Should You Choose a T-Type or Straight-Type Harmonic Joint Module for a Collaborative Robot?

Hello, I’m Theodore Li, Technical Director at HONPINE. Today, I’ll introduce two harmonic joint module solutions for collaborative robotic arms: the TCHL straight-type joint module and the HTS T-type joint module.

Integrated harmonic joint modules are key drive units in collaborative robots (cobots). By integrating a harmonic drive reducer, torque motor, absolute encoders, drive electronics, and an optional holding brake into a compact assembly, they simplify mechanical integration and provide closed-loop servo control.

However, choosing the right harmonic joint module involves more than comparing torque ratings or positioning accuracy. The module’s structural configuration directly affects installation space, arm geometry, internal cable routing, mechanical stiffness, and the risk of interference between moving links.

In this guide, I’ll explain the structural characteristics, advantages, limitations, and typical applications of the TCHL straight-type and HTS T-type harmonic joint modules to help engineers select the right solution for their collaborative robot designs.

1. General Selection Principles for Collaborative Robot Joint Modules

Selecting a joint module for a collaborative robotic arm requires a comprehensive evaluation of five factors: torque and reduction ratio, encoder performance, communication protocol, environmental conditions, and mechanical configuration.

1.1 Match Torque and Reduction Ratio to the Operating Conditions

The first step is to determine the continuous torque and peak torque required at each joint. The selected module must accommodate the actual load profile, including acceleration, deceleration, external forces, and short-duration dynamic loads.

The joint module series described in this guide covers multiple frame sizes, with outer diameters of 70, 80, 90, 110, 142, and 170 mm. Available reduction ratios include 51:1, 81:1, 101:1, and 121:1, depending on the specific model.

In general, a higher reduction ratio provides greater torque multiplication but reduces output speed for a given motor speed. A lower reduction ratio allows higher output speed but provides less torque multiplication.

The final choice should be based on the actuator’s torque-speed characteristics, the robot’s motion profile, and the required duty cycle.

Important: Avoid operating a joint module beyond its permitted torque limits, particularly for prolonged periods. Overloading can accelerate wear or damage the reducer and other components, potentially compromising joint control and reliability.

For vertically mounted joints that must hold a load against gravity, consider a brake-equipped configuration. A suitable holding brake can help prevent unintended shaft rotation when power is removed. For horizontal joints with limited gravitational loading, a brake-free configuration may be appropriate, depending on the application’s safety requirements.

1.2 Evaluate Encoder Configuration and Positioning Accuracy

The joint modules described in the original specifications use a 17-bit single-turn and 16-bit multi-turn magnetic absolute encoder configuration. Battery backup preserves multi-turn position information when power is removed, allowing the system to retain position data without requiring a reference-homing procedure after every power cycle, provided the encoder backup system is correctly maintained.

The output-side angular resolution is influenced by the encoder configuration and reduction ratio. A higher reduction ratio can provide finer theoretical output-angle increments for a given motor-side encoder resolution. However, actual positioning accuracy also depends on reducer transmission errors, mechanical deformation, backlash characteristics, mounting precision, and control performance.

When selecting a module, engineers should also check the following mechanical accuracy parameters:

• Output flange face runout

• Output shaft radial runout

• Coaxiality

• Parallelism

These tolerances influence the alignment and repeatability of the assembled robotic arm. HONPINE’s measured mechanical accuracy data for each model should be checked during the design and validation process.

1.3 Choose the Appropriate Communication Protocol

The communication protocol is another important consideration when integrating multiple joint modules into a collaborative robotic arm.

For the product configurations described here, the model suffix identifies the communication option: C for CANopen and E for EtherCAT. Confirm the exact suffix and available protocol for the selected TCHL or HTS model before ordering.

EtherCAT (E)

EtherCAT supports high-speed, deterministic communication and synchronized control across multiple axes. It is suitable for collaborative robots with six or more joints, particularly where coordinated motion and real-time control are important.

It can be integrated with compatible EtherCAT master controllers, including Beckhoff-based systems.

