Rotary encoder training material

Rotary Encoder Technical Information

OMRONCONFIDENTIAL

Version 1 Jan 07

1. What is Rotary Encoder

2. Types of Rotary Encoder

3. Classifications of Rotary Encoder

4. Rotary Encoder Terminologies

5. Application examples of rotary encoder

6. Omron’s Range of Rotary Encoder

7. Guidelines for selection of Rotary Encoder

Topics

1. What is Rotary Encoder?

An encoder is a sensing device that provides feedback from the physical world. It converts motion to an electrical signal which can be read by some type of control device. This signal can be used to control a conditional event.

Many encoder technologies are utilized to create the signal, such as mechanical, magnetic, resistive and optical. Currently, the most common technology employed by encoders is optical. Encoders may produce either incremental or absolute signals.

A rotary encoder is an electro-mechanical device used to convert the angular position of a shaft or axle to a digital code commonly used in robotics, rotating radar platforms & etc.

A rotary encoder uses a slotted wheel with a single LED/ photo-detector pair to generate pulses as the wheel turns.

1. What is Rotary Encoder?

2. Types of Rotary Encoder

2.1 Incremental Type

2.2 Absolute Type

2. Types of Rotary Encoder - Incremental

2.1 Incremental Type

Incremental signals provide a series of high and low waves which indicate movement from one position to the next; there is no special indication provided by the encoder to show the specific position, only an indication that the position has changed. They are devices that provide a series of periodic signals in the form of pulses due to mechanical motion of shaft revolution.

Speed of an object can be measured by counting the pulses for a period of time. To calculate angle or the distance covered, pulses are counted starting from a reference point.

Construction of Incremental Rotary Encoder

Opaque lines


Components inside Incremental Rotary Encoder

The LED emits light beam which passes through a transparent disk patterned with opaque lines.

When the photo sensor receives the light beam, it produces a sinusoidal wave form, which is transformed into a square wave or pulse train.

This pulse signal is then sent to the counter or controller which will then send the signal to produce the desired function.

How Incremental Rotary Encoder works?


2.2 Absolute Rotary Encoder

Absolute encoders use a unique "word" for each position, meaning that an absolute encoder provides both the indication that the position has changed and an indication of the absolute position of the encoder.

Provides information in the form of unique output for every movement of the shaft rotation (in Binary, BCD or Gray Code).

Uses gray code to represent each position.

Advantage over incremental encoder => Position is maintained after a power-down. The absolute position is recovered upon power-up without requiring a home cycle or any shaft rotation.

2. Types of Rotary Encoder - Absolute

Sector Contact 1 Contact 2 Contact 3 Angle

1 off off off 0° to 45°

2 off off on 45° to 90°

3 off on off 90° to 135°

4 off on on 135° to 180°

5 on off off 180° to 225°

6 on off on 225° to 270°

7 on on off 270° to 315°

8 on on on 315° to 360

Binary Coding1

2

4

5

3

6 7

8

2 bits change

1 2 3

Black sectors are ‘ON’


From sector 4 to sector 5, it shows that contact 2 and contact 3 changes from ON to OFF. However in a practical device, the contacts are never perfectly aligned. They will not switch at the same time but at different time, i.e. only 1 bit changes at a time

For example: If contact 1 switches first, followed by contact 3 and then contact 2, for example, the actual sequence of codes will be:

Sector 4: off-on-on (starting position)on-on-on (first, contact 1 switches on)

on-on-off (next, contact 3 switches off) Sector 5: on-off-off (finally, contact 2 switches off)

This behavior is undesirable and could cause the system to fail.

Sector 8Sector 7

Binary Coding


1

2

3

4

5

6 7

8

2 31

Black sectors are ‘ON’

1 bit change from 1 sector to another

Sector Contact 1 Contact 2 Contact 3 Angle

1 off off off 0° to 45°

2 off off on 45° to 90°

3 off on on 90° to 135°

4 off on off 135° to 180°

5 on on off 180° to 225°

6 on on on 225° to 270°

7 on off on 270° to 315°

8 on off off 315° to 360°

3-bit Binary-Reflected Gray code (BRGC)

Gray Coding


A system of binary counting, in which two adjacent codes differ in only one position even from sector 4 to sector 5.

