4. • The self balancing type recorder is also called the potentiometric
recorder.
• T he constructional details are shown in figure.
• The potentiometer is supplied with a reference voltage derived from
an internal power supply.
• It can be seen from the block diagram that the input signal and the
part of the voltage across the potentiometer are in series.
• The difference between these voltages is the error signal
5. • This error signal is available at the input terminals of the amplifier.
• The field coil of the motor is connected to the output of the
amplifier.
• The construction of the motor is such that, it turns in a direction that
rotates the wiper of the potentiometer to reduce the error
• When the error reduces to zero the motor slows down and stops.
• As- the wiper of the potentiometer is driven by the motor's armature
and as a pen is arranged over the wiper, the pen executes motion in
the direction of movement of the wiper.
• As the armature moves in either direction depending on the error
the pen moving in synchronized direction records the waveform.
• The paper feed motor will be synchronized with power line frequency
8. • A strip chart recorder consists of:
• (i) Paper drive systems: It should move the paper at a uniform
speed. A spring wound mechanism may be used but in most of the
recorders synchronous motor is used for driving the paper.
• (ii) Marking mechanism: The most commonly used marking
mechanisms are as following:
• a. Marking with ink filled stylus
• b. Marking with heated stylus
• c. Chopper bar
• d. Electric stylus marking
• e. Electrostatic stylus
• f. Optical marking method
9. • Iii Tracing systems: There are two types of tracing systems used for
producing graphic representations.
• a. Curvilinear system
• b. Rectilinear system
• Working
• A strip chart recorder has:
• (i) A long roll of graph paper moving vertically.
• (ii) A system for driving the paper at some selected speed. A speed
selector switch is generally provided. Chart speed of 1-100 mm/s are
usually used.
• (iii) A stylus for making marks on the moving graph paper. The stylus
moves horizontally in proportional to the quantity being recorded
10. • (iv) A stylus driving system which moves the stylus in a nearly exact
replica or analog of the quantity being recorded.
• (v) A range selector switch is used so that input to the recorder drive
system is within the acceptable level.
• (vi) Most recorder use a pointer attached to the stylus. This pointer
moves over a calibrated scale thus showing the instantaneous value
of the quantity being recorded. An external control circuit for the
stylus may be used.
13. • The block diagram is shown in Figure.
• From the block diagram we find that the X input and Y input are supplied to the
error detector in series with the standard reference voltage offered by the internal
reference source.
• The output of the error detector is given to a chopper.
• The servo amplifier is driven by the chopper.
• The amplifiers output drives the pen.
• The Y amplifier's output drives the arm.
• Square shaped graph paper will be used.
• It is fixed over a pad by electrostatic attraction or by vacuum.
14. Working:
• The input signals are attenuated to around 0.5 mV which is within the
dynamic range of the recorder.
• Both X and Y signals are compared with the internal reference source.
This is done in the balancing or Error detector block.
• The X and Y channel error output will be the DC error, which is the
difference between the input signals and the reference voltage.
• The DC error signal of both channels is used in the choppers to
convert it in to an AC signal.
• The magnitude of the AC output of the choppers is insufficient to
drive the motors of the pen and the arm.
• Therefore the output of the two choppers will be amplified in the
servo amplifiers.
15. • The servomotors drive the pen and the arm.
• The pen and the arm execute motion in proper direction to reduce
the error.
• The movement of the pen and arm is to bring the system to balance.
• The variation of X and Y signals, move the pen and the arm in the
appropriate directions to keep the system in balance.
• This movement produces a record of the signal components on the
paper.
• Both X and Y channels and the total system works simultaneously.
• Input range variable from 0.25 V/cm to 10 V/cm
• Accuracy ± 0.3 % to ± 0.1 % at full scale.
17. BASIC INSTRUMENTATION SYSTEM
It is branch of engineering which deals with various types of instrument to record, monitor, indicate
and control various physical parameters such as pressure, temperature, etc
18. • Primary sensing element
• The primary sensing element is also known as sensor. Basically transducers
are used as a primary sensing element. Here, the physical quantity (such as
temperature, pressure etc.) are sensed and then converted into analogues
signal.
• Variable conversion element
• It converts the output of primary sensing element into suitable form
without changing information. Basically these are secondary transducers.
