Selecting the Right Encoder for Your Application..

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Thursday, 19 July 2012 17:53

Many industrial control systems need position and speed feedback.

 

In the initial stages, the encoders consisted of potentiometers, brush encoders, magnetic encoders and rarely optical encoders and resolvers. Each device had certain limitations. The potentiometers and magnetic encoders had limited resolution. The brush encoders required frequent maintenance. The optical encoders used incandescent lamps, which were large in size and had limited life expectancy. The resolvers could offer better resolution and accuracy, but were very expensive due to the decoding electronics required. The recent technological developments have brought significant improvements in the initial models. Today optical encoders and resolvers are more commonly used in industry. And with the introduction of cost effective Smart-Encoders by Autotech, there will be a paradigm shift in the selection and use of encoders.

 

Types of Encoders::
Absolute and Incremental: The incremental encoder, when it rotates, generates pulses, which are counted
to give position information relative to a known point, whereas an absolute encoder provides a unique value at each position and retains actual shaft position even if power fails and the shaft moves. Incremental encoders are less complex and have fewer outputs, 2 or 3, whereas Absolute encoders typically have 12 outputs, and are generally more expensive. Incremental encoder applications typically require a reset input to zero out the count and start a fresh cycle whereas absolute encoders do not need a reset input as the output is always uniqueand absolute. Absolute encoders are also available as multi-turn units with built-in gear trains suitable for linear applications where it takes several revolutions
of the encoder shaft to complete one machine or process cycle.

Choice of Optical or Resolver::

  • Optical Encoders

The Optical Encoders typically consist of a rotating and a stationary member. The rotor is usually a metal or glass disc mounted on its shaft. The disc has an optical pattern. The stator has an LED block and phototransistors arranged so that the LED light shines through the transparent sections of the rotor disc and received by phototransistors on the other side. Incremental Optical Encoder The incremental optical encoders uses a simple disc pattern. This slotted rotor disc alternately interrupts the light beam between
the LED & phototransistor and thus produces a pulse output. The number of pulses depends on the
number of slots on the disc. The pulses are then fed to a counter, where they are counted to give position information. The pulse rate indicates shaft speed. An additional phototransistor can also determine the direction of rotation. Some models also provide a marker pulse output, which is generated once every revolution at a fixed shaft position and can be used to mark a zero reference point. Many different
pulse configurations are available, but the most commonly known is called the “quadrature”, where two square wave pulses 90° apart from each other are generated. Asssuming 360 count resolution
the encodershown has moved 150 counts clockwise. Incremental Encoder would have given 150pulses to be counted by an external device. If you lost 90° power tothe encoder at this 270° point and assuming the encoder Shaft position moved another 60° before it after power loss came to a stop the external 210°
150°  180° device would not know that the shaft position now is 210°, whereas if it was an absolute  encoder that read shaft position and provided a unique value for this position, it will read 210° upon restoration of power. Optical encoders from other vendors can be either incremental or absolute whereas resolver encoders are typically absolute. Incremental Smart-Encoders are either optical or resolver based whereas absolute Smart-Encoders use resolvers only.

Last Updated on Thursday, 19 July 2012 17:59