Motion Control - Encoder Sensor Technology

CONFIDENTIAL PWB TECHNOLOGIES ITALIA 1

Introduction to Encoder Sensor Technology


Introduction

The aim of this presentation is an overview of the encoder technology

and the main definitions about encoder signals and their characteristics

connected to sensors and media.


Encoders

Encoders provide the information of the angular position or speed of a shaft (rotary encoder) or the linear position or speed (linear encoder). There are two types of encoder:

• absolute encoder• incremental encoder.


Absolute encoders

Absolute encoders provide the information of the absolute angular position.


Incremental encoders

Incremental encoders measure only angular displacements.


Examples of signals output from incremental encoder

Analog sensor

Digital sensor


Sensor technologies

TransmissiveThe encoder media (code-wheel or code-strip) is between the light source (infrared typically) and the photo detector. The transparent windows and the black bars split the photodiode flux to the photo detector.


Transmissive encoder


Reflective encoder

The light source (infrared typically) and the photo detector are on the same side (with respect to the code wheel or code strip). The reflective and the black bars reflect and stop alternatively the photodiode flux to the photo detector.


Reflective encoder


Transmissive vs. reflective technology

Transmissive:- more space required (one opto component on each side).- less sensitivity to the media flatness.- lower sensitivity to the media position.Reflective:

- smaller space required (the components are on the same side).

- higher sensitivity to the encoder wheel flatness.- higher sensitivity to the media position.


Code wheel definitions


CPR Counts per revolution: number of bar/window pairs on the code wheel.

Optical RadiusThe distance from the code wheel center of rotation to the center of the encoder sensor.


PitchThe distance between the corresponding edges in two consecutive bars.

Bar/Window RatioThe ratio between the widths of a bar and a window.


EccentricityDistance between the center of the code wheel (the hole) and the center of the optical pattern.

Run-outThe total movement of the code wheel in a rotation: it is two times the eccentricity


Run-out and Eccentricity


Encoder output signals


One cycle (C)360 electrical degrees (°e), 1 bar/window pairPulse width (P)The number of electrical degrees when output is high during one cycle, nominally 180°or ½ a cycle.

State width (S)The number of electrical degrees between a transition in the output of a channel A and the neighboring transition of channel B.


PhaseThe number of electrical degrees between the center of the high state on channel A and the center of the high state on channel B. This value is nominally 90°.JitterThe jitter (%) measures the maximal deviation of the cycle (period) from its nominal value and it is defined as

Jitter(%)= [(Cmax-Cmin)/Cmedium] *100


Position errorLet imagine the following steps:- collection of the pitch times Ci;

- CM the mean value of Ci;- ∆Ci = [Ci-CM] deviation from CM for each i.

The position error in the position j (where j in the range from 1 to CPR) is

• P.Err(j)= jΣi=1 ∆ci

(expressed in mechanical degrees 360°= 1 round).


Position error


Note about jitterJitter (deviation of the cycle from its nominal value) is the sum of several errors (and electrical noise too) due to:· run-out (“quasi-deterministic” error)· pitch error;· bar/window ratio error;· code wheel flatness;· sensor electrical noise.


Encoder signal informationOne output channel (ch A or ch B) gives information about angular position or speed

Angular position: one cycle output transition means a rotation of 360°/CPR

Angular speed: (output frequency)/CPR = (angular rotations)/s


Phase delay between ch. A and ch. B gives information about rotation direction,

CW or CCW.

A

B

Ch A leads ch B Ch B leads ch A


One cycle (C)360 electrical degrees (°e), 1 bar and window pair corresponding to the peak to peak time.PhaseThe number of electrical degrees between the peak of the high state on ch A and the peak of the high state on ch B. This value is nominally 90°.JitterIt measures the maximal deviation of the cycle (period) from its nominal value:Jitter(%) = [(Cmax-Cmin)/Cmedium]*100


Analog vs. digital encoder

Digital:- more signal robustness against

electrical noise, dirtiness, any kind of “disturbance”

Analog:- low cost (@ low resolution)- higher performances (interpolation)- higher sensitivity to any “disturbance”

Motion Control - Encoder Sensor Technology

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