abstract
with the growing
advancement in technologies industries are trying to come up with ease and
simplicity in the working of machines, the processes which can be handled
easily by the managers even from far off distances,
here in this project
we are trying to reduce the problem of flow measurement in kiln. kiln is a
furnace in which coal is used as a fuel to generate desired amount of heat as
per process requirement. accurate feeding of coal is important for good quality
of end product. coal is taken to kiln in a closed channel because coal is
highly flammable and also it is in powdered form so can not be taken through
open channel. it is difficult to measure the rate of flow of material passing
through such a closed channel. our system named “coal dosing system “makes this
situation of measuring flow rate easier.
diagram:
system basically consists of a hopper which is filled with
powdered coal. feed pipe is feeding the hopper at specific intervals. three
load cells are attached to the body of hopper at 120 degree apart from each
other. this spacing of 120 degrees is done so that even if there is uneven
distribution of material in hopper the averaging of readings of these load
cells nullifies it. reading from load cell are recorded at particular
instances. then difference between two consecutive readings is taken and rate
is calculated. rotary air lock is used to control the flow rate.
system is static in
nature and thus requires low maintenance. static system is preferred for
measuring rate and taking corresponding control action. use of microcontroller and high resolution
adc makes it possible to measure and record the flow rate accurately.
literature survey
introduction :
coal dosing system is used to feed
pulverized coal to kiln at accurate rate. kiln is a furnace in which coal is
used as a fuel to generate desired amount of heat as per process requirement.
accurate feeding of coal is important for good quality of end product. as coal
is in powdered form it cannot be taken to kiln in any sort of open channel.
therefore, it is carried in closed channels like pipes, bins, etc. because of
this the measurement of flow rate becomes difficult. to overcome this problem
and to make accurate measurement and control of the flow rate, this system
called as “coal dosing system “is designed.
literature study :
·
books
on general control systems.
·
brochures
on load cell by manufacturers.
·
internet
sites related to process control.
·
datasheets
of ics.
project overview
features:
·
static
system of measurement
·
low
maintenance
·
more
accurate
·
economic
system
·
digital
display
·
communication
with pc
·
programmable
·
complete
flow profile storage on hourly, daily, monthly, yearly basis
contents:
dosing
system comprises of following components: -
·
inlet device
·
a weighing bin supported on load cell
·
discharge device in the form of rotary air lock
·
electronic evaluating system to measure the flow
rate
·
electrical control system to regulate various
devices.
block diagram:
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load cells are transducers that
convert load acting on them into analog electrical signal. this analog signal
is differential in nature. hence to amplify this, an instrumentation amplifier
is used. instrumentation amplifier amplifies the differential input signal and
brings it to the required level. as this signal is to be applied to the
microcontroller, it is first converted to into digital form with the help of
analog to digital converter. after required manipulation, the output of
microcontroller is given to the display (for visual indication), to leds (to
indicate critical conditions) and to relays.
description of blocks
·
load cell :
a load cell is classified as a force transducer. this device
converts force or weight into an electrical signal. basically it is a passive
bridge type of transducer and requires stable excitation for generating output.
the output is is a differential voltage signal having amplitude in the range of
a few milivolts.
·
instrumentation amplifier :
the output available
from load cell is a weak differential signal having amplitude of a few
milivolts. this weak signal is riding on high level of common mode dc signal
(1/2 of bridge excitation voltage). in order to amplify this differential
signal in the presence of high common mode voltage, an instrumentation
amplifier is required. this is because an instrumentation amplifier is a true
differential gain amplifier and has a high cmrr. other properties such as low
offset voltage, low drift, etc are also useful.
·
analog to digital converter :
signal output from instrumentation amplifier
is amplified version of output of load cell. this signal is to be fed to
microcontroller for further processing. but since this signal is in analog
form, it is required to be converted into digital form to be compatible with
microcontroller. this conversion of analog signal to digital signal is done
using adc. the adc used is a serial adc
with 12bit resolution with high conversion rate.
