DD Motor vs Harmonic Reducer for Collaborative Robot Joints: Which Is Better?

Introduction

Collaborative robots are designed to work in environments where humans and robots share the same workspace. This creates different requirements for robot joint design compared with conventional industrial robots.

A collaborative robot joint needs to provide sufficient torque and positioning accuracy while also maintaining compact dimensions, low weight, smooth motion, responsive control and appropriate force interaction.

This makes the choice of robot joint transmission particularly important.

Two technologies that can be considered for collaborative robot joints are the DD motor (direct-drive motor) and the harmonic reducer. A DD motor drives the joint directly without a mechanical reduction stage, while a harmonic reducer combines a high-speed servo motor with a high-ratio precision transmission.

Neither technology is universally better. The more practical question is which solution provides the right balance of torque density, backdrivability, dynamic response, joint size, rigidity, thermal performance and control requirements for a particular collaborative robot.

This article compares DD motors and harmonic reducers for collaborative robot joints and explains when each technology may be a better choice.

What Does a Collaborative Robot Joint Need?

Before comparing DD motors and harmonic reducers, it is important to understand the requirements of a collaborative robot joint.

Unlike a simple rotary mechanism, a robot joint must repeatedly accelerate, decelerate, reverse direction and maintain precise position while carrying a moving link and payload.

The joint therefore needs to handle several requirements simultaneously.

Torque density is important because the actuator needs to generate sufficient torque without making the robot arm excessively heavy.

Low backlash is valuable for maintaining positioning accuracy and predictable motion.

Backdrivability becomes important when the robot needs to respond naturally to external forces.

Dynamic response affects how quickly the robot can accelerate, decelerate and react to changes in motion.

Rigidity influences the robot's ability to maintain its commanded position under load.

Thermal performance also matters because collaborative robots often operate for long periods in compact joint housings.

These requirements can lead to different transmission choices depending on the robot architecture.

How Does a Harmonic Reducer Work in a Collaborative Robot Joint?

A harmonic reducer is normally combined with a servo motor to form a high-performance robot joint drive.

The motor operates at relatively high speed, while the harmonic transmission provides a high reduction ratio and converts the motor output into the lower-speed, higher-torque motion required by the robot joint.

The compact transmission structure allows a relatively small motor to generate a much higher output torque at the joint.

This is one of the main reasons harmonic reducers are widely considered for robotic joints.

A harmonic reducer can also provide very low backlash, which is valuable for applications requiring precise positioning and coordinated multi-axis motion.

For collaborative robots, this combination of high reduction ratio, compact size, low backlash and high torque density can be particularly useful for shoulder, elbow and other load-bearing joints.

However, a harmonic reducer is still a mechanical transmission.

Its dynamic behavior, efficiency, torsional characteristics and backdrivability depend on the specific reducer design, reduction ratio, load and operating conditions.

Therefore, engineers should evaluate the complete joint rather than assuming that every harmonic reducer provides the same performance.

How Does a DD Motor Work in a Collaborative Robot Joint?

A DD motor takes a different approach.

Instead of placing a reduction mechanism between the motor and the robot joint, the motor directly drives the output.

Because there is no conventional mechanical reduction stage, the system can have fewer transmission components.

This can provide several benefits.

The absence of a mechanical reduction stage eliminates gear backlash associated with the transmission and can provide a direct relationship between motor torque and joint movement.

DD motors can also provide excellent dynamic response.

When the joint needs rapid acceleration, deceleration or frequent changes in direction, direct drive can be attractive because there are fewer mechanical transmission elements between the motor and the load.

Another important characteristic is backdrivability.

A direct-drive joint can generally be easier to backdrive than a highly geared joint, making DD motors interesting for applications where force interaction and compliant motion are important.

The limitation is torque.

Because there is no reduction mechanism multiplying the motor torque, the DD motor itself must generate the required joint torque.

For a high-torque collaborative robot joint, this can require a larger motor with a larger active diameter and greater thermal capacity.

Harmonic Reducer vs DD Motor for Collaborative Robot Joint Torque

Torque density is one of the most important differences between the two approaches.

With a harmonic reducer, the motor does not need to directly generate the final joint torque. The reduction mechanism increases output torque while reducing rotational speed.

This makes it easier to create a compact high-torque joint.

A DD motor must produce the required output torque directly.

