CONTINUOUS ACTUATORS

CONTINUOUS ACTUATORS

John   02:51:00

Continuous actuators allow a system to position or adjust outputs over a wide
range of values. Even in their simplest form, continuous actuators tend to be mechanically
complex devices. For example, a linear slide system might be composed of a motor with
an electronic controller driving a mechanical slide with a ball screw. The cost for such an
actuators can easily be higher than for the control system itself. These actuators also
require sophisticated control techniques that will be discussed in later chapters. In general,
when there is a choice, it is better to use discrete actuators to reduce costs and complexity.

ELECTRIC MOTORS
An electric motor is composed of a rotating center, called the rotor, and a stationary
outside, called the stator. These motors use the attraction and repulsion of magnetic
fields to induce forces, and hence motion. Typical electric motors use at least one electromagnetic
coil, and sometimes permanent magnets to set up opposing fields. When an voltage
is applied to these coils the result is a torque and rotation of an output shaft. There are
a variety of motor configuration the yields motors suitable for different applications. Most
notably, as the voltages supplied to the motors will vary the speeds and torques that they
will provide.
A control system is required when a motor is used for an application that requires

continuous position or velocity.  In any controlled
system a command generator is required to specify a desired position. The controller
will compare the feedback from the encoder to the desired position or velocity to
determine the system error. The controller with then generate an output, based on the system
error. The output is then passed through a power amplifier, which in turn drives the
motor. The encoder is connected directly to the motor shaft to provide feedback of position.

DC BRUSHED MOTORS
In a DC motor there is normally a set of coils on the rotor that turn inside a stator
populated with permanent magnets. The magnetics provide a permanent magnetic field for the rotor to push against. When current is run through the wire loop it creates a magnetic field.
The power is delivered to the rotor using a commutator and brushes. In the figure the power is supplied to the rotor through graphite brushes rubbing
against the commutator. The commutator is split so that every half revolution the
polarity of the voltage on the rotor, and the induced magnetic field reverses to push against
the permanent magnets.

The direction of rotation will be determined by the polarity of the applied voltage,
and the speed is proportional to the voltage. A feedback controller is used with these
motors to provide motor positioning and velocity control.
These motors are losing popularity to brushless motors. The brushes are subject to
wear, which increases maintenance costs. In addition, the use of brushes increases resistance,
and lowers the motors efficiency.

AC SYNCHRONOUS MOTORS

A synchronous motor has the windings on the stator. The rotor is normally a squirrel
cage design. The squirrel cage is a cast aluminum core that when exposed to a changing
magnetic field will set up an opposing field. When an AC voltage is applied to the
stator coils an AC magnetic field is created, the squirrel cage sets up an opposing magnetic
field and the resulting torque causes the motor to turn.
The motor is called synchronous because the rotor will turn at a frequency close to
that of the applied voltage, but there is always some slip. It is possible to control the speed
of the motor by controlling the frequency of the AC voltage. Synchronous motor drives
control the speed of the motors by synthesizing a variable frequency AC waveform.

These drives should be used for applications that only require a single rotation
direction. The torque speed curve for a typical induction motor is shown in Figure 368.
When the motor is used with a fixed frequency AC source the synchronous speed of the
motor will be the frequency of AC voltage multiplied by the number of poles in the motor.
The motor actually has the maximum torque below the synchronous speed. For example a
motor 2 pole motor might have a synchronous speed of (2*60*60/2) 1800 RPM, but be
rated for 1720 RPM. When a feedback controller is used the issue of slip becomes insignificant.

BRUSHLESS DC MOTORS
Brushless motors use a permanent magnet on the rotor, and user wire windings on
the stator. Therefore there is no need to use brushes and a commutator to switch the polarity of the voltage on the coil. The lack of brushes means that these motors require less
maintenance than the brushed DC motors.
To continuously rotate these motors the current in the outer coils must alternate
continuously. If the power supplied to the coils is an AC sinusoidal waveform, the motor
will behave like an AC motor. The applied voltage can also be trapezoidal, which will give
a similar effect. The changing waveforms are controller using position feedback from the
motor to select switching times. The speed of the motor is proportional to the frequency of
the signaL.

