AC instrumentation transducers
Just as devices have been made to measure
certain physical quantities and repeat that information in the form of
DC electrical signals (thermocouples, strain gauges, pH probes, etc.),
special devices have been made that do the same with AC.
It is often necessary to be able to
detect and transmit the physical position of mechanical parts via
electrical signals. This is especially true in the fields of automated
machine tool control and robotics. A simple and easy way to do this is
with a potentiometer:
However, potentiometers have their own
unique problems. For one, they rely on physical contact between the
"wiper" and the resistance strip, which means they suffer the effects of
physical wear over time. As potentiometers wear, their proportional
output versus shaft position becomes less and less certain. You might
have already experienced this effect when adjusting the volume control
on an old radio: when twisting the knob, you might hear "scratching"
sounds coming out of the speakers. Those noises are the result of poor
wiper contact in the volume control potentiometer.
Also, this physical contact between wiper
and strip creates the possibility of arcing (sparking) between the two
as the wiper is moved. With most potentiometer circuits, the current is
so low that wiper arcing is negligible, but it is a possibility to be
considered. If the potentiometer is to be operated in an environment
where combustible vapor or dust is present, this potential for arcing
translates into a potential for an explosion!
Using AC instead of DC, we are able to
completely avoid sliding contact between parts if we use a variable
transformer instead of a potentiometer. Devices made for this
purpose are called LVDT's, which stands for Linear Variable
Differential Transformers. The design of an LVDT looks
like this:
Obviously, this device is a
transformer: it has a single primary winding powered by an external
source of AC voltage, and two secondary windings connected in
series-bucking fashion. It is variable because the core is free
to move between the windings. It is differential because of the
way the two secondary windings are connected. Being arranged to oppose
each other (180o out of phase) means that the output of this
device will be the difference between the voltage output of the
two secondary windings. When the core is centered and both windings are
outputting the same voltage, the net result at the output terminals will
be zero volts. It is called linear because the core's freedom of
motion is straight-line.
The AC voltage output by an LVDT
indicates the position of the movable core. Zero volts means that the
core is centered. The further away the core is from center position, the
greater percentage of input ("excitation") voltage will be seen at the
output. The phase of the output voltage relative to the excitation
voltage indicates which direction from center the core is offset.
The primary advantage of an LVDT over a
potentiometer for position sensing is the absence of physical contact
between the moving and stationary parts. The core does not contact the
wire windings, but slides in and out within a nonconducting tube. Thus,
the LVDT does not "wear" like a potentiometer, nor is there the
possibility of creating an arc.
Excitation of the LVDT is typically 10
volts RMS or less, at frequencies ranging from power line to the high
audio (20 kHz) range. One potential disadvantage of the LVDT is its
response time, which is mostly dependent on the frequency of the AC
voltage source. If very quick response times are desired, the frequency
must be higher to allow whatever voltage-sensing circuits enough cycles
of AC to determine voltage level as the core is moved. To illustrate the
potential problem here, imagine this exaggerated scenario: an LVDT
powered by a 60 Hz voltage source, with the core being moved in and out
hundreds of times per second. The output of this LVDT wouldn't even look
like a sine wave because the core would be moved throughout its range of
motion before the AC source voltage could complete a single cycle! It
would be almost impossible to determine instantaneous core position if
it moves faster than the instantaneous source voltage does.
A variation on the LVDT is the RVDT, or
Rotary Variable Differential Transformer.
This device works on almost the same principle, except that the core
revolves on a shaft instead of moving in a straight line. RVDT's can be
constructed for limited motion of 360o (full-circle) motion.
Continuing with this principle, we have
what is known as a Synchro or Selsyn, which is a device
constructed a lot like a wound-rotor polyphase AC motor or generator.
The rotor is free to revolve a full 360o, just like a motor.