·
microcontroller :
microcontroller is a heart of the system. it
forms an on chip computer with expandable external memory. serial 12 bit
digital data coming from adc is used by microcontroller for calculating flow
rate and taking control action. program
is written in the memory of microcontroller according to which it takes the
action one by one. pc interfacing can also be done so that the whole process
can be controlled from one control room irrespective of the location where
process is going on. back-up memory is provided through serial eeprom so that
at the time of power failure the relevant data can be stored.
- keyboard:
keyboard interfacing is done with
microcontroller so that any parameter can be changed by user directly using
keys. this allows user interface with the microcontroller.
·
dac :
for taking control action we can take out
pwm output from microcontroller which is then converted to analog signal using
dac.
·
display :
seven segment common cathode displays are
used for displaying the flow rate. multiplexing is done to reduce the current
requirement. the display indicates the flow rate in run mode and the program
parameter and its value in program mode.
·
rotary air lock :
rotary air lock is used to control the flow
rate by varying the speed of the motor. as speed of motor increases more and
more material is taken out of the hopper and flow rate increases and as speed
decreases flow rate also decreases. the design of rotary air lock is such that
it doesn’t allow the air from the aerated material inside the bin to escape when
the material is being taken out and hence the name.
design
·
design of circuit :
- load cell and excitation voltage:
load cell is the input transducer which
converts the weight into electrical signal. the selection of load cell is an
important part of the whole system. the load cell should have a good
sensitivity, linearity, accuracy, repeatability and stability against temperature
variation and aging. it should also have a very good protection against ingress
of dust and liquids, as it is exposed to dusty and occasionally humid
atmosphere. considering all these aspects we have selected an hbm make load
cell type z6-4 100 kg having sensitivity of 2mv per volt of excitation at rated
capacity ; i.e. it will give 2mv output for 1v of excitation at rated load of
100 kg.
the selection of excitation voltage is very
important. it should be ripple free, stable and accurate. it should also be
able to provide the required amount of current to the load cell. a higher value
of excitation voltage will give more output signal from the load cell but at
the same time it will also lead to higher power dissipation in load cell
causing self heating and hence undesirable temperature effects. a lower
excitation voltage would mean lower self power dissipation but it would also
mean a lower output voltage at a given load. considering all these aspects and
referring to the upper limit of excitation voltage (18v) specified in the
datasheet, we have selected 10vdc as the excitation voltage for load cell.
design:
double regulation technique is used to
provide stable excitation voltage of 10.00 vdc to the load cell. the first
stage of regulation is provided by the 15vdc regulated supply used for
instrumentation amplifier and op-amps. the second stage is provided by lm317
three terminal adjustable regulators which have 15vdc as input and its output
is adjusted to exact 10.00vdc by proper adjustment of r113, r112 and pr4 trim
pot.
formula:
v out = 1.25 * (1+ (r112+pr4)/r113)
selecting values of r112 as 1k5, r113 as
240e and pr4 as 500e trim pot, we get adjustment range of9 to 11v. the trim pot
is adjusted such that the output is exact 10.00v.
for pr4=500e
v out = 1.25 * (1+ (1k5+500e)/240e)
= 11.6667v
for pr4=0 ohm
v out = 1.25 * (1+ (1k5+0)/240e)
= 9.0625v
capacitor c50 is provided at the input of
the regulator lm317 for further reduction of input ripple and c51 is provided
at output to improve dynamic response.
