How it Works?

What Is A Step Motor?

2011-11-15T16:07:00.001+04:00

A step motor (or stepper motor as they are commonly referred) is a digital device, in that digital information is processed to accomplish an end result, in this case, controlled motion. It is reasonable to assume that a step motor will faithfully follow digital instructions just as a computer is expected to. This is the distinguishing feature of a step motor.

In essence, step motors are electrical motors that are driven by digital pulses rather than a continuously applied voltage. Inherent in this concept is open-loop control, wherein a train of pulses translates into so many shaft revolutions, with each revolution requiring a given number of pulses. Each pulse equals one rotary increment, or step (hence, step motors), which is only a portion of one complete rotation.
Therefore, counting pulses can be applied to achieve a desired amount of shaft rotation. The count automatically represents how much movement has been achieved, without the need for feedback information, as would be the case in servo systems.

Precision of step motor controlled motion is determined primarily by the number of steps per revolution; the more steps, the greater the precision. For even higher precision, some step motor drivers divide normal steps into half-steps or micro-steps. Accuracy of the step motor is a function of the mechanical precision of its parts and assembly. Whatever the error that may be built into a step motor, it is noncumulative. Consequently, it can be negligible.

02K-S523W, Shaft Type 5-Phase Stepping Motor

How Do They Work?

A step motor is an electromagnetic, rotary actuator, that mechanically converts digital pulse inputs to incremental shaft rotation. The rotation not only has a direct relation to the number of input pulses, but its speed is related to the frequency of the pulses.

Between steps, the motor holds its' position (and its' load) without the aid of clutches or brakes. Thus a step motor can be precisely controlled so that it rotates a certain number of steps, producing mechanical motion through a specific distance, and then holds its load when it stops. Furthermore, it can repeat the operation any prescribed number of times. Selecting a step motor and using it advantageously depends on three criteria: desired mechanical motion, speed, and the load.

A10K-S545W-G5, Geared Type Stepping Motor
With the appropriate logic, step motors can be bi-directional, synchronous, provide rapid acceleration, stopping, and reversal, and will interface easily with other digital mechanisms. They are further characterized as having low rotor moment of inertia, no drift, and a noncumulative positioning error.
Generally step motors are operated without feedback in an open-loop fashion and sometimes match the performance of more expensive DC Servo Systems. The only inaccuracy associated with a step motor is a noncumulative positioning error measured in % of step angle.

Basic Types: Variable Reluctance, Permanent Magnet, Hybrid

Variable Reluctance (VR) - VR motors are characterized as having a soft iron multiple rotor and a wound stator. They generally operate with step angles from 5 degrees to 15 degrees at relatively high step rates, and have no detent torque (detent torque is the holding torque when no current is flowing in the motor). In Figure 5, when phase A is energized, four rotor teeth line up with the four stator teeth of phase A by magnetic attraction. The next step is taken when A is turned off and phase B is energized, rotating the rotor clockwise 15 degrees; Continuing the sequence, C is turned on next and then A again. Counter clockwise rotation is achieved when the phase order is reversed.

Permanent Magnet (PM) - PM motors differ from VR's by having permanent magnet rotors with no teeth, and are magnetized perpendicular to the axis. In energizing the four phases in sequence, the rotor rotates as it is attracted to the magnetic poles. The motor shown in Figure 6 will take 90 degree steps as the windings are energized in sequence ABCD. PM's generally have step angles of 45 or 90 degrees and step at relatively low rates, but they exhibit high torque and good damping characteristics.

Hybrid - Combining the qualities of the VR and the PM, the hybrid motor has some of the desirable features of each. They have high detent torque and excellent holding and dynamic torque, and they can operate at high stepping speeds. Normally, they exhibit step angles of 0.9 to 5 degrees. Bi-filar windings are generally supplied (as depicted in Figure 7), so that a single-source power supply can be used . If the phases are energized one at a time, in the order indicated, the rotor would rotate in increments of 1.8 degrees. This motor can also be driven two phases at a time to yield more torque, or alternately one then two then one phase, to produce half steps or 0.9 degree increments.

AH16K-G569W, Hollow Shaft Type Stepping Motor

Where Are They Used?

Although the step motor has been overshadowed in the past by servo systems for motion control, it now is emerging as the preferred technology in more and more areas. The major factor in this trend is the prevalence of digital control, and the emergence of the microprocessor.
Today we have many step motor applications all around us. They are used in printers (paper feed, print wheel), disk drives, photo-typesetting, X-Y plotters, clocks and watches, factory automation, aircraft controls, and many other applications. Ingenuity and further advances in digital technology will continue to extend the list of applications.

A16K-G569-SB, Shaft + Brake Built-in Type Stepping Motor

How Are They Controlled?

Amount, speed, and direction of rotation of a step motor are determined by appropriate configurations of digital control devices. Major types of digital control devices are: Motor Drivers, Control Links, and Controllers. These devices are employed as shown in Figure 8. The Driver accepts clock pulses and direction signals and translates these signals into appropriate phase currents in the motor. The Indexer creates the clock pulses and direction signals. The computer or PLC (programmable logic controller) sends commands to the indexer.

A140K-M599-GB5, Geared + Brake Built-in Type Stepping Motor

How the Stepper motors are made and how they operate

Part 1

Part 2

A50K-M566-RB10, Rotary Actuator + Brake Built-in Type Stepping Motor

What Is Photoelectric Sensor ?

