Download CH4_Switches_Sensors_V1.1 3842KB Nov 30 2015 04:36:47 PM

Control AC Motor
PLC Implementation
Add Indicator Light
PLC Implementation
Add Limit Switch
PLC Implementation
Magnetic Contactor


For handling large current
and voltage.
Can be used for interlock
to prevent false and
dangerous operations.
Magnetic Contactor Circuit
Manual Operated Switches


Controlled by hand
Include toggle switch,
push button, selector
switch, etc.
Selector Switch Circuit
Switches


DIP Switch
Limit Switch
Limit Switch Circuit

For Starting and
stopping a motor
in forward and
reverse with limit
switch providing
over-travel
protection
Limit Switch Symbols
A typical limit switch consists of a switch body and
an operating head. The switch body includes
electrical contacts to energize and de-energize a
circuit. The operating head incorporates some type
of lever arm or plunger, referred to as an actuator.
Principle of Operation
The free position is the position of the actuator when no
external force is applied.
Pre-travel is the distance or angle travelled in moving the
actuator from the free position to the operating position.
The operating position is where contacts in the limit
switch change from their normal state (NO or NC) to their
operated state.
Over-travel is the distance the actuator can travel safely
beyond the operating point.
Differential travel is the distance travelled between
the operating position and the release position.
The release position is where the contacts change
from their operated state to their normal state.
Release travel is the distance travelled from the
release position to the free position.
Momentary Operation
One type of actuator operation is momentary. When the
target comes in contact with the actuator, it rotates the
actuator from the free position, through the pre-travel area,
to the operating position. At this point the electrical
contacts in the switch body change state. A spring returns
the actuator lever and electrical contacts to their free
position when the actuator is no longer in contact with the
target.
Maintained Operation
In many applications it is desirable to have the actuator
lever and electrical contacts remain in their operated state
after the actuator is no longer in contact with the target.
This is referred to as maintained operation. With
maintained operation the actuator lever and contacts return
to their free position when a force is applied to the actuator
in the opposite direction. A fork style actuator is typically
used for this application.
Mounting Considerations
Limit switches should be mounted in locations
which will prevent false operations by normal
movements of machine components and machine
operators. An important aspect of limit switch
mounting is cam design. Improper cam design can
lead to premature switch failure. For lever arm
actuators it is always desirable to have the cam
force perpendicular to the lever arm. For
applications in which the cam is traveling at speeds
less than 100 feet per minute a cam lever angle of
30 degrees is recommended.
Overriding and Non overriding Cam

In overriding cam applications it is necessary to
angle the trailing edge of the cam in order to
prevent the lever arm from snapping back.
Snapping back of the lever arm can cause shock
loads on the switch which will reduce the life of
the switch. Non-Overriding cams are cams which
will not over-travel the actuating mechanism.
Flexible Loop and Spring Rod
Flexible loop and spring rod actuators can be
actuated from all directions, making them suitable
for applications in which the direction of approach
is constantly changing.
Example Application
Temperature Switches

Or called Thermostat, to
sense temperature changes,
actuated by some specific
temperature change
Pressure Switch

Since many machine systems use pneumatic (air)
pressure, vacuum, or hydraulic pressure to
perform certain tasks, it is necessary to be able to
sense the presence of pressure or vacuum


Pressure Switch
To control
pressure of liquid
and gas
Actuated when a
specified pressure
is reached.
Bellow Switch
• The bellows switch is a relatively simple device
that provides a discrete (on or off) signal based
on pressure. Notice that the bellows (which is
made of a flexible material, usually rubber) is
sealed to the end of a pipe from which the
pressure is to be sensed. When the pressure in
the pipe increases, the bellows pushes on the
actuator of a switch.
Strain Gage Pressure Sensor

The strain gage pressure sensor is the most
popular method of making analog measurements
of pressure. It is relatively simple, reliable, and
accurate. It operates on the principle that
whenever fluid pressure is applied to any solid
material, the material deforms (strains).
Strain Gage Pressure Sensor

The basic components of a pressure switch are
illustrated below. Electrical contacts are operated
by the movement of a diaphragm against the
force of a spring. The contacts may be normally
open (NO) or normally closed (NC). The spring
setting determines how much fluid pressure is
required to operate the contacts.
Pressure Switch Circuit
Level Switch



To sense the
height of a liquid.
A float is attached
The rising and
lowering of the
float trips the level
switch
Transducers and Sensors


Transducer convert
energy from one
form to another.
Sensors are
transducer for
detecting and
measuring.
Sensor

One type of feedback frequently needed by
industrial-control systems is the position of one
or more components of the operation being
controlled. Sensors are devices used to provide
information on the presence or absence of an
object.
Sensors

Sensors include limit switches, photoelectric ,
inductive, capacitive, and ultrasonic sensors.
These products are packaged in various
configurations to meet virtually any requirement
found in commercial and industrial applications.
Sensors
Load cells