On the rotor is a single winding connected to a source of AC voltage,
much like the primary winding of an LVDT. The stator windings are
usually in the form of a three-phase Y, although synchros with more than
three phases have been built:
Voltages induced in the stator windings
from the rotor's AC excitation are not phase-shifted by 120o
as in a real three-phase generator. If the rotor were energized with DC
current rather than AC and the shaft spun continuously, then the
voltages would be true three-phase. But this is not how a synchro is
designed to be operated. Rather, this is a position-sensing
device much like an RVDT, except that its output signal is much more
definite. With the rotor energized by AC, the stator winding voltages
will be proportional in magnitude to the angular position of the rotor,
phase either 0o or 180o shifted, like a regular
LVDT or RVDT. You could think of it as a transformer with one primary
winding and three secondary windings, each secondary winding oriented at
a unique angle. As the rotor is slowly turned, each winding in turn will
line up directly with the rotor, producing full voltage, while the other
windings will produce something less than full voltage.
Synchros are often used in pairs. With
their rotors connected in parallel and energized by the same AC voltage
source, their shafts will match position to a high degree of accuracy:
Such "transmitter/receiver" pairs have
been used on ships to relay rudder position, or to relay navigational
gyro position over fairly long distances. The only difference between
the "transmitter" and the "receiver" is which one gets turned by an
outside force. The "receiver" can just as easily be used as the
"transmitter" by forcing its shaft to turn and letting the synchro on
the left match position.
If the receiver's rotor is left unpowered,
it will act as a position-error detector, generating an AC voltage at
the rotor if the shaft is anything other than 90o or 270o
shifted from the shaft position of the transmitter. The receiver rotor
will no longer generate any torque and consequently will no longer
automatically match position with the transmitter's:
This can be thought of almost as a sort
of bridge circuit that achieves balance only if the receiver shaft is
brought to one of two (matching) positions with the transmitter shaft.
One rather ingenious application of the
synchro is in the creation of a phase-shifting device, provided that the
stator is energized by three-phase AC:
As the synchro's rotor is turned, the
rotor coil will progressively align with each stator coil, their
respective magnetic fields being 120o phase-shifted from one
another. In between those positions, these phase-shifted fields will mix
to produce a rotor voltage somewhere between 0o, 120o,
or 240o shift. The practical result is a device capable of
providing an infinitely variable-phase AC voltage with the twist of a
knob (attached to the rotor shaft).
So far the transducers discussed have all
been of the inductive variety. However, it is possible to make
transducers which operate on variable capacitance as well, AC being used
to sense the change in capacitance and generate a variable output
voltage.
Remember that the capacitance between two
conductive surfaces varies with three major factors: the overlapping
area of those two surfaces, the distance between them, and the
dielectric constant of the material in between the surfaces. If two out
of three of these variables can be fixed (stabilized) and the third
allowed to vary, then any measurement of capacitance between the
surfaces will be solely indicative of changes in that third variable.
Medical researchers have long made use of
capacitive sensing to detect physiological changes in living bodies. As
early as 1907, a German researcher named H. Cremer placed two metal
plates on either side of a beating frog heart and measured the
capacitance changes resulting from the heart alternately filling and
emptying itself of blood. Similar measurements have been performed on
human beings with metal plates placed on the chest and back, recording
respiratory and cardiac action by means of capacitance changes. For more
precise capacitive measurements of organ activity, metal probes have
been inserted into organs (especially the heart) on the tips of catheter
tubes, capacitance being measured between the metal probe and the body
of the subject. With a sufficiently high AC excitation frequency and
sensitive enough voltage detector, not just the pumping action but also
the sounds of the active heart may be readily interpreted.
Like inductive transducers, capacitive
transducers can also be made to be self-contained units, unlike the
direct physiological examples described above. Some transducers work by
making one of the capacitor plates movable, either in such a way as to
vary the overlapping area or the distance between the plates. Other
transducers work by moving a dielectric material in and out between two
fixed plates:
Transducers with greater sensitivity and
immunity to changes in other variables can be obtained by way of
differential design, much like the concept behind the LVDT (Linear
Variable Differential Transformer). Here are a few examples of
differential capacitive transducers:
As you can see, all of the differential
devices shown in the above illustration have three wire
connections rather than two: one wire for each of the "end" plates and
one for the "common" plate. As the capacitance between one of the "end"
plates and the "common" plate changes, the capacitance between the other
"end" plate and the "common" plate is such to change in the opposite
direction. This kind of transducer lends itself very well to
implementation in a bridge circuit:
Capacitive transducers provide relatively
small capacitances for a measurement circuit to operate with, typically
in the picofarad range. Because of this, high power supply
frequencies (in the megahertz range!) are usually required to reduce
these capacitive reactances to reasonable levels. Given the small
capacitances provided by typical capacitive transducers, stray
capacitances have the potential of being major sources of measurement
error. Good conductor shielding is essential for reliable and
accurate capacitive transducer circuitry!