2011-11-08T17:11:00.002+04:00

What Is Photoelectric Sensor ?

Photoelectric sensors represent perhaps the largest variety of problem solving choices in the industrial sensor market. Today ’s photoelectric technology has advanced to the point where it is common to find a sensor that will detect a target less than 1 mm in diameter while other units have a sensing range up to 60 m. These factors make them extremely adaptable in an endless array of applications. Although many configurations are available including laser-based and fiber optic sensors, all photoelectric sensors consist of a few of basic components. Each contains an emitter, which is a light source such as an LED(light emitting diode) or laser diode, a photodiode or phototransistor receiver to detect the light source, as well as the supporting electronics designed to amplify the signal relayed from the receiver.
Probably the easiest way to describe the photoelectric operating principal is: the emitter, also referred to as the sender, transmits a beam of light either visible or infrared, which in some fashion is directed to and detected by the receiver. Although many housings and designs are available they all seem to default to the basic operating principal.

Just as the basic operating principal is the same for all photoelectric families, so is identifying their output. “Dark-On” and ”“Light-On” refers to output of the sensor in relation to when the light source is hitting the receiver. If an output is present while no light is received, this would be called a “Dark On ” output. In reverse, if the output is ON while the receiver is detecting the light from the emitter, the sensor would have a “Light-On ” output. Either way, a Light On or Dark On output needs to be selected prior to purchasing the sensor unless it is user adjustable. In this case it can be decided upon during installation by either flipping a switch or wiring the sensor accordingly.
The method in which light is emitted and delivered to the receiver is the way to categorize the different photoelectric configurations. The most reliable style of photoelectric sensing is the through beam sensor. This technology separates the emitter and receiver into separate housings. The emitter provides a constant beam of light to the receiver and detection occurs when an object passing between the two breaks the beam. Even though it is usually the most reliable, it often is the least popular due to installation difficulties and cost. This is because two separate pieces (the emitter and receiver) must be purchased, wired and installed. Difficulties often arise in the installation and alignment of two pieces in two opposing locations, which may be quite a distance apart.

Through beam photoelectric sensors typically offer the longest sensing distance of photoelectric sensors. For example, units are available with a 25 m and more sensing range. Long range is especially common on newly developed photoelectric sensors such as models containing a laser diode as the emitter. Laser diodes are used to increase sensing accuracy and detect smaller objects These units are capable of transmitting a well-collimated beam with little diffusion over the sensing ranges as long as 60 m. Even over these long distances, some through beam laser sensors are capable of detecting an object 3 mm in diameter, while objects as small as .01 mm can be sensed at closer ranges. However, while precision increases with laser sensors the speed of response for laser and non-laser through beam sensors typically remain the same, around 500 Hz. An added bonus to through beam photoelectric sensors is their ability to effectively sense an object in the presence of a reasonable amount of airborne contaminants such as dirt. Yet if contaminants start to build updirectly on the emitter or receiver, the sensor does exhibit a higher probability of false triggering. To prevent false triggering from build up on the sensor face, some manufacturers incorporate an alarm output into the sensor ’s circuitry. This feature monitors the amount of light arriving on the receiver. If the amount light decreases to a certain level without a target in place, the sensor sends a warning out by means of a built in LED and/or an output wire.
A very familiar application of a through beam photoelectric sensor can be found is right in your home. Quite often, a garage door opener has a through beam photoelectric sensor mounted near the floor, across the width of the door. This sensor is making sure nothing is in the path of the door when it is closing. A more industrial application for a through beam photoelectric is detecting objects on a conveyor. An object will be detected anyplace on a conveyor running between the emitter and receiver as long as there is a gap between the objects and the sensors light does not “burn through ” the object. This is more a figurative term than literal. It refers to an object that is thin or light in color and allows the light emitted from the emitter to penetrate the target so the receiver never detects the object.

Dubai Sensor

Diffuse sensors operate under a somewhat different style than retros and through-beams although the operating principle remains the same: diffuse photoelectrics actually use the target as the “reflector”, such that detection occurs upon reflection of the light off the object back onto the receiver as opposed to an interruption of the beam. The emitter sends out a beam of light. Most often it is a pulsed infrared, visible red or laser beam, which is reflected by the target when it enters the detectable area. The beam is diffused off of the target in all directions. Part of the beam will actually return back to the receiver inside of the same housing in which the sensor originally emitted it from. Detection occurs and the output will either turn on or off (depending upon if it is Light On or Dark On) when sufficient light is reflected to the receiver. This can be commonly witnessed in airport washrooms, where a diffuse photo will detect your hands as they are placed under the faucet and the attending output will turn the water on. In this application, your hands act as the reflector.

Due to the operating principle of using the target as the reflector, diffuse photoelectrics are often at the mercy of target material and surface properties; a non-reflective target such as matte-black paper will have a significantly decreased sensing range as compared to a bright white target. But, what seems as a drawback on the surface can actually be a benefit in practice. Because diffuse sensors are somewhat color dependant, certain versions are suitable for distinguishing dark and light targets in applications that require sorting by contrast or quality control. Specialty versions of diffuse sensors are evencapable of detecting different colors. Also, with only the sensor itself to mount, installation of diffuse sensors is usually simpler than for through-beams and retros.
Deviations of sensing distances and false triggers when reflective backgrounds are present led to the development of other diffuse sensors. These new developments, allow the diffuse sensor to “see ” an object while simultaneously ignoring any objects behind it.In the simplest of terms, the sensor is looking out at specific point in the foreground and ignoring anything beyond that point. There are two ways in which this function is achieved, the first and most common is using fixed-field technology. In this technology, the emitter sends out a beam of light like a standard diffuse photoelectric sensor. In turn, the light is received by two receivers and a comparator then evaluates how the light is received. One receiver is focused on the “sweet spot ” or desired sensing location and the other on the background or long range. If the comparator finds the long-range receiver is detecting a higher intensity of reflected light, than the amount on the focused receiver, the output will not turn on. Only when the intensity of light on the focused receiver is above the long-range receiver will an output occur.