STEPPER MOTORS
Stepper motors are designed for positioning. They move one step at a time with a
typical step size of 1.8 degrees giving 200 steps per revolution. Other motors are designed
for step sizes of 2, 2.5, 5, 15 and 30 degrees.
There are two basic types of stepper motors, unipolar and bipolar, as shown in Figure
370. The unipolar uses center tapped windings and can use a single power supply. The
bipolar motor is simpler but requires a positive and negative supply and more complex
switching circuitry.

to be continued

HOW TO FAST CONNECT PROFIBUS CABLE USING FAST CONNECT STRIPPING TOOL

HOW TO FAST CONNECT PROFIBUS CABLE USING FAST CONNECT STRIPPING TOOL

John   02:21:00

PROFIBUS, the number 1 fieldbus in automation technology, has already been on the market for several years. Special types of cable (such as standard cable, trailing cable and halogen-free cable) are available to satisfy the requirements of different fields of application. The FastConnect cabling system from the SIMATIC NET product line now greatly improves the attractiveness of the PROFIBUS. Easy installation coupled with an enormous

reduction in preparation time are the most important advantages of the FastConnect connection system.

The system consists of three optimally matched components:

• Easy-to-install cablewith copper cores

• Stripping tool

• Snap-on connector(under development) with insulation displacement

Today, the cable is stripped carefully layer by layer with a cable knife, that is, the outer sheath must be cut to precisely the right size for the connector to be used.

The braided shield must then be prepared for the respective shield grounding connection.

Finally, the cores must be stripped to a prescribed length.

These steps, all of which are needed to install a PROFIBUS connector, are extremely

time-consuming. The time needed to cut to length and terminate a PROFIBUS cable is currently around 10 to 15 minutes.

It's this VERY easy:

Take the stripping tool in your right hand

Measure the length of PROFIBUS FastConnect Stripping Tool (FCS) cable by placing the cable on the measuring template. Use the index finger of the left hand as end stop. Place the end of the measured piece of cable in the tool. End stop for the cutting depth is the index finger of the left hand.

Clamp the cable end tightly in the stripping tool all the way to the end stop.

To strip the cable, turn the stripping tool four times in the direction of the arrow. Leaving the stripping tool closed, pull it off the cable end. Remove the protective layer from the cores.

Insert cores according to color coding into the connector and push down lock.

Fasten strain relief screws and your are done.

Installation of a PROFIBUS connector is child's play with the new

PROFIBUS FastConnect system. The FastConnect cable's special design makes it possible to use the FastConnect stripping tool for precise, one-step stripping of outer sheath and braided shield, enabling error-free preparation of PROFIBUS connectors in about two minutes. The FastConnect cable can be used both for both snap-on connectors and for the preparation of PROFIBUS connectors with screw-type terminals. Installation of the new Fast- Connect snap-on connectors

is also greatly simplified by their uniform fixing dimensions and connection technology. In addition to savings in both time and costs, the PROFIBUS FastConnect system also reduces the source of errors.

PROFIBUS FastConnect standard cable

The PROFIBUS standard cable from SIMATIC NET, which has proven itself many times over, was modified in its internal design so as to allow use of the

FastConnect stripping tool. The Fast- Connect standard cable also has all the outstanding electrical and mechanical properties of the PROFIBUS standard cable.

PROFIBUS FastConnect stripping tool (FCS)

The FCS was developed as stripping tool for FastConnect cables. The precisely

preset cutting depths and distances between cutters make it possible to cut

back exactly the right amount of the FastConnect cable's outer sheath and braided shield in one step, precluding PROFIBUS connector installation errors due to inexact stripping of the cables.

