達松發可動線圈 (D’Arsonval movement)
http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_moveme
nt.htm
http://www.mikesflightdeck.com/diy_darsonval_instruments.htm
The D'Arsonval Approach
If you're building a sim based on a smaller general aviation plane like a C-172, you might also take
a look at moving coil or D'Arsonval meters. These are the typical meter movements used in
voltmeters and audio systems before LCDs and LEDs came on the scene. Although they can be
manufactured to move a needle through a 270 degree arc, most such meters are simple 90 degree
units. These meters offer a simple approach to simulating the fuel quantity and battery current
gauges often seen on the smaller GA craft.
D'Arsonval movements form the basis for VOR and glide slope needles, as well as things like the
"to-from" flag on the VOR. These meters tend to be rather delicate, but if you're steady of hand,
you might consider adapting the basic movement to a simulated instrument. The units pictured
above both have zero on one side of the scale. However, occasionally you will find one with zero
in the center, making it a good candidate for just such a project. These meters were picked up at a
swap meet for $2US a piece. Similar units show up in the inventory of electronics suppliers
handling surplus at prices ranging from $3US to $12US.
D'Arsonval meters can be easily interfaced with micro controllers running pulse width modulation
software. You won't need a buffer amp as the meter movements are quite sensitive.
http://tpub.com/content/neets/14175/css/14175_34.htm
For higher current ranges (above 50 amperes) ammeters that use external shunts are used. The
external shunt resistor serves the same purpose as the internal shunt resistor. The external shunt is
connected in series with the circuit to be measured and in parallel with the ammeter. This shunts
(bypasses) the ammeter so only a portion of the current goes through the meter. Each external shunt
will be marked with the maximum current value that the ammeter will measure when that shunt is
used. Figure 1-23 shows an ammeter that is designed to use external shunts and a d’Arsonval meter
movement. Figure 1-23(A) shows the internal construction of the meter and the way in which the
external shunt is connected to the meter and to the circuit being measured. Figure 1-23(C) shows
some typical external shunts. Figure 1-23.—An ammeter employing the d'Arsonval principle
and external shunts. A shunt resistor is nothing more than a resistor in parallel with the meter
movement. To measure high currents, very small resistance shunts are used so the majority of the
current will go through the shunt. Since the total resistance of a parallel circuit (the meter
movement and shunt resistor) is always less than the resistance of the smallest resistor, as an
ammeter’s range is increased, its resistance decreases. This is important because the load resistance
of high-current circuits is smaller than the load resistance of low-current circuits. To obtain accurate
measurements, it is necessary that the ammeter resistance be much less than the load resistance,
since the ammeter is connected in series with the load. Q20.
ammeter measure?
What electrical property does an
AC voltmeters and ammeters
http://www.allaboutcircuits.com/vol_2/chpt_12/1.html
AC electromechanical meter movements come in two basic arrangements: those based on DC
movement designs, and those engineered specifically for AC use. Permanent-magnet moving coil
(PMMC) meter movements will not work correctly if directly connected to alternating current,
because the direction of needle movement will change with each half-cycle of the AC. (Figure
below) Permanent-magnet meter movements, like permanent-magnet motors, are devices whose
motion depends on the polarity of the applied voltage (or, you can think of it in terms of the
direction of the current).
Passing AC through this D'Arsonval meter movement causes useless flutter of the needle.
In order to use a DC-style meter movement such as the D'Arsonval design, the alternating current
must be rectified into DC. This is most easily accomplished through the use of devices called diodes.
We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies from a
distorted (or rectified) sine wave. Without going into elaborate detail over how and why diodes work as
they do, just remember that they each act like a one-way valve for electrons to flow: acting as a
conductor for one polarity and an insulator for another. Oddly enough, the arrowhead in each diode
symbol points against the permitted direction of electron flow rather than with it as one might expect.
Arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant
direction throughout all portions of the AC cycle: (Figure below)
Passing AC through this Rectified AC meter movement will be drive it in one direction.
Another strategy for a practical AC meter movement is to redesign the movement without the inherent
polarity sensitivity of the DC types. This means avoiding the use of permanent magnets. Probably the
simplest design is to use a nonmagnetized iron vane to move the needle against spring tension, the
vane being attracted toward a stationary coil of wire energized by the AC quantity to be measured as
in Figure below.
Iron-vane electromechanical meter movement.
