cashewnut shelling machine

1. AUTOMATIC CASHEWNUT
DESHELLING & PACKAGING
MACHINE
-Compiled by
Lal Surajkumar Shrikant
A report on Sensors used
in the Project

2. QUADRATURE
ENCODER
IR SENSOR
LIMIT SWITCHES
PROXIMITY
SWITCH
contents

3. QUADRATURE ENCODER
The most common type of incremental
encoder uses two output channels (A and
B) to sense position. Using two code tracks
with sectors positioned 90 degrees out of
phase, the two output channels of the
quadrature encoder indicate both position
and direction of rotation.
If A leads B, for example, the disk is
rotating in a clockwise direction. If B leads
A, then the disk is rotating in a counter-
clockwise direction.
By monitoring both the number of pulses
and the relative phase of signals A and B,
you can track both the position and
direction of rotation.
Some quadrature encoders also include a
third output channel, called a zero or index
or reference signal, which supplies a single
pulse per revolution. This single pulse is
used for precise determination of a
reference position.

4. Working
The code disk inside a quadrature encoder contains
two tracks usually denoted Channel A and Channel B.
These tracks or channels are coded ninety electrical
degrees out of phase, as indicated in the image below,
and this is the key design element that will provide
the quadrature encoder its functionality.
In applications where direction sensing is required, a
controller can determine direction of movement based
on the phase relationship between Channels A and B.
As illustrated in the figure below, when the quadrature
encoder is rotating in a clockwise direction its signal
will show Channel A leading Channel B, and the reverse
will happen when the quadrature encoder rotates
counterclockwise.

5. Working
Apart from direction, position can also be monitored
with a quadrature encoder by producing another signal
known as the “marker”, “index” or “Z channel”. This Z
signal, produced once per complete revolution of the
quadrature encoder, is often used to locate a specific
position during a 360° revolution.
When to use Quadrature
Encoders?
Quadrature encoders are used in bidirectional position
sensing and length measuring applications. However, in
some unidirectional start-stop applications, it is important
to have bidirectional information (Channel A & B) even if
reverse rotation of the shaft is not anticipated. An error in
count could occur with a single-channel encoder due to
machine vibration inherent in the system. For example, an
error in count may occur with a single-channel encoder in
a start/stop application if it mechanically stops rotating
when the output waveform is in transition. As subsequent
mechanical shaft vibration forces the output back and
forth across the edge the counter will up-count with each
transition, even though the system is virtually stopped. By
utilizing a quadrature encoder, the counter monitors the
transition in its relationship to the state of the opposite
channel, and can generate reliable position information.

6. Achieving higher resolution with
Quadrature Encoders
When more resolution is needed, it is possible for the
counter to count the leading and trailing edges of the
quadrature encoder’s pulse train from one channel,
which doubles (x2) the number of pulses per
revolution. Counting both leading and trailing edges
of both channels of a quadrature encoder will
quadruple (x4) the number of pulses per revolution.
As a result, 10,000 pulses per turn can be generated
from a 2,500 PPR quadrature encoder.

7. IR SENSOR
An Infrared (IR) sensor is used to detect
obstacles in front of the robot or to
differentiate between colors depending
on the configuration of the sensor.
The sensor emits IR light and gives a
signal when it detects the reflected light.
An IR sensor consists of an emitter,
detector and associated circuitry. The
circuit required to make an IR sensor
consists of two parts; the emitter circuit
and the receiver circuit.
The emitter is simply an IR LED (Light
Emitting Diode) and the detector is
simply an IR photodiode which is
sensitive to IR light of the same
wavelength as that emitted by the IR
LED. When IR light falls on the
photodiode, its resistance and
correspondingly, its output voltage,
change in proportion to the magnitude
of the IR light received. This is the
underlying principle of working of the IR
sensor.

8. Working
IR sensors are also used to distinguish between black and white
surfaces. White surfaces reflect all types of light while black
surfaces absorb them. Therefore, depending on the amount of
light reflected back to the IR receiver, the IR sensor can also be
used to distinguish between black and white surfaces

9. LIMIT SWITCHES
A Limit Switch is enclosed in a case to
protect a built-in basic switch from
external force, water, oil, gas, and
dust. Limit Switches are made to be
particularly suited for applications that
require mechanical strength or
environmental resistance.
The shapes of Limit Switches are broadly
classified into Horizontal, Vertical, and
Multiple Limit Switches. The structure of
a typical vertical Limit Switch is shown in
the following figure as an example. Limit
Switches are generally composed of five
components.