Adjustable sensing distance versions are also available. The receiver element in an adjustable-field sensor is accomplished by the use of an array of receivers and a potentiometer to electrically adjust the sensing distance.
Fixed-field and adjustable-field photoelectric sensors operate optimally at their preset “sweet spot ”. They allow for the recognition of small parts and a tight drop-off between thesensed target and cutoff point. They also offer an improvement over a standard diffuse sensors ’ difficulty in sensing different color targets. However, target material surface qualities, such a high gloss, can produce various results. In addition, highly reflective objects outside of the sensing area tend to
send enough light intensity back to the receivers for the output to trigger, especially when the receivers are electrically adjusted.
To combat these limitations, a technology known commonly as true background suppression by triangulation was developed. True background suppression sensors emit a beam of light exactly like a standard diffuse, but unlike fixed-field sensors, which rely on light intensity, background suppression units rely completely on the angle at which the beam returns to the sensor.
To accomplish this, background suppression sensors employ two or more receivers accompanied by a
focusing lens. The receivers remain in a fixed position, while the lens is mechanically adjusted to change the angle of received light. .This configuration allows for an extremely steep cutoff between target and background, sometimes as small as .1mm. Also, this is a more stable method when reflective backgrounds are present, or large target color variations are an issue: reflectivity and color affect the intensity of reflected light, not the angles of refraction used by triangulation-basedbackground suppression photos.

How Capacitive Proximity Sensor Works?

2011-10-23T17:20:00.002+04:00

Capacitive Proximity Sensor
Control Technology

Proximity Sensor

Capacitive Proximity Sensors detect all materials, including liquids, powders and metallic and non-metallic solids. These sensors are often used to control the levels of pellets, liquids and powders in production control.
Like their inductive counterparts, they are manufactured in shielded and non-shielded configurations and are available in both AC and DC power formats. The shielded models are used to detect solid products such as cartons, stacks of paper, wood or liquids through the wall of a non-metallic container. The non-shielded models are used to detect liquids or powders where the product flows around the sensor. Shielded models have a shorter detection range since part of the field is lost in the shielding process.
All detection ranges given on the following pages are for a steel target which is equal to, or larger than the diameter of the sensor. Non-conductive products, such as wood or plastic, will be detected at a reduced detection range. The range at which a product is detected is directly related to the dielectric constant of the material; the greater the dielectric constant, the greater the detection range

Shielded Configuration:		Non-shielded Configuration:

Shielded sensors have a straight-line electrical field. They scan for the presence of solids (e.g. wafers, components, PCB’s, hybrids, cartons, bottles, plastic blocks and stacks of paper) at a distance. Shielded capacitive sensors can also detect liquids through a separating wall (glass or plastic up to a maximum of 4mm thick).		Non-shielded sensors have a spherical electrical field. They are designed to touch the product with their active surface. They are used to detect mainly bulk goods or liquids (e.g. granulate, sugar, flour, corn, sand, oil, water or pastes).

Sensitivity Adjustment

All capacitive models have an adjustable sensitivity that allows the calibration to be made in the field. This allows the sensitivity to be set for the target desired; for example, the sensitivity can be reduced to ignore a glass container but still detect the liquid inside the container. Similarly, the sensitivity can be reduced to ignore a build-up of a viscous product such as honey while still detecting the level when a large amount of the product reaches the sensor. To adjust the sensitivity on a Capacitive Proximity Sensor, mount the sensor in the working position. Allow the target material to reach the position where detection should take place. If the material has not been detected, rotate the sensitivity potentiometer clockwise, with the screwdriver provided, until detection first occurs (LED will illuminate). Continue to rotate the potentiometer for another 1/4 turn. Remove the target material and ensure that the sensor turns OFF (LED will turn off). If the sensor turns OFF, leave the sensitivity at that position. If the sensor remains ON, decrease the sensitivity (counterclockwise rotation) until the sensor turns OFF. For best results, position the sensitivity potentiometer half way between these two points.

Capacitive Sensor Oscillator Circuit Capacitive proximity sensors consist of an RC-oscillator with a special multi-part sensing electrode. The electrode and the oscillator circuit have a tube connected with earth potential for lateral shielding. This enables flush mounting of sensor in metal, since the electrical field is only present in front of the sensing electrode. This field is the active zone of the sensor.
When the conductive material is removed from the active zone, the oscillator is undamped and the oscillation amplitude decreases. The amplifier os the oscillator voltage and the sensitivity of the sensor can be altered by the built-in potentiometer.
The middle electrode together with the built-in re-coupling gives very effective compensation under conditions of humidity, dust or icing. Special circuitry automatically compensates for these influences. The preset sensing distance remains nearly constant. The electrode design, along with the compensating circuitry of capacitive sensors, is a unique design, and provides performance advantages far superior to other capacitive sensors.

