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6 x 6 inch 3D Part
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Machine productivity can often be dramatically improved without increasing optimal process speeds
NEE have been making machine controllers for more than 25years. During this period our controllers have evolved to meet or
exceed our customers needs.
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The design goal of our current AMC motion controller range was to be able to operate the machine at the machine's limit.
What did we mean by this? surely every controller manufacturer would say the same. Besides, I often hear, "surely a given machines
productivity is limited mostly by the process being carried out the machine", well not exactly! Our customers have always wanted us
to make their machines more productive. Machine controllers had limited computing power, limited memory, and the embedded software control algorithms
were very simple in comparison to those used in 'state of the art' systems today.
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When the productivity of a typical machine is analyzed carefully, a number of timewasting events
become apparent, these include:
- The time taken by the operator to load the item being processed
- The time taken to transfer the part program to the machine control
- The setting up time taken by the operator to make the machine operate
- The time taken by the machine to run the part program performing the operation being carried out
- The time taken by the operator to unload the processed part
- The quality and precision of the finished part, poor quality increases the work required subsequently
- The down time of the machine
As a machine control developer and manufacturer we focus mostly on items b), c), d), f) and g).
Items a) and e) are generally the responsibility of the machine designer.
We resolved b) by making motion controllers that can accept data at a very high rate, in 1990
for example when competitive machines could accept part program data at up to 19,200 baud (bits per
second), ours accepted data at up to 115,200 baud. We also designed our controllers to accept the
next part program while the current one was running.
We minimise item c), the time taken by the operator to run the machine, by designing the control
operating sequence to minimise operator keystrokes, and more recently, by enabling our customers
to rapidly design their own PC based custom motion control panels with ‘point’ ‘click’ and ‘drag’
operations using our WinAMC® software.
Addressing item d) the time taken by the machine to run the part programs, is much more complex.
Part programs for numerical machines comprise of a series of machine positions, defining the
required machine path, which are to be executed sequentially. These machine positions are
generally produced by computer aided design ‘CAD’ programs in job files which can be very large
when they define three dimensional ‘3D’ parts.
There are often also machine movements required between machine paths to for example move form one
path to another or to change tools, these must be executed as fast as possible.
Most machine processes have an optimum speed which is usually limited by the process being carried
out, the aim is to operate the machine at this optimum speed as much as possible.
Our early motion controllers were made in 1970’s, these made point to point movements in straight
lines. Between every movement the machine would decelerate and stop so the controller could
calculate the next move. When this was ready the machine would accelerate again to, if the move was
sufficiently long, the required speed, then decelerate so the next calculation could be done. The
result was that machine movement was made with a lot of pauses.
The next step was to join movements together, wherever possible to minimise the pauses. This
required the motion controller to always look ahead by two machine movements and to calculate the
direction or angular change between them while the machine was moving. If the angular change was
small no deceleration was required between the moves. If the angular change was large the machine
would have to be decelerated to a stop at the junction then immediately accelerated to make the
next movement.
A refinement of this was to determine the angle between the moves then decelerate the machine
accordingly, small angles required a smaller deceleration at the join.
In reality an angular change in machine movement involves changes in motor speeds, so motion
controllers work with speed changes at move intersections in all moving motors to minimise time
wasting. However the algorithms used by competing motion controllers vary a lot, the trick is to
design motion controller software that gives the best performance but requires the minimum
computing time.
The next improvement was to look many more moves ahead calculating optimum speeds between each
move pair.
The machine speed at any time is dependent on how far the motion controller can look ahead as it
must always be possible to stop the machine in the event of a large direction change just beyond
the horizon.
Many modern motion controllers look around 10 moves ahead, AMC controllers
however look a sufficient distance ahead so that the optimum machine speed can be achieved wherever possible.
If the motion controller only looks a limited number of small moves ahead the total distance is often
too short to achieve the optimum speed. In complex part programs defining 3D shapes moves are
generally very short, by looking a sufficient distance ahead, which could be 100’s of moves, the
AMC achieves the optimum speed
wherever possible.
A further factor impacting the productivity of machines driven by servomotors are the algorithms
that convert desired machine movement into motor motion. AMC’s have unique but
extremely stable algorithms combining motor velocity and following error, the difference between actual and
desired positions. These algorithms result in extremely smooth machine motion giving the best part finishes
and, at the same time, extremely high part accuracy. An added advantage of good algorithms is that inexpensive
servomotor drives can be used reducing total systems costs. A more technical description of how we achieve
maximum productivity can be found in the machine movement and other articles in
the technical papers section.
A factor in machine reliability is the reliability of the machine controller. Our
AMC system design simplifies system hardware – the electronic components, and makes
maximum use of system software. We also design and build our machine controllers for demanding industrial environments.
In the final analysis the machine’s productivity is what counts, a growing number of customers
are switching to our controller in part because of the often dramatic productivity improves seen.
The benefits for customers section gives many real examples of what
our controllers have achieved.
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