Australia's #1 industrial directory for equipment & suppliers

Selecting the right encoder for your application

Supplier: Uticor AVG
20 July, 2012

Balmoral Technologies offers one of the broadest lines of Autotech encoders in the industry — from a tiny size 15 (housing diameter to be 1.5”), industry standard size 25 (2.5” dia), to explosion proof encoders.

This is along with wide variety of position sensing technologies, electrical characteristics and communication options. This section is written to provide a reference guide to selecting the right encoder for your application

Basics

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 unique and 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 use a simple disc pattern. This slotted rotor disc alternately interrupts the light beam between the LED and 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.

Assuming a 360 count resolution, the encoder shown above (right) has moved 150 counts clockwise.

Incremental Encoder would have given 150 pulses to be counted by an external device. If you lost power tothe encoder at this point and assuming the encoder moved another 60° before it came to a stop the external 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.