Transcript Document

Sensors
ME 586 - Automation
Objectives
• Identify commonly used sensor types
•Where, how and why they are used
•Latest and greatest capabilities
•Bottom Line (cost)
• Where to go to find out more
ME 586 - Automation
Proximity Sensors
•Inductive
•Capacitive
•Ultrasonic
•Photoelectric
ME 586 - Automation
Inductive Sensors
•How they work
•Creates a radio frequency field using
an oscillator and a coil. The presence of
a metal object changes the field and the
sensor is able to detect this.
*Picture compliments of Baumer Electric Ltd.
ME 586 - Automation
Inductive Sensors
•Applications
•Conveyor on/off switches
•Begin machine cycle
•Quality control (sense lids, proper alignment,
etc.)
•Count, determine direction of
motion/rotation, positioning
•Anytime you want to sense metal
ME 586 - Automation
Inductive Sensors
• Advantages
•Can detect metal target even
through non-metallic barriers
•Eliminates need for contact
•Operate in harsh conditions
•Rapid response time
•Long life, virtually unlimited operating
cycles.
ME 586 - Automation
Inductive Sensors
• Limitations
•Can only detect conductive metal
•Relatively short range. Usually used for less than 1”
sensing distance.
•May be affected by metal chips collecting on sensor face.
ME 586 - Automation
Inductive Sensors
• Things
to be Aware Of
•Specified range is for axial approach. If object
approaches from the side, range is decreased.
•Range depends of metal type!!!!
•St37 ( Fe )
1
•Aluminium foil ( Al )
1
•Nickel chromium ( V2A )
0.9
•Mercury ( Hg )
0.6
•Lead, brass ( Pb, Ms )
0.5
•Aluminium ( solid )
0.45
•Copper ( Cu )
ME 586 - Automation
0.4
Inductive Sensors
• Current
Specifications
•Range: up to 40 mm
•Switching Frequency: 25 Hz to 3 kHz.
•Time delay: < 2ms
•Repeatability error: < 1% of range
•Cost: $25 to $250 (typically just under $100)
ME 586 - Automation
Capacitive Sensors
ME 586 - Automation
Capacitive Sensors
•How they work
•Uses two plates to form a linear capacitor (hence the
name). The amount of energy that can be stored between
the plates depends on the material between them. When a
material other than air is present, the sensor can detect it.
ME 586 - Automation
Capacitive Sensors
•Applications
•leak detection
•conveyors
•avoid or jam control
•robotics
•semiconductor manufacturing
•jam protection
•food processing
•positioning
•missing component detection
•parts detection or control
•bottle filling
•indexing
•bottle detection
•bottle cap or can lid detection
•thickness monitoring
•counting
•gaming table chip monitoring
•broken or damaged tool
detection
•missing unit in shipping carton detection
•liquid level control
•volume level control
•bin level in silo detection
•low paper roll monitoring
ME 586 - Automation
Capacitive Sensors
• Advantages
•Can detect aabout anything
•Can detect liquid targets through non-metallic barriers
(glass, plastic, etc.)
•Operate in harsh conditions
•Quick response time
•Can detect difference of object, not just presence
•Long operational life, with virtually unlimited cycles.
ME 586 - Automation
Capacitive Sensors
• Limitations
•Typically short range (less than 15mm)
•Affected by varying temperature, humidity and moisture
conditions
•Not as accurate as inductive proximity sensors
ME 586 - Automation
Capacitive Sensors
• Things
to be Aware Of
•Again, range depends upon direction of approach
•Range also depends on material
•Be sure to check for ambient temperature limits
ME 586 - Automation
Capacitive Sensors
• Current
Specifications
•Range: typically up to 25 mm (can be as high as
150mm!)
•Switching Frequency: up to 200 Hz
•Time delay: <=25 ms
•Repeatability: < 2% of range
•Cost: $25 - $250 (typically $80-100)
ME 586 - Automation
Ultrasonic Sensors
ME 586 - Automation
Ultrasonic Sensors
•How they work
•Sends out sound waves above audible frequencies
(ultrasonic), and listens for the return. Uses the
time delay, and the speed of sound in air to
determine distance to object. Also can be used just
to see if object is there.