- instrumentation amplifier:
instrumentation amplifier has to take weak
signal from transducer and amplify it thus it must have high cmrr, low offset
voltage, low drift, high accuracy. we have chosen ina118 instrumentation
because of its following features:
- low offset voltage : 50 microv max
- low drift : 0.5 microv/degree c max
- high cmrr : 110 db min
- 8-pin plastic dip, so-8
- excellent accuracy
- small size
- wide bandwidth at high gain ( 70khz at g=100)
- variable gain facility
design of
components:
resistors r74, r75: 100k
these resistors
act as an accidental current limiting resistor
zener diode d26: 12v
diodes d26 to d29: in4148*4
these diodes are
used to clamp accidental over voltage.
if input
differential voltage exceeds beyond 12.6v then the over voltage is clamped.
voltage beyond
12v is required to be clamped because supply to ina118 is 15v.
capacitor c42, c43:
these
are supply by-pass capacitors. the type chosen is disc because they have very
low lead inductance and provide a good by-pass for high frequency noise /
transients.
c42
& c43 = 104 disc type capacitor.
- design of instrumentation amplifier for gain:
we
use range switch to select different values of gain resistor rg according to
the sensitivity of the load cell, to make provision of different load cells of
different sensitivities.
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rg
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gain
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r84 = 320 e
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g = 129.2
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r85 = 620e
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g = 81.64
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r86 = 750e
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g = 67.66
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r87 = 1k
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g = 51
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r88 = 1k2
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g = 42.66
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g = 1 +
(50kohm/rg)
there are 5 resistors used as rg, which is normally selected as per the
sensitivity of the load cell used. the resistors should be high precision type
and should have low ppm tempco to match the specifications of the
instrumentation amplifier. a 0.1%, 15ppm type is chosen.
4.
design of tare network:
the tare network comprises of precision
op-amp op07, 4 way dip tare switch, ten turn tare trim pot and associated
resistive network. this network is optionally used for adjusting the tare
manually in case there is a heavy tare burden on the system due to mechanical
limitations. this allows maintaining a high resolution of net weight
measurement.
op07
op-amp is a low drift, low offset, precision op-amp and it is used in summing
mode and it combines the tare voltage in steps from dip switch and continuous
voltage from tare trim pot. the low drift is essential as it is used directly
in conjunction with instrumentation amplifier.
the tare switch and associated potential
divider network provides tare voltage in four equal steps which is further
attenuated by factor of 10 (gain of op07 = 10kohm/100kohm = 0.1) to bring it to
the lower level. this is done to match the signal level available at the pin 5
of instrumentation amplifier.
a)
voltage at switch position 1:
15v*7.2kohm/10.2kohm =10.58v
b)
voltage at switch position 2:
15v*4.8kohm/10.2kohm =7.05v
c)
voltage at switch position 3:
15v*2.4kohm/10.2kohm = 3.53v
d)
voltage at switch position 4:
15v*kohm/10.2kohm = 0v
the continuous voltage available from ten turn
trim pot can be varied from 0 to 1.25v which is also further attenuated by
factor of 10.
both these voltages are summed at summing
junction of the op07. the low output impedance of op07 is essential for
precision operation of instrumentation amplifier.
zener z2 is low tempco precision reference
used to derive continuous tare voltage.
zener chosen is:
lm 336 (2.5v) with tempco of 30ppm.
- analog to digital converter :
analog signal needs to be converted into
digital signal so that it can be further processed in microcontroller. this
conversion is done using mcp 3201 adc.
resolution required:
12bits corresponds to 4k count
50% of it is used for tare weight
thus available count is of 2k
this we get resolution of
(1/2000)*100%=0.05%
this means we can measure 100kg weight with
resolution of 50gm.
thus we get enough resolution using 12 bit
adc whereas 8bit adc gives poor resolution.
serial adc selection:
parallel 12 bit adc requires 12
microcontroller pins so as to get data from adc, but 12 pins of controller were
not available in our case so serial adc was chosen which required only one pin
of microcontroller to take data. though it is slower than parallel adc it
doesn’t affect much in our process. all these requirements were fulfilled by
mcp 3201 adc which is 12bit serial adc with following features:
·
12-bit
resolution
·
on chip
sample and hold
·
single
supply operation : 2.7-5.5v
·
100ksps
max. sampling rate at vdd=5v
·
50ksps
max. sampling rate at vdd=2.7v
·
low
power cmos technology
·
8-pin
pdip package
·
successive
approximation technique of conversion
6.
