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					Electrical indicating and
Test instruments


Digital meters
All types of digital meter are basically modified forms of the digital voltmeter
(DVM),irrespective of the quantity that they are designed to measure. Digital
meters designed to measure quantities other than voltage are in fact digital
voltmeters that contain appropriate electrical circuits to convert current or
resistance measurement signals into voltage signals. Digital multimeters are also
essentially digital voltmeters that contain several conversion circuits, thus
allowing the measurement of voltage, current and resistance within one
instrument.Digital meters have been developed to satisfy a need for higher
measurement accuracies and a faster speed of response to voltage changes than
can be achieved with analogue instruments. They are technically superior to
analogue meters in almost every respect. However, they have a greater cost due
to the higher manufacturing costs compared with analogue meters. The binary
nature of the output reading from a digital instrument can be readily applied to a
display that is in the form of discrete numerals. Where human operators are
required to measure and record signal voltage levels, this form of output makes
an important contribution to measurement reliability and accuracy, since the
problem of analogue meter parallax error is eliminated and the possibility of
gross error through misreading the meter output is greatly reduced.The
availability in many instruments of a direct output in digital form is also very
useful in the rapidly expanding range of computer control applications. Quoted
inaccuracy figures are between š0.005% (measuring d.c. voltages) and š2%.
Additional advantages of digital meters are their very high input impedance
(10M_ compared with 1–20 k_ for analogue meters), the ability to measure
signals of frequency up to 1MHz and the common inclusion of features such as
automatic ranging, which prevents overload and reverse polarity connection etc.


The major part of a digital voltmeter is the circuitry that converts the analogue
voltage being measured into a digital quantity. As the instrument only measures
d.c.quantities in its basic mode, another necessary component within it is one that
performs a.c.–d.c. conversion and thereby gives it the capacity to measure a.c.
signals. After conversion, the voltage value is displayed by means of indicating
tubes or a set of solidstate light-emitting diodes. Four-, five- or even six-figure
output displays are commonly used, and although the instrument itself may not
be inherently more accurate than some analogue types, this form of display
enables measurements to be recorded with much greater accuracy than that
obtainable by reading an analogue meter scale.Digital voltmeters differ mainly in
the technique used to effect the analogue-to-digital conversion between the
measured analogue voltage and the output digital reading. As a general rule, the
more expensive and complicated conversion methods achieve a faster conversion
speed. Some common types of DVM are discussed below.
Voltage-to-time conversion digital voltmeter
This is the simplest form of DVM and is a ramp type of instrument. When an
unknown voltage signal is applied to the input terminals of the instrument, a
negative-slope ramp waveform is generated internally and compared with the
input signal. When the two are equal, a pulse is generated that opens a gate, and
at a later point in time a second pulse closes the gate when the negative ramp
voltage reaches zero. The length of time between the gate opening and closing is
monitored by an electronic counter, which produces a digital display according to
the level of the input voltage signal. Its main drawbacks are non-linearities in the
shape of the ramp waveform used and lack of noise rejection, and these problems
lead to a typical inaccuracy of š0.05%. It is relatively cheap, however.
Potentiometric digital voltmeter
This uses a servo principle, in which the error between the unknown input
voltage level and a reference voltage is applied to a servo-driven potentiometer
that adjusts the reference voltage until it balances the unknown voltage. The
output reading is produced by a mechanical drum-type digital display driven by
the potentiometer. This is also a relatively cheap form of DVM that gives
excellent performance for its price.
Dual-slope integration digital voltmeter
This is another relatively simple form of DVM that has better noise-rejection
capabilities than many other types and gives correspondingly better measurement
accuracy (inaccuracy as low as š0.005%). Unfortunately, it is quite expensive.
The unknown voltage is applied to an integrator for a fixed time T1, following
which a reference voltage of opposite sign is applied to the integrator, which
discharges down to a zero output in an interval T2 measured by a counter. The
output–time relationship for the integrator is shown in Figure 6.1, from which the
unknown voltage Vi can be calculated
Voltage-to-frequency conversion digital voltmeter
In this instrument, the unknown voltage signal is fed via a range switch and an
amplifier into a converter circuit whose output is in the form of a train of voltage
pulses at a frequency proportional to the magnitude of the input signal. The main
advantage of this type of DVM is its ability to reject a.c. noise.


