First, we need to drive the transducer to generate the
proper frequency. As mentioned last month, a piezoelectric
transducer has a natural frequency determined by its
dimensions and its material. The device is resonant and — if
we are to get useful acoustic vibration from it — we need to
operate the transmitter at or very near to this natural
With any driver circuit, we need to know what our load
looks like in terms of the voltage, current, and impedance
requirements. Maximum drive voltages for these
transducers are typically on the order of 20V, but we need
to be a little careful in how we read such a specification. If
the datasheet says 20 Vrms (root mean square) maximum,
then a 0V to 40V square wave would satisfy this limit. On
the other hand, if the datasheet says 20V peak-to-peak,
then our square wave generator should be 0V to 20V.
By the way, the transducer (being a highly resonant
device) will filter out all but the fundamental frequency, so
it doesn’t matter if our generator produces a sine wave or a
square wave. In most applications, the square wave is easier
to generate, so we’ll use that as our wave shape.
For the remainder of this article, a 30V source voltage
will be used with the understanding
that the actual voltage needs to be
tailored to the particular transducer
Looks Like a
Capacitor to the
electrically look like a capacitor. The
capacitance value should be given on
the datasheet. Typical values are 2,000 pF and 3,000 pF.
Since the device is useful to us only at its resonant
frequency, we can immediately determine the reactance as:
where f RES is the transducer’s resonant frequency (for
example, 40 kHz or 25 kHz). So, for a transducer with a
resonant frequency of 40 kHz and a capacitance of 2,400
pF, the device looks like a 1.65 kΩ resistor. A 30V pulse
across the transducer will therefore result in a current of
about 18 mA through the device.
Where Do We Get the
To produce a 30V square wave, we need a DC power
source that generates at least 30V output. So, where do
you find such a voltage on most robots or other small
vehicles? Fortunately, it’s not hard to build a circuit that can
boost the 3.3V or 5V on a board to 30 or more volts. To
see how this is done, take a look at Figure 1.
The circuit is simple: an inductor, a capacitor, a diode,
To start, the switch closes (Figure 2).
Assume that initially, the inductor has
zero current and the capacitor has zero
voltage. With the diode reverse-biased,
the circuit really is just the inductor
connected across the + 3.3V (or whatever
the low voltage is that we’re using as the
energy source). The current through the
inductor increases at the rate:
36 SERVO 03.2018
Figure 1. Basic boost circuit.
Figure 2. Boost circuit charging.
ΔiL =VL Δt L