What Effect Does Gain Have on Performance?
The higher the gain, the less error (E) required to break friction
or maintain velocity. The error required to
break friction will affect position accuracy at the end of a move,
which makes it a major factor in
achieving repeatability. The error to break static friction can be
measured with the loop closed by slowly
changing the command (C) by its least increment while observing the
buildup of the error (E). As noted
earlier, a velocity loop will have a major impact on the error
required to break friction. This test should be
done at several points along the travel since mechanical variations
will cause the breakaway friction to
change.
Another common problem is null hunt, a phenomenon in which an axis
moves back and forth with a
square waveform at a low frequency. This is usually caused by the
breakaway or static friction being
significantly higher than the running friction. Essentially, the
error builds up to break friction, but once
motion starts the error is more than necessary to maintain the
desired velocity so it overshoots the desired
position. This continues to repeat in both directions. It can be
prevented by lowering the gain, however
lowering the gain will also affect accuracy. Lowering the ratio of
static to running friction can be
achieved with roller bearings or, as is more common now, through
the use of a special coating material as
one of the bearing surfaces. A static to running ratio of 1.01 or
less is achievable in this manner.
Accuracy during motion is a concern in many applications. Cutting
metal, routing wood, etching glass,
and grinding silicon wafer edges are examples where extreme
accuracy during motion is required. A
servo with a gain of 1 IPM/MIL will have 0.001" of error when
traveling at 1 IPM, 0.01" at 10 IPM and
0.1" at 100 IPM. It follows that the best accuracy can be achieved
by keeping velocities low and gain
high. This is a good generality, but not always that simple to
achieve.
Configuration of Servo System
The following diagram illustrates a servo system in detail:
(1) Controlled system: Mechanical system for which the position or
speed is to be controlled.This includes a drive system that
transmits torque from a servomotor.
(2) Servomotor: A main actuator that moves a controlled system. Two
types aravailable: AC servomotor and DC servomotor.
(3) Detector: A position or speed detector. Normally, an encoder
mounted ona motor is used as a position detector.
(4) Servo amplifier: An amplifier that processes an error signal to
correct the difference between a reference and feedback data and
operates theservomotor accordingly. A servo amplifier consists of a
comparator, which processes error signals, and a power
amplifier,which operates the servomotor.
(5) Host controller: A device that controls a servo amplifier by
specifying a positionor speed as a set point.
Servo components (1) to (5) are outlined below:
(1) Controlled system
In the previous figure, the controlled system is a movable table
for which the positionor speed is controlled. The movable table is
driven by a ball screw and is connected tothe servomotor via gears.
So, the drive system consists of:
Gears + Ball Screw
This drive system is most commonly used because the power
transmission ratio(gear ratio) can be freely set to ensure high
positioning accuracy. However, play in thegears must be minimized.
The following drive system is also possible when the controlled
system is a movable
table:
Coupling + Ball Screw
When the power transmission ratio is 1 :1, a coupling is useful
because it has noplay.
This drive system is widely used for machining tools.
To develop an excellent servo system, it is important to select a
rigid drive system that has no play. Configure the controlled
system by using an appropriate drive system for the control
purpose.
Timing Belt + Trapezoidal Screw Thread
A timing belt is a coupling device that allowsthe power
transmission ratio to be set freelyand that has no play.
A trapezoidal screw thread does not proviexcellent positioning
accuracy, so can be treated as a minor coupling device.
IN STOCK Mitsubishi Servo Motor HC-RFS103 / HC-RFS153 AC Industrial Motor 1kw
