Galil Adds PVT Mode for Smooth Motion

Galil Adds PVT Mode for Smooth Motion - ServoTrends - October 2009

Galil recently enhanced the firmware of the DMC-40x0 and DMC-18x6 Accelera controllers by adding in true PVT (Position-Velocity-Time) functionality. In the PVT mode, the user can specify an ending velocity in addition to a delta position and delta time. This allows for much smoother motion while moving through specified points as compared to the existing Contour Mode (CM) which only specifies the delta time and position. In the Contour Mode, linear interpolation is used between the position points resulting in constant velocity and infinite acceleration between points. This can lead to "ticking" during motion as the velocity instantly changes between contour points. The PVT mode uses a 3rd order polynomial to interpolate between position points as opposed to linear interpolation. This results in no discontinuities in velocity and creates smooth motion. Figures 1 and 2 below show the velocity profiles between contour and PVT mode, given the same position end points and delta time.

Figure 1: Velocity Profile in Contour Mode

Figure 2: Velocity Profile in PVT Mode

In the example above, it is important to note that even though the velocity and position profile between points are different, the ending positions for each segment are the same between contour mode and PVT mode. This means that in systems where end points are critical, but path between points are not, PVT mode can easily replace contour mode for smoother motion. Galil has also developed a software program that can help convert existing contour programs to PVT programs. Please consult a Galil Applications Engineer for details.

Galil's implementation of PVT mode includes two new commands; PV and BT. The PV command follows the format PVn=p,v,t where n is the axis, p is the relative position, v is the velocity to achieve at the end of the segment, and t is the delta time. Each axis on the controller has a 255 segment buffer for PVT points. The BT command is used to coordinate the start of PVT mode across multiple axes. For full command details please refer to the DMC-40x0 Command Reference.

Example: Consider a two-axis stage for moving a silicon wafer along a specific path for cleaning. In this application it is critical that the wafer be at certain points at specific times, but the path between points is not critical. No jerky motion is allowed as it may displace the wafer. This system is a good candidate for PVT motion.

The required points for the stage are below in Table 1. Notice each segment is required to take 512ms, except the last one which must complete in 200ms.

Table 1: Stage Requirements

Given the data in Table 1, the user must now select velocities for each PVT segment. Since the first two segments in the X axis are the same, it is reasonable to select a non-zero value for the ending velocity of segment one. At the end of each of the subsequent segments, the X axis needs to stop so we set the velocities to zero. Note that these are ending velocities; the velocity between points will be non-zero and will follow a second order polynomial path. The Y axis changes direction at the end of each segment and as a result we will choose to set the ending velocities to zero.

By combining the data from Table 1 and Table 2 we can form a DMC program that will follow our profile. The DMC program can be seen in Figure 3. Graphing out the full X vs. Y profile in Figure 4 we can see that each end point specified is achieved in the allotted time. Figure 5 shows the X axis velocity (in red) along with the Y axis velocity (in orange). Both velocity profiles are much smoother then a similar profile constructed in contour mode.

Table 2: Ending Velocities

Figure 3: DMC Program

Figure 4: X vs. Y Graph of Commanded Position

Figure 5: X and Y Velocities vs. Time

By taking advantage of Galil's new PVT mode of motion the system engineer has the ability to profile much smoother motion. The main drawback of contour mode, namely the discontinuity in velocities, is solved by switching to PVT mode. Both new and existing designs would both see a direct benefit from implementing the PVT mode of motion. If you have questions about PVT mode, or your application, please contact a Galil Applications Engineer at 1-800-377-6329 or download the Accelera Command reference at http://www.galilmc.com/support/manuals.php

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