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Plasma cut 18awg sheet metal
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Blending moves increases machine productivity
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It is not always a smooth path when you use computers to merge previously distinct engineering
functions warns Chris Williams of NEE Controls Ltd
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Computer aided design packages draw using a series of straight line vectors, arcs, bezier and spline
commands which can be blended together to give apparently perfect curves where required. At least that
is how it appears to the designer as he looks at his screen. But if you transfer the CAD files
straight to a CAM environment and use it to drive say a milling cutter, you realise your nice
curves are still vector approximations.
To obtain smooth and continuous axis velocity
profiles a motion planner must be used. This not only optimises the curve but ensures the
required motion is generated within acceleration, deceleration and maximum speed limits.
This prolongs machine and drive system life, reduces tool wear and maintenance, cuts cycle time
and improves workpiece quality.
To achieve this, the motion planner must know what is to be
executed ahead of what is currently being executed. The term for this function is "look-ahead" and
constitutes the main criteria to smooth motion. Some motion controllers leave this to the CNC programmer.
But it is better if this complex problem is resolved by the motion controller as the permissible
velocity changes are a function of the machine being controlled and not the part program.
A simple, yet effective solution is to let the system builder configure a set of parameters for
each axis which set the maximum allowable velocity change at minimum speed and the maximum
allowable velocity change at maximum speed. Using these parameters the control equations can
interpolate to obtain the intermediate permitted velocity changes for any given change of direction.
With these parameters the step change at the intersect can be controlled and it is left with the
system builder to trade part quality against job cycle execution time.
For a continuous motion profile without step changes each vector must be smoothly blended with the next
A further degree of control needs to be embedded within the motion algorithms to enhance part
quality. When decelerating all axes to a stop due to a large intersect angle having been
specified, it is often necessary to pause at the intersect before accelerating off in the new
direction. This pause is required to give the stopping axes time to settle and prevents a wavy
cut being machined. The length of dwell should be proportional to the size of angle change to
minimise the overall effect on the job execution time but still large enough to attain the required
cut quality. A simple way to achieve this is to specify the minimum angle change for which a
minimum pause will be required and specify the pause required for a complete reversal of
direction and then interpolate. To achieve a continuous motion profile without velocity step
changes the vectors must be blended to each other with other curves. The blend used is
parabolic in shape and allows a smooth change of velocity, but not acceleration between successive
vectors.
Whilst blending allows faster overall machine motion, the trade off is part
accuracy since at the blend the tool deviates from the programmed path. The permitted deviation
distance should be a parameter the system builder can configure to permit the trade off to be made.
Before assuming it is not acceptable to deviate from the programmed path consider how
accurately and repeatable the machine can position. Also consider how well the servoing function
can follow the path specified for it and bear in mind the specification of the component being
machined as the tolerance may be greater than the deviation distance. In many cases the extra
amount of productivity offsets the small inaccuracy of the component. In some cases the component
accuracy may be improved due to the velocity profile being smoother.
Once the final velocity profile has been specified by "looking-ahead", limiting intersect velocities and
blending where possible the next stage is to break it up into set points. Set points are either
velocity or positional commands to be executed at specified time intervals. The time interval is
generally termed "trajectory time", typically 1-30ms.
Once a smooth, accurate coordinated set
of velocity or positional command profiles for each axis has been generated all that remains is for
them to be faithfully reproduced by each axis driving motor. This is normally achieved with an optical
encoder mounted on the motor shaft, providing feedback information.
However feedback systems
exhibit lag times between commanded and actual trajectory. One solution is to add extra intelligence
into the compensation loop by modifying the control signal based upon the compensation loop knowing what
the commanded velocity and acceleration/deceleration is, ahead of the current set point. In
the quest to achieve higher precision component manufacture it has become necessary to overcome
mechanical resonance. A digital notch filter placed at the output of the compensator loop may be used
to counteract the physical resonance. Multiple order filters can be implemented by cascading.
Blending Moves article originally published in " Drives & Controls " magazine.
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