ME 586 - Automation
Ultrasonic Sensors
•Different Types
Ultrasonic proximity sensor with
analog output stage
Both current and voltage outputs from the
sensor are proportional to the distance of
the sensor from the target. This allows
simple non-contact measurement
ME 586 - Automation
Ultrasonic Sensors
•Different Types
Ultrasonic retro-reflective sensor
A fixed machine part is used here as a
reflector. The time difference between the
emission and the reception of an ultrasonic
signal (known as propagation time) is
therefore fixed and known. When an object
comes within this sensing distance the
output is activated
ME 586 - Automation
Ultrasonic Sensors
•Different Types
Ultrasonic through beam sensor
These sensors are ideal for applications in
which objects follow each other in quick
succession. They are also recommended
when high switching frequencies are
required, up to 200 Hz
ME 586 - Automation
Ultrasonic Sensors
• Advantages
•Can detect more types of objects than other three types of
sensors (pretty much anything)
•Very good for telling distances
•Longer range than capacitive and inductive sensors
•Can operate in harsh conditions
•Quick response time
•Long operational life, with virtually unlimited cycles.
ME 586 - Automation
Ultrasonic Sensors
• Limitations
•Have a “dead zone” close to the face of the sensor – can’t
detect very close objects
•Can’t detect very small objects (detectable size depends on
wavelength) (except for really high tech ones – 0.076mm!)
•Speed depends on material (cotton, sponge, etc. require slower
frequencies)
•Smooth surfaced objects must be aligned correctly or echo
won’t return to sensor
ME 586 - Automation
Ultrasonic Sensors
• Current
Specifications
•Range: 50mm to 11.3m
•Sampling Frequency: up to 2 kHz (usually about
120 Hz or less, depending on distance and material)
•Maximum Target Speed: up to 400 in/sec
•Time delay: 0.5 ms
•Repeatability: 0.1% of range
•Cost: $75 – several hundred (typically just over
$100)
ME 586 - Automation
Photoelectric Sensors
ME 586 - Automation
Photoelectric Sensors
•How they work
•A photoelectric proximity switch is one in which the light source and
light sensor are housed in the same unit. The sensor picks up the pulse
of the LED (light emitting diode), which is usually in either the infrared
or visible light range, as it reflects off of the object being sensed.
ME 586 - Automation
Photoelectric Sensors
•Thru-Beam
A source unit in one location sends a light beam to a
detector unit in another location. An object is detected
when it passes between the source unit and the detector
unit, interrupting the light beam.
•Reflex
(RetroReflective)
The source and detector are housed in one package and
placed on the same side of the target object’s path.
When the object passes by, the source signal is reflected
back to the detector by a retro-reflector.
•Diffuse
Reflective
The source and detector are housed in one package and
placed on the same side of the target object’s path. When
the object passes by, the source signal is reflected back
to the detector off the target object itself.
•Background
Rejection
This is a special type of diffuse reflective sensor that
includes two detectors. This arrangement allows the
sensor to detect targets reliably within a defined range,
and to ignore objects just outside of this range. Unlike a
standard diffuse reflective sensor, color or reflectivity has
minimal effect on the sensing range.
ME 586 - Automation
Photoelectric Sensors
•Applications (just a few)
•Material Handling A sensor can ensure that products move along
a conveyor line in an orderly manner. The sensor will stop the
operation if a jam occurs. And items can be counted as they move
down the line.
•Packaging Sensors can verify that containers are filled properly,
labeled properly and have tamper-proof seals in place.
•Machine operation Sensors can watch to verify that a machine is
operating properly, materials are present and tooling is not
broken.
•Paper Industry Sensors can detect web flaws, web splice, clear
web and paper presence, while maintaining high web speeds.
