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.
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.
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.
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.
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.
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.
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 up
directly 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.
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 even
capable 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 the
sensed 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 .1
mm. 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-based
background suppression photos.
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.
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
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.
---------------------------------------
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.
-------------------------------------
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.
----------------------------------------------------------
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.
------------------------------------
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.
----------------------------------------------------------------------------
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.
--------------------------------------
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.
---------------------------------------
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.
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.
Proximity Sensor
