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SIMILAR PRODUCTS
SGMAH-02AAA61D-OY
SGMAH-02AAA61D-YO
SGMAH-02AAA6C
SGMAH-02AAA6CD-0Y
SGMAH-02AAA6SD
SGMAH-02AAAG761
SGMAH-02AAAGB61
SGMAH-02AAAH161
SGMAH-02AAAH76B
SGMAH-02AAAHB61
SGMAH-02AAAJ32C
SGMAH-02AAAJ361
SGMAH-02AAA-SB12
SGMAH-02AAAYU21
SGMAH-02AAF4C
SGMAH-02ABA21
SGMAH-02ACA-SW11
SGMAH-02B1A21
SGMAH-02B1A2C
SGMAH-02B1A41
SGMAH-02B1A6C
SGMAH-02BAA21
SGMAH-02BAA41
SGMAH-02BAAG721
SGMAH-02BBA21
SGMAH-03BBA-TH11
SGMAH-04A1A2
SGMAH-04A1A21
SGMAH-04A1A2B
SGMAH-04A1A2C
SGMAH-04A1A41
SGMAH-04A1A4B
SGMAH-04A1A4C
A normal servo, which lags the command, will round the corner as the second axis command builds up and the lag in the first axis dissipates. With feedforward, since the machine is coincident with the command when the corner is reached and the feedforward for the first axis is removed in a step fashion, the first axis will respond similar to its step response.
What About Servo Update Time?
Control vendors update their servos on a regular time basis. Servo update time is the time interval between
the calculations for command (C) minus feedback (F) to give error (E). In other words, it is how often a correction (E) is calculated. Update times vary from microseconds up to 16 ms (milliseconds) for most
controllers. Some vendors initially updated servos once per scan with their programmable controller.
However, variations in scan time caused severe servo problems, especially when axes needed to be
coordinated. Typically the software for closing the servo loop is written in assembly language which is OK
since it is invisible to the user. Assembly language is the best to use because it is the most efficient with
computer time, especially since it interrupts the computer so often. To conserve computer time, it would
seem logical to choose the longest servo update time possible, however, sample data theory would tell one
to choose the shortest. In a study of this conducted by myself and Dr. John Bollinger of the University of
Wisconsin, the conclusion was that a 10 ms servo update time would be more than adequate for heavy duty
industrial machinery. This has proven to be true.
Many vendors are also closing the velocity loop digitally with the computer. Since the velocity loop is
inside the position loop, the velocity bandwidth must be at least five times greater, thereby requiring a
sample time of 2 ms or better.
There are two basic approaches to computer closure of the servo loop. One is to devote a computer to each
axis. The other is to have a central computer that closes all axes. Each vendor will have a list of reasons why his is best, but here are several basic observations.