CANopen (C)

CANopen can provide a practical solution for systems that prioritize relatively simple network architecture and cost-effective integration. It may be suitable for robots with fewer axes or less demanding synchronization requirements.

Compatible CANopen master controllers and PLCs can be used, subject to device profiles and controller support.

The appropriate protocol depends on the complete control architecture, communication cycle requirements, synchronization performance, and controller compatibility.

1.4 Consider Environmental Conditions and Mechanical Constraints

Before selecting a joint module, verify that its operating environment meets the product specifications.

The environmental conditions provided for the series are:

• Operating temperature: 0–40°C

• Storage temperature: −20–60°C

• Relative humidity: 20–80% RH, non-condensing

• Protection rating: IP54

• Recommended operating altitude: below 1,000 m

These values should be confirmed against the datasheet for the exact model and configuration.

Control cabinets should also provide adequate heat dissipation to prevent internal temperatures from exceeding the permitted operating range.

From a mechanical perspective, engineers should check the hollow bore diameter, flange mounting-hole pattern, arm-link length, and potential interference throughout the robot’s complete range of motion.

This is where the structural differences between the TCHL straight-type and HTS T-type modules become particularly important.

1.5 Understand the Structural Difference Between TCHL and HTS

The TCHL straight-type joint moduleand the HTS T-type joint module differ primarily in their output flange geometry and installation arrangement.

The TCHL straight-type uses a straight, cylindrical configuration with the output flange arranged concentrically with the module body.

The HTS T-type uses an offset output flange arrangement that allows the connected mechanical structure to be positioned differently relative to the cylindrical module body.

These differences influence the overall dimensions, mounting arrangement, and packaging of the robotic arm.

When comparing corresponding models, verify the specifications of each configuration rather than assuming that every internal component or electrical characteristic is identical.

2. Advantages of the TCHL Straight-Type Harmonic Joint Module

The TCHL straight-type joint module features a cylindrical body with a concentric output flange. The output flange is arranged around the module’s central axis, creating a straight, compact form factor.

Depending on the model, the product drawings may include configurations such as TCHL-14-70 and TCHL-17-80. Confirm the exact model designation against the current HONPINE product catalogue.

2.1 Compact Radial Dimensions for Space-Constrained Designs

Because the module body and output flange share a common axis, the straight-type configuration has no additional lateral flange offset.

This can help reduce radial space requirements in robotic joints where the module must fit inside a cylindrical arm link or within a nested mechanical assembly.

The hollow center also provides a potential route for internal cables, pneumatic tubing, and other service lines, subject to the available bore diameter and installation requirements.

For collaborative robot designs with limited radial clearance, the TCHL straight-type configuration can simplify packaging and help maintain a compact external profile.

2.2 Concentric Flange Design for Load Transfer

The concentric output flange provides a circular mounting interface around the central bore. Mounting bolts can be distributed around the flange according to the specified hole pattern, helping transfer loads into the reducer output structure.

This arrangement can be advantageous for joints subjected to bending moments and overturning loads, provided the module is correctly sized and mounted.

The actual load capacity and stiffness depend on the reducer, output bearing arrangement, flange geometry, bolt specification, mounting interface, and external load conditions.

For base and shoulder joints that carry substantial bending moments, engineers should compare the module’s permissible radial, axial, and moment loads with the actual loads generated by the robot.

The product manual should be used to verify the correct bolt sizes and tightening torques for each model.

2.3 Simplified Mechanical Design and Manufacturing

The straight cylindrical profile can simplify the design of robotic arm housings and link structures.

In many applications, the arm link can be designed around a cylindrical or concentric structure with mounting interfaces at each end. This may reduce the need for custom offset brackets or complex external contours.

A simpler housing can also make machining, assembly, and maintenance more straightforward, depending on the overall mechanical design.

For manufacturers developing multiple collaborative robot models, a standardized straight-type joint interface may help simplify the reuse of mechanical components.

2.4 Broad Compatibility with Conventional Robotic Arm Architectures

Straight-type joint modules are well suited to robotic architectures in which adjacent links are connected through concentric mounting interfaces.

