Robot Components: The Role and Function of Actuators

This article is designed for anyone curious about the role of actuators, one of the most important robot components. Complex technical concepts are explained in a simple and easy to understand way, making it ideal for students and beginners learning robotics and automation. By subscribing to the official BONSYSTEMS YouTube channel, BONSYSTEMS GLOBAL, you can explore a wide range of content that helps you understand the latest trends and core technologies in the robotics industry.

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Robots that walk, run, and even perform movements similar to acrobatics may seem almost human. Behind these complex and precise motions lies a key technology, the actuator. As a core element among robot components, the actuator plays a critical role in generating motion by converting energy into mechanical movement and enabling robots to perform controlled and dynamic actions.

1. What Is an Actuator?

An actuator is not a component used only in robots. It is widely applied across many industries, including automated equipment, factory production lines, and heavy machinery used in construction sites. As one of the core robot components, actuators are available in various types and forms depending on their application, performance requirements, and operating environment. Each actuator plays an essential role in converting energy into motion, enabling machines and robots to perform precise and controlled movements.

2. Classification by Application and Form

1) Linear Actuator

A linear actuator is a drive device that converts the rotational energy of a motor and gearbox into linear motion. It can deliver large forces in a straight direction with high stability, making it well suited for applications that require high load capacity and high output. Due to these characteristics, linear actuators are commonly used in equipment such as lift systems that require strong pushing or pulling force. They are also effective in parts of humanoid robots where payload demands are high, such as the lower body. As one of the essential robot components, the linear actuator plays an important role in enabling powerful and precise linear movement.

2) Rotary Actuator

A rotary actuator generates rotational motion by using the motor’s rotational force directly or by amplifying torque through a gearbox. It plays a core role as an actuator in a wide range of industrial and robotic applications that require rotational movement, including robot joints, rotary tables, and robotic arm drive units. In humanoid robots, rotary actuators are especially important in joint areas such as the shoulders and elbows, where a wide range of motion and smooth, flexible movement are required. Among robot components, rotary actuators are fundamental to achieving natural and dynamic joint motion.

Robot Actuator BCSA V4 Series (Rotary Type)

3. Classification of Actuators by Power Source

1) Hydraulic Actuator

A hydraulic actuator operates by using the pressure and flow of fluid, typically hydraulic oil, to generate motion. One of its biggest advantages is the ability to produce extremely large force and high torque output. Because of this capability, hydraulic actuators play a critical role of actuator in industrial environments where high torque is required, such as industrial presses, heavy construction equipment, and large scale manufacturing systems.

As an important category among robot components, hydraulic actuators are especially suitable for applications that demand strong and stable power. However, since they rely on liquid based fluids, issues such as leakage or contamination can lead to performance degradation. To ensure reliable operation, regular inspection and maintenance are essential when using hydraulic actuator systems.

2) Electric Actuator

An electric actuator is driven by an electric motor as its power source. Unlike hydraulic systems, it does not require consumable fluids such as hydraulic oil, which eliminates the need for fluid disposal and helps reduce environmental impact. For this reason, electrification is rapidly expanding across many industries as a cleaner and more sustainable alternative.

One of the greatest advantages of electric actuators is their relatively simple maintenance. In addition, their high precision makes it easier to implement systems capable of real time data collection and feedback control. Traditionally, electric actuators produced lower output torque compared to hydraulic systems. However, recent advances in motor technology and gearbox design have led to continuous development of new solutions that overcome these limitations.

As one of the most important robot components, electric actuators play a key role of actuator in modern robotics. They are especially critical in humanoid robot joints, where precise control, responsiveness, and performance are essential for achieving smooth and human like motion.

3) Pneumatic Actuator

A pneumatic actuator converts compressed air inside a cylinder into mechanical motion. This power transmission method first emerged in the early 20th century and has since been widely used across various engineering fields. Rather than generating high torque, pneumatic actuators are better suited for high speed motion.

Because of this characteristic, pneumatic actuators are commonly applied in automation equipment, robotic arms, and grippers where fast and repetitive movements are required. As one of the practical robot components, pneumatic actuators play an important role of actuator in enabling quick and efficient motion in industrial and robotic applications.

4. The Hidden Key Robot Component: The Gear Reducer

In recent years, the robotics industry has seen an increasing trend toward in house development of actuator technologies. This reflects the growing importance of actuators as essential robot components. However, securing actuator technology internally is not an easy task. One of the main reasons is the difficulty of manufacturing the gear reducer, which is a core technology that largely determines actuator performance.

