Incremental vs. Absolute Encoders: Key Differences & How to Choose
Rotary encoder guide: how incremental and absolute encoders work, signal types, and which to choose for motion-control applications.
Incremental vs. Absolute Encoders: Differences & Guide
An incremental encoder outputs pulses relative to a reference point and loses its position after power-off; an absolute encoder outputs a unique position code for every shaft angle and retains position even after power loss. Both are rotary encoders, sometimes called shaft encoders, rotational encoders, or angle encoders, and both exist in magnetic and optical versions. The right choice depends on what happens the moment a machine switches back on.
A line that waits through a homing cycle after every restart, or a machine fitted with absolute positioning it never strictly needed, is a common sign that this choice was made too quickly. It shows up directly in changeover time, safety sign-off, and bill-of-materials cost. This guide covers the principles of measuring of both types of encoders, the technology used, the way to integrate each, and the criteria to choose one of them for four common applications.
What Is an Incremental Encoder?
An incremental encoder measures movement, not position. As the shaft turns, it generates a stream of pulses on two channels, typically labeled A and B, offset by 90° from each other (quadrature). A control system counts these pulses to work out how far the shaft has rotated, and the phase relationship between A and B indicates direction. Many incremental encoders add a third channel, Z, which fires once per revolution as an index or reference mark.
As the encoder can only detect changes, it has no way of knowing the shaft’s position when powering up. The encoder knows the relative movement of the shaft from some arbitrary point where the counting starts. It is why any time the machine is powered off and then back on, whether due to a power loss, emergency stop, or regular startup in the morning, a homing cycle is required. The homing cycle entails moving the axis to a physical reference (limit switch, mechanical stop, or the Z pulse) in order to regain trust in the position count.
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Output signals: Encoders that produce incremental signals are usually HTL (more noise-resistant, often found in industrial environments) or TTL (used when the signal must travel over longer distances and at higher frequencies with smaller voltage swing).
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Advantages: Low cost, simple wiring, high pulse resolution available, well-suited for speed and direction measurements.
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Disadvantages: No absolute value on power up; requires homing cycle; missing pulse will cause real-position error in the end.
What Is an Absolute Encoder?
An absolute encoder assigns a unique digital code to each specific angular position of the shaft. As opposed to the incremental encoder that counts pulses from an arbitrary starting point, the absolute encoder reds the position directly, like a clock shows the time at a glance rather than requiring a count of how far the hands have moved.
Single-turn absolute encoders are designed to encode a certain angular position within a single turn of a shaft, from 0 to 360 degrees.
Multi-turn versions add a mechanism, typically a gear train or a magnetic/battery-backed counting stage, that also tracks the number of full shaft rotations.
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Advantages: Does not require homing procedure, provides immediately valid position at power-up, does not suffer from the accumulation of position errors due to missed pulses, ideal for safety-critical or critical restart axes.
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Disadvantages: Higher unit cost, typically needs a digital communication interface instead of simple pulse connection, multi-turn versions are more complicated compared to single turn.
The most important feature is position memory. Since the code is directly related to the angle of rotation of the shaft instead of accumulated position, the encoder knows exactly where it is when the power is turned back on. No need to home, to locate the reference point, no delays, no risks while the axis is moving without feedback in order to locate its zero position.
Encoder type, installation conditions and environment limitations
Magnetic vs. Optical Encoders: Choosing the Sensing Technology
The choice of whether to use incremental or absolute encoders is the question of what kind of feedback the encoder will provide. Sensing technology is the way it does it and this decision is independent of the first one as there are both kinds of incremental and absolute encoders using magnetic and optical sensing technologies.
Magnetic encoders read a magnetized ring instead of an optical disc, sensing changes in the magnetic field rather than interruptions in light. It means that there is no fragile optical path that can be blocked or contaminated and that is why magnetic sensing technology is much more resistant in washdown, dusty, vibration prone environments and food processing industries. It is the technology used by BEGE in its line of MIG encoders – a patented magnetic design integrated between the motor and gearbox.