A field device that measures a varying value is
typically connected to a transducer. A load cell is
a device that takes a varying value and converts it
to a variable voltage or current output. In this
example the load cell is converting a value of
weight into a 0 - 10 VDC output. This load cell
outputs 0 - 10 VDC for a 0 - 500 Lbs input. The
0 - 10 VDC load cell output is connected to the
input of an analog expansion module.
Sensors used in Manufacturing
Limit Switch Vs Sensors

Limit switches use a mechanical actuator input,
requiring the sensor to change its output when an
object is physically touching the switch. Sensors,
such as photoelectric, inductive, capacitive, and
ultrasonic, change their output when an object is
present, but not touching the sensor.
“No Touch” Sensors
There are four types of “no touch” sensors:
inductive, capacitive, ultrasonic, and photoelectric.
1) Inductive proximity sensors use an
electromagnetic field to detect the presence of metal
objects.
2) Capacitive proximity sensors use an electrostatic
field to detect the presence of any object.
“No Touch” Sensors
3) Ultrasonic proximity sensors use sound waves
to detect the presence of objects.
4) Photoelectric sensors react on changes in the
received quantity of light. Some photoelectric
sensors can even detect a specific color
“No Touch” Sensors
Inductive Proximity Sensors
The sensor incorporates an electromagnetic coil
which is used to detect the presence of a conductive
metal object. The sensor will ignore the presence of
an object if it is not metal.
Inductive Sensors

This type of sensor consists of four elements: coil,
oscillator, trigger circuit, and an output. The oscillator is
an inductive capacitive tuned circuit that creates a radio
frequency. The electromagnetic field produced by the
oscillator is emitted from the coil away from the face of
the sensor. The circuit has just enough feedback from
the field to keep the oscillator going.
When a metal target enters the field, eddy currents
circulate within the target. This causes a load on the
sensor, decreasing the amplitude of the electromagnetic
field.
Cont.

As the target approaches the sensor the eddy currents
increase, increasing the load on the oscillator and further
decreasing the amplitude of the field. The trigger circuit
monitors the oscillator’s amplitude and at a
predetermined level switches the output state of the
sensor from its normal condition (on or off).
As the target moves away from the sensor, the
oscillator’s amplitude increases. At a predetermined level
the trigger switches the output state of the sensor back
to its normal condition (on or off).
Proximity Sensor


To detect the
presence of an object
without physical
contact.
Inductive is actuated
by metal object
Proximity Sensor
Inductive Proximity Sensor
Proximity Sensors Connections
Hysteresis
Shield Proximity Sensor

The ferrite core concentrates the radiated field in
the direction of use. A shielded proximity sensor
has a metal ring placed around the core to restrict
the lateral radiation of the field. Shielded
proximity sensors can be flush mounted in metal.
A metal-free space is recommended above and
around the sensor’s sensing surface. Refer to the
sensor catalog for this specification. If there is a
metal surface opposite the proximity sensor it
must be at least three times the rated sensing
distance of the sensor from the sensing surface.
Unshield Proximity Sensor

An unshielded proximity sensor does not have a metal
ring around the core to restrict lateral radiation of the
field. Unshielded sensors cannot be flush mounted in
metal. There must be an area around the sensing surface
that is metal free. An area of at least three times the
diameter of the sensing surface must be cleared around
the sensing surface of the sensor. In addition, the sensor
must be mounted so that the metal surface of the
mounting area is at least two times the sensing distance
from the sensing face. If there is a metal surface
opposite of the proximity sensor it must be at least three
times the rated sensing distance of the sensor from the
sensing surface.
Capacitive Proximity

Actuated by any dirt
in their environment.

Usually supplement
to the inductive
proximity sensor
when there is no
metal available for
the actuation.
Capacitive Sensors

Capacitive sensing is a noncontact technology
suitable for detecting metals, nonmetals, solids,
and liquids, although it is best suited for
nonmetallic targets because of its characteristics
and cost relative to inductive proximity sensors.
Due to their ability to detect most types of
materials, capacitive sensors must be kept away
from non-target materials to avoid false
triggering. For this reason, if the intended target
contains a ferrous material, an inductive sensor is
a more reliable option.
Magnetic Switch


A magnetic switch
also called reed
switch, composed of
two flat contact
sealed in a glass tube.
As a permanent
magnet approaches,
the ends of the
overlapped contact
tab attract one
another.
Light Sensor


Photovoltaic cell
converts light energy
directly into electric
energy.
Photoconductive
cell also called
photo-resistive cell.
Light energy falling
on the cell will cause
change in resistance
Photoelectric Sensor


Use a LED for the light
source and
phototransistor to sense
the presence of light.
Two main types of
photoelectric :
- Reflective
- Through Beam
Reflective Type

It is used to detect the light beam (visible,
infrared, or laser) reflected from the target.
Through Beam

It is used to measure the change of light quality
caused by the target crossing the optical axis.
Features of Photoelectric Sensor



Non contact
detection – long
service life
Almost any
material – based
quantity of light
received
Long detection
distance –
reflective(1m),
through beam (10m)



