International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Design and Development of Wireless Flow
Transmitter
Hemalatha. J, Nikhil P.S, Shaniya Shaji C and Komanapalli Venkata Lakshmi Narayana, Member, IEEE
School of Electrical Engineering
VIT University
Vellore, India
Abstract — This paper presents the development of a wireless
flow transmitter using rotameter and operational amplifier based
signal conditioning circuit. The rotameter is a linear gravity
based flow measuring device. It consists of a display unit which
directly displays the flow rate. For industrial applications a new
circuitry is required to transmit the measured flow rate to the
control unit at remote areas. A ferromagnetic wire attached to
the float of rotameter acts as a core of an inductance pickup coil
constitutes the flow sensor. The self-inductance of the coil
changes according to the change in flow rate. An operational
amplifier based circuit is used for the measurement of selfinductance. The voltage corresponding to the flow rate is finally
converted into 4-20 mA current. A wireless readout is provided
using a microcontroller unit and a Bluetooth module. The results
show a linear relation between the variation of self-inductance of
a pickup coil as well as transmitter output with respect to flow
rate. The theoretical equations and the simulation results are
added in this paper.
Keywords— Rotameter, Self-inductance, Flow transmitter,
Maxwell’s bridge, Bluetooth module, Wireless readout
E
I.
INTRODUCTION
ffective transmission of measured parameters to the
control rooms, which is situated at some distance apart
from the field, is inevitable during most of the process control
applications. Even though many type of flow meters such as
mass flow meter, turbine flow meter, electromagnetic flow
meter, orifice meter and venturimeter are available for flow
measurement, rotameter has few advantages compared to the
other measuring devices .The response of rotameter is linear
with respect to the flow rate. It has simple construction and
inexpensive too but it does not have a transmission circuitry
integrated along with it, which limits its applications where
long distance flow transmission and data storage is required.
Hence development of a coherent flow transmitter for
rotameter seemed to be a good prospect and has been chosen
for study in this paper.
Many advanced techniques to measure and transmit flow
rate is made known from various parts of the world. Bera et al.
[1] have implemented a capacitance type level transducer for
conducting liquids and it uses a uniform circular cylinder of
insulating material as the sensor. The change in level of liquid
is indicated by the change in capacitance. It is of low cost and
also of non-contact type. Another approach is suggested by
T.Moazzeni et al. [2] for high temperature, radioactive and
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
corrosive environment. It employs cross correlation method
for measuring the flow rate and a thermocouple is used for
sensing the thermal flow signals. Beaulieu et al. [3] have used
the venturi tube and an advanced version of unsteady
Bernoulli equation for the flow meter of unsteady liquid. Bera
and Mandal [4] have designed an orifice transducer and
validated for conductive liquids. The main advantage of this
system is that differential pressure cell is not used; hence the
problems caused by the pressure cells are avoided. The
conventional flow measurement based on hot wire technique
uses Wheatstone bridge circuit has been substituted by an
intelligent digital flow rate measurement system which uses
pulse width modulation concept by Cao et al. [5]. Another
technique has been put forward by Bera and Chakraborty [6]
for the flow measurement of conductive liquids. Here the
Wheatstone bridge network is constructed using four
electrodes kept at a radial distance from the flow tube. Hence
the unbalance state of the bridge is indicated as the change of
flow. Bera et al. [7] have suggested the use of motor load
current as the sensing signal. It is given to thyristor driven
pump for the measurement of flow through the pipe line. A
precise mass flow measurement method of gas with the
measurement of temperature and pressure has been developed
by Poveay and Beard [8]. A semi cylindrical structured
capacitive sensor with interface circuit has been developed for
the flow measurement by Chiang and Huang [9]. It utilises a
switched mode charge transfer approach for capacitance
measurement.
The rotameter is a variable area type flow meter. It consists
of a tapered metering tube and a float which is free to move up
and down within the tube. The metering tube is mounted
vertically with the small end at the bottom.