INSTRUMENTATION AND CONTROL

INSTRUMENTATION AND CONTROL

John   00:04:00

CLICK HERE to get more details on sensors and transducers

SENSORS APPLICATION

SENSORS APPLICATION

John   23:52:00

ULTRASONIC SENSORS

1.Level Measurement in Large Vessels (Tanks,
Silos)
2..Anti corrosion
3.Height measurement
4.Level Measurement in Small Bottles
5.Breakage sensing
6.Quality Control
7.Object sensing
8.Bottle Counting
9.Stack height sensing
10.Vehicle Sensing and Positioning
11.Diameter sensing and strip speed control
12.Contour Recognition
13.People sensing
14.Wire and Rope Breakage Monitoring
15.Loop Control

PHOTOELECTRIC SENSORS

1.verifying objects in clear bottles
2.Flow of Pallets Carrying Bottles
3.Car Wash
4.counting bottles
5.counting cans
6.counting cartons
7.Reading Reference Marks for Trimming
8.detecting persons
9.controlling parking gate
10.End of Roll Detection
11.detecting caps on bottles
12.Detecting Tab Threads
13.Counting packages
14.Detecting Components Inside Metal Can
15.Detecting items of varying height
16.Determining Orientation of IC Chip
17.controlling height of stacks
18.Detecting Orientation of IC Chip
19.detecting jams on a conveyor
20.Counting Boxes Anywhere on a Conveyor
21.counting IC chip pins
22.Batch counting and Diverting Cans Without Labels
23.Detecting reflective objects
24.Detecting Presence of Object to Start a Conveyor
25.Verifying screws are correctly seated
26.Verifying lipstick height before capping
27.Verifying Cakes are Present in Transparent Package
28.Detecting  labels with transparent background
29.Monitoring Objects as they Exit Vibration Bowl

PROXIMITY SWITCHES

1.Detecting milk in cartons
2.Detecting the Presence of a Broken Drill Bit
3.Detecting Presence of Set Screws on Hub for Speed or Direction Control
4.Controlling Fill level of solids in a bin
5.detecting full open or closed valve positions
6.Detecting presence of can or lid
7.Detecting Broken Bit on Milling Machine

 TO BE CONTINUED

PHOTO ELECTRIC SENSORS

PHOTO ELECTRIC SENSORS

John   02:54:00

A photoelectric sensor is another type of position sensing device. Photoelectric sensors, similar to the ones shown below, use a modulated light beam that is either broken or reflected by the target.

The control consists of an emitter (light source), a receiver to detect the emitted light, and associated electronics that evaluate and amplify the detected signal causing the photoelectric’s output switch to change state.
Modulated light increases the sensing range while reducing the effect of ambient light. Modulated light is pulsed at a specific frequency between 5 and 30 KHz. The photoelectric sensor is able to distinguish the modulated light from ambient light. Light sources used by these sensors range in the light spectrum from visible green to invisible infrared. Light-emitting diode (LED) sources are typically used.

CLEARANCE

It is possible that two photoelectric devices operating in close proximity to each other can cause interference. The problem may be rectified with alignment or covers. The following clearances between sensors are given as a starting point. In some cases it may be necessary to increase the distance between sensors.

 EXCESS GAIN

Many environments, particularly industrial applications, include dust, dirt, smoke, moisture, or other airborne contaminants. A sensor operating in an environment that contains these contaminants requires more light to operate properly. There are six grades of contamination:
1. Clean Air (Ideal condition, climate controlled or sterile)
2. Slight Contamination (Indoor, nonindustrial areas, office
buildings)
3. Low Contamination (Warehouse, light industry, material
handling operations)
4. Moderate Contamination (Milling operations, high
humidity, steam)
5. High Contamination (Heavy particle laden air, extreme
wash down environments, grain elevators)
6. Extreme/Severe Contamination (Coal bins, residue on lens)

