Problem

An undersized servo system is one that lacks the ability to accelerate the load at a rate sufficient for the machine. Here, the drive, motor, or motor/drive combination is too small for the application. Undersized servo systems are not traditionally thought of as servo control problems. However, it is covered here because machine designers sometimes encounter the problem when working on servo performance problems. Also, it can appear very much like a servo problem.

Undersized servo systems can only be observed when the system is in saturation (when the drive is at maximum current). For a control system operating inside the limits of the current controller, time to respond to a command is independent of the amplitude of the command. For example, a servo system that responds to a 5 RPM command in 10 mSec will usually respond to a 2 RPM in about that same time period. The response to these low command levels are presummed to be satisfied without saturating the current controller. This is often called small-signal command response.

When the commands get larger, say 2000 RPM, the current controller saturates. In this case, the time to respond to the command is the sum of the acceleration limit and the small-signal response. This is often called large-signal command response.

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 between the two plots). During a large command, the acceleration rate is limited by the size of the motor and drive.

Velocity vs. time for a typical undersized system. Comman is shown above and resonse below. The time scale is 0.02 sec/div. With a 100 RPM command (left) the response appears sluggish. The same system with a 10 RPM command (right) shows the servo is actually responsive. Note the flat slope on the response at left (about 100 RPM in 0.02 sec) inidicates that the current output is clamped and the servo is producing peak acceleration.

Verification

You can verify this problem by observing the current required to make the acceleration in question. If the current amplifier is saturated (clamped at minimum or maximum output) during most of the acceleration, the acceleration time is limited by the amplifier or motor size. Changing servo gains will make minimal improvement on this problem.

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