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【AXILE insight】 What is a torque motor and how does it make AXILE machines more agile and reliable?

January 2022
07
Author: linchiang
Company: Buffalo Machinery Co., Ltd.
【AXILE insight】  What is a torque motor and how does it make AXILE machines more agile and reliable?
Torque motors are a special class of brushless permanent-magnet synchronous motors. Since the payload is directly connected to the rotor without the use of transmission elements, torque motors are classified as direct drive. Torque motor technology allows application of direct drive motion to the fourth and fifth axes that control CNC rotary table tilting and rotation. In some 5 axes machine models, it also applies to the spindle rotation and inclination.
Torque motors are also "frameless" motors. This means they don't have housings, bearings, or feedback devices. In this sense the motor is a kit and meant to be part of the machine structure.

Torque motors have been around since at least 1970 and have a lot to offer. They are deployed as part of several mission-critical, high-profile applications from space exploration to high-production machine tools.
Nowadays, direct-drive technology is recognized as a leading solution for the requirements of high-productivity, improved accuracy and increased dynamics of modern machinery.
 
Technology and operational mode 
 
So torque motors can work as actuators for direct-drive mechanisms to replace traditional setups that include geared electric motors, for example.
Torque motors produce high torque at moderate speeds and even when stationary or “stalled”. Contrary to traditional drives, the sizing and selection of a torque motor is purely based on torque, not power. Fundamentally, the peak torque determines the maximum torque that the motor physically produces and the continuous torque defines the amount of torque the motor can continuously supply.
As mentioned above, a quite common application for torque motors is on indexers and rotary tables. Why is that? Well, many indexing tables make use of traditional servo drives that incorporate mechanical power-transmission linkages and powertrains paired with low-inertia servomotors and high-resolution feedback. But the catch here is that some high-speed indexers and especially precision rotary tables cannot be accurately and quickly indexed or positioned with good stability when there is “lost motion” from mechanical gearing and coupling in the loop. Here, direct-drive torque motors can output high peak torque to position quickly without any lost motion to worry about. Plus the high-resolution feedback device is mounted with the motor directly on the driven shaft — which eliminates a source of loop instability.
 
Main benefits of torque motor technology
 
1. Reduced cost of ownership
Direct coupling of the payload to the rotor eliminates the need for mechanical transmission elements such as gearboxes, timing belts, speed reducers and worm gear drives. And unlike brushed rotary motors, there is no contact between rotor and stator; therefore there is no mechanical wear, resulting in excellent reliability and long lifetimes. Fewer mechanical parts also minimizes maintenance and reduces the system cost. The direct-drive technology intrinsic to a torque motor system results in an efficient and effective gearless assembly.
 
2. Easy integration
The use of magnets and limited air gap results in a large hollow shaft or bore for easy integration of cables, cooling tubes, or other application- related equipment. The ring-like configuration of a torque motor minimizes the volume required for mounting. This gives the machine designer great flexibility in locating the motor to work with bearings, feedback devices, and payload.
 
3. Dynamic performance
Dynamic performance is drastically improved by very high control loop bandwidth. The direct coupling of the load and position feedback to the motor has the advantage of eliminating all phenomena that limit the dynamic performance on non-direct driven machines. Eliminating long-time drift, elasticity, and backlash is a huge advantage for machine performance and lifetime.
 
4. Wide torque-speed range
Direct-drive torque motors deliver high torque over a wide range of speed, from a standstill or low speed condition to high angular velocities.The performance of a torque motor over its velocity range is illustrated in its torque/speed curve.
 
Torque motors and AXILE machines ap-plications
 
An essential part of the agility of our recognized G6 and G8 Gantry type 5 axes machines comes from the use of torque motors in the rotary axis ( C ) and swiveling axis ( A ).We can also use their performance and reliability to offer stable and fast turning applications with a speed rotation of up to 1,000 RPM on the ( C ) axis based on a 800 mm table size and a highly rigid G8 machine structure.

On bigger machines such as our Double Column 5 axis (DC12 model), the choice was made to use torque motor technology in the spindle axes head too. A robust design with monolithic fork helps to supply a very high clamping torque in the B-axis. The large rotary table (1,200 mm) is also driven by torque motor technology to make it the most dynamic and accurate machine package.

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