What is an Optical Encoder & How Does it Work?

Optical encoders play a crucial role in motion control systems, helping track position, speed, and direction with precision. These devices rely on light passing through a coded disk to generate signals, which are then processed to provide movement feedback. Their accuracy and reliability make them indispensable in industries ranging from robotics to medical equipment.

If you’re selecting an optical encoder for your application, understanding how they function and their different types is essential. This guide breaks down everything you need to know about optical encoders from their working principles to their advantages over other encoder types.

What is an Optical Encoder?

An optical encoder is a motion-sensing device that detects position and movement and converts that data into a digital signal. It operates using an LED light, a photosensor, and a coded wheel to generate signals based on light interruption. These signals are then processed to determine an object’s speed, direction, and position in various applications.

Optical encoders are widely used in automation, manufacturing, and robotics due to their high precision and reliability. Whether you need to measure rotational movement in a motor or track linear displacement in a robotic arm, optical encoders provide the accuracy required for smooth operations.

How Does an Optical Encoder Work?

The working principle of an optical encoder revolves around light detection. Here’s a simplified breakdown:

• Light Emission: A beam of light is emitted from an infrared LED source.
• Coded Disk Rotation: This light passes through a rotating coded disk (patterned with transparent and opaque sections).
• Light Detection: A photosensor assembly detects the light variations as the disk rotates.
• Signal Conversion: The detected light patterns are converted into electrical signals.
• Processing Output: A controller or processing unit interprets these signals to determine motion parameters (speed, position, direction).

Since optical encoders use light-based detection, they are less prone to electromagnetic interference, making them a preferred choice in high-precision environments.

What is the Working Principle of an Optical Encoder?

Optical encoders convert the motion detected in the photosensor (via light) to an electrical signal, which is received and then transmitted as a square wave.

These square waves can then be read and turned into a signal by some type of control device in a motion control system, such as a counter or PLC. A PLC is also known as a programmable logic controller.

Broken down in steps, this is how an optical encoder provides motion feedback information:

A beam of light emitted from an LED passes through a Code Disk, a transparent disk patterned with opaque lines (much like the spokes on a bike wheel).

The light beam is picked up by a Photodetector Assembly, also called a photodiode array or a photosensor.

The Photodetector Assembly responds to the light beam, producing a sinusoidal wave form, which is transformed into a square wave or pulse train.

This pulse signal is simple:
light = on
no light = off

The pulse signal is then sent to the counter or controller through the Electronics Board.

The counter or controller (not pictured) then sends the signal to produce the proper function (stop, go, rotate, etc.).

There are different motion control systems that will receive information from the optical encoder’s output signal. These motion control systems include those designed for:

• Position control, meaning they will use the optical encoder’s information to determine where an object is in relation to, for example, a conveyer belt.
• Velocity control can be used to determine how fast an object travels, such as RPMs on a conveyer belt or other similar operations.
• Torque control is rare but can be used in robotics operations. For instance, an optical encoder can tell how much strength robotics equipment uses.

What are the Parts of an Optical Encoder?

An optical encoder is built with precision-engineered components that work together to convert motion into digital signals. Each part plays a crucial role in ensuring accurate feedback for motion control applications.

Let’s take a closer look at the four fundamental components:

1. LED Light Source

At the heart of an optical encoder is the LED light source, which emits a steady beam of infrared light. Unlike the visible LED lights used in homes, this infrared LED operates at specific wavelengths, ensuring optimal interaction with the encoder’s sensor.

The infrared light is directed toward the coded wheel, where it either passes through or is blocked, depending on the pattern of slits. The stability and intensity of the LED light directly impact the encoder’s accuracy, making high-quality infrared LEDs essential for precision applications.

Since optical encoders rely on light-based detection, they are less affected by electromagnetic interference (EMI) compared to magnetic encoders.

2. Photo Sensor Assembly (Light Detection Unit)

The photo sensor assembly is responsible for detecting the light that passes through the coded wheel. This assembly includes photodetectors or phototransistors that convert the received light into an electrical signal.

When light reaches the sensor, it creates a sinusoidal or square wave signal, which is later processed into motion data. The sensor operates at high speeds, allowing real-time tracking of position, direction, and velocity.

High-resolution encoders use advanced sensor arrays to improve signal clarity and reduce errors caused by environmental factors such as dust or vibrations.

3. Coded Wheel (Optical Disk with Slits)

The coded wheel (also called an encoder disk) is a precisely manufactured disk featuring alternating transparent and opaque sections. This rotating disk is crucial for determining position and movement.

