SIGNAL CONDITIONING UNIT (SCU)
The sensors used to measure various
parameters in the farm give their output in terms of change in resistance. We
have to convert this change in resistance to a suitable form to connect with
the LabVIEW through DAQ. The input voltage range of DAQ is from 0v to 10v.To
convert the change in resistance values to voltage in this range we use a
Wheatstone bridge circuit.
Wheatstone bridge is a very simple
circuit usually used to convert impedance variation into voltage variation. One
of advantage of bridge for this task is that it can be designed so the voltage
produced varies around zero. This means that amplification can be used to
increase the voltage level for increased sensitivity to variation impedance,
initially the bridge was in balanced position. This is the resistance offered
by input and output resistors will be same and thus there is no resistance no
potential difference between input and output. Any change in one of the
resistors will lead to imbalance to bridge and thus to change in voltage. Thus
the change in resistance offered by sensors can be effectively measured.
Fig 4.1 Wheatstone bridge
Thus the change in
resistance is converted into voltage. The voltage obtained at the output of the
Wheatstone bridge is directly given to the Data Acquisition card (DAQ). The
voltage that acquired by the DAQ from the Signal Conditioning Unit (SCU) is
directly proportional to the change in the parameter measured by the
corresponding sensor. This acquired voltage is given to the computer for the
further processing. Computer uses the software LabVIEW for the analysing the
signal acquired. The result of the analysis is displayed using the same
software.
According to the result
obtained after the analysis of data, the software decides the control action
need to be taken. In the controlling section, DAQ receives the controlling
signal send by the LabVIEW software. DAQ generates a proportional voltage. In
the case of the NI USB-6212 DAQ, the generated voltage has no enough power to
drive a controlling device. So to overcome this problem, we are using switching
circuit to interface or to condition the signal by switching a voltage source
having enough power.
The switching circuit
used here is a Light Detecting Circuit, because the current from the DAQ is not
enough to switch a transistor. Light Detecting Circuit (LDC) has mainly three
parts, a light source, light detector and transistor switching circuit. The light
source used in the above circuit is an LED and the LDR (Light Dependent
Resistor) is employed as the light detector. A Light Dependent Resistor (LDR,
photoconductor, or photocell) is a device which has a resistance which varies
according to the amount of light falling on its surface.
There are two basic circuits using LDR - the first is activated by
darkness, the second is activated by light. The two circuits are very similar
and just require an LDR, some standard resistors and any small signal transistor.
Here, we are using the circuit activated by light.
When the LED focused to the LDR is
turned on, the resistance of the LDR decreases with respect to the light
intensity. Then the current starts flowing to the base of the transistor, which
causes the transistor to turn on and pass the supply voltage (+V) through it.
The load resistance connected at the emitter terminal create a voltage drop.
This voltage drop collected across the load resistance is taken as the output. As
we said, in this circuit we use this light detecting circuit to interface the
DAQ with the controllers through a relay.
Figure
4.2 Interfacing using LDC
Interfacing using LDC
is done by connecting the light source (LED) to the output port of the DAQ. When
a controlled signal is transferred by the software to the DAQ, it generates a
voltage needed to turn on the LED. The light produced by the LED switches the
voltage source having optimum power by the switching circuit. This voltage is
used to operate the relay.
4.1 DAQ
PC-Based
Data Acquisition (DAQ):
DAQ is data acquisition. It is device
which contains both ADC & DAC in it. It is interface between analog output
of sensor and the PC. The data traditional experiments in it signal from
sensors are sent to analog or digital domain, read by experimenter, and
recorded by hand. In automated data acquisition systems the sensors transmit a
voltage or current signal directly to a computer via data acquisition board.
Software such as LabVIEW controls the acquisition and processing of such data. The
benefits of automated systems are many:
· improved
accuracy of recording
· increased
frequency with which measurements can be taken
· potential
to automate pre and post processing and built in quality control
CONFIGURATION
CONSIDERATION
Here
we have to consider the following properties of the input signal
1.
Sampling Rate
2.
Resolution
3.
Range
4.
Amplification
DEVICE
RANGE
The
device range can be as follows
Minimum
and maximum voltages the ADC can digitize
DAQ
devices often have different available ranges
· 0
to +10 volts
· -10
to +10 volts
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