They can be considered for base, shoulder, and elbow joints, as well as rotary positioning mechanisms and other applications where a straight cylindrical form is beneficial.

When a TCHL module is integrated into a control system, the communication protocol, commissioning tools, and software configuration must match the selected product version. Structural changes between module configurations should not, by themselves, be assumed to require a different control strategy.

2.5 Limitations of the TCHL Straight-Type Configuration

The main limitation of the straight-type configuration appears when the robotic arm requires a compact 90-degree connection between adjacent links.

Because the output flange is arranged concentrically with the module body, the overall assembly may require additional axial space when two links are connected at a right angle.

This can increase the joint assembly’s thickness and extend the distance between adjacent link structures. In a compact wrist mechanism, this additional space may reduce the available motion envelope and increase the risk of self-interference.

An additional adapter bracket may be required, depending on the mechanical layout.

2.6 Typical Applications for TCHL Straight-Type Joint Modules

The TCHL straight-type configuration is worth considering for:

• Base and shoulder joints in collaborative robotic arms

• Elbow joints with concentric mounting arrangements

• Rotary positioning tables and mechanisms

• Applications requiring a compact radial profile

• Robot structures designed around cylindrical arm links

Joints that experience substantial bending moments and require an appropriate concentric mounting interface

The final selection must still be validated against the module’s rated loads, dimensions, and installation requirements.

3. Advantages of the HTS T-Type Harmonic Joint Module

The HTS T-type joint module features an offset output flange arrangement. The flange is positioned laterally relative to the cylindrical module body, creating a configuration that can accommodate right-angle connections between adjacent robotic links.

This geometry can be particularly useful in compact collaborative robot wrists, where several rotational axes must fit within a limited installation space.

3.1 Right-Angle Mounting for Reduced Axial Space

The offset output flange allows the connected link structure to be positioned differently from that of a concentric straight-type module.

In suitable mechanical layouts, this can simplify a 90-degree connection between adjacent arm links and reduce the axial thickness of the assembled joint structure.

The advantage is particularly relevant to the wrist axes of a collaborative robot, commonly referred to as axes 4, 5, and 6.

These joints are often tightly packaged because several rotation axes must be accommodated close to the end effector.

By reducing the space required for the joint connection, the HTS T-type configuration may help create a more compact wrist and reduce the risk of interference when the robot reaches confined workspaces.

The actual improvement depends on the link geometry, output flange dimensions, mounting arrangement, and complete motion envelope.

3.2 Simplified Multi-Axis Wrist Design with Fewer Adapter Components

A conventional straight-type module may require an additional L-shaped adapter bracket to achieve a right-angle connection between adjacent links.

Depending on the design, this bracket can add mass, increase the number of components, and introduce additional mounting interfaces that must be manufactured and assembled accurately.

The HTS T-type configuration integrates the offset into the module’s structural arrangement, potentially reducing the need for separate adapter components.

This can help simplify the mechanical design of compact multi-axis wrist assemblies.

Reducing unnecessary brackets and interfaces may also lower the end-effector assembly’s mass and improve the robot’s payload-to-self-weight ratio, provided the resulting structure meets the required stiffness and strength criteria.

3.3 Hollow-Center Routing for Internal Cables

The T-type configuration can also support internal cable routing through the module’s hollow center, subject to the specific model’s bore dimensions and mechanical arrangement.

Cables and service lines can be routed through the module body and connected to the offset output structure according to the installation design.

Internal routing helps protect cables from external snagging, abrasion, and interference with surrounding equipment.

For collaborative robots that require compact cable management, this can be especially useful around the wrist, where external cable loops may interfere with nearby objects or limit the usable motion range.

Engineers should verify the cable bend radius, connector dimensions, permissible cable twist, and clearance throughout the complete joint motion.

3.4 Limitations of the HTS T-Type Configuration

Despite its packaging advantages, the T-type design is not the best choice for every joint.

Larger lateral envelope

The offset output flange protrudes beyond the central cylindrical profile. This can increase the robot’s overall width or radial envelope.

For base and shoulder joints, where the robot may require a narrow external profile, this additional lateral projection can be a disadvantage.