In this cookie kitchen, fiber optic
photoelectric sensors are placed in a hot
oven. As long as the sensors detect
motion as the trays of cookies move by,
the oven stays on. If the conveyor stops,
the sensors will detect light or dark for too
ME 586 - Automation
Photoelectric Sensors
• Advantages
•Much greater sensing range
•Can tell how far away the object is
•Fast response time
•Typically very accurate (considering sensing range)
ME 586 - Automation
Photoelectric Sensors
• Limitations
•Don’t function well in contaminant environments
•Sometimes too powerful (Excess Gain)
•Reliability depends on object being sensed (can be too
dark, too transparent, etc.)
•More expensive
•Require more power to operate
ME 586 - Automation
Photoelectric Sensors
• Current
Specifications
•Range: up to 130m (typically between 0.5 and
10m)
•Switching Frequency: up to 1 kHz (typically 20 –
60 Hz)
•Time delay: as low as 0.5 ms (typically 8-50 ms)
•Accuracy: as good as 0.5mm or less
•Cost: very low end - $50, typical - $125-150, very
sophisticated = very expensive
ME 586 - Automation
Sensors Summary
Who Sells Them? (Thomas Register lists 120+ vendors)
Rockwell Automation
Cutler-Hammer, Sensor Div.
TURCK, Inc.
Electro Corp.
SICK, Inc.
Stedham Electronics Corp.
Baumer Electric Ltd.
Advance Controls, Inc.
Balluff, Inc.
Altech Corp.
Southern Controls, Inc.
Fargo Controls, Inc.
ME 586 - Automation
Sensors Summary
Where to Find out More?
www.theproductfinder.com/sensors/sensor.htm
(good
source for info about how they work and lists of vendors)
www.ch.cutler-hammer.com/training/slfstudy/sensors/welcome.htm
(excellent website for more technical information about various types of sensor
and their applications)
http://www.thomasregister.com/
(great source for finding vendors of a specific type of sensor)
ME 586 - Automation
Encoders
ME 586 - Automation
Objectives
•Present background and function of encoders
•Discuss where, when, and why encoders are used
•Introduce types, models, and current technology of encoders
• Delineate benefits and limitations
• Cite references and locations of further information
ME 586 - Automation
Background
A vast number of sensor products exist to detect all types of
events. There are sensors to detect the presence of objects, the
speed, the size, the structure, the color, the exact dimensions, the
location, etc. Once the detection occurs, there is also a wide variety
of ways a sensor can communicate, or convert, this information.
Analog-to-digital conversion begins with sampling, or
measuring the amplitude of the analog waveform at equally spaced
discrete instants of time.
ME 586 - Automation
As the signal is sampled the
amplitude at each interval is
quantized, and the values are mapped
into a series of binary digits, or bits.
The information is then transmitted
as a digital signal to the receiver,
where it is decoded and the analog
signal reconstituted.
ME 586 - Automation
8
2 =256
In order for a sampled signal
to be stored or transmitted in digital
form, each sampled amplitude must
be converted to one of a finite number
of possible values, or levels. For ease
in conversion to binary form, the
number of levels is usually a power of
2--that is, 8, 16, 32, 64, 128, 256, and
so on, depending on the degree of
precision required. In the figure, an
analog waveform is shown being
quantized on an 8-level scale (0
through 7).
ME 586 - Automation
Encoder example – An absolute optical encoder has 8
rings, 8 LED sensors, and 8 bit resolution. If the output
pattern is 10010110, what is the shaft’s angular position?
Ring
1
2
3
4
5
6
7
8
Angle (deg)
180
90
45
22.5
11.25
5.625
2.8125
1.40625
Pattern Value (deg)
1
180
0
0
1
22.5
0
1
5.625
1
2.8125
0
Angular Position = 180 + 22.5 + 5.625 + 2.8125
Total = 210.94
ME 586 - Automation
Absolute Encoders
The term absolute defines the
type of information that is
relayed to the processor. There
are only two options available
here, either absolute or
incremental.
The absolute encoder differs from the
incremental encoder in that each angular
location is represented by a different
digital word.
ME 586 - Automation
Absolute Encoders
In the case of the incremental encoder, it is only possible
to know your location relative to another location. The absolute
encoder solves this problem by making each
angular position unique. (An image of an
absolute encoder disk is shown to the right.)