For low-torque joints, this may not be a significant limitation.

However, as the required joint torque increases, the motor size, current and thermal requirements can increase significantly.

For this reason, harmonic transmission can be attractive for collaborative robot joints that need high torque within a limited mechanical envelope.

DD motors become more interesting when the required torque is moderate and the design places greater emphasis on direct drive, high dynamic response and backdrivability.

Which Provides Better Backdrivability: DD Motor or Harmonic Reducer?

Backdrivability is particularly important in collaborative robotics.

A robot may need to detect external forces, react to contact and operate safely around people.

A highly geared transmission can resist external movement because the load must drive the motor through the transmission.

A DD motor does not have this same reduction mechanism.

The load is directly connected to the motor, which can make the joint naturally responsive to external forces.

This is one of the strongest arguments for DD motors in force-sensitive robotic applications.

However, this does not mean that harmonic robot joints cannot support force control.

A harmonic joint can use high-resolution encoder feedback, motor current information or an integrated torque sensor to support force and torque control.

The actual performance therefore depends on the complete joint architecture.

For applications requiring highly sensitive force interaction, engineers should evaluate not only the transmission type but also the encoder configuration, torque sensing, control algorithm and mechanical friction.

Which Is More Compact for Collaborative Robot Joints?

Compactness is another area where harmonic reducers can have an advantage.

A harmonic reducer can provide a high reduction ratio in a relatively small package

This allows the motor and transmission to be integrated into a compact joint housing.

For a six-axis collaborative robot, this can be important because every joint contributes to the overall size and weight of the robot.

Reducing the mass of the upstream joints can also reduce the torque required from the joints further down the robot arm.

A DD motor can eliminate the reducer, but this does not necessarily mean the complete joint will be smaller.

The motor needs to produce the required torque directly, so a high-torque DD motor may require a relatively large diameter.

Therefore, removing the reducer does not automatically result in a smaller robot joint.

The correct comparison should be based on the complete actuator envelope, including the motor, bearings, housing, encoder and cooling structure.

Harmonic Reducer vs DD Motor for Robot Joint Accuracy

Both technologies can support high-precision robot joints, but they achieve it differently.

Harmonic reducers are attractive because they can provide extremely low backlash and a high reduction ratio.

This makes them suitable for applications where precise angular positioning is important.

DD motors eliminate mechanical reduction backlash altogether.

However, the absence of gearbox backlash does not guarantee higher system accuracy.

The final robot joint accuracy is also affected by:

• Encoder resolution

• Bearing accuracy

• Motor control

• Structural deformation

• Thermal expansion

• Mechanical stiffness

For a collaborative robot, these factors need to be evaluated together.

A DD motor with excellent encoder feedback but insufficient structural rigidity may not outperform a well-designed harmonic joint.

Similarly, a harmonic reducer with low backlash cannot compensate for poor encoder feedback or excessive structural deformation.

Which Is Better for High-Speed Collaborative Robot Motion?

DD motors have a natural advantage when high-speed continuous rotary motion and rapid dynamic response are important.

Because the motor directly drives the load, there is no reduction stage that needs to transmit the motion.

This can be valuable in applications requiring frequent acceleration and deceleration.

Harmonic reducers can also support fast robot motion, but the maximum output speed is constrained by the transmission and selected reduction ratio.

For collaborative robot joints that need high output speed but relatively low torque, DD motors can therefore be an attractive option.

For joints that require high torque at relatively low output speed, harmonic transmission may provide a more practical solution.

Harmonic Reducer vs DD Motor for Collaborative Robot Force Control

Force control is becoming increasingly important in collaborative robotics.

Applications such as assembly, polishing, insertion, inspection and human-robot interaction require the robot to respond to contact forces rather than simply follow a predefined position trajectory.

DD motors are naturally attractive for these applications because the motor is directly connected to the output.

The mechanical relationship between motor torque and joint torque is relatively direct.

However, a harmonic joint can also be designed for force-controlled applications.

For example, an integrated robot joint can use an output-side encoder or torque sensor to obtain more direct information about joint movement or torque.

This can significantly improve the ability of the control system to distinguish between motor-side motion and actual output motion.

For high-performance collaborative robot joints, the question should therefore not simply be:

DD motor or harmonic reducer?

A better question is:

How accurately can the complete joint measure and control output torque?