Capacitive Switch Block Diagram

Applications for Capacitive Proximity Sensors

Liquid Level Control

Carton Detection

Bin Level Control

Resist Liquid Level

Capacitive Proximity Sensors reliably detect liquid levels.

The packaging industry relies on Capacitive sensors to detect paper and cardboard cartons.

Capacitive Sensors can detect liquids, powders, plastic pellets and pastes for level control.

Use Capacitive Sensors to monitor resist liquid levels in pipes.

How Inductive Proximity Sensor Works?

2011-10-23T14:33:00.001+04:00

OPERATING PRINCIPLES FOR INDUCTIVE PROXIMITY SENSORS

Proximity Sensors

Inductive proximity sensors are used for non-contact detection of metallic objects. Their operating principle is based on a coil and oscillator that creates an electromagnetic field in the close surroundings of the sensing surface. The presence of a metallic object (actuator) in the operating area causes a dampening of the oscillation amplitude. The rise or fall of such oscillation is identified by a threshold circuit that changes the output of the sensor. The operating distance of the sensor depends on the actuator's shape and size and is strictly linked to the nature of the material (Table 1).

Table 1. Sensitivity when different metals are present. Sn = operating distance.
Fe37 (Iron)	1 x Sn
Stainless steel	0.9 x Sn
Brass - Bronze	0.5 x Sn
Aluminum	0.4 x Sn
Copper	0.4 x Sn

Outputs:

DC Voltage

2 wire DC: These sensors contain an output amplifier with the function N.O. or N.C. that can pilot a load connected in series. In this system a residual current flows through the load even when in the open state and a voltage drop occurs to the sensor when it is in the closed state. Attention must be paid to these restrictions when selecting relays or electronic controls to be used with these sensors. They are compatible with P.L.C. units.

3 & 4 wire DC: These amplified D.C. sensors contain an output amplifier. They are supplied as 3 wire with function N.O. or NC and as 4 wire with complementary outputs (NO + NC) in the types NPN and PNP. Standard version include protected against short circuit, protected against polarity and peaks created by the disconnection of inductive loads. They are compatible with P.L.C. Units

Analog & Linear: In these 3 wire amplified sensors a current or voltage output varies in proportion to the distance between the sensor and a metallic object.

NAMUR: These are 2 wire non-amplified sensors whose current varies in the presence of a metallic object. The difference between these sensors and traditional sensors is the absence of amplifier trigger stages. Their current and voltage limits allow them to be used in hazardous (explosive) environments when used with approved amplifiers. In standard applications (normal atmospheres) the sensor must be used with amplifier units ALNC, ALN2 or similar.

AC Voltage

2 wire AC: These are two-wire sensors that contain a thyristor output amplifier. In this system a residual current flows through the load even when in the open state and a voltage drop occurs to the sensor when it is in the closed state. Attention must be paid to the minimum switching current, residual current and voltage drop when selecting low consumption relays or high impedance electronic controls to be used with these sensors. They are compatible with P.L.C. Units

Definitions:
NO (normally open): A switch output that is open prohibiting current flow when an actuator is not present and closes allowing current flow when an actuator is present.
NC (normally closed): A switch output that is closed allowing current flow when no actuator is present and opens prohibiting current flow when an actuator is present.
NPN Output: Transistor output that switches the common or negative voltage to the load. The load is connected between the positive supply and the output. Current flows from the load through the output to ground when the switch output is on. Also known as current sinking or negative switching.
PNP Output: Transistor output that switches the positive voltage to the load. The load is connected between output and common. Current flows from the device's output, through the load to ground when the switch output is on. Also known as current sourcing or positive switching.
Operating Distance (Sn): The maximum distance from the sensor to a square piece of Iron (Fe 37), 1mm thick with side's = to the diameter of the sensing face, that will trigger a change in the output of the sensor. Distance will decrease for other materials and shapes. Tests are performed at 20ºC with a constant voltage supply. This distance does include a ± 10% manufacturing tolerance.
Power Supply: The supply voltage range that sensor will operate at.
Max Switching Current: The amount of continuous current allowed to flow through the sensor without causing damage to the sensor. It is given as a maximum value.
Min Switching Current: It is the minimum current value, which should flow through the sensor in order to guarantee operation.
Max Peak Current: The Max peak current indicates the maximum current value that the sensor can bear in a limited period of time.
Residual Current: The current, which flows through the sensor when it is in the open state.
Power Drain: The amount of current required to operate a sensor.
Voltage Drop: The voltage drop across a sensor when driving the maximum load.
Short Circuit Protection: Protection against damage to a sensor if the load becomes shorted.
Operating Frequency: The maximum number of on/off cycles that the device is capable of in one second. According to EN 50010, this parameter is measured by the dynamic method shown in fig. 1 with the sensor in position (a) and (b). S is the operating distance and m is the diameter of the sensor. The frequency is given by the formula in fig. 2.
Repeatability (%Sn): The variation between any values of operating distance measured in an 8 hour period at a temperature between is 15 to 30ºC and a supply voltage with a <= 5% deviation.
Hysteresis (%Sn): The distance between the "switching on" point of the actuator approach and the "switching off" point of the actuator retreat. This distance reduces false triggering. Its value is given as a percent of the operating distance or a distance. See Fig. 3
Flush Mounting: For side by side mounting of flush mount models refer to Fig. 4a. Non-flush mount models can be embedded in metal according to Fig. 4b. for side by side refer to fig. 4c. Sn = operating distance.
Protection Degree: Enclosure degree of protection according to IEC (International Electrotechnical Commission) is as follows:
IP 65: Dust tight. Protection against water jets.
IP 67: Dust tight. Protection against the effects of immersion

