Technical solutions abound, but a consulting engineer must work within cost constraints
For ten years I was a principal in a consulting
Universal Sonics Corporation. We had many customers from all over the
and we specialized in ultrasonic imaging.
One day I got a call from a
was not familiar with that was manufacturing ultrasonic toothbrushes.
a problem: some of the toothbrushes worked fine, and others had low-output
power. They could not understand what the problem was. Could we help
asked the customer to send me the schematics, and a sample of a good
and a bad one. I then reviewed the schematic, and noticing that the
the piezoelectric transducer was at a fixed frequency; I took the
from the good and bad toothbrushes and measured their impedance plots
resonance with a vecto- impedance meter.
The good one resonated fairly
the drive frequency, and the other one was about 10% off. The Q of
fairly high, so the 10% frequency shift caused the drive voltage to be
enough off from resonance, that sufficient acoustic output would no
generated. For a low-cost consumer product, specifying the resonant
to a higher tolerance would be too expensive. If I could adjust the
frequency to match the self-resonant frequency the problem would be
Manually tuning the device was also not cost effective.
became obvious: I needed to design an oscillator with sufficient drive
used the piezoelectric transducer as the resonant electrical element.
I designed a circuit; bread boarded and tested it, which worked as
sample and the design documents were quickly delivered to the customer,
thrilled because his problem was solved in an inexpensive, easy- to-produce
The design change was immediately put into production. As a
thousands of toothbrushes were produced without further incident.
Howard Fidel has BE, MS and MBA degrees. He is currently a senior engineer at Schick Technologies in New York, and has received 8 patents during his 36 year career. He can be reached at email@example.com.