Peekel Instruments User manual PICAS V2.6.1
Page 5 of 59
1.2 General
PICAS is a tabletop stand-alone compact amplifier system from Peekel Instruments B.V.
It can be connected as one node (station) in a larger, decentralized system by using the
integrated RS-485 bus connection.
PICAS is delivered with an LCD-front and pushbuttons for the operation of the system.
The PICAS can be also be connected to a external PC through a RS232 or USB interface.
It is designed to be used for high-accuracy experimental and industrial measurements and
can be used with a variety of Wheatstone bridge-based sensors.
PICAS can hold 2 input cards. These input cards can be the CA2CF or CA4AI cards.
The CA2CF card comprises 2 high-accurate galvanic isolated carrier frequency amplifier
channels each with its own analog output. On these channels a variety of resistive
straingauge configurations can be connected for experimental materials testing. Also Load
cells can be connected for industrial weighing and force measurements. LVDT’s (Linear
Variable Differential Transformers) can be used for measuring linear or angular
displacements and also Capacitive Transducers can be connected.
The CA4AI card comprises 4 input channels for voltage, current or resistor measurements.
PICAS contains a control board which comprises a microprocessor which controls the
amplifier settings, keyboard and display handling and the communication to external
systems through the serial communication channels.
1.3 The Carrier Frequency principle
High-accuracy measuring at the output of passive transducers is usually configured into
some sort of a Wheatstone Bridge circuit which always needs some form of reference
(bridge supply) voltage.
DC bridge supply is by far the most popular for resistive transducers, but when it comes to
the highest sensitivity, DC might introduce different spurious voltages which makes the
measuring unreliable. In the late 50’s PEEKEL already developed the Carrier Frequency
principle for these applications, where an AC voltage is being used for the supply, which
eliminates most of these spurious and misleading signals. Furthermore, AC bridge supply
can be also used together with capacitive and inductive transducers.
If dynamic signals are being measured, the AC bridge supply voltage will be “modulated” by
the measuring signal and by “detecting” this signal, the output signal becomes available.
This way of measuring, through modulation of a carrier frequency with detection in a later
step, is similar to the principle of AM radio. Hence, the term “Carrier Frequency” is being
used.
The inherent use of isolation transformers assures a complete isolation between the
sensing circuit and the rest of the system.
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