Fig. 1

Fig. 2

Fig.3

PH meter

2011-10-19T12:30:00.002+04:00

PH meters

From Wikipedia, the free encyclopedia

ApH meter

A pH meter is an electronic instrument used for measuring the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.
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The probe

The pH probe measures pH as the activity of hydrogen cations surrounding a thin-walled glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that is measured and displayed as pH units by the meter. For more information about pH probes, see glass electrode.
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The meter

The meter circuit is no more than a voltmeter that displays measurements in pH units instead of volts. The input impedance of the meter must be very high because of the high resistance — approximately 20 to 1000 MΩ — of the glass electrode probes typically used with pH meters. The circuit of a simple pH meter usually consists of operational amplifiers in an inverting configuration, with a total voltage gain of about −17. The inverting amplifier converts the small voltage produced by the probe (+0.059 volt/pH) into pH units, which are then offset by seven volts to give a reading on the pH scale. For example:

At neutral pH (pH 7) the voltage at the probe's output is 0 volts. 0 * 17 + 7 = 7.
At basic pH, the voltage at the probe's output ranges from +0 to +0.41 volts (7 * 0.059 = 0.41). So for a sample of pH 10 (3 pH units above neutral), 3 * 0.059 = 0.18 volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
At acid pH, the voltage at the probe's output ranges from −0.41 volts to −0. So for a sample of pH 4 (3 pH units below neutral), −3 * 0.059 = −0.18 volts, the output of the meter's amplifier is −0.18 * 17 + 7 = 4.

The two basic adjustments performed at calibration (see below) set the gain and offset of the inverting amplifier.
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Calibration and use

For very precise work the pH meter should be calibrated before each measurement. For normal use calibration should be performed at the beginning of each day. The reason for this is that the glass electrode does not give a reproducible e.m.f. over longer periods of time.
Calibration should be performed with at least two standard buffer solutions that span the range of pH values to be measured. For general purposes buffers at pH 4 and pH 10 are acceptable. The pH meter has one control (calibrate) to set the meter reading equal to the value of the first standard buffer and a second control (slope) which is used to adjust the meter reading to the value of the second buffer. A third control allows the temperature to be set. Standard buffer sachets, which can be obtained from a variety of suppliers, usually state how the buffer value changes with temperature.
For more precise measurements, a three buffer solution calibration is preferred. As pH 7 is essentially, a "zero point" calibration (akin to zeroing or taring a scale or balance), calibrating at pH 7 first, calibrating at the pH closest to the point of interest ( e.g. either 4 or 10) second and checking the third point will provide a more linear accuracy to what is essentially a non-linear problem. Some meters will allow a three point calibration and that is the preferred scheme for the most accurate work. Higher quality meters will have a provision to account for temperature coefficient correction, and high-end pH probes have temperature probes built in.
The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale. After each single measurement, the probe is rinsed with distilled water or deionized water to remove any traces of the solution being measured, blotted with a scientific wipe to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in another solution.
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Storage conditions of the glass probes

When not in use, the glass probe tip must be kept wet at all times to avoid the pH sensing membrane dehydration and the subsequent dysfunction of the electrode.
A glass electrode alone (i.e., without combined reference electrode) is typically stored immersed in an acidic solution of around pH 3.0. In an emergency, acidified tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water "sucks" ions out of the probe membrane through diffusion, which degrades it.
Combined electrodes (glass membrane + reference electrode) are better stored immersed in the bridge electrolyte (often KCl 3 M) to avoid the diffusion of the electrolyte (KCl) out of the liquid junction.
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Cleaning and troubleshooting of the glass probes

Occasionally (about once a month), the probe may be cleaned using pH-electrode cleaning solution; generally a 0.1 M solution of hydrochloric acid (HCl) is used,^[1] having a pH of about one.
In case of strong degradation of the glass membrane performance due to membrane poisoning, diluted hydrofluoric acid (HF < 2 %) can be used to quickly etch (< 1 minute) a thin damaged film of glass. Alternatively a dilute solution of ammonium fluoride (NH₄F) can be used. To avoid unexpected problems, the best practice is however to always refer to the electrode manufacturer recommendations or to a classical textbook of analytical chemistry.
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Types of pH meters

A simple pH meter

pH meters range from simple and inexpensive pen-like devices to complex and expensive laboratory instruments with computer interfaces and several inputs for indicator and temperature measurements to be entered to adjust for the slight variation in pH caused by temperature. Specialty meters and probes are available for use in special applications, harsh environments, etc.
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History

The first commercial pH meters were built around 1936 by Radiometer in Denmark and by Arnold Orville Beckman in the United States. While Beckman was an assistant professor of chemistry at the California Institute of Technology, he was asked to devise a quick and accurate method for measuring the acidity of lemon juice for the California Fruit Growers Exchange (Sunkist). Beckman's invention helped him to launch the Beckman Instruments company (now Beckman Coulter). In 2004 the Beckman pH meter was designated an ACS National Historical Chemical Landmark in recognition of its significance as the first commercially successful electronic pH meter.^[2]
In the 1970s Jenco Electronics of Taiwan designed and manufactured the first portable digital pH meter. This meter was sold under Cole-Parmer's label.

how ph meters work ?