Electrostatic attraction between two metal plates separated by an air gap is an alternative
mechanism for generating a needle-moving force proportional to applied voltage. This works just as
well for AC as it does for DC, or should I say, just as poorly! The forces involved are very small,
much smaller than the magnetic attraction between an energized coil and an iron vane, and as such
these “electrostatic” meter movements tend to be fragile and easily disturbed by physical movement.
But, for some high-voltage AC applications, the electrostatic movement is an elegant technology. If
nothing else, this technology possesses the advantage of extremely high input impedance, meaning
that no current need be drawn from the circuit under test. Also, electrostatic meter movements are
capable of measuring very high voltages without need for range resistors or other, external
apparatus.
When a sensitive meter movement needs to be re-ranged to function as an AC voltmeter,
series-connected “multiplier” resistors and/or resistive voltage dividers may be employed just as in
DC meter design: (Figure below)
Multiplier resistor (a) or resistive divider (b) scales the range of the basic meter movement.
Capacitors may be used instead of resistors, though, to make voltmeter divider circuits. This
strategy has the advantage of being non-dissipative (no true power consumed and no heat produced):
(Figure below)
AC voltmeter with capacitive divider.
If the meter movement is electrostatic, and thus inherently capacitive in nature, a single “multiplier”
capacitor may be connected in series to give it a greater voltage measuring range, just as a
series-connected multiplier resistor gives a moving-coil (inherently resistive) meter movement a
greater voltage range: (Figure below)
An electrostatic meter movement may use a capacitive multiplier to multiply the scale
of the basic meter movement..
The Cathode Ray Tube (CRT) mentioned in the DC metering chapter is ideally suited for measuring
AC voltages, especially if the electron beam is swept side-to-side across the screen of the tube while
the measured AC voltage drives the beam up and down. A graphical representation of the AC wave
shape and not just a measurement of magnitude can easily be had with such a device. However,
CRT's have the disadvantages of weight, size, significant power consumption, and fragility (being
made of evacuated glass) working against them. For these reasons, electromechanical AC meter
movements still have a place in practical usage.
With some of the advantages and disadvantages of these meter movement technologies having been
discussed already, there is another factor crucially important for the designer and user of AC
metering instruments to be aware of. This is the issue of RMS measurement. As we already know,
AC measurements are often cast in a scale of DC power equivalence, called RMS
(Root-Mean-Square) for the sake of meaningful comparisons with DC and with other AC
waveforms of varying shape. None of the meter movement technologies so far discussed inherently
measure the RMS value of an AC quantity. Meter movements relying on the motion of a mechanical
needle (“rectified” D'Arsonval, iron-vane, and electrostatic) all tend to mechanically average the
instantaneous values into an overall average value for the waveform. This average value is not
necessarily the same as RMS, although many times it is mistaken as such. Average and RMS values
rate against each other as such for these three common waveform shapes: (Figure below)
RMS, Average, and Peak-to-Peak values for sine, square, and triangle waves.
Since RMS seems to be the kind of measurement most people are interested in obtaining with an
instrument, and electromechanical meter movements naturally deliver average measurements rather
than RMS, what are AC meter designers to do? Cheat, of course! Typically the assumption is made
that the waveform shape to be measured is going to be sine (by far the most common, especially for
power systems), and then the meter movement scale is altered by the appropriate multiplication
factor. For sine waves we see that RMS is equal to 0.707 times the peak value while Average is
0.637 times the peak, so we can divide one figure by the other to obtain an average-to-RMS
conversion factor of 1.109:
In other words, the meter movement will be calibrated to indicate approximately 1.11 times higher
than it would ordinarily (naturally) indicate with no special accommodations. It must be stressed
that this “cheat” only works well when the meter is used to measure pure sine wave sources. Note
that for triangle waves, the ratio between RMS and Average is not the same as for sine waves:
With square waves, the RMS and Average values are identical! An AC meter calibrated to
accurately read RMS voltage or current on a pure sine wave will not give the proper value while
indicating the magnitude of anything other than a perfect sine wave. This includes triangle waves,
square waves, or any kind of distorted sine wave. With harmonics becoming an ever-present
phenomenon in large AC power systems, this matter of accurate RMS measurement is no small
matter.
The astute reader will note that I have omitted the CRT “movement” from the RMS/Average
discussion. This is because a CRT with its practically weightless electron beam “movement”
displays the Peak (or Peak-to-Peak if you wish) of an AC waveform rather than Average or RMS.