The bridge circuit is not the only way to
effectively interpret the differential capacitance output of such a
transducer, but it is one of the simplest to implement and understand.
As with the LVDT, the voltage output of the bridge is proportional to
the displacement of the transducer action from its center position, and
the direction of offset will be indicated by phase shift. This kind of
bridge circuit is similar in function to the kind used with strain
gauges: it is not intended to be in a "balanced" condition all the time,
but rather the degree of imbalance represents the magnitude of the
quantity being measured.
An interesting alternative to the bridge
circuit for interpreting differential capacitance is the twin-T.
It requires the use of diodes, those "one-way valves" for electric
current mentioned earlier in the chapter:
This circuit might be better understood
if re-drawn to resemble more of a bridge configuration:
Capacitor C1 is charged by the
AC voltage source during every positive half-cycle (positive as measured
in reference to the ground point), while C2 is charged during
every negative half-cycle. While one capacitor is being charged, the
other capacitor discharges (at a slower rate than it was charged)
through the three-resistor network. As a consequence, C1
maintains a positive DC voltage with respect to ground, and C2
a negative DC voltage with respect to ground.
If the capacitive transducer is displaced
from center position, one capacitor will increase in capacitance while
the other will decrease. This has little effect on the peak voltage
charge of each capacitor, as there is negligible resistance in the
charging current path from source to capacitor, resulting in a very
short time constant (τ). However, when it comes time to discharge
through the resistors, the capacitor with the greater capacitance value
will hold its charge longer, resulting in a greater average DC voltage
over time than the lesser-value capacitor.
The load resistor (Rload),
connected at one end to the point between the two equal-value resistors
(R) and at the other end to ground, will drop no DC voltage if the two
capacitors' DC voltage charges are equal in magnitude. If, on the other
hand, one capacitor maintains a greater DC voltage charge than the other
due to a difference in capacitance, the load resistor will drop a
voltage proportional to the difference between these voltages. Thus,
differential capacitance is translated into a DC voltage across the load
resistor.
Across the load resistor, there is both
AC and DC voltage present, with only the DC voltage being significant to
the difference in capacitance. If desired, a low-pass filter may be
added to the output of this circuit to block the AC, leaving only a DC
signal to be interpreted by measurement circuitry:
As a measurement circuit for differential
capacitive sensors, the twin-T configuration enjoys many advantages over
the standard bridge configuration. First and foremost, transducer
displacement is indicated by a simple DC voltage, not an AC voltage
whose magnitude and phase must be interpreted to tell which
capacitance is greater. Furthermore, given the proper component values
and power supply output, this DC output signal may be strong enough to
directly drive an electromechanical meter movement, eliminating the need
for an amplifier circuit. Another important advantage is that all
important circuit elements have one terminal directly connected to
ground: the source, the load resistor, and both capacitors are all
ground-referenced. This helps minimize the ill effects of stray
capacitance commonly plaguing bridge measurement circuits, likewise
eliminating the need for compensatory measures such as the Wagner earth.
This circuit is also easy to specify
parts for. Normally, a measurement circuit incorporating complementary
diodes requires the selection of "matched" diodes for good accuracy. Not
so with this circuit! So long as the power supply voltage is
significantly greater than the deviation in voltage drop between the two
diodes, the effects of mismatch are minimal and contribute little to
measurement error. Furthermore, supply frequency variations have a
relatively low impact on gain (how much output voltage is developed for
a given amount of transducer displacement), and square-wave supply
voltage works as well as sine-wave, assuming a 50% duty cycle (equal
positive and negative half-cycles), of course.
Personal experience with using this
circuit has confirmed its impressive performance. Not only is it easy to
prototype and test, but its relative insensitivity to stray capacitance
and its high output voltage as compared to traditional bridge circuits
makes it a very robust alternative.
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