2011-10-19T12:21:00.000+04:00

PH meters

by Chris Woodford. Last updated: July 22, 2011.

If it turns pink, it's acid I think—you probably learned thatuseful phrase once upon a time, along with the second half of thesame rhyme: "and if it turns blue, it's an alkali true." Measuringacids and alkalis (bases) with litmus paper is something pretty mucheveryone learns how to do in school. It's relatively easy to compareyour little strip of wet paper with the colors on a chart and figureout how acidic or alkaline something is on what's called thepH scale. But sometimes that's too crude a measurement. If you keeptropical fish, for example, or you're a gardener with specimens thatlike soil of a certain acidity or alkalinity, getting things wrongwith the litmus risks killing off your prized pets or your plants.That's why many people invest in a meter that can measure pHdirectly. What are pH meters and how do they work? Let's take acloser look!
Photo: US naval hospital technicians test a water sample for acidity, alkalinity, and chlorine levels. This sophisticated two-probe, digital meter is made by Hach. It can be hooked up to a computer with a USB cable to download data from its internal memory, which can store 500 measurements. Photo by Nick De La Cruz courtesy of Defense Imagery.

What is acidity?

If you're interested in measuring acidity, it helps if you knowwhat it is before you start! Most of us have only the faintest ideawhat an acid or an alkali really is. We know it's a substance thatcan "burn" our skin (though it's a chemical burn, not a heatburn), but that's about it. What's even more confusing is that we cansafely eat some acidic things (lemons, for example, contain citricacid) but not others (drinking a chemical like sulfuric acid would beextremely dangerous).
Photo: Some acids, such as lemon juice, are perfectly safe to handle;others will burn your skin and can do painful, permanent damage.
Acids and alkalis are simply chemicals that dissolve in water toform ions (atoms with too many or too few electrons). An aciddissolves in water to form positively charged hydrogen ions (H+),with a strong acid forming more hydrogen ions than a weak one. Analkali (or base) dissolves in water to form negatively chargedhydroxide ions (OH−). Again, stronger alkalis (which can burn you asmuch as strong acids) form more of those ions than weaker ones.

What does pH actually mean?

The pH (always written little p, big H) of a substance is anindication of how many hydrogen ions it forms in a certain volume ofwater. There's no absolute agreement on what "pH" actually standsfor, but most people define it as something like "power ofhydrogen" or "potential of hydrogen." Now this is where it getsconfusing for those of you who don't like math. The proper definitionof pH is that it's minus the logarithm of the hydrogen ionactivity in a solution (or, if you prefer, the logarithm of the reciprocal of thehydrogen ion activity in a solution). Gulp. Whatdoes that mean?
It's simpler than it sounds. Let's unpick it a bit at a time.Suppose you have some liquid sloshing about in your aquarium and youwant to know if it's safe for those angelfish you want to keep. Youget your pH meter and stick it into the "water" (which in realityis a mixture of water with other things dissolved in it). If thewater is very acidic, there will be lots of active hydrogen ions andhardly any hydroxide ions. If the water is very alkaline, theopposite will be true. Now if you have a thimble-full of the waterand it has a pH of 1 (it's unbelievably, instantly, fish-killinglyacidic), there will be 10 million times (10 to the power of 7, written 107) more hydrogen ionsthan there would be if the water were neutral (neither acidic noralkaline), with a pH of 7. There will be 100 million million (1014)more hydrogen ions than if the water were extremely alkaline, with apH of 14. Maybe you can start to see now where those mysterious pHnumbers come from?

Photo: The pH scale relates directly to the concentration of hydrogen ions in a solution, but notin a simple linear way. The relationship is what we call a "negative exponential": the higher the pH (lower the acidity), the fewer the hydrogen ions—but there are vastly fewer ions at high pH than at low pH.
Suppose we decide to invent a scale of acidity and start it off atvery acidic and call that 1. Then something neutral will have far fewer(one 10 millionth or 10−7 times as many hydrogen ions) andsomething alkaline will have fewer still (that's one 100 trillionth,or one 100 million millionth, or 10−14 times as many). Dealing withall these millions and billions and trillions is confusing and daftso we just take a logarithm of the number of hydrogen ions and referto the power of ten we get in each case. In other words, the pH meanssimply looking at the (probably gigantic) number of hydrogen ions,taking the power of 10, and removing the minus sign. That gives us apH of 1 for extremely acidic, pH 7 for neutral, and pH 14 forextremely alkaline. "Extremely alkaline" is another way of sayingincredibly weakly acidic.

How does a pH meter work?

If you're using litmus paper, none of this matters. The basic ideais that the paper turns a slightly different color in solutionsbetween pH 1 and 14 and, by comparing your paper to a color chart,you can simply read off the acidity or alkalinity without worryinghow many hydrogen ions there are. But a pH meter has to somehowmeasure the concentration of hydrogen ions. How does it do it?
An acidic solution has far more positively charged hydrogen ionsin it than an alkaline one, so it has greater potential toproduce an electric current in a certain situation—in other words,it's a bit like a battery that can produce a greater voltage. A pHmeter takes advantage of this and works like a voltmeter: it measuresthe voltage (electrical potential) produced by the solution whose acidity we'reinterestred in, compares it with the voltage of a known solution, and uses the differencein voltage (the "potential difference") between them to deduce the differencein pH.

