User Manual

Page 45
Figure 53. Snap-action switch
In switch applications where dc response is required, piezo film
in combination with a snap dome
provides a high voltage pulse.
When the snap device actuates, the film is rapidly strained,
typically generating a 10 volt pulse into a one megohm circuit
as shown in Figure 53. This concept is especially well suited for
wakeup switches, where an electronic device can be dormant
for long periods without power consumption until the snap
action device is actuated. The piezoelectric pulse turns on the
electronics. Battery operated parking meters, where battery life
is very critical, are an example of a piezo snap action switch
application. A thermal snap action device also employs this
principle.
Impact Sensors
Impact Printers
High speed impact printers require very accurate print head timing. Impact must occur the instant that
a high speed revolving steel band, embossed with print characters, is properly positioned in front of the
print hammer. Any advance or delay in energizing the print hammer will result in an offset print of the
desired character.
Piezo film sensor strips, built into the printer platen, monitor the impact timing and force of the bank
of print heads, and transmit the information to the controller. Automatic adjustment is made in the
actuator timing to accommodate any minor change in print head timing. The very high speed of the
embossed steel ribbon, about 300 inches per second, requires a very fast switch response. Alternative
impact switches are quickly destroyed by the large impact forces of the print head. Piezo film switches
have been in use in this application for more than five years without failure.
Sports Scoring
Piezo film sensors can be used to measure impact time, location (accuracy) and force. These parameters
are desirable in several sports scoring applications. The energy of a 90 mph pitch has instantaneous
power of about 50,000 watts! The great challenge in this application is target ruggedness without the
introduction of severe bounceback into the design.
A second sports scoring application is electronic dartboards, where piezo film monitors the many
impact zones in the game. Scoring is electronically recorded.
Musical Instruments
The popularity of electronics for musical instruments presents a special problem in drums and pianos.
The very high dynamic range and frequency response requirements for drum triggers and piano
keyboards are met by piezo film impact elements. Laminates of piezo film are incorporated in foot
pedal switches for bass drums, and triggers for snares and tom-toms. Piezo film impact switches are
force sensitive, faithfully duplicating the effort of the drummer or pianist. In electronic pianos, the
piezo film switches respond with a dynamic range and time constant that is remarkably similar to a
piano key stroke.