Still, a similar problem arises: how do you determine the RMS value of a waveform from it?
Conversion factors between Peak and RMS only hold so long as the waveform falls neatly into a
known category of shape (sine, triangle, and square are the only examples with Peak/RMS/Average
conversion factors given here!).
One answer is to design the meter movement around the very definition of RMS: the effective
heating value of an AC voltage/current as it powers a resistive load. Suppose that the AC source to
be measured is connected across a resistor of known value, and the heat output of that resistor is
measured with a device like a thermocouple. This would provide a far more direct measurement
means of RMS than any conversion factor could, for it will work with ANY waveform shape
whatsoever: (Figure below)
Direct reading thermal RMS voltmeter accommodates any wave shape.
While the device shown above is somewhat crude and would suffer from unique engineering
problems of its own, the concept illustrated is very sound. The resistor converts the AC voltage or
current quantity into a thermal (heat) quantity, effectively squaring the values in real-time. The
system's mass works to average these values by the principle of thermal inertia, and then the meter
scale itself is calibrated to give an indication based on the square-root of the thermal measurement:
perfect Root-Mean-Square indication all in one device! In fact, one major instrument manufacturer
has implemented this technique into its high-end line of handheld electronic multimeters for
“true-RMS” capability.
Calibrating AC voltmeters and ammeters for different full-scale ranges of operation is much the
same as with DC instruments: series “multiplier” resistors are used to give voltmeter movements
higher range, and parallel “shunt” resistors are used to allow ammeter movements to measure
currents beyond their natural range. However, we are not limited to these techniques as we were
with DC: because we can to use transformers with AC, meter ranges can be electromagnetically
rather than resistively “stepped up” or “stepped down,” sometimes far beyond what resistors would
have practically allowed for. Potential Transformers (PT's) and Current Transformers (CT's) are
precision instrument devices manufactured to produce very precise ratios of transformation between
primary and secondary windings. They can allow small, simple AC meter movements to indicate
extremely high voltages and currents in power systems with accuracy and complete electrical
isolation (something multiplier and shunt resistors could never do): (Figure below)
(CT) Current transformer scales current down. (PT) Potential transformer scales voltage down.
Shown here is a voltage and current meter panel from a three-phase AC system. The three “donut”
current transformers (CT's) can be seen in the rear of the panel. Three AC ammeters (rated 5 amps
full-scale deflection each) on the front of the panel indicate current through each conductor going
through a CT. As this panel has been removed from service, there are no current-carrying
conductors threaded through the center of the CT “donuts” anymore: (Figure below)
Toroidal current transformers scale high current levels down for application to 5 A full-scale AC
ammeters.
Because of the expense (and often large size) of instrument transformers, they are not used to scale
AC meters for any applications other than high voltage and high current. For scaling a milliamp or
microamp movement to a range of 120 volts or 5 amps, normal precision resistors (multipliers and
shunts) are used, just as with DC.
REVIEW:
 Polarized (DC) meter movements must use devices called diodes to be able to indicate
AC quantities.
 Electromechanical meter movements, whether electromagnetic or electrostatic, naturally
provide the average value of a measured AC quantity. These instruments may be ranged
to indicate RMS value, but only if the shape of the AC waveform is precisely known
beforehand!
 So-called true RMS meters use different technology to provide indications representing
the actual RMS (rather than skewed average or peak) of an AC waveform.
Ammeters
http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Ammeters/Ammeters.html
The analogue ammeter is a basic meter movement with
a shunt placed in parallel across it. The movement goes
full scale with only a milliampere or so of current
through it, and the shunt passes the extra current around
the meter movement. The fraction of the overall current
which passes through the movement is a function of the
resistance of the movement and the much lower
resistance of the shunt. The basic mechanism is that
developed by Edward Weston in the last few years of the
19th century.
That being said, almost all 20th century ammeters look
alike. Here are three meters which have unique cases. At
the right is a Current Indicator made by Whitney
Electrical Instrument Company of New Hampshire; the
earliest patent date is May 16, 1893. This instrument is in
the Kenyon College collection.
The two ammeters below are in private collections and
date from the early years of the 20th century..
Richard Zitto
Thomas Greenslade
And then there is the Volt-Ammeter. This probably has an internal shunt for use as an ammeter
and an internal multiplier for use as a voltmeter. The two instruments below have an unusual
upright configuration on a horizontal base. The one at the left, by the Ziegler Electric Co. of Boston
is in the museum room at the Physics Department of Washington and Lee University; the example
by Knott on the right appeared in a eBay auction.