Excess gain represents the amount of light emitted by the transmitter in excess of the amount required to operate the receiver. In clean environments an excess gain equal to or greater than 1 is usually sufficient to operate the sensor’s receiver. If, for example, an environment contained enough airborne contaminants to absorb 50% of the light emitted by the transmitter, a minimum excess gain of 2 would be required to operate the sensor’s receiver. Excess gain is plotted on a logarithmic chart. The example shown below is an excess gain chart for an M12 thru-beam sensor. If the required sensing distance is 1 m there is an excess gain of 30. This means there is 30 times more light than
required in clean air hitting the receiver. Excess gain decreases as sensing distance increases. Keep in mind that the sensing distance for thru-beam sensors is from the transmitter to the receiver and the sensing distance for reflective sensors is from the transmitter to the target.

SWITCHING ZONE
Photoelectric sensors have a switching zone. The switching zone is based on the beam pattern and diameter of the light from the sensor’s emitter. The receiver will operate when a target enters this area.

SCAN TECHNIQUES

A scan technique is a method used by photoelectric sensors to detect an object (target). In part, the best technique to use depends on the target. Some targets are opaque and others are highly reflective. In some cases it is necessary to detect a change in color. Scanning distance is also a factor in selecting a scan technique. Some techniques work well at greater distances while others work better when the target is closer to the sensor.

THROUGH BEAM

 Separate emitter and receiver units are required for a thru-beam sensor. The units are aligned in a way that the greatest possible amount of pulsed light from the transmitter reaches the
receiver. An object (target) placed in the path of the light beam blocks the light to the receiver, causing the receiver’s output to change state. When the target no longer blocks the light path
the receiver’s output returns to its normal state. Thru-beam is suitable for detection of opaque or reflective objects. It cannot be used to detect transparent objects. In addition, vibration can cause alignment problems. The high excess gain of thru-beam sensors make them suitable for
environments with airborne contaminants. The maximum sensing range is 300 feet.

THROUGH BEAM EFFECTIVE BEAM

The effective beam of a photoelectric sensor is the region of the beam’s diameter where a target is detected. The effective beam on a thru-beam sensor is the diameter of the emitter and receiver lens. The effective beam extends from the emitter lens to the receiver lens. The minimum size of the target should equal the diameter of the lens.

REFLECTIVE OR RETROEFLECTIVE SCAN

Reflective and retroreflective scan are two names for the same  technique. The emitter and receiver are in one unit. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. A normal or a corner-cube reflector can be used. When a target blocks the light path the output of the sensor changes state. When the target no longer blocks the light path the sensor returns to its normal state. The maximum sensing range is 35 feet.

RETROREFLECTIVE SCAN EFFECTIVE BEAM

The effective beam is tapered from the sensor’s lens to the edges of the reflector. The minimum size of the target should equal the size of the reflector.

REFLECTORS

Reflectors are ordered separately from sensors. Reflectors come in various sizes and can be round or rectangular in shape or reflective tape. The sensing distance is specified with a particular reflector. Reflective tape should not be used with polarized retroreflective sensors

RETROREFLECTIVE SCAN AND SHINY OBJECTS

Retroreflective scan sensors may not be able to detect shiny  objects. Shiny objects reflect light back to the sensor. The sensor is unable to differentiate between light reflected from a shiny object and light reflected from a reflector.

POLARIZED RETROREFLECTIVE SCAN

A variation of retroreflective scan is polarized retroreflective  scan. Polarizing filters are placed in front of the emitter and receiver lenses. The polarizing filter projects the emitter’s beam in one plane only. This light is said to be polarized. A corner-cube reflector must be used to rotate the light reflected back to the receiver. The polarizing filter on the receiver allows rotated light to pass through to the receiver. In comparison to retroreflective scan, polarized retroreflective scan works well when trying to detect shiny objects.

