 mV threshold leaves room above small vibrations while keeping the 0.200 V tap visible.
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Retrieval practice

Recall check 1 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q1What physical stimulus does a piezoelectric sensor fundamentally respond to?

AA constant applied force, producing a steady voltage indefinitely.
BAmbient light falling on the crystal.
CA changing mechanical deformation (force that varies with time).
DA change in supply voltage.
Show answer

Answer: C Piezoelectric output tracks changing strain, like a rapidly adapting mechanoreceptor; steady force gives no lasting output.

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Retrieval practice

Recall check 2 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q2A piezoelectric element with d = 30 pC/N is deformed by a dynamic force of 5 N and read by a charge amplifier with Cf = 1.5 nF. What is the peak output voltage?

A0.1 V after charge is divided by Cf.
B150 V, using Q = d×F directly as a voltage.
CAbout 45 V, multiplying d, F, and Cf together.
D0 V, because piezoelectric sensors produce no output.
Show answer

Answer: A Compute the charge from d×F, then divide by the feedback capacitance.

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Retrieval practice

Recall check 3 of 3

Physics Phoebe says: answer from memory, then check your reasoning.

Q3An engineer tries to use a piezoelectric film to measure the steady weight of an object resting on it, but the output decays to zero within seconds. Why, and what is the biomimetic fix?

AThe film ran out of charge and must be recharged from a supply.
BThe weight is too heavy, saturating the sensor permanently.
CA stronger charge amplifier would let it hold a DC level indefinitely.
DThe charge from a constant force leaks away through finite resistance.
Show answer

Answer: D No DC response is inherent to piezoelectrics; combine with a static-capable sensor, mirroring rapidly- and slowly-adapting receptors in skin.

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Print reference

Answers

Answer key.

  1. C · Piezoelectric output tracks changing strain, like a rapidly adapting mechanoreceptor; steady force gives no lasting output.
  2. A · Compute the charge from d×F, then divide by the feedback capacitance.
  3. D · No DC response is inherent to piezoelectrics; combine with a static-capable sensor, mirroring rapidly- and slowly-adapting receptors in skin.
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