As the disk rotates, it intermittently allows or blocks the infrared light, creating a distinctive pulse pattern. The number and arrangement of slits on the disk determine the resolution of the encoder. A higher number of slits means finer resolution and more accurate measurements.

Material composition coded wheels can be made from:

• Glass: Offers high precision but is fragile.
• Metal: Durable and resistant to wear, ideal for harsh environments.
• Resin or Plastic: Cost-effective and lightweight, used in compact encoders.

4. Signal Processing Circuitry

Once the photo sensor assembly converts light into electrical pulses, the signal processing circuitry refines and amplifies these signals for accurate motion tracking. The circuitry ensures the generated signals are clean, noise-free, and compatible with controllers such as PLCs (Programmable Logic Controllers) or motor drivers.

Some advanced optical encoders use interpolation techniques to enhance resolution by processing partial light variations into additional signals. This component also helps eliminate errors such as jitter (unintended signal fluctuations), ensuring smooth and precise feedback.

Types of Optical Encoders

Optical encoders come in different types, each designed for specific applications:

1. Absolute Optical Encoders

• An absolute encoder provides a unique position value for each movement.
• Ideal for applications requiring precise positioning, such as robotics and medical devices.

2. Incremental Optical Encoders

• An incremental encoders measure changes in position relative to a reference point.
• Commonly used in conveyor systems and industrial machinery for speed and direction control.

Transmissive vs. Reflective Encoders

• Transmissive Encoders: Light passes through the coded disk. Used in high-accuracy applications.
• Reflective Encoders: Light reflects off the disk, making them compact and suitable for space-constrained systems.

Key Differences Between Optical and Magnetic Encoders

A magnetic encoder is a type of motion sensor that detects changes in position using a rotating magnetic field and a magneto-resistive sensor. Instead of light-based detection like optical encoders, it relies on magnetic flux variations to generate signals.

While both optical encoders and magnetic encoders serve the same purpose, tracking position, speed, and direction the way they detect motion differs significantly. Understanding these differences helps industries choose the right encoder for their specific applications.

Advantages of Optical Encoders

Optical encoders are widely used due to their superior precision and reliability. Here’s why they stand out:

• High Accuracy: Optical encoders offer higher resolution compared to magnetic encoders.
• No Magnetic Interference: They work well in environments with strong electromagnetic fields.
• Fast Response Time: Their ability to generate rapid signals makes them ideal for high-speed applications.
• Versatility: Optical encoders can be used in robotics, medical equipment, manufacturing, and more.

However, optical encoders may be sensitive to dust and require proper enclosure in harsh environments.

Higher resolution and higher accuracy

The resolution of an optical encoder is basically determined by the number of slits in the code wheel. Therefore, it is necessary to increase the number of slits in the code wheel in order to achieve high resolution, but it is necessary to reduce the area of each slit in order to be compatible with the miniaturization of the encoder.

As a result, high precision is required for assembling the components, and physical limits are reached somewhere. In order to further improve the resolution, there is a method of “electrical interpolation” the A phase and B phase of the output signal by using a pseudo sine wave signal instead of a pulse signal.

In this way, the optical encoder can realize high resolution and high accuracy by optimizing the structure of LED, code wheel, photo sensor, etc. and reducing the distortion of the pseudo sine wave.

Where are Optical Encoders Used?

The optical encoder has the advantage that it is easy to improve accuracy and resolution by devising the shape of the slit because it has a mechanism that detects whether light passes through the slit or not. Therefore, it is used for servo control and hollow through shaft type motor control that require high precision.

In addition, since it is not affected by the surrounding magnetic field, it can be used in applications where a strong magnetic field is generated.

Optical encoders are used across various industries where precision motion tracking is required. Some key applications include:

• Manufacturing & Automation: Used in conveyor belts, robotic arms, and cutting equipment.
• Medical Equipment: Found in CT scanners, lab test machines, and precision surgical devices.
• Aerospace & Defense: Essential for radar positioning and satellite tracking.
• Automotive: Used in electronic throttle controls, power steering systems, and ABS braking systems.
• Renewable Energy: Helps in solar tracking systems to optimize energy capture.

Conclusion

Optical encoders are the backbone of modern motion control systems, offering unmatched precision and reliability. Whether you need accurate positioning in robotics, speed control in conveyor systems, or precise feedback in medical equipment, choosing the right optical encoder is crucial.

Understanding the working principles, types, and advantages of optical encoders allows businesses to optimize their machinery for improved efficiency and performance.

As industries continue to evolve, optical encoders will remain a key component in driving innovation and automation forward.