A gear reducer, as the name suggests, is a mechanical device that reduces speed. It converts the high rotational speed of a motor into increased force, or torque. The higher the reduction ratio, the greater the torque that can be generated, but the rotational speed becomes slower. On the other hand, a lower reduction ratio results in faster rotation but lower torque.

Because of this trade off, selecting the appropriate type of gear reducer and reduction ratio requires careful consideration of the intended application, required performance, specifications, and functional requirements. As a critical element that supports the role of actuator, the gear reducer plays a decisive role in determining how effectively robot components perform in real world applications.

5. Types of Gear Reducers Used in Robot Actuators

Among the key technologies that support the role of actuator in robot motion, the gear reducer is one of the most essential elements. Gear reducers commonly used in robot actuators can generally be classified into the following types.

1) Planetary Gear Reducer

A planetary gear reducer consists of a central sun gear, surrounding planet gears, and an outer ring gear. Its simple structure and high efficiency make it widely used in industrial robots, automation equipment, and servo systems. As a core element among robot components, planetary gear reducers offer a good balance between performance, durability, and compact design.

2) Strain Wave Gear Reducer

A strain wave gear reducer operates by deforming a thin, flexible gear to transmit motion. This structure allows it to achieve very high reduction ratios and excellent positional accuracy. With almost no backlash, it is commonly used in industries where precise control is critical, such as medical devices and semiconductor equipment.

3) Two Stage Reduction Structure (Planetary + Cycloidal)

This two stage reduction structure combines a planetary gear reducer with a cycloidal reducer. It is widely used in the automation and robotics fields because it provides both high output torque and strong durability. Along with strain wave gear reducers, this structure is one of the leading reduction technologies that dominate the current gear reducer market for robot actuators.

4) Cycloidal Gear Reducer

A cycloidal gear reducer transmits power using a disc shaped cam with a cycloidal curve and a pin mechanism. Multiple teeth engage simultaneously, allowing the load to be distributed evenly. This design enables high torque density and excellent durability. In addition, various reduction ratios can be achieved in a relatively compact size, making cycloidal gear reducers well suited for robot actuators and high load drive systems.

When selecting a gear reducer, it is important to consider differences in performance and specifications based on the structure and operating principle. Careful selection of this key robot component has a significant impact on overall actuator performance and the effectiveness of robotic systems.

6. Key Insights

Let us summarize what we have covered so far. The role of an actuator is to convert energy into mechanical motion so that robots and machines can perform their intended functions. As one of the most important robot components, actuators enable movement, force, and control across a wide range of robotic and mechanical systems.

At the core of actuator technology lies the gear reducer. This component plays a decisive role in determining actuator performance by balancing speed and torque. If you have followed the discussion up to this point, you should now have a solid understanding of the hardware driven motion principle centered on the relationship between gear reducers, actuators, and robots.

From here, we will briefly introduce several additional concepts that can help you better understand recent trends in the robotics industry. You can think of this section as a supplementary appendix. For readers who want to explore these topics in more depth, detailed explanations and reference links will be provided for further study.

1) Torque Density

Torque density is a performance metric that indicates how much torque an actuator can generate relative to its size or weight. In other words, it quantitatively measures how strong a compact and lightweight actuator can be. Torque density is commonly calculated using the following formulas:

– Weight based torque density = Maximum torque ÷ Weight
– Volume based torque density = Maximum torque ÷ Volume

A higher torque density allows robots to achieve lighter and more agile structures. This directly contributes to improved motion performance, better energy efficiency, and longer battery life, especially in humanoid robots and multi joint robotic systems. For these reasons, torque density has become one of the most important design indicators in modern robot actuators.

As actuator performance strongly influences overall robot capability, torque density plays a key role of actuator design and is now considered a critical factor among advanced robot components.

Learn more: Why Torque Density Matters in Robot Design

2) Backlash

Backlash refers to the small gap or clearance between meshing gears. When the direction of rotation changes, this gap can cause a slight delay before the output responds, often perceived as a brief “free play” or idle movement. Backlash can occur in any gear or reduction system, and minimizing it is especially important in applications that require precise position control and high repeatability.

However, backlash is not always undesirable. In some cases, a certain amount of clearance can provide mechanical tolerance and help enable smoother operation, reducing stress on components. In actuator systems, managing backlash is a key design consideration, as it directly affects the performance and reliability of robot components and influences the overall role of actuator in delivering accurate and stable motion.

3) Backdrivability

Backdrivability refers to a phenomenon in which force is transmitted in reverse, from the output side of a gear reducer or actuator back to the input side. In other words, when an external force is applied, the output shaft can transfer that force to the motor side, allowing the mechanism to move in the opposite direction.

