"The U.K.'s foremost motion control manufacturer.
Leading the world in motion control systems"
 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. |