Direct drive torque motor circular grating

Circular (angle*) encoders can be used on a wide variety of machines and equipment. A rotary encoder consists of a position measurement readhead and a precise scale engraved on the cylindrical or disc surface of the rotary encoder. The readhead measures position by optically sensing regularly spaced scale markings and transmitting it as an analog or digital signal out of this information. Subsequently, the signal is converted into a position reading via a digital display (DRO) or motion controller.

Precise rotary motion is required by many modern automation systems, such as rotary computer-to-plate (CTP) pre-presses, machine tool A, B, and C axes, surface mount machines, shape measurement systems, wafer handling and inspection equipment, and goniometers. Different applications require different combinations of encoder performance and features to optimize their functionality – some require accuracy, while others require repeatability, high resolution, or low cyclic error for speed loop control. Choosing an encoder that offers the best balance between technical specifications and functionality is challenging, and few encoders meet all requirements.

Precision motion control depends on the accuracy and dynamic response of the system. Accurate position measurement is important, but the system will not work properly without precise position control. Direct drive rotary motors, or torque motors, offer high torque and precision servo control over a very small angular range. Since the load is coupled directly to the drive motor, there is no need to install transmission components that can cause backlash, hysteresis, gear errors, or belt stretching, resulting in an excellent dynamic response. While the frameless construction of large bore torque motors does not have an obvious coupling available to mount the shaft encoder, the ring encoder provides a simple solution. In addition, the rotary encoder can be rigidly coupled to the drive motor like a load, eliminating unnecessary gaps in the system. In any measurement or control system, it is desirable to have the encoder as close as possible to the drive motor, which helps to minimize potential shaft resonances that affect servo performance, especially as the servo bandwidth increases.

Rotary encoders are an excellent solution for providing precise angular position feedback. As with selecting a motor, choosing the right rotary encoder requires an understanding of the factors that affect the accuracy of the encoder and a good understanding of how to overcome performance shortcomings based on actual specifications. When choosing a rotary encoder, it is wise to consider a range of parameters such as data rate, system size, complexity, and cost, in addition to accuracy and resolution. Today, linear gratings can measure with accuracy and resolution of tens of nanometers, while rotary gratings can measure within a corner second. A dime second is a very small angle:
• It can be expressed as the angle corresponding to the arc length of 1 μm at a radius of 206.25 mm.
• It can be expressed as the angle between the 30 m distance on the surface and the center of the earth.
• Resolves to a data rate of 1.3 MHz at 1 rpm.

It is useful to consider accuracy, resolution, and repeatability when determining the required measurement performance:
For applications with high reproducibility requirements (e.g. pick-up devices), the repeated stops of the system at the same grating counting position are more important than the accuracy of the individual table angles.
For continuous smooth motion, the selected encoder resolution and accuracy do not allow jitter errors to occur within the control servo bandwidth.
For slow-moving devices, such as astronomical telescopes, accurate angle measurements are more important than the system's maximum data rate.
For high-speed systems, it may be necessary to make a trade-off between speed and positioning accuracy: Thick pitch (fewer ticks) gratings are suitable for high data rates, but fine pitch (more ticks) gratings typically have lower subdivision errors.