DIFFERENCES IN ABSOLUTE AND INCREMENTAL ENCODERS
This article will cover the different types of encoders available and how they can be used to perform which functions.
Table of Contents
Technologies and Types of Encoders
Although there are many types of encoders, they all fall under two main sensing methods. Those being:
There are many different types of encoder measurement types within these categories such as:
There are many electromechanical technologies, such as:
It can be difficult to understand the Encoder’s information.
Use words like absolute, incremental, linear, optical, magnetic, and rotary.
To help you understand the basics, we touch on some key points.
Let’s break down these categories a bit and then explain some of the configurations.
The Linear Encoder first uses a transducer for measuring the distance between two points. These encoders may use a rod or cable to measure the distance between two points.
The transducer collects data from the cable or rod to create an output signal that corresponds to the object’s movements.
The Linear Encoder measures the distance and uses that information to determine the object’s position.
A CNC milling machine that requires precise measurement of movement is an example where a Linear Encoder could be used.
Linear encoders can be either “Absolute” (or “Incremental”) This article will briefly touch upon Absolute and Incremental measures.
Encoder for Rotary (Shaft).
The Rotary Encoder is a device that collects data and gives feedback on the object’s rotation.
Sometimes called “Shaft Encoders”, Rotary Encoders can also be known as “Shaft Encoders”. This encoder type converts an object’s angular location or motion based on the rotation of the shaft. It depends on what measurement type is used.
Absolute Rotary Encoders can measure angular positions, while Incremental Rotary Encoders can measure distance, speed, position, and more.
Rotor encoders can be used in many areas, including robotics and computer input devices such as trackballs and mice.
As previously mentioned, rotary or shaft encoders may be “Absolute” (or “Incremental”) as described.
The next encoder is called a “Position Encoder”. It is used to determine the mechanical location of an object. This is the “absolute” position.
They can also be used to determine the position change between the object and encoder. A change in position relative to the object or encoder would represent an incremental change.
For sensing tooling position and multi-axis positioning, Position Encoders are used extensively in the industry.
The Position Encoder may also be Absolute, Incremental or both.
“Optical” encoders can interpret data in pulses of light, which can then be used for determining such things as velocity, direction, and position.
A shaft is a rotating disc that has opaque segments that correspond to a specific pattern. These encoders are able to determine the movement of an object in “rotary” and “shaft” applications, as well as the precise position in “linear”, functions.
Optic encoders can be used in a variety of applications, including printers, CNC milling machines, robotics, and other types of machinery.
These encoders can be either Absolute or Incremental.
After explaining the main groups you might be able to see a pattern.
All encoders do the same thing: they produce an electric signal that can be translated into speed, position, angle, and so on.
Absolute Encoder vs. Absolute Encoder
Let’s now discuss the differences between Absolute and Incremental encoder measurements, after we have broken down each group.
We will use the Rotary encoder type to illustrate the difference between incremental and absolute measurements.
A slotted disc mounted on a shaft is used with a stationary pick-up device in a Rotary “Absolute” measurement type encoder. A unique code pattern is created when the shaft rotates. This means that every position on the shaft has a unique code pattern, and this pattern can be used to determine its exact location.
If power was lost to the encoder and the shaft is rotated, power will be restored and the encoder will continue to record absolute position. This can be shown by the unique pattern transmitted from the disc and received at the pickup.
This measurement is best for applications that require a high degree of certainty, such as safety. The encoder is able to determine its position at any time based on the unique pattern it produces.
Absolute measurement encoders are available
For measurements of short distances, single-turn encoders can be used. Multi-turn encoders would be better suited for long distances and more complicated positioning requirements.
In incremental measure encoders, the output signal is generated each time the shaft turns a specified amount. The number of revolutions that the shaft rotates is used to interpret this output signal.
When powered on, the incremental encoder starts counting at zero. There are no safeguards for the position, unlike absolute encoders.
The incremental encoder starts its count at zero during startup or power disruption. It is therefore necessary to establish a reference point for all tasks that require positioning.
Encoders for Counting Applications
This article describes the use of an encoder to count. It is an example of an incremental encoder.
Assume power is not interrupted. Turn on the conveyor and put the machine into setup mode.
The controller receives counts as the encoder turns. Let’s assume that the count range is between 0 and 10000.
This encoder is incremental so we don’t know the absolute position. We just know that every revolution registers a count of 10000.
The object will be placed on the conveyor. Once the entrance photo-eye sensor detects it, the current encoder count will be captured. Let’s assume that this number is 5232.
The count will be captured with the object exiting, and the exit photo-eye detecting it. The number is 6311. We will say that the number is 6311.
This example shows that although we don’t know the exact location of the object, the travel count from the entry to the exit is 1079.
This doesn’t mean that the object is within three inches of the exit.
We just know that the object is going to enter. A count will be taken and the object will leave. The count was again captured.
If we do not see the object exit within the allowed travel count plus or minus a Deadband, the machine will fault, and the process will be stopped.
There are many different types of encoders available and we could spend hours discussing them all.
We hope you have a better understanding of the types available and when it might be best to pick one over another.