Compensating for Eccentric Loads during High Speed Machining
As part complexity and production demands continue to grow, demand for high-speed, multi-axis machine tools has never been higher. Because of this demand the market has been moving towards machine designs that achieve very rapid acceleration. In order to achieve the rapid acceleration required the use of direct drive motor technology has increased. High-power, rare-earth magnets in direct drive rotary motors and the use of low inertia rotary and linear motors have made top speeds and acceleration rates much higher than were possible even a couple years ago. Increasing the speeds and acceleration rates that the machine is capable of is important to improving productivity but in order to realize this increase - accuracy must be maintained.
The increase in acceleration has introduced additional challenges when trying to maintain part accuracy. One such challenge is the affect of an eccentric load located on the table. It is very common for a part and fixture mounted on the rotary table to be off-center. This results in an eccentric load on the table. When the rotary table with eccentric load is located on top of one of the linear axes there is an interactive force between these axes. When the rotary axis accelerates, there is a centrifugal force and reaction force due to the acceleration applied to the linear axis. The same is true of acceleration of the linear axis; interactive force is applied to the rotary axis during acc/dec of the linear axes. The interaction between axes can be seen as a reduction in accuracy - in the form of a tool mark, step or oscillation at the tool tip. This is especially troublesome during 5-axis, high-speed machining cycles.
Interactive force is not easily compensated due to the variable nature load and speed. The only mechanical solution is to center the load on the table; this is not always practical. The other solution would be to reduce the feeds and speeds of the cutting cycle but at the expense of productivity. A better solution is to apply compensation in the servo system.
Interactive Force Compensation
The servo system of a FANUC 30/31/32i-A CNC (series 30i) is able to control up to 32 servo axes simultaneously via the FANUC serial servo bus (FSSB). This high-speed fiber optic communication means that all servo data and control loops are closed in the CNC at extremely high speed. This highspeed communication allows for compensation features not previously available. The recent addition of “Interactive Force Compensation” (IFC) addresses the problem of force interaction between linear and rotary axes. Once setup IFC enables highly accurate positioning control by compensating for mutual interactive force between axes in the servo software. The effect of this function is easily seen by improved speed and accuracy in 5-axis machining.
IFC is a standard feature on the series 30i CNC that only requires parameter setup and tuning utilizing FANUC Servo Guide PC based servo-tuning software. Various axis configurations are supported including; single rotary-to-linear interaction, rotary-to-rotary interaction, tandem/synchronous axes and compound axes. It is also possible to set IFC such that the interactive force between two axes can be compensated for each. This way the force from acceleration of a linear axis to a rotary is compensated and also the force due to acceleration of the same rotary to the linear can be compensated.
Interactive Force Compensation Example
IFC requires proper parameter settings of relationship between “moving axis” and “compensated axis” according to the actual mechanical configuration. In this example the machine has three axes for which compensation will be set. The rotary table sits on top of a compound X-Y linear axis.
- C axis (rotary axis) that has an eccentric load mounted on X axis (linear axis), and X axis is mounted on Y axis (linear axis).
- There is no interactive force between the X and Y axis, because X axis is orthogonal to Y axis.
- Not only are the interactive force from C axis to X- and Y-axis compensated but also interactive force from X- and Y-axis to C axis is also compensated.
Tuning for Interactive Force
Interactive Force Compensation requires some servo parameter tuning. This is due to the fact that the amplitude of force changes according to the position of the eccentric load on the table and the effect of the force differs between mechanical systems and axis arrangements. Because of the individual nature of the forces on a particular machine an angular offset and compensation gain must be tuned using PC based Servo Guide tuning software.
- At the point A and C in the figure, no interactive force acts on rotary axis as the linear axis accelerates.
- At the point A and C in the figure, interactive force which is produced by acceleration of rotary axis does not act on the linear axis, but interactive force which is produced by centrifugal force of rotary axis acts on the linear axis.
- At the point B and D in the figure, interactive force that is produced by centrifugal force of rotary axis is traverse to the linear axis and does not act on linear axis, but interactive force that is produced by acceleration of rotary axis acts on linear axis.
Effect of Interactive Force Compensation
The result of acceleration and centrifugal forces from the moving axis are seen as a position deviation in the compensated axis. Once the tuning of the angular data and compensation gain is completed IFC will eliminate the effect of interactive force on the compensated axis. A significant reduction in position error can be seen resulting in better part accuracy, reduction in tool marks and reduced chatter.
During high-speed machining utilizing both rotary and linear axes an eccentric load can result in significant position error due to the interactive forces. The centrifugal force and acceleration force applied is not easily compensated in a conventional CNC machine – typically, these forces limit the speed and accuracy. FANUC has developed Interactive Force Compensation as a means of reducing position error due to eccentric loads during high-speed machining, expanding the limits by allowing for higher production rates and better accuracy from high speed and 5-axis machining systems.
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