CHARARCTERISTICS OF ELECTROMECHANICAL TRANSDUCERS1. Aim of the
experiment Here the purpuse of the experiment is to measure the
position using Reflective Opto Transducers , Gray Coded Disc , and
Servo Potentiometer and also we will use Inductive Transducers ,
and Tacho-Generator for speed measurement.2.1.1. Experiment
This is the known values upward and here the experimental
resultsPosition CBA
0000
1001
2011
3010
4110
5111
6101
7100
As you see here the experimental values are all same with the
known value.We found the experimental values with rotating the
shaft of the motor .We can easily understand that light emitted led
means 1 and the dark one 0.So we filled the table according to this
information.Table 2.2OutputOutput Voltage
LED OFFLED ON
A0,64,85
B1,34,88
C0,654,85
Here in table 2.2 as you see ,we found output voltages of
leds.We turned the shaft until we saw the maximum and minimum
voltage values.
Question 1.1The electrical circuit arrangement for the
reflective opto transducer is like that ;
The digiac 1750 (D1750) unit is a comprehensive transducer and
instrumentation trainer with examples of a full range of input and
output transducers, signal conditioning circuits and display
devices. The unit is self contained and enables the characteristics
of individual devices to be studied and also their interconnection
to form complete closed loop systems. The only additional items
recommended are an electronic voltmeter and an oscilloscope.
Question 1.2The reflective opto sensor is a device employing an
infrared L.E.D. and photo transistor.The components are arranged so
that the beam is returned if a reflective surface is placed at the
correct distance.The reflective surface is a gray coded adhesive
overlay on a aluminum disk which revolves with the drive shaft.
Gray code is a sequence of binary numbers where only one bit
changes at a time. Marching through the integer sequence then only
requires flipping one bit at a time which in certain applications
drastically reduces any errors. With Gray code only one bit is
changing from OFF to ON and therefore there is no error. In
automation it's particularly good for position transducers used to
measure the angle of a shaft. This application benefits from the
cyclic nature of Gray codes, because the first and last values of
the sequence are different by only one bit. That is, if you feel
like going round and round in circles.In an optical absolute
encoder there is a disk with a Gray code pattern like so... Black
areas produce a low voltage output from the associated reflective
opto sensor while white areas reflect infa-red produce a high
voltage.A beam of light, LED or laser, is shot through the disk and
the light is picked up on the other side by a gang of
phototransistors. This light is converted into electrical signals
which can be read as Gray code or converted to BCD. As the shaft
(and therefore the disk) rotate the light coming through the holes
changes but only slightly based on the Gray code. Thus the
controller reading the phototransistors knows the exact position of
the shaft.
Question 1.3
If the signals in all parts of a control system are continuous
functions of time, the system is Classified as continuous time
feedback control system. Typically all control signals are of low
frequency and if these signals are unmodulated, the system is known
as a d.c.control system.These systems use potentiometers as error
detectors, d.c amplifiers to amplify the error signal, d.c. servo
motor asactuating device and d.c tachometers or potentiometers as
feedback elements.If the control signal is modulated by an a.c
carrier wave, the resulting system is usually referred to as an a.c
control system.These systems frequently use synchros as error
detectors and modulators of error signal, a.c amplifiers to amplify
the error signal and a.c servo motors as actuators. These motors
also serve as demodulators and produce an unmodulated output
signal.
2.2.1. Experiment: Characteristics of a Servo Potentiometer
Output Voltage(v)Dial Setting(degree)
5.00163
4.67150
3.78120
2.890
1.960
0.9430
0.050
-0.84-30
-1.68-60
-2.56-90
-3.44-120
-4.26-150
-5.00185
Table 4: output voltage servo potentiometer
Here we changed the values of the degrees on the servo
potentiometer clockwise and also counterclockwise to find the
maximum negative and maximum positive value of the voltage.Then
other samples of voltages were recorded according to the control
dial setting.
Question 2.1
-As we know from the experiment input voltages of it changes
between 5 and 5 volt.So we should observe that maximum values
should be 5 and 5.-Error-sensingnegative feedbackto correct the
performance of a mechanism.-It applies only to systems where
thefeedbackor error-correction signals help control mechanical
position, speed or other parameters.
Question 2.2
The charateristic of servo potentiometer around 0 degree is ;
output voltage is too small nearly zero and the value will not be
measured with perfect accuracy.
Question 2.3
The output voltage is approximately 5 volt.And this output is
the max voltage value.After 180 degree ; voltage decreasing with
time.
2.3.1.Experiment: Characteristics of an Inductive Transducer
The maximum output voltage when the slot is immediately above
the inductive sensor = 6.5VOutput voltage with the slot over the
inductor = 6.73 voltMaximum value of output voltage = 7.14 volt
Counting or speed measurement :
Speed of shaft recorded with the inductive sensor = 6 rev/secThe
speed of the shaft recorded without Low Pass Filter = 195
rev/sec
Table 2.6 The shaft speed for comparison
Motor VoltageShaft Speed (rev/sec)2V4V7V10V
Inductive Transducer6142831
Slotted Opto Transducer5142740
2.4.1 experiment : Characteristics of a DC Permanent Magnet
Tacho-Generator
Table 2.8 The output voltage against shaft speed
Output Voltage(Moving Coil Meter) , VShatft Speed(rev/sec) ,
n
25
2.510
5.020
7.030
9.540
Question 4.1The output voltages versus to shaft speed graph
explains that chacteristic of tacho-genearator is nearly linear
because voltage is doubled and speed of shaft is nearly doubled and
voltage is four times to first voltage and then speed of the shaft
is approximately eight times greater than first speed. When the
values are getting bigger , the linearity gets worse.Calibration of
the moving coil meter to indicate speed directly :
Table 9: The motor speed
Shaft Speed (rev/min)600100012001600
Calculated Shaft Speed (rev/sec)10172026
Shaft Speed from Counter (rev/sec)11172228
Question 4.2
Using counter is easier than using calibrated Moving Coil Meter
for setting the motor speed, because you don't need to calibrate
the counter.
CONCLUSONGenerally ; in this experiment we measured position and
speed using various components of DIGIAC 1750 and also using them
we construct the circuit which is needed.We learned the properties
of the most of the components of DIGIAC 1750 as expressed in the
answers of the questions.