High response
speed – 50us
Color Discrimination
– based on
absorption of its
color
Highly accurate
Barcode Technology


Bar code system consists of : bar code symbol, a
scanner , and a decoder.
Bar code symbol contains up to 30 characters
encoded in a machine-readable form.
Bar code system

Mostly based on UPC (universal product code)
UPC type (1 char)
 Manufacturer or vendor ID (5 chars)
 Item number (5 chars)
 Check digit (1 char)

Barcode Scanner



A light source illuminates the bar code symbol,
those bar absorb light , and spaces reflect light.
A photo detector collects this light in the form of
an electronics signal pattern.
The decoder received the signal and converts
these data into character data.
Ultra sonic Sensor


An ultrasonic sensor
operates by sending
sound waves toward the
target and measuring
the time it takes for the
pulses to bounce back.
Its bounce back time
proportional to the
distance
Strain/Weight Sensor


A strain gauge transducer converts a mechanical
strain into an electric signal.
Strain gauge are based on the principle that the
resistance of a conductor varies with length and
cross-sectional area.
Temperature Sensor
Thermocouple



Most commonly used for temperature
measurement.
It is a pair of dissimilar conductors welded or
fused together at one end to form the “hot”,
junction with the free ends available for
connection to the “cold” or reference junction.
When temperature difference occurs, a small dc
voltage is generated.
Non contact Temperature Sensor

Infrared radiation from the object surface
generates hot spot on the thermocouple
Flow Measurement



Converts the kinetic energy
that the fluid has into some
other measurable form.
Turbine flow meter’s blades
turn at a rate proportional to
the fluid velocity.
Magnetic flow meter offers no
restriction to flow. A coil set
up a magnetic field, if a
conductive liquid flows
through this magnetic field, a
voltage is induced
Flow


The flow, Q, of a fluid in a pipe is directly
proportional to the velocity of the fluid, V, and
the cross sectional area of the pipe, A. Therefore,
we can say Q = V x A.
Therefore, we may conclude that if we wish to
increase the flow of a fluid in a pipe, we have two
choices - we can increase the fluid’s velocity, or
install a larger diameter pipe. Fluid flow is
measured in many different ways.
Drag Disk Flow Switch

This unit consists of a case containing a snapaction switch, a switch lever arm extending from
the bottom of the case, and a circular disk
attached to the end of the lever arm. In this case,
the device is threaded into a “T” connector
installed in the pipe and oriented such that the
drag disk is perpendicular to the direction of
flow. As flow rate increases it increases the force
on the drag disk. At a predetermined high flow
rate the force on the drag disk is sufficient to
push the lever arm to the right and actuate the
switch.
Inline Flow Meter

This device has it’s drag disk mounted inside a plastic or
glass tube with an internal spring to counteract the force
applied to the disk by fluid flow. A toroidal permanent
magnet is mounted on the downstream side of the drag
disk and is held in place by the spring force. A magnetic
reed switch is mounted on the outside of the tube with
it’s vertical position on the tube being set by an
adjustment screw. As fluid flow increases the disk and
piggy-back magnet will rise in the tube. When the
magnet aligns with the reed switch the switch will
actuate.
Thermal Dispersion Flow Switch
Thermal Dispersion Flow Switch

A method to indirectly measure flow is by
measuring the amount of heat that the flow
carries away from a heater element that is
inserted into the flow. By setting a trip point, we
can have an electronic temperature switch actuate
when the temperature of the heated probe drops
below a predetermined level
Cont.

One problem associated with this technique is
that, since we may not know the temperature of
the fluid in the pipe, it is difficult to determine
the flow based simply on the temperature of the
heated probe.
Paddlewheel Flow Sensor

One method to directly measure flow velocity is
to simply insert a paddlewheel into the fluid flow
and measure the speed that the paddlewheel
rotates.
Cont.

It provides an output of pulses, with the pulse
rate being proportional to flow velocity. Some of
the more elaborate models will also convert the
pulse rate to a DC voltage (an analog output).
Keep in mind that as with many types of flow
sensors, the device actually measures fluid speed,
not flow rate. However, flow rate can be
calculated if the pipe diameter is also known.
Turbine type flowmeter.
Cont.

Turbine-type flowmeters are a popular means of
measurement and control of liquid products in
industrial, chemical, and petroleum operations.
Turbine flowmeters, like windmills, utilize their
angular velocity (rotation speed) to indicate the
flow velocity. The bladed rotor rotates on its axis
in proportion to the rate of the liquid flow
through the tube.Fluid passing through the flow
tube causes the rotor to rotate, which generates
pulses in the pickup coil.
Velocity/RPM Sensor

The output of a
generator varies with
the speed at which
the generator is
driven.
Tachometer Generator
Tachometer generators provide a convenient means
of converting rotational speed into an analog
voltage signal that can be used for motor speed
indication and control applications.
Optical Encoder

An encoder is used to convert linear or rotary
motion into a binary digital signal. Encoders are
used in applications where positions have to be
precisely determined.