Eccentric loading considerations

Because the output flange is offset relative to the module body, the mechanical design must account for the resulting load path and any additional bending moments generated by the external structure.

For heavy loads or large bending moments, engineers should verify the module’s allowable loads and ensure that the housing and mounting interfaces provide sufficient strength and stiffness.

More complex housing geometry

The offset shape may require a customized arm-link housing rather than a simple cylindrical enclosure.

This can increase machining and mechanical design complexity, depending on the robot’s architecture and manufacturing process.

The HTS T-type configuration should therefore be selected when its packaging benefits justify the additional design considerations.

3.5 Typical Applications for HTS T-Type Joint Modules

The HTS T-type configuration is worth considering for:

• Compact wrist assemblies in collaborative robotic arms

• Right-angle connections between adjacent robotic links

• Multi-axis wrist mechanisms with limited axial space

• Robotic arms that must reach into confined work areas

• Designs where reducing adapter brackets and distal joint mass is a priority

• Applications that benefit from internal cable routing through a compact joint structure

The selected model must still satisfy the required torque, speed, stiffness, and permissible load specifications.

4. TCHL Straight-Type vs. HTS T-Type: How to Choose

The most important difference between these two configurations is their mechanical layout, not simply their torque rating.

4.1 When Should You Choose the TCHL Straight-Type Module?

Choose the TCHL straight-type configuration as a starting point when the robot’s mechanical architecture benefits from a concentric cylindrical module.

It is particularly worth evaluating for base and shoulder joints, rotary positioning applications, and arm links where radial clearance is limited.

The straight-type design may also simplify housing design when the arm structure uses cylindrical components and conventional concentric mounting interfaces.

4.2 When Should You Choose the HTS T-Type Module?

Choose the HTS T-type configuration as a starting point when the robotic arm requires a compact right-angle connection or when the axial thickness of a multi-axis wrist assembly must be minimized.

Its offset output flange can simplify mechanical packaging and potentially reduce the need for separate adapter brackets.

However, engineers should also evaluate the increased lateral envelope, load path, housing design, and available clearance throughout the robot’s range of motion.

4.3 Can Both Configurations Be Used in the Same Collaborative Robot?

Yes. A collaborative robot can use different joint module configurations at different positions when the mechanical architecture requires it.

For example, a robot may use TCHL straight-type modules at the base and shoulder, where concentric mounting is advantageous, and HTS T-type modules in a compact wrist assembly, where a right-angle layout helps reduce packaging space.

This mixed configuration can provide greater mechanical design flexibility, provided that the selected modules meet the required torque, speed, communication, and installation specifications.

5. Final Recommendations for Collaborative Robot Joint Module Selection

Choosing between a TCHL straight-type harmonic joint module and an HTS T-type harmonic joint module should begin with the mechanical structure of the robotic arm.

The TCHL straight-type configuration is generally worth considering when a concentric cylindrical design, compact radial profile, and straightforward mounting arrangement are priorities.

The HTS T-type configuration is particularly worth considering when right-angle link connections, compact wrist packaging, and reduced axial space are important.

Neither configuration is universally superior. The appropriate choice depends on the joint position, available installation space, external loads, required motion range, and overall robot architecture.

Before finalizing a design, engineers should verify the exact model dimensions, torque-speed performance, encoder specifications, communication protocol, permissible loads, and brake options against the applicable product documentation.

6. Contact HONPINE for Collaborative Robot Joint Module Solutions

At HONPINE, we understand that collaborative robot designers need more than a compact actuator. They need a joint module configuration that fits the mechanical architecture, supports the required motion performance, and simplifies system integration.

The TCHL straight-type and HTS T-type harmonic joint modules provide two different structural approaches to collaborative robot joint design.

Whether you are developing a six-axis collaborative robotic arm, a compact robotic wrist, or a customized automation system, selecting the appropriate configuration early in the design process can help reduce mechanical complexity and improve space utilization.

Contact HONPINE to discuss your collaborative robot joint module requirements and identify a suitable configuration for your application.