User Manual

Page 44
Figure 51. Switch for pinball machine
Figure 52. Beam switch
The "digital" piezo film switch features a simple MOSFET circuit that consumes no power during the
normally-open state. In response to a direct contact force, the piezo film beam momentarily triggers the
MOSFET. This provides a momentary "closure" for up to a 50 V maximum voltage. The output of this
low profile contactless switch is well suited to logic-level switching. The unit does not exhibit the
corrosion, pitting or bounce that are normally associated with contact switches.
The company has tested these switches in excess of 10 million cycles without failure. The switch solves
the nagging problem of fouled contacts in pinball machines, a significant source for machine downtime
and lost revenue. The simplicity of the design makes it effective in applications which include:
! Counter switches for assembly lines and shaft rotation
! Switches for automated processes
! Impact detection for machine dispensed products
! Panel switches
! Foot pedal switches
! Door closure switches
The cantilever beam that carries the piezo film can be modified to adjust
switch sensitivity for high to low impact forces. Figure 51 shows the
construction of the digital switch.
Beam Switch
Piezo film switches can be used to measure the amplitude, frequency
and direction of an event and are useful in object detection and
recognition, counting, wakeup switches and bidirectional encoding
applications. The construction of the beam-type switch is shown in
Figure 52.
Note that the piezo film element is laminated to a thicker substrate on
one side, and has a much thinner laminate on the other. This moves the
neutral axis of the structure out of the piezo film element, resulting in a
fully tensile strain in the piezo film when deflected downward, and a
fully compressive strain when deflected in the opposite direction. Were
the neutral axis in the center of the piezo film element, as would be the
case if the two laminae were of equal thickness, the top half of the piezo film would be oppositely
strained from the bottom half under any deflection condition, and the resulting signals would therefore
be canceled.
Beam switches are used in shaft rotation counters in natural gas meters and as gear tooth counters in
electric utility metering. The beam switch does not require an external power source, so the gas meter is
safe from spark hazard. Other examples of applications for the beam switch include a baseball target
that detects ball impact, a basketball game where a hoop mounted piezo film sensor counts good
baskets, switches inside of an interactive soft doll to detect a kiss to the cheek or a tickle (and the
sensor is sewn into the fabric of the doll), coin sensors for vending and slot machines and as digital
potentiometer for high reliability.
Snap-Action Switches
Piezoelectric materials do not have a true dc response. Very slow events, 0.0001 Hz, for example, are
not normally possible to detect with piezoelectric film.