How to Choose Harmonic Joint Actuators for a 7-DOF Robotic Arm？

Hello, I’m Theodore Li, Technical Director at HONPINE. In this article, I’ll explain how to select harmonic joint actuators for a seven-degree-of-freedom robotic arm.

A seven-degree-of-freedom, or 7-DOF, robotic arm has one additional degree of freedom beyond the six generally required to control an end effector’s position and orientation in three-dimensional space. This additional degree of freedom is known as kinematic redundancy.

With appropriate mechanical design and control algorithms, this redundancy provides greater motion flexibility, supports obstacle avoidance and singularity avoidance, and enables compliant movements that resemble those of a human arm.

These capabilities make 7-DOF robotic arms valuable for research and development in medical robotics, precision assembly, flexible processing, and operations in challenging environments.

So, how should you choose the joint modules for a 7-DOF robotic arm?

How to Select Harmonic Joint Actuators for a 7-DOF Robotic Arm？

Match Each Actuator to the Joint’s Load Requirements

Within the same actuator series, a larger frame size generally offers greater output torque and can support higher joint loads. However, the actuator should be selected according to the requirements of the individual joint rather than its model number alone.

In many robotic-arm configurations, joints farther from the end effector must support the payload together with more of the arm’s downstream components. As a result, these joints often require greater output torque.

The shoulder joint is a typical example. Because it supports much of the arm and operates at a considerable distance from the end-effector load, a larger harmonic joint actuator is often selected for this position.

Wrist joints, by contrast, require careful consideration of wrist bending and rotation, tooling loads, and the overall mass of the distal assembly. Smaller, lighter joint modules are often appropriate when they can meet the required torque and motion specifications.

The selection principle is to calculate the load requirements of each joint and match the actuator accordingly—not simply to use the same module throughout the arm.

Example: HONPINE TCHL Harmonic Robotic Actuators

To illustrate this approach, consider a 7-DOF robotic-arm configuration using HONPINE TCHL Series harmonic robotic actuators.

In this example, the shoulder uses a TCHL 20 joint module, while the wrist uses a TCHL 14 joint module.

The TCHL 14 features a compact, lightweight design with a hollow structure and a passage for cable routing. These characteristics make it a candidate for space-constrained wrist and robotic neck assemblies, where reducing distal mass is an important design objective.

This configuration illustrates the selection approach; it is not a universal recommendation for every 7-DOF robotic arm.

Why Do 7-DOF Surgical Robotic Arms Require More Advanced Harmonic Joint Modules?

A seven-degree-of-freedom redundant design, combined with suitable motion-planning and obstacle-avoidance techniques, gives a surgical robotic arm greater freedom to maneuver within a constrained workspace.

This flexibility helps the system address the positioning and access requirements of complex surgical procedures.

The robotic-arm capabilities discussed here also include hand-guided movement, motor-driven operation, API-based remote control, and automatic payload identification and compensation.

Supporting these functions places demanding requirements on joint sensing, control response, and system integration.

Torque Sensing and Responsive Force Control

For the force-controlled surgical-arm configuration discussed here, integrated high-precision torque sensors are an important part of the joint-module design.

These sensors provide feedback for force-sensitive control and collision detection. The performance figures described for this application include force-control accuracy of 0.5 N and a control update rate above 1,000 Hz.

The intended capability is a millisecond-scale response to contact forces, supporting delicate tasks such as insertion and controlled screw-driving.

These figures need to be associated with a defined configuration and test method. They should not be treated as specifications shared by every joint module or every complete robotic arm.

High-Bandwidth Torque Control and Compliant Motion

High-bandwidth force control can be summarized as:

High-bandwidth closed-loop torque control + compliant control algorithms

The objective is to make the system’s torque or force response sufficiently fast and accurate for the intended task.

An integrated high-resolution torque sensor provides the feedback needed for this control approach. The complete response also depends on how sensing, actuation, and control algorithms are integrated.

Redundancy-Based Motion Planning and Obstacle Avoidance

Redundancy-based obstacle avoidance can be described through three key elements:

Kinematic redundancy + null-space projection + hierarchical task-priority optimization

The additional degree of freedom gives the arm more options for adjusting its configuration while maintaining the required end-effector task.

Motion planning can coordinate several objectives with different priorities, such as:

1. Respecting joint limits to support safe operation.

2. Minimizing unnecessary joint motion and joint speeds, with energy efficiency as an optimization objective.

3. Maintaining a suitable arm configuration and managing proximity to singularities.

When sufficient redundant motion is available, the controller has greater freedom to accommodate secondary tasks such as obstacle avoidance. Whether all objectives can be satisfied still depends on the available workspace, joint limits, and task constraints.

HONPINE HAG Series Harmonic Joint Modules

For demanding motion-control applications, HONPINE offers the HAG Series harmonic joint modules.

The HAG Series is intended for applications requiring precise motion control and advanced force-sensing capabilities. Safe Torque Off (STO) is available as an optional feature, subject to the selected model and configuration.

This option can be considered as part of the complete robotic system’s safety design. It should not be interpreted on its own as confirmation that a joint module or robotic arm is approved for a particular medical application.

When evaluating an HAG harmonic joint module, confirm the required sensing performance, control interfaces, and available safety functions for the specific configuration.

Learn More About Harmonic Actuators for 7-DOF Robotic Arms

Choosing a harmonic joint actuator starts with understanding what each joint needs to do. Shoulder, elbow, and wrist joints can have different requirements for torque, weight, installation space, sensing, and control.

Contact HONPINE for more information about harmonic rotary actuators and joint modules for 7-DOF robotic arms.