DIFFUSE SCAN
The emitter and receiver are in one unit. Light from the emitter strikes the target and the reflected light is diffused from the surface at all angles. If the receiver receives enough reflected
light the output will switch states. When no light is reflected back to the receiver the output returns to its original state. In diffuse scanning the emitter is placed perpendicular to the target. The receiver will be at some angle in order to receive some of the scattered (diffuse) reflection. Only a small amount of light will reach the receiver, therefore, this technique has an effective range of about 40”

DIFFUSE SCAN CORRECTION FACTOR

The specified sensing range of diffuse sensors is achieved by  using a matte white paper. The following correction values may be applied to other surfaces. These values are guidelines only
and some trial and error may be necessary to get correct operation.
Test Card (Matte White) 100%
White Paper 80%
Gray PVC 57%
Printed Newspaper 60%
Lightly Colored Wood 73%
Cork 65%
White Plastic 70%
Black Plastic 22%
Neoprene, Black 20%
Automobile Tires 15%
Aluminum, Untreated 200%
Aluminum, Black Anodized 150%
Aluminum, Matte (Brushed Finish) 120%
Stainless Steel, Polished 230%

DIFFUSE SCAN WITH BACKGROUND SUPPRESSION

Diffuse scan with background suppression is used to detect objects up to a certain distance. Objects beyond the specified distance are ignored. Background suppression is accomplished
with a position sensor detector (PSD). Reflected light from the target hits the PSD at different angles, depending on the distance of the target. The greater the distance the narrower the angle of the reflected light.

DIFFUSE SCAN EFFECTIVE BEAM

The effective beam is equal to the size of the target when located in the beam pattern.

 OPERATING MODES

There are two operating modes: dark operate (DO) and light operate (LO). Dark operate is an operating mode in which the load is energized when light from the emitter is absent from the
receiver.

Light operate is an operating mode in which the load is energized when light from the emitter reaches the receiver.

FIBER  OPTIC
Fiber optics is not a scan technique, but another method for transmitting light. Fiber optic sensors use an emitter, receiver, and a flexible cable packed with tiny fibers that transmit light. Depending on the sensor there may be a separate cable for the emitter and receiver, or it may use a single cable. When a single cable is used, the emitter and receiver use various methods to distribute emitter and transmitter fibers within a cable. Glass fibers are used when the emitter source is infrared light. Plastic
fibers are used when the emitter source is visible light.

Fiber optics can be used with thru-beam, retroreflective scan, or diffuse scan sensors. In thru beam light is emitted and received with individual cables. In retroreflective and diffuse scan light is emitted and received with the same cable (bifurcated). Fiber optics is ideal for small sensing areas or small objects. Fiber optics have a shorter sensing range due to light losses in the fiber optic cables.

LASERS

Lasers are sometimes used as sensor light sources. Siemens uses Class 2 lasers which have a maximum radiant power of 1 mW. Class 2 lasers require no protective measures and a laser
protection officer is not required. However, a warning notice must be displayed when laser sensors are used. Laser sensors are available in thru-beam, diffuse scan, and diffuse scan with background suppression versions. Lasers have a high intensity visible light, which makes setup and adjustment
easy. Laser technology allows for detection of extremely small objects at a distance. The Siemens L18 sensor, for example, will detect an object of 0.03 mm at a distance of 80 cm. Examples
of laser sensor applications include exact positioning, speed detection, or checking thread thickness of 0.1 mm and over

ULTRASONIC PROXIMITY SENSORS

ULTRASONIC PROXIMITY SENSORS

John   02:14:00

Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals. When a target enters the beam the sound is reflected back to the switch, causing it to energize or deenergize the output circuit.

PIEZOELECTRIC DISK

A piezoelectric ceramic disk is mounted in the sensor surface. It can transmit and receive high-frequency pulses. A highfrequency voltage is applied to the disk, causing it to vibrate at the same frequency. The vibrating disk produces high-frequency sound waves. When transmitted pulses strike a sound-reflecting object, echoes are produced. The duration of the reflected pulse is evaluated at the transducer. When the target enters the preset operating range, the output of the switch changes state.
When the target leaves the preset operating range, the output returns to its original state.

