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