Glossary of Encoder
Terminology
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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
arcseconds 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.
ANGULAR MISALIGNMENT
is the maximum deviation in perpendicularity between the encoder
shaft and the face of the mounting surface. It is the total of
shaft misalignment, shaft runout and mounting face runout.
BIT
is an abbreviation for BInary digiT; it refers to the smallest
element of RESOLUTION. (See QUANTUM)
CPR
can mean either cycles/rev or counts/rev. To avoid confusion,
this term should not be used. (see
Understanding Qudrature)
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.
(cycles/rev) x (rev/sec)/1000 = kHz
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)
mks -
also referred to as µs
PPR = Pulses per revolution.
Commonly (but mistakenly) used instead of cycles/rev when
referring to QUADRATURE square wave output. (See the technical
articles
Square Waves and Pulses: A Clarification and
Understanding Quadrature)
QUADRATURE
refers to the 90-electrical-degree phase relationship between the
A and B channels of incremental encoder output. (See the
technical article
Understanding Quadrature)
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. (See the
technical article
Understanding Quadrature)
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.)
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.