Harmonic Joint Modules Can Integrate More Than the Reducer

A modern robot joint module is not simply a harmonic reducer.

A complete joint module may integrate:

• Servo Motor

• Harmonic Reducer

• Encoder

• Driver

• Brake

• Torque Sensor

• Housing

• Bearings

This level of integration can reduce the engineering work required to build a robotic joint.

For collaborative robot manufacturers, an integrated harmonic joint module can also provide a more standardized mechanical and electrical interface.

This can be useful when developing multiple robot models with different payload capacities.

The main advantage is not simply the harmonic reducer itself, but the ability to integrate the transmission, motor, sensing and control components into a compact actuator.

 When Should You Choose a Harmonic Reducer for a Collaborative Robot?

A harmonic reducer is worth considering when the collaborative robot joint requires a combination of high torque, compact dimensions and low backlash.

It can be particularly suitable for joints where the actuator needs to support significant load while remaining relatively lightweight.

Typical applications may include:

• Collaborative robot arms

• Six-axis collaborative robots

• Humanoid robot joints

• Mobile manipulators

• Precision robotic arms

For these applications, the harmonic transmission can provide a practical balance between torque density, precision and package size.

However, the reducer should be selected according to actual torque, speed, load inertia, duty cycle and thermal conditions rather than simply choosing the highest reduction ratio available.

When Should You Choose a DD Motor for a Collaborative Robot?

A DD motor may be more suitable when direct drive and high dynamic response are the primary design requirements.

Typical considerations include:

• High backdrivability

• Fast acceleration and deceleration

• Continuous rotation

• Direct torque control

• Low mechanical transmission complexity

DD motors can be especially attractive for selected wrist joints, force-sensitive mechanisms and specialized collaborative robot architectures.

They may be less attractive when the joint needs very high torque within an extremely compact housing.

In those cases, the additional motor size required to generate the torque directly can become a significant design constraint.

DD Motor vs Harmonic Reducer: Which Is Better for Different Robot Joints?

The best technology may also vary between different axes of the same robot.

A shoulder or base joint generally needs high torque because it supports a large portion of the robot structure and payload.

A harmonic joint module can therefore be attractive for these positions.

An elbow joint also needs significant torque and compactness, making harmonic transmission a practical option in many designs.

Wrist joints often have different requirements.

Lower weight, high speed and dynamic response can become more important, so a DD motor may be worth evaluating depending on the robot architecture.

This means that a robot manufacturer does not necessarily need to use exactly the same actuator architecture for every joint.

Different joint positions can have different torque, speed, inertia and force-control requirements.

How to Choose Between a DD Motor and Harmonic Reducer for a Cobot

The selection can be simplified by evaluating the actual design priorities.

If the joint requires high torque density, compact dimensions, high reduction ratio and very low backlash, a harmonic reducer is often a strong candidate.

If the joint prioritizes direct drive, backdrivability, high dynamic response and low mechanical transmission complexity, a DD motor may be more attractive.

If the application requires highly responsive force control, both technologies should be evaluated together with the sensing architecture.

A torque sensor, dual encoder configuration and suitable servo control can have a significant influence on the final performance of the robot joint.

Conclusion: DD Motor or Harmonic Reducer for Collaborative Robots?

There is no universal winner between a DD motor and a harmonic reducer.

A harmonic reducer is generally more attractive when a collaborative robot joint requires high torque density, compact dimensions, low backlash and a high reduction ratio.

A DD motor becomes more attractive when direct drive, backdrivability, high dynamic response and continuous rotary motion are the primary requirements.

For many compact collaborative robot arms, harmonic joint modules offer a practical combination of torque, precision and integration. For specialized joints that emphasize direct force interaction and dynamic response, DD motors can provide important advantages.

The most important point is to evaluate the complete robot joint, not just the motor or reducer.

Motor torque, reduction ratio, encoder feedback, bearing capacity, structural rigidity, thermal performance, force sensing and control architecture all contribute to the final performance.

For robot manufacturers developing collaborative robots, the right solution may therefore be a harmonic joint module for high-torque axes and a direct-drive architecture for selected high-speed or force-sensitive joints, depending on the robot's mechanical and control requirements.

HONPINE provides harmonic reducers, harmonic rotary actuators, robot joint modules and DD motor solutions for different robotic motion requirements, allowing engineers to select the appropriate transmission architecture according to the actual application.