06 Numerical Control 2 - Interpolation Flashcards
Input Signals of an NC
- NC-Program
- Tool data
- Offset Tables
- Sensor Signals
- Override
Processing of NC and drive units
o Interpreter: Works as a syntax analysator (parser) -> translates the different formats of NC programs or input data to an internal, machine readable format
o Geometry Processing: Calculates reference points for control with all corrections and data based on the position of the path edge points
o Control: Cascaded control loops have the drives follow the commanded values
Output of an NC
In the central drive unit pulse modulated signals are converted to voltages that power the motors and create rotational movements
Tool Path Correction - Indirect Programming
Contour of the workpiece is programmed, not the path of the tool
o Compensation needed, realized by the controller
o Less programming effort
o Parameters of tool (diameter, length, geometry) need to be very precise
Tool Path Correction - Direct Programming
Exact path of the tool is programmed
o Higher programming effort
Collision Avoidance
o NCs offer a functionality for collision detection -> Do not only execute the current NC line, but also take a look at the following lines and check if these lines might cause collisions.
o If a possible collision with current segment is detected, the segment’s length is reduced and the tool movement is continued with the found, formerly intersecting segment
Pitch Error Compensation for spindles of a lathe
o Spindles and the turret of a lathe suffer from wear and manufacturing tolerances
o Lead/pitch of thread is not constant for the whole length of a spindle -> This error can be measured and reported in tables and the error can be compensated by NC
Kinematic Transformation
Transformation of the programmed tool path to axis movements in the corresponding machine coordinate system
- NC programming is usually performed using the part coordinate system to avoid complex calculations NC must perform a transformation of the workpiece coordinate system (WCS) to the machine coordinate system (MCS)
- The kinematic of a machine is the structure of the machines motion axes
- Transformation from WCS to MCS is called inverse transformation -> Must be performed for every point that is interpreted by the interpolator and during every interpolation cycle
o No real-time behavior required -> Usually used to calculate and display current workpiece position data mostly for informational purpose
Serial Kinematics
Straight line movements in one coordinate of the machine coordinate system (MCS) can be executed by moving only one axis
Parallel Kinematics
A spindle platform is moved through simultaneous, parallel movements of multiple machine axes (e.g. hexapod)
Velocity Control
Adjusts velocity and acceleration according to the programmed feed rate under the surrounding conditions.
- Important constraints: Dynamical limits (e.g. acceleration) -> may cause damage when exceeded
o Compromise when selecting the limits: High workpiece quality & short manufacturing times - Typical acceleration of an interpolation: Constant acceleration -> Linear increase of speed -> Quadratic increase of the position
- Modern controls also allow to limit jerk while calculating the velocity profile
o With jerk limitation the jerk is not pulse-like but instead limited as a rectangular profile -> More phases that allow a smoother machining
Look-Ahead Function
- Analyses multiple, usually 10-100, NC blocks in advance regarding the resulting speed and direction changes of the path.
o Modified speed profile can be calculated for the given blocks through a model of dynamic behavior of the machine and its feed axes - Gives controller the information that e.g. there will be no need to decelerate in the next n blocks
- Recognize direction changes at high feed rates and optimally adjust the velocity profile in time
- Make feed adjustments to fit the required cutting conditions
- Reduces unnecessary decelerations between NC blocks
- Faster and more precise machining
Tolerance-Based Interpolator
Instead of passing exactly through all points defined by the NC program or generated by the rough interpolator, a slight error is accepted. Not generally valid but can be switched on by the NC program
Advantages of a tolerance-based interpolator
o Allows smooth and faster movements
o Saves time
o Improves the surface quality of the workpiece
o Reduces stops and accelerations and therefore reduces vibrations
Linear Interpolation
o Active tool is moved along a straight line from the current starting point is moved to the programmed destination with a well-defined velocity. All axes can be moved simultaneously
o Usually performed by directly calculating the axis values
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