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Servo systems are known for being able to produce rapid motion in response to a command. Unfortunately, some servo systems do not provide the response the original machine designer expected. Problems if insuffcient responsiveness to a command is normally seen in one of three ways:
Low bandwidth is where the servo system tuning only supports modest response rates. The servo responds to rapid commands in a sluggish manner. Low bandwidth is most easily demonstrated with small-amplitude commands, where the current loops remain out of saturation.
Typical slow response (left) compared to rapid response (right) in a servo system.
(Square-wave velocity command above and response is below.)
Slow settling is where the servo is sluggish when coming to rest. Otherwise, the servo may be considered responsive. For example, the response to a 0 to 100 RPM command may be acceptable, but the response to return to 0 RPM may be too sluggish. Slow settling problems usually occur below 5 or 10 RPM.
An undersized servo system is one that lacks the ability to accelerate the load at a rate sufficient for the machine. Here, the drive or motor is too small for the application. This problem is seen with large-amplitude commands. The responsiveness of the servo may be excellent with small-amplitude commands, as shown in the figures below (note vertical scale changes bewteen the two plots). During a large command, the acceleration rate is limited by the size of the motor and drive.
Typical response of an undersized system (left).
The small signal response (right) demonstrates the servo gains are resonsive.
(Note vertical scale is 100 RPM/div at left and 10 RPM/div at right).