Optical encoders measure position using a glass or film disc marked with patterns, breaking a beam of light as it rotates. This gives extremely fine resolution in clean and controlled environments; however, the disc and light beam itself can be compromised by any dirt, condensation, oil mists or vibrations causing misalignment.
Practical guideline: use optical when high resolution in a clean environment is needed; use magnetic when the problem is the environment.
Are Magnetic Encoders Affected by Motors Nearby?
Encoders are typically installed deep inside a machine, near the motor, and this raises a fair question: do strong motor magnetic fields interfere with the reading? The answer is that modern magnetic encoders are designed to resist these external interferences.
The signal exchange between the two parts of the encoder happens at a certain frequency, which avoids confusing external magnetic fields of the nearby motor with the measurement signal. It is another reason for the ability of magnetic encoders to be manufactured thin and light, which is very important for high acceleration applications such as robotics, where every extra weight plays an important role.
Flange-Mounted vs. Hollow-Shaft Encoders
Rotary encoders may also be differentiated depending on how they mount to the drive train. A flange mounted rotary encoder integrates directly between the motor and the gearbox, coupling onto the shaft flange rather than sitting on a separate output shaft.
A hollow shaft rotary encoder slides directly over the output shaft and held in place by a torque arm. BEGE offers a range of MIG encoders which are flange-mounted in both incremental and absolute versions: no additional length is added to the drive train, the output shaft remains free and unobstructed, and the encoder is well protected between the motor and the gearbox.
Considering a flange encoder for your machine or installation? Contact our engineers and find out what the best solution is for your application.
How to Choose an Encoder: A Decision Guide
Follow these questions in order:
| Is it necessary for the machine to know its exact position immediately after power restoration? | Yes → use absolute
No → incremental may be taken into account due to the price |
| Is the homing run after each reset dangerous, slow, or physically impossible? (Robotics arms in close proximity to personnel, cranes, lifts, axes, which cannot move without sight). | Yes → use absolute
No → both types of encoders can be used; continue to question 3 |
| Is the application fast and cost-sensitive with the homing cycle being unproblematic? (Conveyors, indexing, speed regulation). | Yes → incremental is generally the best solution No → if the budget permits and greater flexibility is important in the long term, then absolute is the best fit |
| Is the environment dusty, wet, vibrating, or a washdown/ hygiene zone? | Yes → magnetic sensing No → magnetic still performs well in a clean environment; optical becomes worth considering only if the application needs the very highest resolution |
Answering these four questions in sequence will solve both issues at once: whether the encoder should be incremental or absolute, and whether it should be magnetic or optical.
When Choosing for an Incremental Encoder?
Incremental is the right solution if a homing move of a short duration after each power up is acceptable and simplicity and economy outweigh the need for the absolute position memory. These include most cases of speed regulation, conveyors and general indexing applications: where the axis has a safe repeatable method to return to the reference position (limit switches, mechanical stops, Z pulse) and loss of position in case of power down does not lead to safety concerns. In the case when the controller already reads simple pulse-train inputs and a digital bus interface is not needed, incremental is also the easier integration.
When Choosing for an Absolute Encoder?
The absolute encoder will suit applications where the precise position of the axis is required immediately after power up: no homing runs, no reference seeking and no window where the machine moves blindly. This is important for applications where the axis is safety critical (robotic arms and all other equipment that works in close proximity to personnel), hoists and lifts where moving without the reference could be dangerous and all the processes where homing cycle after each stop will incur additional downtime costs more than the higher cost of the encoder.
Find a detailed comparison of features between MIG Basic, MIG Nova+, AST and MIG INOX at our MIG encoder comparison page.
Connecting Encoders to PLCs and Servo Drives
Incremental encoders typically connect via HTL or TTL quadrature signals. They are detected by the counter card, the PLC high speed input channel, or the encoder input of the servo drive.
In case of the absolute encoders, the protocol used will generally be digital in nature. Interfaces that are commonly used include SSI and BiSS. EtherCAT or PROFINET can be used for networking on a fieldbus system to link the encoder with drives and I/O.
Ensuring that the controller or drive supports the specific interface is worth doing before specifying an absolute encoder, as this in a common integration mismatch.
Need help comparing and selecting the right encoder for your specific industrial application?