Parts

A typical pH meter has two basic components: the meter itself,which can be a moving-coil meter ora digital meter (one with a numeric display), and either one or two probes that you insert intothe solution you're testing. If you have two probes, each one is anelectrode (you always need two electrodes to make a completeelectrical circuit); if you have only one probe, both of the twoelectrodes are built inside it in one handy unit. The electrodesaren't like normal electrodes (simple pieces of metal wire); each oneis a mini chemical set in its own right. The electrode that does themost important job, which is called the glass electrode, has asilver-based electrical wire suspended in a solution of potassiumchloride, contained inside a bulb made from a very specialglass coated with silica and metal salts. The other electrode iscalled the reference electrode and has a potassium chloride wiresuspended in a solution of potassium chloride.

Photo: How a pH meter works: 1 = Solution being tested; 2 = Glass electrode, coated with special silica glass, and containing potassium hydroxide; 3 = Silver electrode; 4 = Hydrogen ions interact with silica glass bulb; 5 = pH meter converts voltage (potential difference) into pH reading; 6 = Reference electrode.

Operation

How does it work? When you dip the two electrodes (or one probecontaining the two electrodes) into your solution, some of thehydrogen ions in the solution move toward the glass electrode andreplace some of the metal ions in its special glass coating. Thiscreates a tiny voltage across the glass that the silver electrodepicks up and passes to the voltmeter. Broadly speaking, the other(reference) electrode acts as a baseline or reference for themeasurement—or you can think of it as simply completing the circuit.The voltmeter measures the voltage generated by the solution anddisplays it as a pH measurement. An increase in voltage means morehydrogen ions and an increase in acidity, so the meter shows it as adecrease in pH; in the same way, a decrease in voltage means fewerhydrogen ions, more hydroxide ions, a decrease in acidity, anincrease in alkalinity, and an increase in pH.

How Rotary Encoder Work?

2011-10-05T00:30:00.000+04:00

Rotary Encoders
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed elsewhere into information such as speed, distance, RPM and position.
The output of absolute encoders indicates the current position of the shaft, making them angle transducers.

Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses, computer input devices (such as optomechanical mice and trackballs), and rotating radar platforms.

There are two main types: absolute and incremental (relative).

Absolute rotary encoder

Construction

Digital absolute encoders produce a unique digital code for each distinct angle of the shaft. They come in two basic types: optical and mechanical.

Mechanical absolute encoders

A metal disc containing a set of concentric rings of openings is fixed to an insulating disc, which is rigidly fixed to the shaft. A row of sliding contacts is fixed to a stationary object so that each contact wipes against the metal disc at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall in the gaps where the metal has been cut out. The metal sheet is connected to a source of electric current, and each contact is connected to a separate electrical sensor. The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source (i.e. switched on) and others are not (i.e. switched off).

Optical absolute encoders

The optical encoder's disc is made of glass or plastic with transparent and opaque areas. A light source and photo detector array reads the optical pattern that results from the disc's position at any one time.
This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft.
The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm).

Incremental rotary encoder

An incremental rotary encoder provides cyclical outputs (only) when the encoder is rotated. They can be either mechanical or optical. The mechanical type requires debouncing and is typically used as digital potentiometers on equipment including consumer devices. Most modern home and car stereos use mechanical rotary encoders for volume control.
Due to the fact the mechanical switches require debouncing, the mechanical type are limited in the rotational speeds they can handle. The incremental rotary encoder is the most widely used of all rotary encoders due to its low cost and ability to provide signals that can be easily interpreted to provide motion related information such as velocity and RPM.
The fact that incremental encoders use only two sensors does not compromise their accuracy. One can find in the market incremental encoders with up to 10,000 counts per revolution, or more.
There can be an optional third output: reference, which happens once every turn. This is used when there is the need of an absolute reference, such as positioning systems.

The optical type is used when higher RPMs are encountered or a higher degree of precision is required.
Incremental encoders are used to track motion and can be used to determine position and velocity. This can be either linear or rotary motion. Because the direction can be determined, very accurate measurements can be made.

	Rotary Encoder
	ENA Series Rotary Encoder
	ENC Series Rotary Encoder
	ENH Series Rotary Encoder

What is a thermocouple?

2011-10-02T13:29:00.000+04:00

Temperature Sensor

The American Society for Testing and Materials (ASTM) has defined the term thermocouple as follows:

Thermocouple, n. - in thermometry, the sensor of a thermoelectric thermometer, consisting of electrically conducting circuit elements of two different thermoelectric characteristics joined at a junction. [Vol. 14.03, E 344 - 02 § 3.1 (2007).]

Put differently, a thermocouple occurs when any two different kinds of metals joined at a junction are exposed to a temperature gradient. When the two different metals are exposed to a temperature gradient they generate a very small electrical charge, commonly measured in millivolts, that correlates to the temperature to which the elements are exposed. This phenomenon is sometimes referred to as the Seebeck effect.

Thermocouples can be made of very common materials such as iron and nickel. Thermocouples can also be made of rare and expensive materials such as platinum and rhodium.