microcontroller:
microcontroller is required to calculate the
flow rate and then take the control action accordingly. microcontroller
executes the program written in its memory and takes action as per the
instructions. memory requirement for our program is about 64kbytes. we needed
12 bit adc and microcontroller with inbuilt adc provides only 8 bit conversion,
so choice of inbuilt adc is out of question. for taking control action pwm
output is also one of the requirements of microcontroller.
considering all these requirements 89c51 rd2
was the best choice for our project. it has got following features :
·
on-chip
flash memory of 64kb
·
ram
expandable externally to 64kb
·
four
8-bit i/o ports
·
fully
static operation
·
power
control modes
·
programmable
counter array
pins used:
port 0
p0.0 to p0.7 – interfaced to external ram
for expanding memory if required.
port 1
p1.0 to p1.7 – microcontroller gives out
data to latches u6 and u7 (74hc374) through this port.
port 2
p2.0 (le1) – latch enable signal for u6 is
given through this pin by microcontroller
p2.1 (le2) – latch enable signal for u7 is
given through this pin by microcontroller
p2.2 (en1) – signal to enable either one of
u4 or u5 decoder (74als138) is given through this pin.
p2.3 to p2.5 (dsel1 to dsel 3) – these are
select lines for both the decoders which select one of the output of the
decoder as per the address on it.
p2.6 – unused
p2.7 (key return) – key return line which is
common for all the user keys is acknowledged at this pin by microcontroller.
port 3 :
p3.0 (rxd) – this pin receives data while
communicating with pc.
p3.1 (txd) – this pin transmits data while
communicating with pc.
p3.2 (cs^) – microcontroller gives chip
select signal to adc through this pin
p3.3 (clk) – clock pulses to adc are given
through this pin
p3.4 – unused
p3.5 (d out) – output from adc is received
at this pin
p3.6 (pwmo) – pwm output from
microcontroller is taken at this pin
p3.7 – unused
- decoders:
decoders are used to make economical use of
port pins. two decoders u4 and u5 are used in the circuit. u4 selects one of
the digits of the display as per the requirement. u5 selects one of the user
keys. there are total 4 user keys namely increment, decrement, next and enter.
- display:
six
digit multiplexed display is used in our project and the division of digits is
as follows:
·
four
digits are used to indicate the process value in run/normal mode
·
same
four digits are used to indicate program parameter value in program mode
·
two
digits are used to indicate the name of the parameter in program mode
since
the display is multiplexed all the identical segments are connected together
(eg: all a-a-a, b-b-b… etc) and the seven common lines representing seven
segments are connected to segment driver npn transistors (bc 547). the
resistors connected in the collector circuit of this driver transistors (r28,
r31, etc) control the segment current and hence the intensity. the digit
drivers transistors are pnp (bc 557) drive the individual digits in sequence as
per scanning frequency.
·
design of software:
introduction:
the total hardware is based on the microcontroller 89c51. accordingly,
the hardware is first developed and made available for operations. the
operations of hardware are dependent on the programming of the microcontroller.
run mode:
by default, the system is in run mode. in
this mode, the flow rate value is continuously monitored and displayed.
program
mode:
in program mode, values of various
parameters can be programmed.
functions of various keys in prg mode are:
·
inr
key- by pressing this key, we can increment the value of parameter.
·
dcr
key-by pressing this key, we can decrement the value of the parameter.
·
nxt
key- by pressing this key, we can change the parameter.
·
ent
key-it stores the adjusted value of parameter in some memory.
flow
chart :
flow chart for main
program:-
flow chart for run mode:-
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applications
- used for measurement of flow of coal required for heating purpose in the industry.
- used in cement industry to measure flow rate of material entering the kiln
- it can also be used for safe and accurate measurement in hazardous area.
- used in cement industry to measure the rate of raw material going to the mill.
- this system is used in the godowns for storage of large amount of grains.
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