Each separate location can be represented
by a binary number, determined by the
sequence of light transmission or blockage
as you progress inward to the center.
ME 586 - Automation
Absolute Encoders
Contrary to incremental encoders, absolute encoders
supply a clear code (information) in each angular position.
This process offers the advantage that even in case of a
power failure the actual position will be transmitted to the
evaluation electronics. Furthermore, errors of measurement
due to missing pulses and cumulative errors are excluded.
The primary advantage of the absolute encoder is
that the position is not lost in the case of power loss or
noise bursts. The largest disadvantage is added complexity
and price.
ME 586 - Automation
Absolute Encoders
Top of the Line – AstroCODER 150
• The only programmable absolute encoder that allows the user to
change programs on the fly.
• Industry leading 680 µsecond scan time virtually eliminates error,
allowing for faster machine speeds while maximizing
productivity.
• Built-in scalable resolution displays user defined units between
16 and 4096.
• Includes resolver based transducers enhancing ruggedness while
maintaining absolute position even after loss of power.
• Accepts inputs from one or two transducers providing independent
dual axis control.
• Position data available in three user selected forms: Serial Digital, Parallel Digital and Analog
Voltages.
• Factory installed Astro data latch reacts to signal from PLC thereby accommodating any
predetermined scan rate.
• Available with Windows® or DOS® based start-up software
ME 586 - Automation
Incremental Encoders
Like any other position feedback device, the incremental
encoder is used to determine rotary or linear position. The term
“incremental” describes the type of information that the encoder
sends out, being either incremental or absolute.
The encoder provides relative position information. As
rotation or linear translation occurs, the incremental encoder sends
out one pulse for each set incremental distance of travel. These
pulses can be counted to determine the linear or rotary position
relative to another position. Motion is quantified by a certain
number of pulses.
ME 586 - Automation
Incremental Encoders
Usually, the incremental encoder will come with three
channels, referred to as A, B, and Z. A and B are placed 90' out of
phase. With these two channels, the
processor determines the distance
traveled by the number of steps, and
the direction traveled by the leading
wave form. The third channel is the
reference. Usually the Z channel will have only one pulse per
revolution or per length of the encoder, so it can be used to
determine an actual location, rather than just an incremental
number. These encoders can be either magnetic, optical,
contacting, or capacitive.
ME 586 - Automation
Incremental Encoders
The disadvantage of the incremental encoder is that it is
unable to determine its location upon start-up, but this problem can
be overcome by taking the time to do a homing or reference pulse
sequence, and then moving the desired amount of steps from there.
The added expense and setup time of an absolute encoder should
be avoided unless completely necessary.
Another benefit of the
incremental encoder is the large
range of possible sizes and the high
degree of compatibility.
ME 586 - Automation
Incremental Functions: Quadrature
Incremental optical encoders generate two data
signals that are electrically 90° out of phase with each other,
as shown below. The term quadrature refers to this 90°
phase relationship. Since each full cycle contains four
transitions, or edges, an encoder that generates 2500
cycles/rev, for example, provides 10,000 edges per
revolution.
ME 586 - Automation
Quadrature
20+ years ago, the prevalent electronic circuitry of
the day was based on "edge detection". The transitions
coming from the encoder would act as the "trigger" to
cause a count. At each transition, the electronics not only
generates a count, but also determines direction of travel
so that it knows whether to count up or down. This is
done by establishing whether the transition is going high
or going low, and what the state of the other signal is.
High
Low
ME 586 - Automation
Quadrature
However, modern electronics looks not at transitions, but at changes of state.
Basically, the user's electronics contains a high-speed clock and constantly samples
the states of A and B. When it sees a change, it counts up or down based on the
following table, where 0,1 represents the states of A and B, respectively. Instead of
waiting for a triggering event from the encoder, the electronics generates its own
triggering based on its detection of a state change
FROM
TO
FROM
TO
from the encoder. A subtle difference, but critical to
the operation of modern digital circuitry.
