Achieving Positioning Accuracy Goals
- Friday, 01 December 2006
Repeatability and Accuracy
Motion systems are usually specified to perform any one or a combination of three different types of moves with consequential backlash and hysteresis. These include unidirectional, point-to-point bi-directional, and contouring modes. Unidirectional moves require point-to-point repeatable moves with the destination point being approached from only one direction. But, compared to repeatability, accuracy for these same systems is more difficult to achieve, because it must also serve as a measurement system. However, highly repeatable, unidirectional systems usually are also highly accurate.
Bi-directional repeatability is more difficult to achieve because of backlash and hysteresis. Many motion controllers can compensate for highly repeatable gear backlash, but fail to handle other components that have less predictable backlash. Supplying a secondary encoder for the position information can compensate for backlash. High accuracy/ repeatability systems often use a position sensor outside of the motor. Care must be taken with these systems since the hysteresis or dead-band would be enclosed within the position loop.
Hysteresis is evident when the system is commanded to reach the same destination from opposite directions. A rotary encoder coupled to the motor would indicate that the load reached the same destination, but the actual position difference is larger than the backlash alone. This hysteresis is caused by unseen clearances and elastic deformations. A linear encoder can compensate for backlash and hysteresis in a screw-driven positioning system. The hysteresis must still be minimized to avoid the aforementioned control problems with oscillation, and systems with hysteresis potentials must have friction minimized.
Some of the less obvious but critical errors come from the support structure (machine base), certain types of bearings, and couplings. The errors are specified as roll, pitch, yaw, straightness and flatness, straight-line accuracy, and resolution. A machine base must be machined to critical straightness and flatness specifications and ground to eliminate angular errors and ensure the drive and encoder accuracy. Aluminum, steel, stainless steel, and cast iron are typical materials used. Aluminum’s advantages include easy fabrication, having a light weight, and a relatively low cost. Stainless steel is used for applications needing high corrosion resistance. Cast iron is best for damping vibration in machine tool applications.
Two major types of errors must be considered in multiple axes systems: orthogonal and stack-up. Orthogonal errors come from axes that are not truly perpendicular. Stack-up errors occur when one motion axis supports another, such as the X-axis riding on the Y-axis. Although the angular errors of one axis affect the other axis, they can be compensated because they are highly repeatable. The critical thing to know is that they are present.
Another factor to consider is the effect of one axis on another. In a three-axis system, with the X-axis components riding on the Y-axis and Z-axis, numerous displacements between them arise from torque, skew, and lead screw errors. The errors in each axis may be as little as five or six microns, but they add up. The mechanical structure of the system supporting all the components must be rigid enough to prevent excessive distortion and consequential inaccuracies.
When specifying multiple axes, collision avoidance must also be dealt with. Many times, interacting multiple axis systems can have an overlooked error that the path to two specific “safe” positions can create a collision as well as the standard collision avoidance of ensuring that two axes cannot simultaneously occupy the same space. It may require an undertaking of the machine software, the motion controller software, or both.