For example, if a powered wheelchair can still be pushed and moved by hand when the power is turned off, it is considered backdrivable. If it cannot be moved, it is not backdrivable. Backdrivability can provide flexibility and safer human interaction in certain applications. However, in equipment where holding a fixed position is critical, backdrivability can become a safety risk. For this reason, whether to allow backdrivability must be carefully considered during actuator and system design.

As an important characteristic related to the role of actuator, backdrivability directly affects how robot components respond to external forces and how safely they interact with their environment.

Learn more: Backlash and Backdrive: What Gear Design Really Needs

4) Quasi Direct Drive

QDD (Quasi Direct Drive) is an actuator design that integrates a compact gear reducer inside a hollow shaft motor with an open center. This structure is highly effective in reducing the overall size of the actuator while achieving high torque density.

By minimizing mechanical complexity and shortening the power transmission path, QDD actuators enhance responsiveness and efficiency. The dynamic and agile movements demonstrated by MIT’s Mini Cheetah robot are a well known example of what this quasi direct drive approach can achieve.

As an advanced actuator architecture, QDD highlights the evolving role of actuator design in modern robotics. It represents a key direction in the development of high performance robot components, especially for robots that require fast, lightweight, and highly responsive motion.

Learn more: Why focus on the QDD Actuator (Quasi Direct Drive Actuator)?

5) Encoder

An encoder is a key sensor that converts the motion state of a motor or mechanical system such as position, speed, and direction into electrical signals and provides feedback to the control system. In robot control, encoders play a crucial role in verifying whether commanded motions are executed accurately in real time and in correcting errors when deviations occur.

– Single Encoder: A single encoder is installed only on the output side and provides feedback on the final position reached by the motor or actuator. With a simple structure, it is suitable for basic position and speed control applications.

– Dual Encoder: A dual encoder system uses sensors on both the input shaft (motor side) and the output shaft (end effector side). This allows simultaneous comparison of motor rotation and actual output motion. As a result, error sources can be identified more precisely, enabling accurate compensation. Dual encoders are especially advantageous in robotic applications that require high precision and strong repeatability.

As part of advanced feedback control, encoders strongly influence the role of actuator in achieving accurate motion and are considered essential sensing elements among modern robot components.

Learn more: What is an Encoder? The Difference Between Single Encoder and Dual Encoder

6) How to Calculate Torque

A gear reducer lowers the rotational speed of a motor while amplifying torque. The output torque can be calculated by multiplying the motor’s rated torque by the reduction ratio and the efficiency of the gear reducer.

Output torque = Motor rated torque × Reduction ratio × Gear reducer efficiency

For example, if a motor has a rated torque of 1.3 N·m, a reduction ratio of 29:1, and a gear reducer efficiency of 75 percent, the output torque is calculated as follows.

1.3 × 29 × 0.75 ≈ 28.3 N·m

As the reduction ratio increases, rotational speed decreases while torque increases proportionally. To accurately estimate real world output torque, it is essential to consider gear reducer efficiency, which typically ranges from 70 to 90 percent.

Understanding torque calculation is fundamental to selecting and designing actuators. It directly impacts the role of actuator in motion generation and is a key factor in optimizing the performance of robot components.

Learn more: How is torque calculated in a motor gearbox system?

7) Operating Principle of a Cycloidal Gear Reducer

A cycloidal gear reducer transmits rotational power using rolling motion, unlike conventional gear systems that rely on direct tooth to tooth meshing. This mechanism is based on a unique geometric shape known as the cycloidal curve.

When the input shaft rotates, the motion is transferred through an eccentric shaft to the cycloidal disc. The disc then moves along a set of fixed pins in a rolling like motion, similar to how an object rolls across a surface. During this process, speed reduction and torque amplification occur simultaneously.

Thanks to this structure, friction and wear are significantly reduced. As a result, cycloidal gear reducers can achieve high reduction ratios and high torque output even in a compact form factor. These characteristics make cycloidal reducers highly suitable for actuators that require durability and high performance. As a key supporting technology for the role of actuator, the cycloidal gear reducer is an important element among advanced robot components.

Learn more: Cycloidal gearbox design: principles, structure, and working mechanismworking

7. Frequently Asked Questions (FAQ)

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※ References

1. Why Electric Actuators are Replacing Hydraulics | POWER&MOTION (2024.05.13.)
2. What are Pneumatic Actuators? Principles, Types, and Applications | WEVOLVER (2024.6.17.)
3. Tesla Bot | Actuators Team | Tesla (2023.1.21.)