Forward
0,1
1,1
0,1
0,0
1,1
1,0
1,0
1,1
1,0
0,0
1,0
1,1
0,0
0,1
0,0
1,0
Reverse
ME 586 - Automation
Quadrature – Pulses
(For the interested reader)
Back when people were counting edges, it was often convenient to
have the encoder vendor provide an output that not only identified a specific
number of edges per cycle (1, 2 or 4), but also gave direction information
directly. Pulse output was introduced for this purpose. Pulses differ from square
waves in 2 important ways:
•
•
Pulse widths are of fixed time duration, whereas the width of a square
wave ON state is a function of speed. (The distance between pulses is,
of course, a function of position.)
"Quadrature" has no meaning with pulse output; you get FWD pulses
on one line, and REV pulses on another. (Or pulses on one line and
direction information on the other.)
Pulse output options were fairly popular at one time, but it's been
dwindling for quite a while. With quad decode chips that are available, the
requirement has pretty much become obsolete.
ME 586 - Automation
rd
A3
Method!?
Although many companies have attempted to develop a new method of
encoding, time and again they have returned to the absolute and incremental
methods. Until now….
A new type of encoder is currently being researched by Gurley Precision
Instruments. A Gurley Virtual AbsoluteTM encoder is absolute in essence or effect
without being formally recognized as such. (That's what virtual means.) In reality,
it is neither an incremental encoder, nor an absolute encoder. It is a whole new
kind of encoder based on pseudorandom encoding technology, which has certain
details of construction similar to an incremental encoder, and certain kinds of
behavior similar to an absolute encoder. Pseudorandom output codes directly
from the disc or scale are not especially useful, so they've invented means for
decoding those signals into a natural binary format you can use like any other
encoder. This decoder (patent pending) stands in place of the quadrature decoder
and up/down counter used with an incremental, so total cost need not be much
more than an incremental encoder of comparable resolution. Yet it's effectively
absolute!
ME 586 - Automation
rd
A3
Method!?
A Virtual Absolute™ encoder
uses just cyclic and index tracks, like an
incremental encoder. However, the index
track is a serial code similar to a bar code
instead of just a single line. You do not
know position immediately upon start-up,
as you do in a conventional absolute, but
after a very short travel, in either direction
and starting from anywhere, you know
exactly where you are. In a rotary VA™
encoder, this initialization angle is
typically about one degree, depending on
the encoder's line count; in a linear VA™
encoder, about 1/2 mm motion is needed.
In a sense, from then on the encoder is
truly absolute.
ME 586 - Automation
A Virtual Absolute Encoder
rd
A3
Method!?
Advantages of the Virtual Absolute™ technology are:
•
The initialization distance or angle is a fixed and very small motion, regardless of the
starting position or direction of travel. Just "bump" it to find out where you are.
•
The encoder contains inherent built-in-test functions not found in any conventional
encoder. It reports not only various encoder malfunctions, but can also help detect
system problems such as too high a temperature or excessive speed.
•
The encoder generates the same whole-word information as a conventional absolute,
so it is very easy to interface to computers, PLC's, servo controls, etc.
•
With its simpler optics, a rotary VA™ encoder can be smaller than a conventional
absolute of equal resolution. And you can use a linear VA™ encoder for applications
where a suitable conventional absolute linear would be very hard to find.
•
Because of its simpler electronics, reduced parts count, and less critical internal
alignments, a VA™ encoder is inherently more reliable than a conventional absolute.
•
A VA™ encoder is usually dramatically less expensive than a conventional absolute.
ME 586 - Automation
Principal Types of Encoders
•Rotary (77 Companies)
•Linear (42 Companies)
•Optical (69 Companies)
•Magnetic (17 Companies)
List obtained from www.plantautomation.com
ME 586 - Automation
Rotary Encoders
ME 586 - Automation
Rotary Encoders
•How they work
Most actuator systems contain some form of rotary motion.
Often times, it is necessary to accurately locate the rotary position
of that motion. One way of accomplishing this is with a rotary
encoder. This device is used to
convert a pattern on a rotary disc into
an electrical signal which can be
processed to determine angular
position.