Digital multimeter
This is an extension of the DVM. It can measure both a.c. and d.c. voltages over
a number of ranges through inclusion within it of a set of switchable amplifiers
and attenuators. It is widely used in circuit test applications as an alternative to
the analogue multimeter, and includes protection circuits that prevent damage if
high voltages are applied to the wrong range.


Analogue meters
Analogue meters are relatively simple and inexpensive and are often used instead
of digital instruments, especially when cost is of particular concern. Whilst
digital instruments have the advantage of greater accuracy and much higher input
impedance,analogue instruments suffer less from noise and isolation problems. In
addition, because analogue instruments are usually passive instruments that do
not need a power supply,this is often very useful in measurement applications
where a suitable mains power supply is not readily available. Many examples of
analogue meter also remain in use for historical reasons.Analogue meters are
electromechanical devices that drive a pointer against a scale.They are prone to
measurement errors from a number of sources that include inaccurate.
Measurement and Instrumentation Principles
scale marking during manufacture, bearing friction, bent pointers and ambient
temperature variations. Further human errors are introduced through parallax
error (not reading the scale from directly above) and mistakes in interpolating
between scale markings.Quoted inaccuracy figures are between š0.1% and š3%.
Various types of analogue meter are used as discussed below.
Moving-coil meters
A moving-coil meter is a very commonly used form of analogue voltmeter
because of its sensitivity, accuracy and linear scale, although it only responds to
d.c. signals.As shown schematically in Figure 6.2, it consists of a rectangular coil
wound round a soft iron core that is suspended in the field of a permanent
magnet. The signal being measured is applied to the coil and this produces a
radial magnetic field. Interaction between this induced field and the field
produced by the permanent magnet causes a torque, which results in rotation of
the coil. The amount of rotation of the coil is measured by attaching a pointer to
it that moves past a graduated scale. The theoretical torque produced is given by:




where B is the flux density of the radial field, I is the current flowing in the coil,
h is the height of the coil, w is the width of the coil and N is the number of turns
in the coil. If the iron core is cylindrical and the air gap between the coil and pole
faces of the permanent magnet is uniform, then the flux density B is constant, and
equation can be rewritten as:


i.e. the torque is proportional to the coil current and the instrument scale is linear.
As the basic instrument operates at low current levels of one milliamp or so, it is
only suitable for measuring voltages up to around 2 volts. If there is a
requirement to measure higher voltages, the measuring range of the instrument
can be increased by placing a resistance in series with the coil, such that only a
known proportion of




the applied voltage is measured by the meter. In this situation the added
resistance is known as a shunting resistor.
Whilst shows the traditional moving-coil instrument with a long U-shaped
permanent magnet, many newer instruments employ much shorter magnets made
from recently developed magnetic materials such as Alnico and Alcomax. These
materials produce a substantially greater flux density, which, besides allowing the
magnet to be smaller, has additional advantages in allowing reductions to be
made in the size of the coil and in increasing the usable range of deflection of the
coil to about 120. Some versions of the instrument also have either a specially
shaped core or specially shaped magnet pole faces to cater for special situations
where a non-linear scale such as a logarithmic one is required.




Moving-iron meter
As well as measuring d.c. signals, the moving-iron meter can also measure a.c.
signals at frequencies up to 125 Hz. It is the cheapest form of meter available
and, consequently,this type of meter is also commonly used for measuring
voltage signals. The signal to be measured is applied to a stationary coil, and the
associated field produced is often amplified by the presence of an iron structure
associated with the fixed coil. The moving element in the instrument consists of
an iron vane that is suspended within the field of the fixed coil. When the fixed
coil is excited, the iron vane turns in a direction that increases the flux through it.
The majority of moving-iron instruments are either of the attraction type or of the
repulsion type. A few instruments belong to a third combination type. The
attraction type, where the iron vane is drawn into the field of the coil as the
current is increased,is shown schematically in Figure 6.3(a). The alternative
repulsion type is sketched in. For an excitation current I, the torque produced that
causes the vane to




where M is the mutual inductance and _ is the angular deflection. Rotation is
opposed by a spring that produces a backwards torque given by:
At equilibrium, T D Ts, and _ is therefore given by:




