Motion control and CNC accuracy

CNC applications need accurate movements, rapid changes in direction, and aggressive acceleration and deceleration. A virtual sensor system can be used to improve control performance and decrease CNC contour errors.
By Derrick Stacey November 4, 2014

Contour errors decrease with analysis of virtual sensor technology from B&R. Courtesy: B&R Industrial AutomationCNC applications often require extremely accurate movements, rapid changes in direction, and aggressive acceleration and deceleration all while using mechanical configurations that make this quite challenging. Motion control schemes are available to make this possible, but in many cases require the use of load side encoders, which may not be mechanically feasible. As a result, only the motor encoder is used for servo control loop feedback, possibly resulting in slower feed rates or less accuracy due to excessive oscillations in the system. Neither result is optimal for machine builders or their end users. 

Virtual sensor technology

With this in mind B&R has developed virtual sensor technology to enhance the controllability of these physical systems. This technology uses a mathematical model of a simple two-mass system and runs a state observer on the servo drive using the physical parameters of the system taken from mechanical design software (such as SolidWorks from Dassault Systèmes) to calculate load side feedback. By running this state observer on the servo drive at sub-millisecond cycle times, potential fieldbus communication delays to the PLC can be eliminated.

The resulting control performance can greatly reduce contour errors between the planned path and the actual tool path even with challenging mechanical arrangements with inertia mismatches as high as 200:1.

– Derrick Stacey is solutions engineer, B&R Industrial Automation; edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske@cfemedia.com.

Key concepts

  • CNC applications need accurate movements, rapid changes in direction, and aggressive acceleration and deceleration.
  • A virtual sensor system can be used to improve control performance and decrease CNC contour errors.

Consider this

How can smarter feedback analysis increase accuracy in your motion application?

ONLINE extra

For more about using encoders to increase motion control accuracy, see the related Control Engineering articles below.