The measuring
fluid is entered via bottom of the tube passes upward around
the float and out at the top. The movement of the float directly
indicates the flow rate with the help of the indicator scale
mounted on it. The only limitation of the rotameter is that
distant transmission of measured flow rate is not possible. An
advanced transmission technique is needed for the
transmission of the sensed data to remote areas where flow
rate is analysed and controlled. The few works have been
carried out by different researchers to add the transmission
capability to the rotameter. Li et al. [10] have proposed the
replacement of normal float by a magnetized float which
behaves the same as that of normal float and the use of a
rotatable cam type arrangement made of magnetic material for
the sensing element. The position of the float is sensed using
the magnetic property of the float. The limitation observed in
this approach is frictional losses of the cam arrangement and
magnetic losses of the float at higher temperatures. A noncontact inductive approach is proposed by Belforte et al. [11].
In this approach, the inductor coil is placed around the
rotameter which act as inductive pickup coil. The change in
flow in turn displaces the float, which varies the selfinductance.
In this paper, a new method for flow rate transmission is
proposed using rotameter as sensor and an operational
amplifier based circuit for signal conditioning. Moreover a
wireless communication is established between the transducer
and the display module using ATmega328 microcontroller
unit and a Bluetooth module
L(h) = C2 (l1 + h) + L(0)
(2)
where C2 = ( µ - µ0 ) A1n 2 ; L(0) = µ0 An 2 l
where A1 is the cross sectional area of the wire and l is the
length of the coil
L(h) = L(1) + ΔL
(3)
where L(1) = C2l1 + L0 and ΔL = K3 h
The sensor is designed so that cross sectional area of the coil
is very much greater than the cross sectional area of the wire
and the total length of the coil is greater than the insertion
length of the wire.
2.1. Signal Conditioner
II.
METHODOLOGY
The position of the float in the rotameter changes with the
flow rate. In order to detect this change, a thin wire is attached
to the top of the float. The self-inductance of the inductor
mounted on top of the rotameter changes with float position,
as the core position changes with flow rate of the liquid. The
variation in self-inductance is sensed by an inductance bridge
and corresponding to different float positions, voltage is
obtained. The operational amplifier based signal conditioning
circuit is used to convert a dc voltage of transducer into a form
suitable to convert into a current. The block diagram of the
flow transmitter is shown in Fig.1.The linearity of the
transducer and the transmitter are verified experimentally and
the output is tabulated.
Float
Position
Two operational amplifiers with high impedance and high
signal to noise ratio are connected to the arms of the bridge as
shown in Fig.2. The points B and D are at ground potential
and the effect of stray capacitance is minimised.
Using Kirchhoff’s current law in nodes B and D, we get
VRG
Vout =
(4)
( Z3 Z 2 − Z 4 Z1 )
Z1Z 3 Z 4
Considering the balance conditions and substituting the
expression for inductance (4) becomes
ω jVC2 RG
(5)
( l1 + h )
Z3 Z1
From (5) the absolute value of output voltage can be written as
Vout =
Vout = ( C4 + C6 h )
Transducer
Signal
Conditioner
V to I
Converter
C3 =
ωVC2 RG
C5
and from (2) and (3)
Fig.1. Block diagram of the flow transmitter
Let h be the height of the float for a flow rate of Q. Then
height of the float is proportional to the flow rate. Hence
h = C1Q
where
(1)
When the float is at the bottom, the inductor will have a
base inductance since some length of the wire will be inserted
to the coil .The inductor has an initial inductance of 60 mH.
With the change in flow rate the inductance changes from the
initial value. Let l1 be the insertion length of the wire, when
the float is at the bottom and l be the length of the coil. So
with change in flow rate the insertion length changes from l1
to l1+h where h is the float position. The inductance change of
the coil is given by
l1
(6)
C6 =
ωVC2 RG
C5
C5 = Z 3 Z1
Finally, from (1) and (5)
Vout = ( C4 + C8Q )
(7)
where C8 = C6C1
and
 20 − 4 
I out = 
Q + 4
 Qm 
(8)
The signal conditioner is designed to obtain an output of 2
to 10 V peak to peak value. The values of R2 and R4 selected
as 1 KΩ and the gain resistance Rg is found to be 1 KΩ. The
fixed inductance is 50 mH. Rg is found to be 1 KΩ. For the
fixed inductance in the bridge, 50 mH inductor is used. OP07
IC’s are used in the circuit because of their high input
impedance and signal to noise ratio.
