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What are the best practices for selecting and integrating motion encoders in robot motion control?

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Motion encoders are essential devices for robot motion control, as they provide feedback on the position, speed, and direction of the robot's joints and end-effector. However, choosing and integrating the right encoder for your robot can be challenging, as there are many factors to consider, such as resolution, accuracy, bandwidth, interface, mounting, and environmental conditions. In this article, you will learn some best practices for selecting and integrating motion encoders in robot motion control, based on the principles of control engineering and the requirements of robot motion planning and control.

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Hassan Harb

Senior Consultant Industy 4.0 ?| Operations & Projects @newboot.io??| Digital Twins | Edge Computing | Artificial…

1 Choose the right encoder type

There are two main types of motion encoders: incremental and absolute. Incremental encoders generate pulses as the encoder shaft rotates, and the number of pulses indicates the relative position and speed of the shaft. Absolute encoders, on the other hand, output a unique code for each position of the shaft, and can retain the position information even when the power is off. The choice between incremental and absolute encoders depends on the application and the budget. Incremental encoders are cheaper, simpler, and faster, but they require a reference point or a homing routine to determine the absolute position. Absolute encoders are more expensive, complex, and slower, but they eliminate the need for a reference point or a homing routine, and can provide higher accuracy and reliability.

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Hassan Harb

Senior Consultant Industy 4.0 ?| Operations & Projects @newboot.io??| Digital Twins | Edge Computing | Artificial Intelligence | Business & Engineering
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Absolute encoders are advantageous in applications where maintaining the position information during power cycles is crucial. If the system needs to resume operation without re-referencing after a power outage, absolute encoders are preferred. Incremental encoders may be preferred in applications where re-referencing or homing routines are easily implemented and do not pose a significant operational challenge.

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Kuldeep Singh

Global Business Leader | Emerging Technologies | Driving Innovation, Digital Transformation & $1B+ Business Growth
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In my experience, selecting the right type of encoder is fundamental to the performance of the robotic system. For example, absolute encoders are invaluable in applications where the robot must maintain awareness of its position even after a power loss.

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2 Select the appropriate resolution and accuracy

The resolution and accuracy of the encoder determine how precisely the encoder can measure the position and speed of the robot. The resolution is the smallest change in position that the encoder can detect, and it is usually expressed in bits, pulses per revolution (PPR), or counts per revolution (CPR). The accuracy is the difference between the actual position and the position reported by the encoder, and it is usually expressed in degrees, arc minutes, or arc seconds. The resolution and accuracy of the encoder should match the specifications of the robot and the control system, as well as the desired performance and precision of the robot motion. A higher resolution and accuracy can improve the feedback quality and reduce the tracking error, but they can also increase the cost, complexity, and data processing of the encoder.

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Hassan Harb

Senior Consultant Industy 4.0 ?| Operations & Projects @newboot.io??| Digital Twins | Edge Computing | Artificial Intelligence | Business & Engineering
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Take into account the operating environment of the robot. Harsh conditions, such as extreme temperatures, vibrations, or contaminants, may impact the performance of the encoder. Choose an encoder with appropriate ruggedness and environmental protection.

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Kuldeep Singh

Global Business Leader | Emerging Technologies | Driving Innovation, Digital Transformation & $1B+ Business Growth
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An example of the importance of resolution and accuracy can be seen in robotics dealing with precise measurements, such as CNC machines, where high-resolution encoders are crucial for intricate detailing.

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3 Consider the bandwidth and interface of the encoder

The bandwidth and interface of the encoder affect how fast and how well the encoder can communicate with the control system. The bandwidth is the maximum frequency of the encoder signal that can be transmitted without distortion, and it is usually expressed in hertz (Hz) or kilohertz (kHz). The interface is the protocol or standard that defines how the encoder signal is encoded, decoded, and transmitted. The bandwidth and interface of the encoder should be compatible with the sampling rate and the communication bus of the control system, as well as the speed and dynamics of the robot motion. A higher bandwidth and a more advanced interface can enable faster and more reliable data transmission, but they can also require more hardware, software, and power resources.

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Kuldeep Singh

Global Business Leader | Emerging Technologies | Driving Innovation, Digital Transformation & $1B+ Business Growth
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I agree that bandwidth and interface are critical, but it’s also important to consider future-proofing. For instance, selecting an encoder with a high-speed interface like EtherCAT may offer better long-term value through compatibility with faster, future communication standards.

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4 Optimize the mounting and alignment of the encoder

The mounting and alignment of the encoder influence how accurately and consistently the encoder can measure the position and speed of the robot. The mounting is the method or device that attaches the encoder to the robot's joint or end-effector, and it can be either direct or indirect. Direct mounting means that the encoder shaft is coupled to the robot's shaft, while indirect mounting means that the encoder shaft is connected to the robot's shaft through a gear, belt, or chain. The alignment is the process or procedure that ensures that the encoder shaft and the robot's shaft are parallel and concentric, and that the encoder zero position corresponds to the robot's zero position. The mounting and alignment of the encoder should be chosen and performed carefully, as they can affect the accuracy, repeatability, and backlash of the encoder.

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Kuldeep Singh

Global Business Leader | Emerging Technologies | Driving Innovation, Digital Transformation & $1B+ Business Growth
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In my experience, improper mounting can lead to premature wear and signal errors. For example, encoders in robotic arms should be mounted to avoid any misalignment due to joint flexing or vibration.

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5 Adapt to the environmental conditions of the encoder

The environmental conditions of the encoder refer to the external factors that can affect the performance and durability of the encoder, such as temperature, humidity, vibration, shock, dust, oil, water, and magnetic fields. The environmental conditions of the encoder should be compatible with the specifications and ratings of the encoder, as well as the operating and storage conditions of the robot. If the environmental conditions of the encoder are harsh or variable, the encoder should be protected or compensated accordingly, by using appropriate materials, coatings, seals, filters, housings, or sensors.

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Kuldeep Singh

Global Business Leader | Emerging Technologies | Driving Innovation, Digital Transformation & $1B+ Business Growth
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An example of adapting to environmental conditions is the use of rugged encoders in robots that operate in harsh environments, such as those found in outdoor agricultural applications or in factories with a lot of dust or moisture.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best practices for selecting and integrating motion encoders in robot motion control?

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1 Choose the right encoder type

2 Select the appropriate resolution and accuracy

3 Consider the bandwidth and interface of the encoder

4 Optimize the mounting and alignment of the encoder

5 Adapt to the environmental conditions of the encoder

6 Here’s what else to consider

Control Engineering

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