KTM5900 Hollow Joint Robot Application Solution

In the R&D and design of collaborative robots and humanoid robots, internal wiring of hollow joints and high-precision position feedback have always been a dual challenge for structural and embedded engineers. Traditional encoder solutions often struggle to balance compact space utilization with ultimate measurement precision. Addressing this industry pain point,Conntek has launched an off-axis magnetic encoder solution based on the KTM5900 chip. With 24-bit high resolution and a flexible off-axis installation architecture, it provides a new choice for position sensing in hollow joint robots that combines high space utilization with high precision.

I. Architecture Optimization and High-Precision Requirements of Hollow Joints

The mechanical design of collaborative robot joints demands extremely high space utilization. The motor's hollow shaft usually needs to pass through multiple sets of cables, such as power lines, encoder lines, brake lines, and temperature sensor lines. Due to its structural limitations, traditional optical encoders often occupy the space at the end of the hollow shaft, compressing the wiring channel or forcing complex side-wrapping designs, which increases the overall volume and assembly difficulty of the joint.

The off-axis solution combining a magnetic ring and a magnetic encoder chip naturally fits the hollow structure: the magnetic ring is sleeved on the rotating shaft, and the chip is installed off-axis on the outside, completely freeing up the hollow shaft space for wiring. However, collaborative robot joints have strict requirements for position accuracy (typically at the 0.01° level), which traditional low-resolution magnetic encoders can hardly meet. Leveraging its 24-bit absolute angle resolution and excellent off-axis installation capability, the KTM5900 successfully achieves high-precision position feedback in hollow joint scenarios, perfectly balancing space and performance.

II. Core Principles and Parameter Configuration of Off-Axis Installation

The KTM5900 fully supports both on-axis and off-axis installation methods. For hollow joint applications, an off-axis magnetic ring solution is recommended: a multi-pole radial magnetized magnetic ring is sleeved on the rotor's hollow shaft, the KTM5900 chip is installed off-axis on the outside of the ring, the typical air gap is controlled at about 0.5mm, and the number of pole pairs is flexibly configured via the M.CPR register.

This solution demonstrates extremely high tolerance in mechanical assembly. According to the recommended parameters in the datasheet, the air gap range for the off-axis solution is -0.5mm to 5mm. This means the chip can be installed externally or sunk inside the magnetic ring (negative air gap), offering significantly better installation flexibility than the on-axis solution. Meanwhile, the off-axis installation has a mechanical deviation tolerance of up to ±5mm, greatly reducing the requirements for mechanical assembly precision and improving mass production efficiency and structural design freedom.

III. Hardware Design and Multi-Pole Configuration Strategies

1. Ultra-Compact FPC Flexible Board Solution

Given the extremely tight PCB space inside hollow joints, it is recommended to use an FPC (Flexible Printed Circuit) to mount the KTM5900 on the joint housing, with the chip facing the magnetic ring, and communicate with the main control board via the SPI interface. The KTM5900 uses a QFN32-5×5mm package and requires no peripheral sensor chips to achieve full functionality. The axial thickness of the entire encoder module (chip + FPC + magnetic ring) can be controlled within 3mm, providing strong support for joint lightweight and miniaturization.

2. Multi-Pole Configuration and Absolute Position Synchronization

Off-axis magnetic rings are usually designed with a multi-pole structure. The more pole pairs, the higher the angle subdivision accuracy within a single pole pair. For collaborative robot joints with speeds typically below 100rpm, a configuration of 32 or 64 pole pairs is recommended to achieve the best accuracy performance.

Addressing the challenge of absolute position synchronization upon power-up in multi-pole applications, the KTM5900 offers two solutions: First, writing the current number of turns and pole pairs via the M.CC turn counter command; Second, using programmable UVW incremental output (1-256 pole pairs) to simulate Hall sensor signals for direct use in motor commutation. In typical applications, the absolute position is written after the initial calibration, and the system can quickly restore the absolute position by reading the SPI angle and turn counter after subsequent power cycles.

IV. Multi-Dimensional Calibration Strategies

Due to factors like magnetic ring eccentricity and uneven air gaps, off-axis installation usually has relatively large nonlinear errors. The KTM5900 provides a comprehensive calibration mechanism to adapt to different application scenarios:

On-site Deployment (No Drag Test Conditions): Supports one-click self-calibration. The motor rotates at a constant speed of 100rpm for 5 seconds, and the chip automatically fits compensation parameters and writes them to MTP memory, quickly correcting installation errors.

Production Line Fine Calibration (With Optical Encoder Drag Test): Supports 256-point fine calibration. By pairing with an optical encoder drag test to obtain a full-cycle error table and writing it into the chip's internal registers, ultimate precision of ±0.025° across the full temperature range can be achieved.

Wide Temperature Environment Adaptation: Supports continuous automatic linear calibration, where the system continuously fits linear parameters in real-time to effectively cope with thermal drift in wide temperature environments from -40°C to 125°C.

With its 24-bit high precision, compact QFN32-5×5mm package, flexible off-axis installation architecture, and convenient one-click self-calibration function, the KTM5900 off-axis magnetic ring solution provides a highly matching position sensing solution for hollow joint robots. This solution not only effectively frees up the wiring space of the hollow shaft and reduces mechanical assembly difficulty but also achieves industrial-grade standards in precision and reliability. For robot R&D companies dedicated to optimizing joint structures and enhancing product integration, the KTM5900 off-axis solution is an ideal choice for achieving product lightweight and high performance.