ME 586 - Automation
Rotary Encoders
Rotary encoders can be classified by two different characteristics:
1) technology used to convert rotary position to an electrical
signal
2) type of electrical output
Several technologies are now used to convert rotary
information into an electric signal. The original method was
through physical contacts. This created obvious limitations in
speed, resolution, and life expectancy. This led to the evolution of
optical, magnetic, and capacitive techniques. The two most
commonly used encoders today are the optical encoders and the
magnetic encoders.
ME 586 - Automation
Rotary Encoders
•Applications
The rotary encoders are most often mounted to the back of a
motor to determine the shaft position, but they are definitely not
limited to this. They can be mounted to rotary positioning tables,
screw drives, gearheads, machining tools, or any other application
where a rotary actuator exists. Many drives and motion controllers
can process common rotary encoder signals. Since
the range of rotary encoders is so broad, there is one
for almost every application requiring position
feedback.
ME 586 - Automation
Rotary Encoders
•Current Specifications
•Measurement range of up to 360°
•Contactless : no wear, no friction, high reliability
•Magnetic : high mechanical ruggedness
•Temperature range from -40°C to +85°C
•Provides absolute position
•Accuracy range of 1° to 0.05°
•Digital or analog output
•Low cost
•Built-in self-test
ME 586 - Automation
Rotary Encoders
Top of the Line – MicroE Systems G1400
FEATURES
•Miniature Sensor Package
•Line Counts from 82K to 2.68B CPR
•Safe Transmissive Design
•Broad Alignment Tolerances
APPLICATIONS
•Servo Track Writers
•Head/Media Testers
•Precision Stage Feedback
•Grating Period: 5 µm
•Resolution from 76.6 µrad to 2.37 nanoradians
•Signal Period: 2.5 µm
•Power Supply: VDC +/- 5% @100 mA, 12 VDC +/- 5% @1 mA
•Speed: 1714 rpm
ME 586 - Automation
Linear Encoders
ME 586 - Automation
Linear Encoders
•How they work
This device is used to convert
linear position information into an
electrical output signal. The linear
encoder consists of a linear tape scale
made up of glass or steel, a light source
(e.g. LED, laser), and a photoreceptor.
The light source, photoreceptor, and
additional scale are usually housed together. This housing either
surrounds the tape scale in through beam encoders or resides on
one side of the tape scale in reflective linear encoders.
ME 586 - Automation
Linear Encoders
Light is projected through or off the tape scale and is
detected by the photoreceptor. The fixed scale modulates the light
as the receptor and light source progress. The
receptor detects these modulations and
converts the input into an electrical output
usually in the form of a quadrature signal
(shown here). The two channels are always
90' out of phase. The direction of the motion
can be determined by the leading channel.
The output is the same as that of the
incremental encoder.
ME 586 - Automation
Linear Encoders
Top of the Line - GEL 221 Linear Scale IP66 - motor technology
Features
•
Magnetic sensing principle

Corrosion resistant 12 mm measuring rod

Easy mounting and adjustment

0.01 mm resolution (w/ external edge-evaluation)

200kHz maximum output frequency

Temperature range 0...+70°C or -20...+85ºC

Supply voltage 5VDC±5% or 10...35VDC

IP66 protection
ME 586 - Automation
Optical Encoders
ME 586 - Automation
Optical Encoders
•How they work
This feedback device is used to detect rotary or linear position and convert it to
an electrical output. A light source, usually either an LED or a laser, is projected through
thin slits in a rotary disc for rotary encoders, or a thin tape scale for linear. The LED is
adequate for most applications, although the laser has found niches in several high
precision, high resolution applications.
The disk and tape can either be made of
covered glass with thin etchings in the
cover, or thin metal with etchings
through it. Each has appropriate
applications. As light is transmitted, a
photo receptor on the opposite side of
the disc or tape detects the light and
converts it to an electrical output.
Different optical encoders can create a
wide range of signals, (e.g. silicon cell,
analog, sinusoidal).
ME 586 - Automation
Optical Encoders
Advantages
Optical encoders offer a higher resolution
and accuracy than all other encoders. Some
can offer in excess of 1 million counts per
Revolution (cpr). Often times the best way
to decide what feedback device you should use for
your application is to determine what type of
information your controller, PLC, smart drive, or other
processor that you are using is capable of processing
without too much trouble. Frequently many types of
feedback will fit your needs, but only a couple will be
simple to integrate. Due of the different signal options
and versatility of the optical encoder, this is a very
popular position feedback device.