THERMOCOUPLE TECHNICAL REFERENCE DATA

Thermocouples are temperature sensors suitable for use with any make of instrument designed or programmed for use with the same type of thermocouple. Thermocouples are based on the principle that when two dissimilar metals are joined a predictable voltage will be generated that relates to the difference in temperature between the measuring junction and the reference junction (connection to the measuring device). The selection of the optimum thermocouple type (metals used in their construction) is based on application temperature, atmosphere, required length of service, accuracy and cost. When a replacement thermocouple is required, it is of the utmost importance that the type of thermocouple type used in the replacement matches that of the measuring instrument. Different thermocouple types have very different voltage output curves. It is also required that thermocouple or thermocouple extension wire, of the proper type, be used all the way from the sensing element to the measuring element. Large errors can develop if this practice is not followed.
Wire Size of Thermocouple: Selecting the wire size used in the thermocouple sensor depends upon the application. Generally, when longer life is required for the higher temperatures, the larger size wires should be chosen. When sensitivity is the prime concern, the smaller sizes should be used.
Length of Thermocouple Probe: Since the effect of conduction of heat from the hot end of the thermocouple must be minimized, the thermocouple probe must have sufficient length. Unless there is sufficient immersion, readings will be low. It is suggested the thermocouple be immersed for a minimum distance equivalent to four times the outside diameter of a protection tube or well.
Location of Thermocouple: Thermocouples should always be in a position to have a definite temperature relationship to the work load. Usually, the thermocouple should be located between the work load and the heat source and be located approximately 1/3 the distance from the work load to the heat source.

Thermocouple Type	Names of Materials	Useful Application Range
B	Platinum30% Rhodium (+) Platinum 6% Rhodium (-)	2500 -3100F 1370-1700C
C	W5Re Tungsten 5% Rhenium (+) W26Re Tungsten 26% Rhenium (-)	3000-4200F 1650-2315C
E	Chromel (+) Constantan (-)	200-1650F 95-900C
J	Iron (+) Constantan (-)	200-1400F 95-760C
K	Chromel (+) Alumel (-)	200-2300F 95-1260C
N	Nicrosil (+) Nisil (-)	1200-2300F 650-1260C
R	Platinum 13% Rhodium (+) Platinum (-)	1600-2640F 870-1450C
S	Platinum 10% Rhodium (+) Platinum (-)	1800-2640F 980-1450C
T	Copper (+) Constantan (-)	-330-660F -200-350C

GLOSSARY OF TERMS
Cold Junction or Reference Junction - The junction generally at the measuring device that is held at a relatively constant temperature.
Cold Junction Compensation - Measures the ambient temperature at the connection of the thermocouple wire to the measuring device. This allows for accurate computation of the temperature at the hot junction by the measuring device.
Dual Element - Two thermocouple elements housed within one thermocouple hardware assembly.
Extension Wire - Wires which connect the thermocouple itself to a reference junction, i.e. controller, receiver, recorder, etc. Extension wire must be of the same type as the thermocouple. Special plugs and jacks made of the same alloys as the thermocouple should be used if a quick disconnect is required for the application.
Grounded Junction - The internal conductors of this thermocouple are welded directly to the surrounding sheath material, forming a completely sealed integral junction.
Ungrounded Junction - Although the internal thermocouple conductors are welded together they are electrically insulated from the external sheath material and are not connected to the sheath in any way. Ungrounded junction thermocouples are ideal for use in conductive solutions or wherever circuit isolation is required. Ungrounded junctions are required where the measuring instrumentation does not provide channel to channel isolation.
Exposed Junction - The thermocouple junction or measuring point is exposed without any protection assembly or tube. Exposed junction thermocouples due to their design, offer the user the fastest response time.
Hot Junction - The measuring junction.
Immersion Length - The portion of the thermocouple which is subject to the temperature which is being measured.
Measuring Junction - The junction in a thermocouple which actually measures the temperature of the object. Often referred to as the Hot Junction.
Protection Tube - A tube like assembly in which the thermocouple is installed in order to protect the element from harsh environments.
RTD - Abbreviation for Resistance Temperature Detector. It is a sensor which operates on the principle that the resistance increases with an increase in temperature at a specific rate. Commonly manufactured using a platinum resistance element. More accurate and more linear than most thermocouples and generally much more costly and slower responding.
Thermocouple - A temperature sensor based on the principle that a voltage is produced when two dissimilar metals. The junction produces a voltage in proportion to the difference in temperature between the measuring junction and the reference junction.
Thermowell - A threaded or flanged closed end tube which is mounted directly to the process or vessel, designed to protect the thermocouple from the process surroundings.

Thermocouple Color Codes:

Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown.

Bj Photoelectric Sensor How it Work

2011-10-02T13:03:00.000+04:00

Bj Serios Photoelectric Sensor

Autonics BJ Series allows more diverse user convenience by adding its line-up with connector type models to existing long sensing distance type with cable outgoing models. Connector type model is ideal for easy maintenance and simple wiring work comparing to cable outgoing type models; in addition, high performance can be guaranteed with IP67 rated waterproof structure.

BJ Series realizes long sensing distance due to newly developed optical lens and superior noise-resistance characteristics. Also, the series implements world-best class sensing performance by minimizing the effect of inverter disturbance light. In addition, it makes adjacent installation possible with mutual interference prevention function and its compact size perfectly supports narrow space installation. A wide range of model line-up including long distance detection type, BGS reflective type, micro spot type and transparent glass sensing type make possible to support more various user’s applications.


BJ Series	BJ Series