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Absolute
Encoder
Type of encoder providing a unique binary word for each position. |
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Absolute
Waveform
A binary output waveform which provides a unique parallel word
code pattern for every position increment of the encoder shaft, corresponding
to the resolution of the encoder. Typical parallel waveforms are Gray
Code, Natural Binary, and Binary Coded Decimal |
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Accuracy
This is the deviation between the actual position and the theoretical
position of each bit edge. The base accuracy of an encoder is traceable
to the encoding disc. Accuracy is different from and is not directly
related to resolution. |
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Angular Resolution
The resolution of a rotary encoder expressed as a portion of a circle
(i.e. a resolution of 360 cycles per turn, is the same thing as an
angular resolution of 1 degree) |
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Bandwidth
The upper usable frequency range of operation for an encoder. Usually
determined by the encoder resolution, type of output driver, the type
and impedance of the cable, and the type of output receiver. |
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Binary Coded Decimal
Decimal number represented in natural binary. For example, 360 would be represented as the 12-bit binary number 0011 0110 0000. |
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Bit
In incremental encoders a bit refers to 1 quantum of data or 1 increment
of digital code. In absolute encoders a bit refers to the number of
tracks which normally equate to the power of 2 of the final resolution,
e.g., 8 bits equals 2 to the 8th power (256 positions), 12 bits equals
2 to the 12th power (4096 positions), etc. |
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Cable - Data Cable Reels
High quality custom data cable reels from BEI consist of low capacitance twisted pairs; provide excellent noise immunity; and are used with a variety of sensors, including encoders, proximity sensors, flow meters, and pressure sensors. |
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Cable-Extension Transducer
see Linear Cable Transducer |
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Complement
The inverse of a digital signal. When a data channel transitions from
the HI state to the LO state, it’s complement will transition
from LO to HI. Complementary output signals, when used with a differential
receiver can utilize common mode noise rejection (CMNR) to reduce
noise susceptibility. |
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Connectors
High quality connectors from BEI are sealed conductors and known as screw-on, military-style connectors; they accept up to 16 gage wire and have a built-in clamping strain relief; as well as solder cups on the back of the pins; they also have an environmentally-sealed backshell for waterproofing. |
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Count
Multiplication
A technique of using leading and trailing edges of incremental
output data channels to increase the resolution of the encoder. This
technique, also called quadrature detection, results in a net multiplication
of the base count by a factor of four. |
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CPT
Counts per turn, or Cycles per turn (see Resolution). |
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CPR
Cycles per Revolution (see Resolution) |
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Draw Wire Sensor
see Linear Cable Transducer |
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Dual Encoder Channels
With incremental encoders, a standard convention is to output a second data channel of position information. For simplicity, these dual channels are usually called the A channel and the B channel. A and B channels are arranged with a preset offset of 1/4 of a cycle, or quadrature. This means that the A channel goes from LO to HI and then, while it is still HI, the B channel goes from LO to HI. Shortly after the A channel goes from HI to LO, the B channel goes from HI to LO. This relationship is called A leads B |
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Edge Detection
Also called quadrature detection. A technique used in incremental
quadrature encoders whereby the controller counts the edge transitions
on the encoder signals. Counting the transitions on a single channel
doubles the resolution (two transitions for every cycle). Counting
transitions on two channels in quadrature results in four times the
base resolution. |
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Encoder
An electro-opto-mechanical sensor that attaches to a shaft and
provides angular position information to a counter or controller.
The purpose of an encoder is to act as a position feedback device
as part of an integrated control system |
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Excess Gray Code
Gray code that does not begin at zero. |
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Explosion-proof
Absolute Encoder  (PDF, 6 pgs/445k)
Explosion-proof
Incremental Encoder (PDF, 4 pgs/429k)
An encoder in which the housing has been designed to contain an
explosion in a hazardous environment. Explosion proof ratings are
provide and tested by international standards organizations. |
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Express Encoder
A trademark for BEI encoders indicating standard encoders that
are available with a short lead time. |
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Gray Code
A common absolute encoder output format based on binary counting, but it avoids the multiple transition problem entirely, It differs from Natural Binary in that only one bit can transition at a time. |
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Hollow
Shaft Encoder
A rotary encoder in which the rotor portion of the encoder is tubular.
The inner diameter of the rotor tube is sized to accept and clamp
to standard shaft sizes. |
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Hysteresis
A dead band purposely introduced in the encoder electronics. This
helps to prevent transition ambiguities if the system happens to dither
on a transition. |
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Incremental
Encoder
A type of encoder signal providing logic states "0" and
"1" alternately for each successive cycle of resolution. |
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Incremental Waveform
Two series of square wave cycles offset by 90 degrees whose period
is determined by the resolution and rotational speed of the encoder.
Typically a once-per-revolution index pulse as well as the inverse
of the index and data lines are also part of the standard waveform
of an incremental encoder. |
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Index
A single, separate output on an incremental encoder providing one
count per revolution. This is often gated to one or both of the count
channels, so that the index maintains the same relationship with the
count channels on every revolution. |
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Interpolation
An electronic multiplication technique for increasing encoder resolution.
Certain interpolation techniques can also increase the encoder bandwidth.
