New analogue
interface for LVDTs
A new
type of programmable analogue interface for LVDTs could be handy for
machine builders. JON SEVERN reports
Linear variable
displacement transducers (LVDTs) provide a simple and reliable means
of converting mechanical displacement into an electrical signal.
They do not suffer from electrical hysteresis and can be adapted to
harsh environments. And because there are no discontinuities in the
output signal, LVDTs have practically infinite resolution.
Unfortunately
LVDTs require supporting analogue circuitry - an AC stimulus and
output signal conditioning - that differs for each LVDT model.
Ready-to-go solutions are available, but they add significant cost
impact to the finished system. Now, however, all the LVDT support
circuitry can be accomplished in a single inexpensive programmable
analogue device, the flexible field programmable analogue array
(FPAA).
An FPAA
contains a matrix of 'configurable analogue blocks' consisting of
switched-capacitor op-amp cells, surrounded by a programmable
interconnect and I/O structure (see screenshot below). Common
analogue signal conditioning functions can be implemented using just
one cell, with more complex functions being implemented using two or
more. In addition an FPAA contains internal voltage references and
programmable clocks. Circuits are designed using a free CAD tool -
AnadigmDesigner - without needing to know underlying circuit
techniques and without breadboarding. Instead, users simply select
analogue building blocks that are specified purely in terms of their
functions, and enter the particular characteristic they need.
AnadigmDesigner offers over 50 parameterisable analogue functions in
its library including gain stages, amplifiers, sample and hold,
filters, oscillators, rectifiers, comparators, DC references,
limiters, peak detectors, Schmitt triggers, integrators,
differentiators, and waveform generators. Experienced analogue
designers can also use the chip's resources to create their own
custom designs. The circuit's configuration is held in on-chip SRAM,
which is initialised on power-up from EPROM, or through the chip's
microprocessor peripheral interface.
An LVDT is a
cylindrical transformer with one primary and two secondary windings
(think of it as three solenoid windings stacked end to end). All the
windings are wrapped around a hollow open-air core containing a
non-contacting ferrous core. The displacement of an object can be
measured if it is mechanically linked to the movable core.
If an AC signal
is applied to the primary side and the ferrous core moved towards
the 'B' side secondary windings, the coupling from the primary to
the B side secondary will increase and the coupling to the A side
secondary will decrease. It is possible to read the changes caused
by this displacement by monitoring only Vb or Va, but it is better
to perform a ratiometric measurement: Vout = (Va - Vb)/(Va + Vb).
This removes the effects of stray noise, and fluctuations in the
driving amplitude (which of course results in a larger signal being
presented on both of the secondary windings, with no net
gain).
Benefits of
FPAAs
To build
a typical circuit using conventional analogue components would need
hours of design, construction, debug, trimming and tuning.
Production would require a significant number of components to
implement, consuming valuable board space and increasing
manufacturing costs. And the design would be 'fixed forever'.
Consider the
situation where an increase in sensitivity is required. With a
discrete solution, you would be forced to re-design, restock
inventory and possibly scrap work in progress. With the FPAA, only
the receiver's gain settings need be adjusted to get more
sensitivity.
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