The instrument thus has a square-law response where the deflection is
proportional to the square of the signal being measured, i.e. the output reading is
a root-mean-squared (r.m.s.) quantity.The instrument can typically measure
voltages in the range of 0 to 30 volts. However,it can be modified to measure
higher voltages by placing a resistance in series with it, as in the case of moving
coil meters. A series resistance is particularly beneficial in a.c. signal
measurements because it compensates for the effect of coil inductance by
reducing the total resistance/inductance ratio, and hence measurement accuracy is
improved. A switchable series resistance is often provided within the casing of
the instrument to facilitate range extension. However, when the voltage measured
exceeds about 300 volts, it becomes impractical to use a series resistance within
the case of the instrument because of heat-dissipation problems, and an external
resistance is used instead.


Electrodynamic meters


Electrodynamic meters (or dynamometers) can measure both d.c. signals and a.c.
signals up to a frequency of 2 kHz. As illustrated in Figure 6.4, the instrument
has a moving circular coil that is mounted in the magnetic field produced by two
separately wound, series-connected, circular stator coils. The torque is dependent
upon the mutual inductance between the coils and is given by:




where I1 and I2 are the currents flowing in the fixed and moving coils, M is the
mutual inductance and _ represents the angular displacement between the coils.
When used as an ammeter, the measured current is applied to both coils. The
torque is thus proportional to current2. If the measured current is a.c., the meter
is unable to follow the alternating torque values and it displays instead the mean
value of current2.By suitable drawing of the scale, the position of the pointer
shows the squared root of this value, i.e. the r.m.s. current.Electrodynamic meters
are typically expensive but have the advantage of being more accurate than
moving-coil and moving-iron instruments. Voltage, current and power can




all be measured if the fixed and moving coils are connected appropriately. When
used for voltage measurement, the instrument can typically measure voltages in
the range of 0 to 30 volts. However, it can be modified to measure higher
voltages by placing a resistance in series with it, as in the case of moving-coil and
moving-iron meters. Also, as in the moving-iron meter, a series resistance is
particularly beneficial in a.c. signal measurements because it compensates for the
effect of coil inductance by reducing the total resistance/inductance ratio, and
hence measurement accuracy is improved. This series resistance can either be
inside or outside the instrument case, as discussed above for the case of moving-
iron meters.


Electronic analogue voltmeters
Electronic voltmeters differ from all other forms of analogue voltmeters in being
active rather than passive instruments. They have important advantages compared
with other analogue instruments. Firstly, they have a high input impedance that
avoids the circuitloading problems associated with many applications of
electromechanical instruments.Secondly, they have an amplification capability
that enables them to measure small signal levels accurately.The standard
electronic voltmeter for d.c. measurements consists of a simple directcoupled
amplifier and a moving-coil meter, as shown in Figure 6.9(a). For measurement
of very low-level voltages of a few microvolts, a more sophisticated circuit,
known as a chopper amplifier, is used, as shown in Figure 6.9(b). In this, the d.c.
input is chopped at a low frequency of around 250 Hz, passed through a blocking
capacitor, amplified,passed through another blocking capacitor to remove drift,
demodulated, filtered and applied to a moving-coil meter. Three versions of
electronic voltmeter exist for measuring a.c. signals. The averagerespondingtype
is essentially a direct-coupled d.c. electronic voltmeter with an additional
rectifying stage at the input. The output is a measure of the average value of the
measured voltage waveform. The second form, known as a peak-responding type,
has a half-wave rectifier at the input followed by a capacitor. The final part of
the circuit consists of an amplifier and moving-coil meter. The capacitor is
charged to the peak value of the input signal, and therefore the amplified signal
applied to the moving-coil meter gives a reading of the peak voltage in the input
waveform.Finally, a third type is available, known as an r.m.s.-responding type,
which gives an output reading in terms of the r.m.s. value of the input waveform.
This type is essentially a thermocouple meter in which an amplification stage has
been inserted at the input.

				
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