Bridge
Output
Rectifier
and Filter
Difference
Amplifier
Voltage
Follower
V/I
Converter
Fig.3. Block diagram of signal conditioning circuit
2.2 Experimental Setup
In the experimental setup flow varies from 0-100 LPM. The
float position changes up to a maximum height of 20 cm. The
length of the float is 5 cm. The base inductance of the coil is
50 mH. The core of the coil is fitted to a screw gauge. The
core is made to move through the coil. The change in
inductance is found to be 50 mH-110 mH. The output is
tabulated and the linearity of the sensor and the transducer is
analysed.
Fig.4. Structural layout of the experimental set up
Fig.2. Operational amplifier based signal conditioner
The block diagram of the signal conditioning circuit is
shown in Fig.3. The bridge output is rectified using a precision
full wave rectifier. The output of the rectifier is fed to a filter.
The rectified and filtered output is given to a difference
amplifier. The resistors of the difference amplifier are chosen
to give a 1 to 5 V at the output. The difference amplifier has
span adjust and zero adjust resistors. The output of the
difference amplifier is given to a voltage follower. The voltage
follower increases the input impedance of the V to I converter.
The NI Multisim simulation of the transmitter circuit is shown
in Fig.5.
2.3 Sensor Specification
A ferromagnetic galvanized iron wire of 1.5 mm diameter
and 40 cm length is used as a flow sensing element. The wire
is brazed exactly at the centre of the steel float of the
rotameter. The other end of the wire is guided through a hole
to an aluminium casing of diameter 20 mm and length of 30
cm placed at the top of rotameter. At maximum flow rate, the
length of the wire movement should not exceed half the length
of the coil. The inductance coil is made of 5000 turns and it is
mounted on the top of the rotameter. To prevent the flow of
water into the casing at high pressure the hole is properly
sealed using Teflon and free movement of the wire through
the seal is ensured.
Fig.5. Simulation diagram of a proposed signal conditioning circuit of flow transmitter
2.4 Wireless Readout
The ATmega328 microcontroller along with Bluetooth
module is used to read the flow rate wirelessly. The flow rates
are displayed in the concerned person’s system. The DC
voltage is fed to the analog input pin in Arduino board and it is
sent to the microcontroller. The Bluetooth module HC-06 is
connected to Arduino using an application. Fig.6 shows the
connecting diagram of Bluetooth module with Arduino.
conditioning circuit as shown in Fig.8. The current and
inductance relationship is also linear as shown in Fig. 9, but
the linearity is more prominent in the region between 60 mH
and 150 mH. An accuracy of 89% is obtained between the
transmitter and calculated output. It has good repeatability.
The sensitivity of the transducer can be improved by
increasing the turns of the inductance pick up coil. The weight
of the ferromagnetic wire plays a major role in determining
the accuracy of the transducer. The weight of the former
should be at least one tenth the weight of the float for the
transducer to be linear and accurate. The wireless
communication using Bluetooth provides effective
communication within 30 m.
Fig.6. Connection diagram of Bluetooth module to Arduino board
III.
RESULTS AND DISCUSSION
The photograph of the developed flow transmitter is shown
in Fig.7. The efficiency of the transmitter is studied
experimentally and verified through the prototype. The
experimental results show that the linear relationship exists
between the change in self-inductance of the sensing coil and
the output voltage of the operational amplifier based signal
Fig.7. Photograph of the developed flow transmitter
REFERENCES
[1]
Fig. 8. Inductance vs. voltage characteristic of a flow sensor
Fig. 9. Inductance vs. current characteristic of a flow transmitter
IV.
CONCLUSION
The wireless flow transmitter has been developed based on
both operational amplifier based signal conditioning circuit
and rotameter. As a result of the experimentation done for the
developed transmitter, the possibility for the design of a flow
sensor on the basis of a rotameter connected to operational
amplifier circuit with inductance pick up coil has been
presented. The developed transmitter exhibits excellent
linearity and accuracy compared with the methods reported in
the literature. The proposed transmitter is particularly suitable
for the practical applications requiring remote measurement of
flow rate with high accuracy and less pressure drop when
compared with commercially available flow measuring
systems. The key feature of the proposed transmitter is a lowcost and having the remote readout capability. This opens up
for near future studies and applications of developing
dynamic, wireless sensor module for physical quantities
measurement and automation
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