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Motion design for packaging machines

Motion design for packaging machines
As soon as something moves in a machine, then there is a need to specify a motion for it. More typically, a number of motions must to be weaved together. Kevin Stamp of PS Motion looks at good and bad motion design.

Motion Design for packaging machines has been around for as long as... well, as long as there have been packaging machines. There has always been a need to mechanically move, fold, tuck, lift, twist, transfer or in some way manipulate packaging and the product to give a finished package. Fortunately, the great thing about motion, and specifically motion-design, is it is free, unlike the physical machine components.

Unfortunately, poor motion is also free. When poor motions are specified and then incorporated into packaging machine mechanisms, servos or cams, the machine will jam more often, need maintaining more often, not run as fast, be noisier and not last as long. But if you specify, then apply, good motions to an identical machine with the same physical components, the machine can often be transformed into an efficient, reliable, quiet and long-lasting machine.

Motion design can mean different things to different machine designers. One definition is "move this from here to there, stay there, come back here again, repeat". Another is "Motion design for machines encapsulates the ideal functional design specification of how the tooling should move and interact with the packaging and product to produce the package. The specification should define the motion of all the tooling in the machine, throughout the machine cycle. It also specifies the 'ideal' motion."

Many packaging machine designers are taught a little about motion design. Most often, engineering classes consider a motion axis in isolation, and typically use the so-called 'Dwell-Rise-Dwell' motion or similar. Also, they will usually use the easier design route of forward kinematics. In other words, nearly all servo-motion or cam design classes apply the motion to the servo axis or the cam-follower as an oscillating or reciprocating output. The mechanism is then 'bolted on'. Using this route, the machine designer often finds the tooling motion is significantly different to the motion specified, especially when a complex mechanism is needed.

In an ideal world, the machine designer needs software to design the motion at the tooling, not the motion of the servo-axis or cam follower - that is, to apply inverse kinematics. In addition, to inverse kinematics, he needs to design the motions of all the motion axes in a machine, not just one. Further, it would be good to view a precise machine model, with all the mechanisms embedded in it. He would want to make changes to motion and even the mechanism design as the machine is cycling, on the fly.

There are many reasons why this is so important. For starters, it can be difficult to need to balance the motion requirements of one axis against another unless you can see them move. In addition, inverse kinematics is required when there is a need to match precisely two or more motion axes while they interact the packaging or product. For example, wrapping machines must match the motion of a product pusher with the motion of a 'steady' while it holds the wrapping film. Similarly, if you need to transfer a product to a moving conveyor, then you will need to coordinate the motion of two or three axes.

If the machine designer does not have the tools to visualise the machine cycling, then often he must play safe. The inevitable consequence is the tooling, and all the machine elements, move faster, with resultant higher accelerations and jerk, thereby creating higher forces and inducing vibrations needlessly into the machine, while needing motors with higher torques, larger bearings and more substantial cams.

All of these examples require the machine and motion designer to be able to design the motion at the tooling and to derive the motion of the cam follower or servomotor. To get the best out of the machine, the designer must design all of the motions together. And if you really want to get the best from the machine you need to be able to design and instantly play back any design changes to see the whole machine and the mechanisms interacting.

With good motion design, you can almost always improve machine performance. You just need the right tools and motion design knowledge. But worryingly, the opposite is certainly true. No matter how well the machine components are specified, without the right motion design and mechanism modelling tools, it is all too easy for a machine builder to end up with a machine that will cost more to build, cost the client more to run and service, and not even do what it says on the tin. Will that machine builder get the repeat order?
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