The
instruments which directly indicate the value of the electrical quantity at the
same time when it is being measured are called indicating instruments. In these
instruments a pointer moves over a graduated scale and gives the value of the
quantity being measured.
Parts
of indicating instruments:
Methods of Supporting Moving
Systems: There are two methods of
supporting the moving system
(1)
by pivoting and
(2)
by thread
suspension.
In pivoted system the moving
system is mounted on a spindle of hardened steel, having the ends conical and
highly polished to form pivots. The ends of the spindle fit into conical holes
in jewels which form the bearings on the fixed part of the instrument. Sapphire
is commonly used for the manufacture of jewels. This type of support is robust;
of low frictional resistance and is sensibly independent of whether the
instrument is used in a vertical or horizontal position.
Suspension system is usually
used for galvanometers, where leveling of instruments is essential before use.
In this system the moving system is suspended by means of a fine ribbon shaped
metal strip from the top. The suspension system is advantageous when the
operating forces are very small as compared to the weight of the moving system.
This system eliminates the friction at the bearings. Such suspensions are
delicate and are required to be protected against mechanical vibrations and
shocks etc. The suspension system has been recently improved by introducing a
taut-ribbon. The taut ribbon is also used for carrying the current to the coil
in moving coil instruments in addition to providing the controlling
torque. Instruments of this type do not require as much careful leveling
as the former type but still need careful handling
Permanent Magnets: Special alloy steels are used in instrument magnet
construction. Tungsten steel has been popular since the early days of
electrical instruments. Cobalt-chrome steels have also been used because of
their higher coercively and where economy of weight and space is
important. But in recent years, alnico magnet has been used to an
increasing extent in instrument applications. In most of the
applications, the field strength may be expected to be between0.05 and 0.25 T
in the air gap of 1.5 – 2.5 mm length; depending on the size and type of
instrument. For efficient design, the x-section of the magnet may be
large and its length small for materials of high coercively and low permanence
whereas the length of the magnet should be large and x-section small for
materials of low coercivity and high permanence.
Scales: The scale is usually printed on the enameled surface of
the metal plate, or on paper or card board firmly cemented to a metal backing
plate. In switchboard and panel instruments, scale markings are usually
broad and for easy reading, the scale is usually subdivided into 2.5 and 10
parts between major scale divisions. In portable instruments, division
lines are laid out individually for each instrument in contrast to scales of
switch-board of panel instruments which are usually pre-printed. This
procedure is necessary because of higher accuracy expected in the indications
of the portable instruments.
In most of the instruments,
the moving system rotates through an angle of 90° for full-scale defection
though some instruments have been designed to give full-scale deflection
through an angle of 120° or even greater. The greater the angle for
full-scale deflection more clear is the scale of the instruments. The
length of scale in most of the instruments is about 15 cm.
Covers or Cases: Covers or cases of the instruments must be dust and
moisture proof in construction. These are usually made of hard wood, brass,
Bakelite, cast iron or pressed steel. The steel cases are preferred in the case
of instruments affected by external magnetic fields due to generators, current
carrying conductors and earth field. The steel case bypasses practically the
entire flux and thus prevents it from affecting the sensitive portions of the
instruments. The smaller the opening in the shell, the more effective the
screening. When the steel covers are used, the moving system is mounted in
a position as far away from the case as possible in order to avoid errors due
to hysteresis and eddy currents effects in the cases.
Commonly used terms in electrical measurements and instrumentation are
defined below:
Measured
Variable: A variable is that quantity of
characteristic which is the object of measurement in an instrumentation
/control system. Variable may be measurement variable, instrumentation
variable or process variable. The physical quantity, property or condition
which is to be measured is referred as the measured and common measured
variables are displacement, force, speed, pressure, temperature, rate of flow,
thickness etc.
Input
Signal: It is a signal applied to a
device, element or system such as pressure applied to the input connection of a
pressure transmitter.
Output
Signal: It is a signal delivered by a
device, element or system.
Range: The region between the limits within which a quantity
is measured, received or transmitted, expressed by stating the lower and upper
limits, is called the range such as 0-50 A, 50-500 V, -20°C-80°C.
Span: The algebraic difference between the upper and lower
range values is called the span. For example for range 0 to 50 A spans is
50 A, for range of -20°C to 80° the span is 100°C.
Calibration: It is to ascertain output of a device corresponding to
a series of the quantity the device is to measure, receive or transmit.