The sequence of incorrect codes shown in the previous illustration cannot happen here.

Gray Coding


BCD Coding


BCD is Binary Coded Decimal; the output is represented by decimal numbers (integers) where each digit is signified by four bits

BCD Coding


Features/ model E6C3-AB5C

Resolution 6, 8, 12

Output code BCD

Output code Resolution (P/R) Code number

BCD 6 0 to 5

8 0 to 7

12 0 to 11

0

45

90

135

180

225

270

0

12

3

4

56

7315

Example: Resolution (P/R): 8 steps

In this case, each code represents rotation of 360/8= 45.


E6C-NMulti-turn model

1. By Type

Absolute

Incremental

E6A2-CCompact low cost model

E6H-CHollow Shaft Model

E6B2-CGeneral Purpose Model

E6C3-ASpace-saving model


E6A2-C

E6F-C

2. By Size

25mm dia – E6A2-C

40mm dia – E6B2-C

50mm dia – E6C2-C, E6C3-C, E6C-N, E6CP-A, E6C3-A

55mm dia – E6D-C

60mm dia – E6F-C, E6F-A

E6C-N

3. By Shaft Diameter

4mm – E6A2-C

6mm – E6B2-C, E6C2-C, E6D-C, E6CP-A

8mm – E6C3-C, E6H-C, E6C-N, E6C3-A

10mm – E6F-C, E6F-A

http://www.omron-ap.com/product_info/E6A2/rotary_E6A2_pic01_06july2001.jpg


5. By Maximum Permissible Speed

1000 r/min , 1500r/min, 5000 r/min, 6000 r/min, 10000 r/min , 12000 r/min

6. By Power Supply Voltage

5 VDC, 5 to 12 VDC, 5 to 24 VDC, 12 VDC, 12 to 24 VDC

4. By Resolution / Pulses per Rotation

Incremental Type => 10, 20, 30, 40, 50, 60, 100, 200, 300, 360, 500, 600, 720, 800, 1000, 1024, 1200, 1500, 1800, 2000, 2048, 2500, 3600, 5000, 6000

Absolute Type

i) Single-turn

ii) Multi-turn

7. By Output

Incremental => NPN, PNP, Complementary Outputs (NPN/PNP), Voltage Output, Line Driver Output, Open-collector Output

Absolute => BCD, Binary, Gray Code,


4. Rotary Encoder TerminologiesTerms Explanation

Absolute Code(See table)

1) Binary code

A pure binary code, expressed in the format 2n. Multiple bits may

change when an address changes.

2) Gray code

A code wherein only one bit changes when an address changes.

The code plate of the rotary encoder uses gray code.

3) Remainder gray code

This code is used when expressing resolutions with gray code that are

not 2n such as 36, 360, and 720. The nature of gray code is such that

when the most significant bit of the code changes from "0" to "1" and

the same size of area is used for both the larger value and the smaller

value of objects, the signal only changes by 1bit within this range when

changing from the end to the beginning of a code. This enables any resolution that is an even number to be set with gray code. Note that in

this case, the code does not begin from place 0, but from an Intermediate code, and thus when actually using a code it must first be

Shifted so that it starts from "0". The example in the code table shows

36 divisions. With respect to the change from place 31 to 32 here,


when 18 places each are taken for the objects, the code extends from

place 14 to place 49. When changing from place 49 to place 14, only

one bit changes, and we can see that the characteristic of gray code is

preserved. By shifting the code 14 places, it can be converted to a code

that starts from place 0.

4) BCD code

Binary Coded Decimal Code.

Each digit of a decimal number is expressed using a binary code.

Ambient Temperature

The ambient temperature that meets the specifications, consisting of the permitted values for the external air temperature and the temperature of the parts that contact the rotary encoder.

Backup-type Absolute Encoder (E6C-M)

A rotary encoder with an internal counter IC that can detect multiple rotation quantity. The E6C-M uses an internal capacitor and the E63-SR5C uses an internal lithium battery to back up data when the main power is off.

CW Clockwise rotation of rotation.

Viewed from the shaft axis, the shaft rotates to the right.

