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 Teknic’s drive configuration software, 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. This is especially important when using an inexpensive controller (or indexer) that generates only trapezoidal (with linear acceleration) motion profiles.
  2. Because the resultant moves are so smooth, the tracking accuracy is improved. This typically results in reduced settling time as well. Additionally, the motor will run cooler (or its size can be reduced).
  3. The RAS function effectively interpolates the incoming velocity profile and its derivatives at a high degree of resolution thus 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 screen captures).
    4. Commanded velocity, tracking error and actual torque used without RAS
    Figure 1: Commanded velocity, tracking error and actual torque used without RAS

    This scope shot shows the commanded velocity (top: 3000RPM/div), tracking accuracy (center: 2 degrees/div) and the actual torque used (bottom: 25% of max torque/div) without the RAS engaged. Notice that the torque required during this move has significant "chatter". With 200ms of dwell between moves, reciprocating this move required 95% of the motor’s RMS capability.

    Same commanded velocity with RAS applied<
    Figure 2: Same commanded velocity with RAS applied

    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 motion so quiet. Also, the RMS load on the motor was reduced by 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, 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., enabling 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 with 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.
    6. RAS smoothes step discontinuities in velocity commands<
    Figure 3: RAS smoothes step discontinuities in velocity commands

    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—Teknic 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, contact your Teknic Applications Engineer for a copy of the SSt Soft Controller app note.

The RAS feature (together with the MoveDone output, Hard Stop Homing, Limit Switch Homing, Torque/Force Foldback 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.

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