HEDS-8910-004 datasheet(3/15 Pages) HP

Part #	HEDS-8910-004

Description	Quick Assembly Two and Three Channel Optical Encoders

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Manufacturer	HP [Agilent(Hewlett-Packard)]

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Theory of Operation

The HEDS-5500, 5540, 5600,

5640, and HEDM-5500, 5600

translate the rotary motion of a

shaft into either a two- or a three-

channel digital output.

As seen in the block diagram,

these encoders contain a single

Light Emitting Diode (LED) as its

light source. The light is

collimated into a parallel beam by

means of a single polycarbonate

lens located directly over the

LED. Opposite the emitter is the

integrated detector circuit. This

IC consists of multiple sets of

photodetectors and the signal

processing circuitry necessary to

produce the digital waveforms.

The codewheel rotates between

the emitter and detector, causing

the light beam to be interrupted

by the pattern of spaces and bars

on the codewheel. The

photodiodes which detect these

interruptions are arranged in a

pattern that corresponds to the

radius and design of the

codewheel. These detectors are

also spaced such that a light

period on one pair of detectors

corresponds to a dark period on

the adjacent pair of detectors. The

photodiode outputs are then fed

through the signal processing

circuitry resulting in A, A, B and B

(also I and I in the HEDS-5540

and 5640). Comparators receive

these signals and produce the

final outputs for channels A and

B. Due to this integrated phasing

technique, the digital output of

channel A is in quadrature with

that of channel B (90 degrees out

of phase).

In the HEDS-5540 and 5640, the

output of the comparator for I

and I is sent to the index

processing circuitry along with

the outputs of channels A and B.

Block Diagram

revolution.

Pulse Width (P):

The number of

electrical degrees that an output

is high during 1 cycle. This value

is nominally 180

°e or 1/2 cycle.

Pulse Width Error (

∆P): The

deviation, in electrical degrees, of

the pulse width from its ideal

value of 180

°e.

State Width (S):

The number of

electrical degrees between a

transition in the output of channel

A and the neighboring transition

in the output of channel B. There

are 4 states per cycle, each

nominally 90

°e.

State Width Error (

∆S): The

deviation, in electrical degrees, of

each state width from its ideal

value of 90

°e.

Phase (

φ): The number of

electrical degrees between the

center of the high state of channel

A and the center of the high state

of channel B. This value is

nominally 90

°e for quadrature

output.

Phase Error (

∆φ): The deviation

of the phase from its ideal value

of 90

°e.

The final output of channel I is an

index pulse PO which is generated

once for each full rotation of the

codewheel. This output PO is a

one state width (nominally 90

electrical degrees), high true

index pulse which is coincident

with the low states of channels A

and B.

Definitions

Count (N):

The number of bar

and window pairs or counts per

revolution (CPR) of the

codewheel.

One Cycle (C):

360 electrical

degrees (

°e), 1 bar and window

pair.

One Shaft Rotation:

360

mechanical degrees, N cycles.

Position Error (

∆Θ): The

normalized angular difference

between the actual shaft position

and the position indicated by the

encoder cycle count.

Cycle Error (

∆C): An indication

of cycle uniformity. The differ-

ence between an observed shaft

angle which gives rise to one

electrical cycle, and the nominal

angular increment of 1/N of a

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HEDS-8910-004 Datasheet(PDF) 3 Page - Agilent(Hewlett-Packard)

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