10. Working
Drive Mechanism of Limit Switch
The drive mechanism of the Limit switch is an important part of
the Limit Switch and is directly linked to seal performance and
operating characteristics. Drive mechanisms are classified into
three types, as shown in the following figure

11. Working
(1) Plunger
There are two types of plunger (types A and B in the figure)
depending on the sealing method. With type A, an O-ring or a
rubber diaphragm is used for sealing. The rubber seal is not
externally exposed, and so resistance is provided against cutting
debris from machine tools, but sand and fine shavings may become
stuck on the sliding surface of the plunger. With type B, sand and
fine shavings will not become stuck, and the sealing performance is
superior to type A, but hot cutting debris striking the switch may
damage the rubber cap.

12. Working
Whether type A or type B is required depends on the location
in which the Switch is to be used. With the plunger drive, the
movement of the plunger piston enables air to be
compressed and taken in. Therefore, if the plunger is left
pushed in for a long time, the air in the Limit Switch will
escape and the internal pressure will become equivalent to
atmospheric pressure.
This will cause the plunger to tend to reset slowly even if an
attempt is made to quickly reset it. To prevent this problem
from occurring, design the system to limit the amount of air
compressed by pushing in the plunger to 20% or less of the
total air pressure in the Limit Switch. To extend the service life
of the Limit switch, the plunger drive includes an OT
absorption mechanism that absorbs the remaining plunger
movement using an OT absorption spring and stops the
movement of an auxiliary plunger that pushes the Built-in
switch according to the movement of the plunger.
(2) Hinge Lever
The amount of plunger movement is increased at the end of
the lever (i.e., roller) by the lever ratio, and so an absorption
mechanism is generally not used.
(3) Roller Lever
The structure of the WL is shown as a typical example. Other
drives include those in which the plunger performs the
function of the reset plunger and those in which a coil spring
is used for the reset force and a cam is used to move the
auxiliary plunger.

13. PROXIMITY SWITCHES
A high-frequency magnetic field is
generated by coil L in the oscillation circuit.
When a target approaches the magnetic
field, an induction current (eddy current)
flows in the target due to electromagnetic
induction. As the target approaches the
sensor, the induction current flow
increases, which causes the load on the
oscillation circuit to increase. Then,
oscillation attenuates or stops. The sensor
detects this change in the oscillation status
with the amplitude detecting circuit, and
outputs a detection signal.

14. Working
At the heart of an
Inductive Proximity Sensor
(“prox” “sensor” or “prox
sensor” for short) is an
electronic oscillator
consisting of an inductive
coil made of numerous
turns of very fine copper
wire, a capacitor for
storing electrical charge,
and an energy source to
provide electrical
excitation.
The size of the inductive
coil and the capacitor are
matched to produce a self-sustaining sine wave
oscillation at a fixed frequency.
The coil and the capacitor act like two electrical
springs with a weight hung between them,
constantly pushing electrons back and forth
between each other. Electrical energy is fed into the
circuit to initiate and sustain the oscillation. Without
sustaining energy, the oscillation would collapse due
to the small power losses from the electrical
resistance of the thin copper wire in the coil and
other parasitic losses.

15. Working
The oscillation produces an electromagnetic field in
front of the sensor, because the coil is located right
behind the “face” of the sensor. The technical name
of the sensor face is “active surface”.
When a piece of conductive metal enters the zone
defined by the boundaries of the electromagnetic
field, some of the energy of oscillation is transferred
into the metal of the target. This transferred energy
appears as tiny circulating electrical currents called
eddy currents. This is why inductive proxes are
sometimes called eddy current sensors.
The flowing eddy currents encounter electrical
resistance as they try to circulate. This creates a
small amount of power loss in the form of heat (just
like a little electric heater). The power loss is not
entirely replaced by the sensor’s internal energy
source, so the amplitude (the level or intensity) of
the sensor’s oscillation decreases. Eventually, the
oscillation diminishes to the point that another
internal circuit called a Schmitt Trigger detects that
the level has fallen below a pre-determined
threshold.
This threshold is the level where the presence of a
metal target is definitely confirmed. Upon detection
of the target by the Schmitt Trigger, the sensor’s
output is switched on.

16. Working
The short animation to the right shows the effect of
a metal target on the sensor’s oscillating magnetic
field. When you see the cable coming out of the
sensor turn red, it means that metal was detected
and the sensor has been switched on. When the
target goes away, you can see that the oscillation
returns to its maximum level and the sensor’s output
is switched back off.