The small meter at the left is in author's
collection. He bought it at a yard sale near
Boston ca. 1985 for only a dollar or two.
The 1916 catalogue of the L. E. Knott
Apparatus Co., Boston, describes this as a
"Horizontal Galvanometer, D'Arsonval
movement, jeweled bearing, 0 center. This is a
commercial type of instrument in horizontal or
laboratory form. The range is such as to make
it of the greatest value in general laboratory
practice. Quick action, quick reading, adapted
to a wide range of experiments, such as
Induction, Polarization of Cells, Measurements
of the Wheatstone Bridge, where an accuracy
equal to ½ millimeter on the bridge is
considered sufficient. Owing to the form of the
pole pieces the scale is proportional to the
amount of current going through, thus giving
the instrument a range of usefulness far grater
than its sensibility would
indicate .....................................$7.50"
This little galvanometer in the
Greenslade Collection is only 10 cm
high. It was made by the
"Thompson-Levering Co., Makers of
Scientific Instruments, Philadelphia,
Pa." and is marked on the top, in ink, "2
µA/div", with 10 divisions on either
side of zero.
A very similar instrument was sold by
Leeds and Northrup of Philadelphia. In
their 1907 catalogue this is listed as a
portable d'Arsonval galvanometer and
priced at $20.00. The sensitivity is the
same as the Thompson-Levering
instrument.
The range of a basic ammeter movement may be extended to lower values by adding low-resistance
shunts across it. These shunts by Weston Electric probably date from the nineteen twenties. The current
leads are the heavy connections at the top, and the ammeter leads are the black ones at the bottom. These
shunts were probably used with the ammeter below.
The large Weston laboratory standard
ammeter at the left was probably used
with the shunts shown above. This
meter includes a thermometer for
temperature corrections, a mirrored
scale and a bubble level.
It is retired from the laboratories at the
University of Texas in Austin.
This second example of
a "Weston Direct
Reading Laboratory
Standard Milli-Volt
Meter" is at Westminster
College in western
Pennsylvania.
It has an 1890 patent
date, is 40 cm square,
and is serial number
621,
This early ammeter, dating from the very beginning of
the twentieth century, is included because its wood has
acquired a very nice patina over the years. The case is
marked "Keystone Electrical Instrument Co.,
Philadelphia, Pa, DC Milliammeter" and the writing on
the scale says "Made for the Central Scientific Co.,
Chicago, Ill."
It is in the collection of historical instruments at Kenyon
College in Gambier, Ohio.
The meter at the left has a basic galvanometer
movement. On the front panel are shunts to enable the meter
to measure currents up to 25 A and multipliers to allow
potentials up to 125 V to be measured.
The 1916 catalogue of the C.H. Stoelting Co. of Chicago notes
that "this instrument is designed for the lecture table, but is
equally well adapted for students' use, having all its mechanism,
internal and external, exposed to view. The movement
employed in it is of the well known Weston Standard patented
movable coil type."
A damping resistor must be permanently connected, as the
pointer will come to rest without oscillations.
This instrument is in the Greenslade Collection.
This massive ammeter, with a thick, cast-brass
front, has a full-scale reading of 44 Amperes. The
lower portion of the scale is non-linear, suggesting
that it was used to measure alternating current. On
the front is cast "Fort Wayne [Indiana] Electric
Works", "'Wood' Am-Meter" and "Pat'd Oct 8,
1889; May 22, 1894".
It is in the Greenslade Collection.
The handsome ammeter at the left was made
by the American Instrument Company of
Newark, New Jersey. It bears patent dates from
1906 and 1907, and is in the collection of
Westminster College in western Pennsylvania.
Its twin is on the voltmeter page.
The device at the left, made by the Brush Electric
Company of Cleveland, Ohio, is a completely different
approach to the problem of measuring electric current.
Here, the current passes through a pair of coils. The
magnetic field, and hence the magnetic force of
attraction for the pair of soft-iron rods, is proportional to
the magnitude of the current.
The apparatus is at Case Western Reserve University in
Cleveland.
At the left is another unusual ammeter from Case
Western Reserve University. This was made by the
Edison General Electric Company of Schenectady,
New York.
The curved iron wire is drawn up into the curved
solenoid when current passes through the coil. The
needle is attached to the wire at the same point as
the wire's pivot. The scale is reasonably linear.
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