Data so obtained are used for (a) determination of location at which scale
graduations are to be placed (b) adjustment of output to bring it to the
desired value within a specified tolerance and (c) ascertaining the error by
comparing the device output reading against a standard.
Accuracy: It is a degree of conformity of an indicated value to a
recognized accepted standard value or ideal value. Conformity may be
conceived as the maximum difference, over the range of the instrument, between
indicated value and the true value under measurement.
Error: The algebraic difference between the indicated value
and the true value of the measured signal is called the error i.e. Error =
Indicated value – true value. +ve error denotes that the indicated value is
higher than true value.
It is
helpful to use the word error in this concept only and to describe it as an
instrumental error meaning the difference between the average of a series of up
and down readings, as indicated by the instrument output, and the corresponding
true values of input.
Zero
Error: It is an error of a device
operation under the specified conditions of use when the input is at the lower
range-value. The term zero-shift is often used to represent a change or drift
in zero error with time.
Span
Error: The difference between the actual
span and the ideal span is called the span error and it is usually expressed as
a percentage of ideal span.
Correction: The algebraic difference between the true value and the
indicated value of the measured signal is called the correction i.e. correction
= true value – indicated value. Correction is a quantity which is added
algebraically to the indicated value so as to have true value.
Hysteresis: It is that property of an element evidenced by the
dependence of the value of the output, for a given excursion of the input,
on the history of prior excursions and the direction of the current traversed.
It is
usually determined by subtracting the value of the dead band from the maximum
measured separation between up-scale going and down-scale going indications of
the measured variable (during a full range traverse, unless otherwise
specified) after transients have decayed. This measurement is sometimes called
hysteresis error.
Dead
Band: It is the range through which an
input can be varied without initiating observable response and is usually
expressed in percentage of span.
Repeatability: The closeness of agreement among a number of
consecutive measurements of the output for the same value of the input under
the same operating conditions, approaching from the same direction for full
range traverses is called the repeatability.
Deviation: It is a departure from a desired or expected value or
pattern and may also be described as the difference between measured value and
true value for a particular input value. The deviation is given a plus or minus
sign, depending on whether the measured values are above or below the true
value.
Linearity: It is the closeness to which a curve approximates a
straight line. It is usually measured as a non-linearity and expressed as
linearity e.g. a maximum deviation between an average curve and a straight
line. The average curve is determined after making two or more full range
traverses in each direction. The value of linearity is referred to the
output unless otherwise specified.
Damping: The progressive reduction or suppression of
oscillations in a device or system is called the damping.
Noise: It is an unwanted component of a signal or variable
which obscures the information content.
Signal
to Noise Ratio: Ratio of signal
amplitude may be peak or rms. For non-sinusoidal signals, peak values
should be used.
Dynamic
Response: The behavior of the output
device as a function of the input, both with respect to time is called the
dynamic response.
Accuracy
is a closeness with which the instrument reading approaches the true value of
the variable under measurement while precision is a measure of the
reproducibility of the measurements i.e. precision is a measure of the degree
to which successive measurements differ from one another. IN brief,
accuracy can be defined as conforming to truth and precision can be defined as
sharply or closely defined.
The
proper operation of the indicating instruments depends on the three torques
listed below:
Deflecting
(Operating) torque.
Controlling
(Restoring) torque.
Damping
torque.
Deflecting
Torque:
It
utilises the effects of electric current or voltage and causes the movement of
the moving system and thereby the pointer from the zero/initial position. For
producing deflecting torque, Td it is necessary to connect the
instrument in the circuit to measure the given electrical quantity. This is
achieved by magnetic effect in moving iron instruments and by electro-dynamic
effect in moving coil instruments. The deflecting torque causes the moving
system and hence the pointer from zero position over a graduated scale to
indicate the value of the electrical quantity being measured.
Controlling
Torque:
With
deflecting torque the pointer would swing indefinitely and go over the maximum
deflected position irrespective of the magnitude of the electrical quantity
being measured. To ensure that the pointer does not go beyond the range of the
scale, a controlling or opposing torque, Tc is required which
opposes the deflecting torque. The pointer comes to a rest or standstill when
the deflecting and controlling torques are equal to each other.
Tc
increases with the deflection of the moving system so that the pointer on the
scale will be according to magnitude of the quantity being measured.
It
restores the pointer to zero position when Td is removed.
Methods:-
Spring
control.
Gravity
control.