ME 586 - Automation
Optical Encoders
Pos. /Description
1 Circlip
2 Washer
3 Spacer
4 Ball bearing
5 Housing
6 LED support
7 LED
8 Spacer ring
9 Codewheel
10 Stator disk
11 Printed circuit
12 Cover
13 Ribbon cable
14 Connector
ME 586 - Automation
Optical Encoders
Top of the Line - S5S single-ended optical shaft encoder
Features
•Small size
•Low cost
•Positive finger-latching connector
•2-channel quadrature,
•TTL squarewave outputs 3rd channel index option
•Tracks from 0 to 100,000 cycles/sec
•Ball bearing option tracks to 10,000 RPM
•-40 to +100°C operating temperature
The S5S single-ended optical shaft encoder is a non-contacting rotary to digital converter.
Useful for position feedback or manual interface, the encoder converts real-time shaft
angle, speed, and direction into TTL-compatible quadrature outputs with or without index.
The encoder utilizes an unbreakable mylar disk, metal shaft & bushing, LED light source,
and monolithic electronics. It may operate from a single +5VDC supply.
ME 586 - Automation
Magnetic Encoders
ME 586 - Automation
Magnetic Encoders
•How they work
This device is used to convert position information into an electrical
output that can be interpreted by a system controller. The two main components
of a magnetic encoder are the read
head and
the magnetic disc. The
read head contains a
magneto resistive
sensor, which is basically
an inductor that
detects changes in the
magnetic flux.
The disc is magnetically
coded. The
magnetic code is interpreted by
the
sensor as a series of on and off states.
One magnetic code is interpreted as a 0 bit
value and the next as a 1 bit value. Through
this combination the magnetic encoder is
able to transmit pulses representing
incremental rotary motion.
ME 586 - Automation
Magnetic Encoders
•Advantages
•The magnetic encoder offers good resolution
•can operate in a wide variety of conditions
•requires low power for operation
•Disadvantages
•they cannot achieve very high speeds
ME 586 - Automation
Magnetic Encoders
Pos. /Description
1 DC-Micromotor
2 Terminals
3 End cap
4 Housing
5 Magnet disk
6 Hall sensor
7 Printed circuit
8 Isolation
9 Cover
10 Ribbon cable
11 Connector
ME 586 - Automation
Who Sells Them? (Thomas Register lists 100+ vendors)
>ACC
>ATS
>AVG Automation
>Astrosystems Automation
>Balluff Inc
>Baumer Electric Ltd.
>Computer Conversions Corp.
>Dynamics Reseach Corp.
>Eastern Air Devices
>Globetron Electronics
>Gurley Precision Instruments
>MicroE
>Motor Technology UK Limited
>NC Servo Technology
>Omron Electronic Inc.
>Ormec Systems Corp.
>Parvex Inc.
>Quin Systems Ltd.
>Southern Power Inc
>Space Age Control Inc
>Stegmann Inc.
>U.S. Digital Corporation
ME 586 - Automation
Where to Find out More?
www.theproductfinder.com/sensors/sensor.htm
(good
source for info about how they work and lists of vendors)
http://www.gpi-encoders.com/
(excellent website for more technical information about various types of
encoders and their applications. Also source of VA encoders.)
http://www.microesys.com/
(source of several leading encoders)
http://www.thomasregister.com/
(great source for finding vendors of a specific type of sensor)
ME 586 - Automation
Glossary of Encoder Nomenclature
ACCURACY is a measure of how close the output is to where it should be. It is usually
expressed in units of distance, such as ±30 arc seconds or ±0.0001 inch. If it's expressed
as a percent, make sure to state whether it's a percent of full scale (not usually meaningful
with a rotary encoder) or a percent of nominal resolution.
BIT is an abbreviation for Binary digit; it refers to the smallest element of resolution.
CPR can mean either cycles/rev or counts/rev. To avoid confusion, this term should not be
used.
