ENCODERS

June 30, 2014


OK, what is an encoder?  Who cares?  What do they do?  Why should I know about them?  How are they used?   Let’s first start by defining the process of encoding in general.  According to the Merriam-Webster dictionary the definition of encoding is:

“to convert (as a body of information) from one system of communication into another; especially:  to convert (a message) into code”. 

Now that we have the definition, are there devices mechanical or otherwise, allowing for encoding of information from one system of communication to another system of communication?   A resounding YES!   For our purposes, an encoder is an electromechanical device that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security or compressions.  Encoders are sensors for monitoring position, angle and speed of moving mechanisms.   There are applications requiring very precise placement of components relative to a datum or mating surface.  Essentially, encoders can be categorized as rotary or linear.  Rotary encoders are sub-divided into incremental and absolute encoders.   There are many processes that require exact positioning of mechanisms, either linear or rotary.  In some applications, such as remote surgery using robotic systems, position and angle are absolutely critical.  Encoders provide this information to software and controllers.

LINEAR ENCODERS:

Linear encoders are sub-divided into wire draw and non-contact types.    A linear encoder is for frictionless length measurement and determining position and is a sensor, transducer or reading-head linked to a scale that specifies position of a part relative to a datum point.  The sensor reads the scale and converts position into an analog or digital signal that is transformed into a digital readout. Movement is determined from changes in position with time. Both optical and magnetic linear encoder types function using this type of method. However, it is their physical properties which make them different.

The JPEGs below will indicate the “hardware” typically used relative to linear encoders.

LINEAR ENCODER SPECIFICS

LINEAR (2)

ROTARY ENCODER:

 A rotary encoder also called a shaft encoder or magnetic encoder, converts angular position or motion of a shaft or axle to an analog or digital code.   A rotary encoder consists of two parts: a rotor and a sensor. The rotor turns with the shaft and contains alternating evenly spaced north and south poles around its circumference. The sensor detects these small shifts in the position N>>S and S>>N. There many methods of detecting magnetic field changes, but the two primary types used in encoders are: Hall Effect and Magneto resistive.  Hall Effect sensors work by detecting a change in voltage by magnetic deflection of electrons. Magneto resistive sensors detect a change in resistance caused by a magnetic field.

Two rotary encoder configurations may be seen as follows:

ROTARY (1)

This type of encoder would require a shaft coupling to operate.

ROTARY (2)

 

For this encoder, the shaft would be fitted into the opening shown and secured with key-seat or other fastening mechanism.

In each case, electrical connections are necessary to send encoded data to a software package then to a controller mechanism.

TYPICAL USES FOR ENCODERS:

The mechanical world would be a very different place if it were not for linear and rotary encoders.  Let’s take a look at real-life uses for both.

  • Automotive GPS and radios
  • Medical equipment
  • Audio/visual recording/mixing equipment
  • Avionics
  • Transportation equipment
  • Fitness equipment
  • Test and measurement equipment
  • Agricultural equipment
  • Construction equipment
  • Oscilloscopes
  • Pulse/signal generators

As with any technology, there are advantages and disadvantages as follows:

 ADVANTAGES:

Highly reliable and accurate
Low-cost feedback
High resolution
Integrated electronics
Fuses optical and digital technology
Can be incorporated into existing applications
Compact size

DISADVANTAGES:

Subject to magnetic or radio interference (Magnetic Encoders)
Direct light source interference (Optical Encoders)
Susceptible to dirt, oil and dust contaminates –

I might note the disadvantages can be compensated for by applying appropriate shielding and components to the overall assembly.

SUMMARY:

Sophisticated robotic systems use encoders in many places to ensure accuracy when the need to accurately position mechanisms is paramount. Users of equipment are usually oblivious to their presence.  They work silently to perform predetermine tasks as dictated by software.

 

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