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Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
Optical Encoders
-^
Any transducer that generates a coded reading of ameasurement can be termed an
encoder
-^
Shaft Encoders
are digital transducers that are
used for measuring angular displacements andvelocities.
-^
Relative advantages of digital transducers overtheir analog counterparts:^ โ High resolution (depending on the word size of the
encoder output and the number of pulses per revolutionof the encoder)
- High accuracy (particularly due to noise immunity of
digital signals and superior construction)
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
- Relative ease of adaptation in digital control systems
(because transducer output is digital) with associatedreduction in system cost and improvement of systemreliability
-^
Shaft Encoders can be classified into twocategories depending on the nature and method ofinterpretation of the output:^ โ Incremental Encoders^ โ Absolute Encoders
-^
Incremental Encoders^ โ Output is a pulse signal that is generated when the
transducer disk rotates as a result of the motion that isbeing measured.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
- At a given instant, the magnitude of each pulse signal
will have one of two signal levels (i.e., a binary state)as determined by a level detector. This signal levelcorresponds to a binary digit (0 or 1). Hence, the set ofpulse trains gives an encoded binary number at anyinstant.
- The pulse windows on the tracks can be organized into
some pattern (code) so that each of these binarynumbers corresponds to the angular position of theencoder disk at the time when the particular binarynumber is detected.
- Pulse voltage can be made compatible with some form
of digital logic (e.g., TTL)
- Direct digital readout of an angular position is possible.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
- Absolute encoders are commonly used to measure
fractions of a revolution. However, completerevolutions can be measured using an additional trackthat generates an index pulse, as in the case of anincremental encoder.
-^
Signal Generation
can be accomplished using any
one of four techniques:^ โ Optical (photosensor) method^ โ Sliding contact (electrical conducting) method^ โ Magnetic saturation (reluctance) method^ โ Proximity sensor method
-^
Method of signal interpretation and processing isthe same for all four types of signal generation.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
Schematic Diagram of an
Absolute Encoder Disk
Pattern
(a) Binary code(b) Gray code
In Binary Code, bit switching may not takeplace simultaneously.
Ambiguities in bit switching can be
avoided by using gray code. However, additional logic is neededto covert the gray-coded number to a
corresponding binary number.
Absolute
Encoders must be
powered andmonitored only when a reading istaken. Also, if areading is missed,it will not affectthe next reading.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
(Electrically Insulating Material)
Schematic Representation of a Sliding Contact Encoder
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
Proximity sensor
Magnetic induction
ferromagnetic material
Schematic Representation of a Proximity Probe Encoder
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
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Elements of the Optical Encoder^ โ The optical encoder uses an opaque disk (code disk)
that has one or more circular tracks, with somearrangement of identical transparent windows (slits) ineach track.
- A parallel beam of light (e.g., from a set of light-
emitting diodes) is projected to all tracks from one sideof the disk.
- The transmitted light is picked off using a bank of
photosensors on the other side of the disk that typicallyhas one sensor for each track.
- The light sensor could be a silicon photodiode, a
phototransistor, or a photovoltaic cell.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
- In contrast, absolute encoder disks have several rows of
tracks, equal in number to the bit size of the output dataword.
Furthermore, the track windows are not equally
spaced but are arranged in a specific pattern on eachtrack so as to obtain a binary code (or gray code) for theoutput data from the transducer.
- It follows that absolute encoders need as least as many
signal pick-off sensors as there are tracks, whereasincremental encoders need one pick-off sensor to detectthe magnitude of rotation and an additional sensor at aquarter-pitch
separation
(pitch
center-to-center
distance between adjacent windows) to identify thedirection
of
rotation,
i.e.,
the
offset
sensor
configuration
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
14
designs
of
incremental
encoders
have
two
identical tracks, one a quarter-pitch offset from theother, and the two pick-off sensors are placed radiallywithout any circumferential offset, i.e., the
offset track
configuration
- A pick-off sensor for a reference pulse is also used. -^
Signal
interpretation
depends
on
whether
the
particular optical encoder is an incremental deviceor an absolute device.^ โ We will focus on the incremental optical encoder.^ โ The
output
signals
from
either
the
offset
sensor
configuration or the offset track configuration are thesame.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
Incremental Optical Encoder Disk
Offset-Sensor Configuration
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
Incremental Encoder Pulse Signals
(a) CW rotation
(b) CCW rotation
(c) reference
Clockwise (CW) rotation:
V
1
lags V
2
by a quarter of a cycle
(i.e., a phase lag of 90
ยฐ)
Counterclockwise (CCW) rotation:V
1 leads V
2 by a quarter of a cycle
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
-^
Two methods are available for determiningvelocities using an incremental encoder:^ โ
pulse-counting method
pulse-timing method
-^
Pulse-Counting Method^ โ The pulse count over the sampling period of the digital
processor is measured and is used to calculate theangular velocity. For a given sampling period, there isa lower speed limit below which this method is not veryaccurate.
Actuators & Sensors in Mechatronics
Optical Encoders
K. Craig
- To compute the angular velocity
ฯ
, suppose that the
count during a sample period
T
is
n
pulses. Hence, the
average time for one pulse is
T/n
. If there are
N
windows on the disk, the average time for onerevolution is
NT/n
. Hence
(rad/s) = 2
n/NT
-^
Pulse-Timing Method^ โ The time for one encoder cycle is measured using a
high-frequency clock signal. This method isparticularly suitable for measuring low speedsaccurately.
- Suppose that the clock frequency is
f
Hz. If
m
cycles of
the clock signal are counted during an encoder period(interval between two adjacent windows), the time forthat encoder cycle (i.e., the time to rotate through oneencoder pitch) is given by
m/f
