Position Encoders - NEX ROBOTICS FIREBIRD V ATMEGA 2560 Hardware Manual

Fire Bird V ATMEGA2560 Hardware Manual

3.9 Position Encoders

Position encoders give position / velocity feedback to the robot. It is used in closed loop to
control robot's position and velocity. Position encoder consists of slotted disc which rotates
between optical encoder (optical transmitter and receiver). When slotted disc moves in between
the optical encoder we get square wave signal whose pulse count indicates position and time
period / frequency indicates velocity.
Optical encoder MOC7811 is used as position encoder on the robot. It consists of IR LED and
the photo transistor mounted in front of each other separated by a slot and encased in black
opaque casing and facing each other through narrow window. When IR light falls on the photo
transistor it gets in to saturation and gives logic 0 as the output. In absence of the IR light it gives
logic 1 as output. A slotted encoder disc is mounted on the wheel is placed in between the slot of
MOC7811. When encoder disc rotates it cuts IR illumination alternately because of which photo
transistor gives square pulse train as output. Output from the position encoder is cleaned using
Schmitt trigger based inverter (not gate) IC CD40106.
Figure 3.25: DC geared motors and position encoders
© NEX Robotics Pvt. Ltd. and ERTS Lab, CSE, IIT Bombay, INDIA 35