Encoders

Introduction:

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The TAMUBot version 3.0 uses either an optical encoder or a hall effect encoder to obtain odometry information of the robot. The Parker01 robot uses an optical encoder to measure the wheel rotations and speed for each wheel.
Robots other than Parker01 have a Hall Effect encoder attached to end of the actuator.

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Parts List:

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Grayhill 63R256
Specifications:
       Standard 5 Pin, High-Resolution
       256 Cycles

Shayang Ye Industrial Co Ltd. (Magnetic) Hall effect encoder
 

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Data Sheets and Ordering Information:

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The parts are listed on the website for the manufacturer Grayhill
The page listing the planetary geared motors can be found at this link: Optical Encoder Machine Interface

This part can be ordered online with the above given specifications at the following link

The data sheets for the Optical Encoder from Grayhill and the Hall Effect Encoder can be found in the images shown below

The PDF can be found at the following links:
    Optical Encoder Series 63R from Grayhill
    Magnetic Hall Effect Encoder from Shyang Ye Industral Co. Ltd.

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    Optical Encoder:





    Magnetic Hall Effect Encoder:



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Features of Component - Optical Encoder:

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1. 256 cycles per revolution
2. 5V operating voltage at maximum of 30mA
3. Handles voltages from 5 V+/- 0.25V  to 50 V for motor control
4. Connected to the axle of the Motor assembly with a geared belt system (3:1 ratio)
5. Life of 300 million revolutions
6. Uses a Quadrature Encoder thus has two outputs
Both the sensors used

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Working of Component - Optical Encoder

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We firstly will go though the mounting process for the optical encoder.
The Optical Encoder is connected to the Parker01 using a belt system.

The parts used for the system are:
1. Optical Encoder Series 63R from Grayhill
2. Mounting bracket
3. Two gears with a 3:1 ratio
4. Belt connecting the gears

The gear mounted on the axle of the motor is three times as large as the gear attached to the optical encoder. Thus for every one rotation of the motor and axle the optical encoder rotates three times giving it a 3:1 ratio.

The figure below shows this clearly.

Figure 1: Encoder Assembly

1) The larger of the two gears is attached to the axle and is held in place by two aluminum spacers. This prevents slippage and improves the accuracy of the reading.
The gear attached to the axle is mounted during the hardware assembly and cannot be done after the motor and drive axle are in place.
The details of this mounting process is given in the Hardware Assembly section of the guide.

2) Attach the optical encoder to the mounting bracket
3) Attach the smaller gear to the optical encoder.
4) Measure the distance required by the belt to the optical encoder in order to keep the belt taunt
5) Keep the belt on the gears while mounting the bracket to the body of Parker01 while making sure the belt stays taunt

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Features of Component - Hall Effect Encoder:

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1. Supply Voltage - Min: 3.5 V Max 20 V at 5 - 10 mA
2. 5V operating voltage at maximum of 30mA
3. Reference Rise/Fall time = 0.3us
4. Max Rise/Fall times = 1.5us

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Working of Component - Hall Effect Encoder

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The Hall Effect Encoder comes mounted to the back of the motor used in another version of the robot.
The mounted Hall effect encoder saves assembling time but care must be taken while mounting the entire assembly as the entire assembly is longer than a motor without a encoder attached

Figure 2: Hall Effect Encoder Attached to Motor

The Hall Effect encoder also has quadrature output. It uses a magnetic effect called the Hall effect to measure the rotation of the motor.

 


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Working of Component - Quadrature Encoder

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The quadrature encoder used on the Parker01 is also called an Incremental Rotary encoder.


Figure 3: Circuitry and Quadrature information (The Greyhill Optical encoder uses a 5 channel pin. As shown in the image above this a pin for voltage, a ground, a no contact pin and two output pins

Both the encoders used on the Parker01 robot use quadrature encoders. These encoders allow not only measurement of rotation but also of direction of rotation
namely clockwise and counter clockwise. The Quadrature encoder is so called because it outputs two pulses that are 90 degrees out of phase. (360 degrees can be divided into 4 parts of 90 degrees each).

Two output pins refer to two channels A and B of the quadrature output.

Obtaining the position is a matter of reading the number of pulses. The resolution of the pulses is given by the number of cycles the encoder can read during each revolution.
In the case of the optical encoder this is 256 cycles per revolution where as the resolution hall effect encoder gives us approximately 10 readings per second (6 us total taken for rise and fall time).

Using these two out of phase waveforms the encoders can give us information about the direction of rotation. For example:

If A leads B, the disk is rotating in a clockwise direction.
If B leads A, then the disk is rotating in a counter-clockwise direction.

The Following table gives us a complete list of what the output means relative to the last reading.



| Reading point
V
___ ___ ___ ___ ___ ___ ___ _
Stream A|___| |___| |___| |___| |___| |___| |___| |___|
___ ___ ___ ___ ___ ___ ___ ___
Stream B__| |___| |___| |___| |___| |___| |___| |___|

Counter-clockwise <--|--> Clockwise

Current Output
Previous | (0;0) (0;1) (1;0) (1;1)
Output |-------------------------------------------------------------------------
|
(0;0) | No Change Clockwise Counter Clockwise Ignore (error)
(0;1) | Counter Clockwise No Change Ignore (error) Clockwise
(1;0) | Clockwise Ignore (error) No Change Counter Clockwise
(1;1) | Ignore (error) Counter Clockwise Clockwise No Change
Figure 4: Quadrature outputs and their meaning  (http://www.sxlist.com/techref/io/sensor/pos/enc/quadrature.htm)

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