• Variable manipulation element
• The output of transducer may be electrical signal i.e. voltage, current or
other electrical parameter. Here, manipulation means change in numerical
value of signal. This element is used to convert the signal into suitabl
19. • Data transmission element
• Sometimes it is not possible to give direct read out of the quality at a
particular place (Example – Measurement of temperature in the
furnace). In such a case, the data should transfer from one place to
another place through channel which is known as data transmission
element. Typically transmission path are pneumatic pipe, electrical
cable and radio links. When radio link is used, the electronic
instrumentation system is called as telemetry system.
• Data presentation or controlling element
• Finally the output is recorded or given to the controller to perform
action. It performs different functions like indicating, recording or
controlling.
20. • Open Loop Control System
• In open loop control system, the output does not affect the control
action of the system. In other words, the system whose working
depends on time is known as the open loop control system. The open
loop system is free from the feedback.
•
21. • Let’s understand this with the help of the few examples.
• open-loop-systemExample 1: Consider the clothes dryer whose
control action is done manually by the operator. Depending on the
wetness of the clothes suppose the operator set the timer for 30
minutes. After 30 minutes the timer will stop even after the clothes
are wet.
22. • Closed Loop Control System
• The closed-loop control system means the output of the system
depends on their input.
• The system has one or more feedback loops between its output and
input.
• The closed-loop system design in such a way that they automatically
provide the desired output by comparing it with the actual input.
• The closed-loop system generates the error signal which is the
difference between the input and output.
23.
24. • The air conditioner is the example of the closed-loop system.
• The air conditioner regulates the temperature by comparing it with
the surrounding temperature.
• The comparison of temperature is done with the help of the
thermostat.
• When the AC provides the error signal which is the difference
between the surrounding temperature and room temperature the
thermostats turn on or off the compressor.
25. • The closed loop system is more reliable and accurate. But this system
is very expensive and requires high maintenance
26. • DATA ACQUISITION SYSTEMS
• Data acquisition system (DAS) is a computerized system that collects
data from the real world, converts it into the form of electrical signals
and do required processing on it for storage, and presentation on
computers.
27. • The function of each block is as under:
Transducers: They are converting physical quantities (such as
temperature, pressure, etc.) into electrical quantities, or measuring
electrical quantities directly. They collect data from physical world.
The most commonly used transducers are:
• RTDs, thermocouples, and thermistors for temperature measurements.
• Signal Conditioning Unit: The signal produced by the transducers
may or may not be very suitable for our system to work properly.
• It may be very weak, very strong or may have some noise.
28. • To convert this signal into the most suitable form, amplification, and
filtration is done respectively by signal conditioning unit. So the signal
conditioning unit converts electrical signals in the most suitable form.
• Multiplexer: The multiplexer receives multiple analog inputs and
provides a single output signal according to the requirements.
• If a separate channel is used for each quantity, the cost of installation,
maintenance, and periodic replacement becomes high. Therefore, a
single channel is used which is shared by various quantities.
• Analog to Digital (A/D) Converters: The data is converted into digital
form by A/D converters.
• After the conversion of data into digital form, it is displayed with the
help of oscilloscopes, numerical displays, panel meters to monitor the
complete system.
29. • The data acquisition system can be divided into two types:
• Analog data acquisition system
• Digital data acquisition system
• The analog data acquisition system gives an analog output whereas the
digital data acquisition system gives a digital output.
• Analog DAS is used when wide frequency width is required or when lower
accuracies can be tolerated.
•
• Digital DAS is used when physical quantity being monitored has a narrow
bandwidth (i.e. when the quantity varies slowly). Also, high accuracy and
low per channel cost are required. These are more complex than analog
DAS.
• The digital data have more advantages over analog data.
30. • Role of Telemetry in Instrumentation System
• Telemetry is the automatic measurement and wireless transmission of data from
remote sources.
• In general, telemetry works in the following way: Sensors at the source measure
either electrical data (such as voltage or current) or physical data (such as
temperature or pressure).
• These measurements are converted to specific electrical voltages.
• A multiplexer combines the voltages, along with timing data, into a single data
stream for transmission to a remote receiver.
• Upon reception, the data stream is separated into its original components and
the data is displayed and processed according to user specifications.
31.
32. • Figure shows a block diagram of a distinctive radio telemetry single
channel system.
• From the block diagram, the Biological signal from patient is
transmitted to Transducer and sensor, There it is transmitted to Signal
conditioner and then to the Transmitter.
• This transmitted original signal is transferred by Signal conditioner
and then to the Display and readout.
• The radio frequency utilized in the system differs from a hundred KHz
to some MHz.
• AM is not adopted for biotelemetry system since the presence of
noise can affect the measured variables severely.
• That is why we usually adopt either frequency modulation or pulse
modulation techniques to transmit the bio-
