Bob Arnold
Feb 21, 2003, 03:22 PM
Perhaps this machining problem has been solved by members of this
group. All references to tool would be replaced by "propeller hub".
Anyways the questions are;
1. Does anyone know of a tutorial or survey of vibration control
methods used in servo systems.
2. What are the advantages/disadvantages of single/multi axis
vibration control.
3. Are there examples of either type of control.
Thanks for your help.
The vibration control techniques defined (my defs. just to help me put
the question) are;
Single-axis control – vibration is sensed (preferably near the tool
point of contact with the work) and corrected by actuating the servo
"closest" the tool. Although used by machine tools, this type of
control might be analogous to controlling vibration of the space arm
or an airplane.
Multi-axis control – vibration is sensed (preferably near the tool
point of contact with the work) and corrected by actuating multiple
servos controlling multiple axes.
In the dialog below the quoted sections are comments from another
person.
I have partially built a six axis CNC wood milling machine whose
dimensions are 10' X 5' X 4' . The tool platform is designed to
control forty-five degrees of attitude in all directions. The target
precision of the machine will be twenty-five thousands of an inch. I
built several prototype motor controllers. Digital hardware/software
consists of closed loop PID control code embedded in PIC
micro-controllers. I have added on board DRAM to a 20MHz PIC chip to
be able to store and process measurement history quickly.
As the machining head is not rigidly attached to the base (and the
workpiece) but rather through 4X8 and 4X4 square tubular steel arms
and bearings, I wanted to control vibration. If you want to control
displacement (horizontal or vertical), you have to know its value. You
can either measure it directly with an accelerometer, or attempt to
infer it from other data and an elaborate and changing model.
"A cutting tool causes sideways pressure and deflection. In general,
all axes will need to respond to maintain the location if the cutting
point. Changes of rotational speed that might be minimized by a
flywheel are probably not important anyway, but lateral flexing hurts.
Lack of stiffness makes the milling attachment for my 6" lathe
unsuitable for fine work." It's pretty clear that if you want to
control the motion of a part, you need to know where it is. How far
away you can reasonably measure it depends on the stiffness of the
part and its mounts, and on the needed accuracy. It is sometimes
simpler and cheaper to use massive precision parts than lighter ones
with precision guidance, but the history of ruling engines now points
the other way. Whether or not you are on the right track, you are
certainly in tune with the times."
"At low cutting speeds, a flywheel assist may be impractical, and the
servo system has to supply the stiffness. The implication is that the
servo's response must take place in a small part of a turn. The servos
I alluded to above can be much more nimble than PIDs, and might help
if a stiffer drive is needed. Whatever paradigm you use, a tach
directly on the spindle will help to wash out stretch and slippage in
the drive. Of course, with direct drive, that question doesn't arise
(although if the servo is snappy enough and the spindle limber enough,
the performance may depend which the end of the spindle is measured)."
"All servo milling designs that I know -- as opposed to NC machines --
measure position at or very close to the cutting point, thereby
loosening the constraints on backlash, leadscrew error, and way
straightness needed to achieve a specified accuracy. It is relatively
easy in one dimension, practical in two, but I've not seen it in
three."
group. All references to tool would be replaced by "propeller hub".
Anyways the questions are;
1. Does anyone know of a tutorial or survey of vibration control
methods used in servo systems.
2. What are the advantages/disadvantages of single/multi axis
vibration control.
3. Are there examples of either type of control.
Thanks for your help.
The vibration control techniques defined (my defs. just to help me put
the question) are;
Single-axis control – vibration is sensed (preferably near the tool
point of contact with the work) and corrected by actuating the servo
"closest" the tool. Although used by machine tools, this type of
control might be analogous to controlling vibration of the space arm
or an airplane.
Multi-axis control – vibration is sensed (preferably near the tool
point of contact with the work) and corrected by actuating multiple
servos controlling multiple axes.
In the dialog below the quoted sections are comments from another
person.
I have partially built a six axis CNC wood milling machine whose
dimensions are 10' X 5' X 4' . The tool platform is designed to
control forty-five degrees of attitude in all directions. The target
precision of the machine will be twenty-five thousands of an inch. I
built several prototype motor controllers. Digital hardware/software
consists of closed loop PID control code embedded in PIC
micro-controllers. I have added on board DRAM to a 20MHz PIC chip to
be able to store and process measurement history quickly.
As the machining head is not rigidly attached to the base (and the
workpiece) but rather through 4X8 and 4X4 square tubular steel arms
and bearings, I wanted to control vibration. If you want to control
displacement (horizontal or vertical), you have to know its value. You
can either measure it directly with an accelerometer, or attempt to
infer it from other data and an elaborate and changing model.
"A cutting tool causes sideways pressure and deflection. In general,
all axes will need to respond to maintain the location if the cutting
point. Changes of rotational speed that might be minimized by a
flywheel are probably not important anyway, but lateral flexing hurts.
Lack of stiffness makes the milling attachment for my 6" lathe
unsuitable for fine work." It's pretty clear that if you want to
control the motion of a part, you need to know where it is. How far
away you can reasonably measure it depends on the stiffness of the
part and its mounts, and on the needed accuracy. It is sometimes
simpler and cheaper to use massive precision parts than lighter ones
with precision guidance, but the history of ruling engines now points
the other way. Whether or not you are on the right track, you are
certainly in tune with the times."
"At low cutting speeds, a flywheel assist may be impractical, and the
servo system has to supply the stiffness. The implication is that the
servo's response must take place in a small part of a turn. The servos
I alluded to above can be much more nimble than PIDs, and might help
if a stiffer drive is needed. Whatever paradigm you use, a tach
directly on the spindle will help to wash out stretch and slippage in
the drive. Of course, with direct drive, that question doesn't arise
(although if the servo is snappy enough and the spindle limber enough,
the performance may depend which the end of the spindle is measured)."
"All servo milling designs that I know -- as opposed to NC machines --
measure position at or very close to the cutting point, thereby
loosening the constraints on backlash, leadscrew error, and way
straightness needed to achieve a specified accuracy. It is relatively
easy in one dimension, practical in two, but I've not seen it in
three."