With the incremental type, the A phase normally leads the B phase in this rotation direction.


With the absolute type, this is the direction of code increase.

The reverse of CW rotation is counterclockwise (CCW) rotation.

Hollow Shaft Type

The rotating shaft is hollow, and the drive shaft can be directly

connected to the hollow hole to reduce length along the direction of the

shaft. A leaf spring is used as a buffer to absorb vibration from the drive

shaft.

Maximum Response Frequency

The maximum frequency at which the signal can respond.

Metal Disk The rotating slit desk in the encoder is made of metal for higher shock

tolerance than glass. Due to slit machining limitations, the metal disk

cannot be used for high-resolution functions.

Moment of Inertia

The inertia of a rotating body. This expresses the magnitude of inertia

when starting and stopping.

Output Circuit 1) Open Collector Output

An output circuit where the emitter of the output circuit transistor is common and the collector is open.


2) Voltage Output

An output circuit where the emitter of the output circuit transistor is common and a resistor is inserted between the collector and the power supply to convert the output from the collector to a voltage.

3) Line Driver Output

An output method that uses a special IC for high-speed, long-distance data transmission, which complies with the RS-422-A standard. The signal is output as a differential second signal, and thus is strong with respect to noise. A special IC called a line receiver is used to receive the signal output from a line driver

Output Duty Ratio

The ratio of the duration of H level during one period to the average

period of pulse output when the shaft is rotated at constant speed.

Output Phase The output signal count in the case of the incremental type.

Types include a 1-phase type (A phase), 2-phase type (A phase, B

phase) and a 3-phase type (A phase, B phase & Z phase). The Z

phase is a zero position signal that is output once a revolution.


Output Phase Difference

The time difference between the rise and fall of A phase & B phase

signals when the shaft is rotated. Expressed as a proportion of the

period of one signal, or as an electrical angle where one signal period

equals 360°. The difference between A phase & B phase as an

electrical angle is normally 90°.

Resolution The pulse count of an incremental signal output when the shaft

revolves once, or the absolute address count.

Rise Time/

Fall Time

The elapsed time from 10% to 90% of the output pulse.

Serial Transmission

In contrast to parallel transmission where multiple bits of data are simultaneously output, this output method outputs data serially from a single transmission line, enabling the use of less wires. The receiving device converts the signals into parallel signals.

Servo Mount This is a method of mounting the encoder whereby the servo mount fittings are used to clamp down the flange of the encoder. The position of the encoder in the direction of rotation can be adjusted, and thus this method is used to temporarily mount the encoder for adjustment to the zero position.


Shaft Capacity This is the load that can be applied to the shaft. The radial load is the load that is perpendicular to the shaft, and the thrust is the load in the direction of the shaft. Both are permitted on the shaft during rotation, and the size of the load affects the life of the bearings.

Startup Torque

The torque needed to rotate the shaft of the rotary encoder at startup.

The torque during normal rotation is normally lower than the startup torque. A shaft that has a waterproof seal has a higher startup torque.

Absolute Code Table

5. Application example of Rotary Encoder

6. Omron’s Range of Rotary Encoder

7. Guidelines for Selection of Rotary Encoder

1. Incremental Type / Absolute Type Select a type that is suitable in terms of cost vs capacity, return (or not) to home

position at startup, speed limit & noise tolerance.

2. Resolution RequiredSelect the optimal model in view of required precision and cost of machine equipment. It recommends selecting the resolution from ½ to ¼ in integrated precision of a machine.

3. External Dimensions

4. Permitted Shaft Load Take into consideration how the mounting method affects the load on the shaft and

mechanical life.

5. Maximum Permitted Number of Revolutions

6. Maximum Response Frequency

7. Degree of Protection IP50 – Dust proof only IP52 – Existence of oil & water IP64 – Oil & water resistance

8. Startup Rotational Torque of Shaft

9. Output Circuit Type For long distance transmission, line driver output is recommended.

7. Guidelines for Selection of Rotary Encoder

Maximum Response Frequency = Number of rotations

60 x Resolutions

Rotary encoder training material

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incremental encoder

selection of rotary

encoder technologies

absolute position

absolute type

absolute signals

starting position

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