See also, Edge Detection. |
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IP Rating
The initials IP stand for Ingress Protection, as defined by the International Electrotechnical Commission (international standard IEC 60529) "Degrees of protection provided by enclosures". The standard outlines a classifcication system for the sealing effectiveness of enclosures of electrical equipment against solid objects into the equipment. The first digit is the degree of dust sealing and the second digit is the degree of water sealing. For example, an IP66 rating would mean the unit is totally protected from dust and is sealed against powerful water jets from any direction. |
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Line Count
This is often used to mean resolution. It refers to the number of
lines that are contained in the code pattern of the code disk on an
incremental encoder. |
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Line Driver
A type of encoder output. This is the general term used for a differential
output driver intended for use with a differential receiver. Line
drivers can be used to source or sink current, have a low output impedance
and, when used with a differential receiver, have a high noise immunity,
even when used with particularly long cable runs. |
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Linear Cable Transducer
A device that provides digital position and speed information by utilizing a flexible cable, a spring-loaded spool, and an incremental optical encoder to precisely measure linear position. Cable extension transducers are also known in the industry as draw wire sensors, draw wire transducers, yo yo pots, linear position string pots, string encoders, and string potentiometers.
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Motion Control
Refers to the use of prime movers (motors), feedback (encoders)
and signal processing (controllers), generally in a closed loop system
to automate a process. |
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Natural Binary Code
A common absolute encoder output format, it is constructed so the code counts up using natural sequence of binary counting. Example, the counting goes 000, 001, 010, 011, 100, and so on. The drawbacks to using this code sequence is that at several count positions, the code will have transitions on multiple bits simultaneously. Because of the normal variations caused by gate delays, line impedances, and other factors, the actual transitions don't occur simultaneously. Reading data during one of these times could result in an erroneous reading. Control systems can overcome this issue by taking multiple readings. |
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Open Collector
A type of encoder output driver that utilizes an NPN transistor. Electrically
it is a current sink and requires a pull-up resistor from the supply
voltage to the signal high line. This resistor can be internal or
external to the encoder. |
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Optical Encoder
An optical encoder is an electro-opto-mechanical sensor that attaches to a shaft and provides angular position information to a counter or controller. The optical encoder uses a sensing technology that relies on the interruption of light to detect the encoder's movement.
See A Bit About the Optical Encoder |
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Parallel Absolute Output
Each bit of the data word is output in parallel on a separate data line. For example, an 8-bit parallel output would have eight distinct data lines. Together these eight lines are capable of representing up to 256 unique positions. Resolution for absolute encoders is expressed in bits and counts, where each successive bit increases the resolution by a factor of two. 10 bits = 210 = 1,024 counts per revolution. |
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PPR
Pulses per Revolution (see Resolution). |
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Position Encoder
Another term for a rotary optical encoder. |
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Position Sensing
For encoders, this is the detection of the angular position of
the shaft relative to a known reference, usually an index pulse. |
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Position Servo
A closed loop control system in which position, rather
than velocity or acceleration is the control parameter. |
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Quadrature
Two incremental output channels that are deliberately out of phase
by 90 electrical degrees. The relationship between the two channels
is set within the encoder. A typical quadrature relationship would
be described as: A leads B for counterclockwise (CCW) rotation of
the input shaft. Quadrature allows for discrimination between direction
of movement (CW versus CCW), error detection in high vibration environments
and higher resolution by using edge detection. |
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Repeatability
The deviation of the actual encoder position produced between subsequent
identical code readings. The ability to repeat exact bit placements.
Repeatability is unrelated to resolution and is usually 4 to 10 times
better than accuracy. |
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Resolution
The number of bits or words contained in the complete code. For incremental
and tachometer encoders, resolution is defined as cycles per revolution.
For absolute single turn encoders, it is called counts per turn. Multiturn
encoders are specified as positions per turn of the input shaft and
the number of turns of the internal gear ratio. Resolution is not
the same as accuracy. |
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Rotary Optical Encoder
See A Bit About the Optical Encoder |
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Serial Synchronous Interface (SSI)
SSI is a means to transmit encoder position information on a single pair of conductors instead of one conductor per bit. A clock pulse train from a controller tells the encoder when to send out data bits: one bit of position data is transmitted to the controller per one clock pulse received by the sensor. |
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Shaft Seal
A flexible rotary sealing device incorporated into the encoder shaft
that prevents or limits environmental intrusion of dust and moisture |
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Shock-proof
Absolute Encoder (PDF, 5 pgs/448k)
Shock-proof
Incremental Encoder (PDF, 4 pgs/383k)
An encoder which has been internally shock-mounted in a heavy
duty housing to allow its use in extremely demanding environments
such as steel mills, pulp and paper processing plants, fluidized beds,
etc. |
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State
Logic level (HI, 1, true) (LO, 0, false). |
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String Encoder
see Linear Cable Transducer |
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String Pot or Potentiometer
see Linear Cable Transducer |
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Tachometer Encoder
A type of incremental encoder that generally uses only one data track
for speed indication. |
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Transition
Change of state of the digital word. This occurs at the bit or word
edge. |
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Yo Yo Pot
see Linear Cable Transducer |
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