Regressive Auto Spline (RAS)

This feature automatically fits a modified, high-order rational spline curve to the incoming digital command stream, before it is passed to the position/velocity compensator. The duration of the RAS spline segment on the incoming profile is configurable using QuickSet™.

This smoothing action has several benefits:

  1. The RAS actively morphs the velocity into a "double" jerk-limited profile (enforcing not only a limit in the rate of change of acceleration or "jerk," but also in the rate of change of the jerk). So you can get all the smoothness benefits of jerk-limited movement—reduced wear on your mechanics, lower acoustical noise, less jostling of the load, minimized ringing of the mechanism at the end of moves, etc. using an inexpensive controller (indexer) that generates only trapezoidal (linear acceleration) type moves.

  2. Because the resultant moves are so smooth, the tracking accuracy is increased which typically results in reduced settling time as well. The motor will also run cooler (or its size can be reduced).

  3. The RAS function effectively interpolates the incoming velocity profile and its derivatives to a high degree of resolution, providing extremely smooth inputs to the position/velocity compensator for use in acceleration and velocity feed-forward calculations. This allows the feed-forward gains to be increased to their optimum level without creating the acoustical noise or torque chatter problems that other digital positioning servo drives suffer from, and which limit their use of feed-forward gains to inadequate levels. (See the following two scope photos).

This scope photograph shows the commanded velocity (top: 3000RPM/div), tracking accuracy (center: 2 degrees/div.) and the torque used (bottom: 25% of max./div.) without the RAS is engaged. Notice that the torque required has significant "chatter". With 200mS of dwell between moves, this move required 60% of the motor’s RMS capability.
The effect of the RAS on the same move’s tracking accuracy and torque usage is dramatic. Notice the freedom from "torque chatter" when the RAS is engaged—This is one reason why the RAS makes axes so quiet. The RMS load on the motor was reduced to 37%!

4. Ultra-smooth motion with high top speed capability can be attained with very inexpensive controllers that have a low maximum pulse rate. This is because even with the SST servo drive configured for a low-resolution input (low number of steps per revolution), the RAS will generate a smooth, high-resolution internal profile for minimum noise and torque chatter. Because of this, the SST servo systems will operate silky smooth even when used with inexpensive pulse outputs from low cost PLCs, "soft" parallel port indexers, etc. leading to a low cost of control.

5. The RAS will smooth out step discontinuities or drop-outs present in a controller’s velocity profiles. These problems are very common in in-house built step motor controllers that may be interrupted by other machine control functions or where the step generation algorithm was not completely perfected. When an SSt servo drive is presented with these less-than-perfect velocity profiles, it will fill in dropouts and smooth large step changes in speed, increasing the finesse of these profiles, extending the useful life of your existing control solution and control software.

Notice that even this rather choppy profile is made into an ultra-smooth move by the RAS.

6. The RAS feature enables a new mode of high resolution "zero-cost" soft control—WorldServo has developed a method of trajectory (velocity profile) generation that allows high-resolution digital command streams to be generated by a CPU with only software and a basic I/O port. If you are interested in a description of this technique and performance benchmarks, download the following application note:
SST Soft Controller Ap-Note [PDF]

The RAS feature (together with the MoveDone output, Hard Stop Homing, Limits Switch Homing, Torque Fold-back modes and flexible input resolution) helps enable the use of low cost controls that lack encoder feedback inputs, even in machines that require state-of-the-art performance.

Benefits Supported
 Ultra-smooth Motion  Faster Moves and Settling Time
 Excellent Tracking Accuracy  Reduced Wear on Mechanics
 Reduced Acoustical Noise  Reduced Motor Size
 Cooler Running Motors  Enhanced Design Flexibility
 Works Well With Low Cost Controls  
 Simplified Control Software Development
 High Performance Drop-in Replacement for Stepper Motors
 Master Features/Benefits Menu