Piezoelectric actuators (benders, stacks, chips, and so on.) are wonderful quick and exact means for technology and management of micro, nano, and even atomic scale motion on millisecond and quicker timescales. Sadly, they’re additionally wonderful high-Q resonators. Determine 1 reveals what you may anticipate in the event you’re in a rush to maneuver a piezo and easily hit it with a unit step. End result: an enormous (practically 100%) overshoot with extended follow-on ringing.
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Determine 1 Typical piezo actuator response to squarewave drive with ringing and ~100% overshoot.
Don’t fear. It’ll get there. Finally. However don’t maintain your breath. Clearly one thing must be performed to switch the drive waveshape if we’re in any respect desirous about velocity and settling time. Many prospects exist, however Determine 2 illustrates a remarkably easy but efficient trick that really takes benefit of the piezo’s pure 2x overshoot: Half and Half step drive.
Determine 2 Half &Half drive step with half amplitude and half resonance interval kills overshoot and ringing.
The surprisingly easy trick is to separate the drive step into an preliminary step with half the specified motion amplitude and a length of precisely half the piezo resonance interval. Therefore: “Half & Half”(H&H) drive. The half-step is then adopted by software of the complete step amplitude to carry the actuator in its new place.
The physics underlying H&H depend on kinetic vitality imparted to the mass of the actuator through the first quarter cycle to be simply adequate to beat actuator elasticity through the second quarter, this bringing the actuator to a swish cease at half cycle’s finish. The drive voltage is then stepped to the complete worth, holding the actuator stationary on the remaining place.
Proven in Determine 3 is H&H would work for a sequence of arbitrary piezo strikes.
Determine 3 Instance of three arbitrary H&H strikes: (T2 – T1) = (T4 – T3) = (T6 – T5) = ½ piezo resonance interval.
If carried out in software program, the H&H algorithm could be simplicity itself and look one thing like this:
Let DAC = present contents of DAC output register
N = new content material for DAC required to provide desired piezo movement
Step 1: substitute DAC = (DAC + N) / 2
Step 2: wait one piezo resonance half-period
Step 3: substitute DAC = N
Achieved
If carried out in analog circuitry, H&H may appear like Determine 4. Right here’s the way it works.
Determine 4 The analog implementation of H&H.
The C1, R1, C2, R2||R3 voltage divider performs the half-amplitude division operate of the H&H algorithm, whereas dual-polarity comparators U2 detect the vanguard of every voltage step. Step detection triggers U3a, which is adjusted by way of the TUNE pot to have a timeout equal to half the piezo resonance interval, giving us the opposite “half”.
U3a timeout triggers U3b, which activates U1, outputting the complete step amplitude, finishing the transfer. The older metallic gate CMOS 4066 is used attributable to its superior low-leakage Roff spec’ whereas the parallel connection of all 4 of its inner switches yields an adequately low Ron.
U4 is only a place keeper for an appropriate piezo drive amplifier to translate from the 5-V logic of the H&H circuitry to piezo drive voltage and energy ranges.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Piezoelectric Know-how: A Primer
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- Improve piezoelectric transducer acoustic output with a easy circuit
- Scale back acoustic noise from capacitors
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