The emitted pulse is actually a set of 30 pulses at an amplitude of 200 Kvolts. The echo can be in microvolts.

BLIND ZONE

A blind zone exists directly in front of the sensor. Depending on the sensor the blind zone is from 6 to 80 cm. An object placed in the blind zone will produce an unstable output.

RANGE DEFINITION

The time interval between the transmitted signal and the echo is directly proportional to the distance between the object and sensor. The operating range can be adjusted in terms of its width and position within the sensing range. The upper limit can be adjusted on all sensors. The lower limit can be adjusted only with certain versions. Objects beyond the upper limit do not produce a change at the output of the sensor. This is known as “blanking out the background” . On some sensors, a blocking range also exists. This is between the lower limit and the blind zone. An object in the blocking range prevents identification of a target in the operating range. There is a signal output assigned to both the operating range and the output range.

RADIATION PATTERN

The radiation pattern of an ultrasonic sensor consists of a main cone and several neighboring cones. The approximate angle of the main cone is 5°.

FREE ZONES

Free zones must be maintained around the sensor to allow for neighboring cones. The following examples show the free area required for different situations.

OPERATING MODES

Sonar sensors can be setup to operate in several different modes: diffuse, reflex, and thru-beam.

DIFFUSE MODE

This is the standard mode of operation. Objects, traveling in anydirection into the operating range of the sound cone, will cause the sensor output to switch states. This mode of operation is similar to a proximity sensor.

REFLEX MODE

The reflex mode uses a reflector located in the preset operating range. The operating range is adjusted for the reflector. The pulses are bounced off the reflector and the echo pulses are returned to the sensor. When a target blocks the echo pulses the output is activated. Typically used in applications where the target is not a good sound absorber.

THROUGH BEAM MODE

Thru-beam sensors consist of a transmitter, which emits ultrasonic pulses, and a receiver. If the beam between the transmitter and the receiver is interrupted the output of the
receiver switches state.

ENVIRONMENTAL INFLUENCES

Sound travel time can be affected by physical properties of the air. This, in turn, can affect the preset operating distance of the sensor.

CAPACITIVE PROXIMITY SENSORS

CAPACITIVE PROXIMITY SENSORS

John   01:48:00

Capacitive proximity sensors are similar to inductive proximity sensors. The main difference between the two types is that capacitive proximity sensors produce an electrostatic field
instead of an electromagnetic field. Capacitive proximity switches will sense metal as well as nonmetallic materials such as paper, glass, liquids, and cloth.

The sensing surface of a capacitive sensor is formed by two concentrically shaped metal electrodes of an unwound capacitor. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. As a result, the oscillator
begins oscillating. The trigger circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the sensor changes. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the sensor output back to its original state.

STANDARD TARGET AND DIELECTRIC CONSTANT
Standard targets are specified for each capacitive sensor. The  standard target is usually defined as metal and/or water. Capacitive sensors depend on the dielectric constant of the target. The larger the dielectric number of a material the easier it is to detect. The following graph shows the relationship of the dielectric constant of a target and the sensor’s ability to detect the material based on the rated sensing distance (Sr).

The following table shows the dielectric constants of some materials. If, for example, a capacitive sensor has a rated sensing distance of 10 mm and the target is alcohol, the effective sensing distance (Sr) is approximately 85% of the rated distance, or 8.5 mm.

DETECTING THROUGH BARRIER
One application for capacitive proximity sensors is level detection through a barrier. For example, water has a much higher dielectric than plastic. This gives the sensor the ability to “see through” the plastic and detect the water.

SHIELDING

All capacitive sensors are shielded. These sensors will detect conductive material such as copper, aluminum, or conductive fluids, and nonconductive material such as glass, plastic, cloth, and paper. Shielded sensors can be flush mounted without adversely affecting their sensing characteristics. Care must be taken to ensure that this type of sensor is used in a dry environment. Liquid on the sensing
surface could cause the sensor to operate.