ERROR is the algebraic difference between the indicated value and the true value of the
input.
FREQUENCY RESPONSE is the encoder's electronic speed limit, expressed in kilohertz (1
kHz = 1000 Hz = 1000 cycles/sec). For calculations, rotational speed must be in rev/sec
(rps = rpm/60); linear speed must be either in/sec or mm/sec, depending on the scale line
count.
ME 586 - Automation
Glossary of Encoder Nomenclature
INDEX SIGNAL is a once-per-rev output used to establish a reference or return to a known
starting position; also called reference, marker, home, or Z
INTERPOLATION involves an electronic technique for increasing the resolution from the
number of optical cycles on the disc or scale to a higher number of quadrature square
waves per revolution or per unit length. These square waves can then be quadrature
decoded.
MEASURING STEP is the smallest resolution element; it assumes quadrature decode. (see
also QUANTUM)
PPR (pulses per revolution) Commonly (but mistakenly) used instead of cycles/rev when
referring to quadrature square wave output.
QUADRATURE refers to the 90-electrical-degree phase relationship between the A and B
channels of incremental encoder output.
ME 586 - Automation
Glossary of Encoder Nomenclature
QUADRATURE DECODE (or 4X Decode) refers to the common practice of counting all 4
quadrature states (or square wave transitions) per cycle of quadrature square waves.
Thus, an encoder with 1000 cycles/rev, for example, has a resolution of 4000 counts/rev.
QUANTIZATION ERROR is inherent in all digital systems; it reflects the fact that you
have no knowledge of how close you are to a transition. It is commonly accepted as
being equal to ±1/2 bit.
QUANTUM (plural is “quanta”) = BIT. It is the smallest resolution element. (quanta and bit
are more commonly used with absolute encoders; counts/rev or measuring steps are
more common with incremental encoders.)
REPEATABILITY is a measure of how close the output is this time to where it was last
time, for input motion in the same direction. It's not usually specified explicitly, but it is
included in the accuracy figure. (As a rule of thumb, the repeatability is generally around
1/10 the accuracy.)
ME 586 - Automation
Glossary of Encoder Nomenclature
RESOLUTION is the smallest movement detectable by the encoder. It can be expressed in
either electrical terms per distance (e.g., 3600 counts/rev or 100 pulses/mm) or in units of
distance (e.g., 0.1° or 0.01 mm).
SLEW SPEED is the maximum allowable speed from mechanical considerations. It is
independent of the maximum speed dictated by frequency response.
ME 586 - Automation
Conversion Factors
ANGULAR MEASURE
1 revolution = 360° = 21,600 minutes = 1,296,000 seconds » 2pi radians (rad)
1° = 60 minutes (min) = 3600 seconds (s) » 0.0175 rad
1 min = 60 s = 0.0167° » 0.291 mrad
1 s = 0.0167 min = 0.000278° » 4.85 µrad
1 rad » 57.3°; 1 mrad » 3.48 min; 1 µrad » 0.206 s
Sometimes the terms "arcminutes" and "arcseconds" are used to differentiate the units of angle
from the units of time. If the context makes the meaning clear, the "arc" prefix need not be used.
Occasionally, the symbols ' and " are used to indicate arcminutes and arcseconds, respectively.
Because they can be confused with feet and inches, they should not be used.
ME 586 - Automation
Conversion Factors
LINEAR MEASURE
1 foot (ft) = 12 inches (in) = 304.8 millimeters (mm)
1 in = 25.4 mm
0.001 in = 25.4 micrometer (µm)
1 meter (m) » 3.281 ft » 39.37 in
1 mm » 0.0394 in
1 µm » 39.37 µin
The terms "mil" (= 0.001 in; short for milli-inch) and "micron" (= 1 µm) should not
be used.
ME 586 - Automation
Conversion Factors
SPEED
1 rev/min (rpm) = 1/60 rev/s (rps)
1 rad/s » 57.3 deg/s » 0.159 rev/s
1 in/min » 0.423 mm/s
1 mm/min » 0.000657 in/s
ME 586 - Automation