Event-driven strategies enable adaptive control

Mechanical cams, once a mainstay of automation, have been largely replaced by servomotors. The reasons are easy to understand: Mechanical simplicity and system configurability are the two most common motives, but improved synchronization, easier machine maintenance, and elimination of wear parts are also frequent objectives.

That said, even when not physically present, the influence of mechanical cams is still there. Cams continue to shape the way design engineers approach machine building and the way automation systems are programmed; designers tend to think in terms of “electronic cams” and their associated cam tables. To some extent, this makes sense. Packaging machines, for example, often require repetitive motions and in these cases, the cam paradigm works well. However, in many situations, engineers fail to take full advantage of process information already present in the system. The result? Machines are repetitive when they could be adaptive.

Cam model versus event-driven model

We will now compare the traditional cam-based model to the event-driven model with a common example from the packaging world — the vertical form-fill-seal machine (VFFS). We will then look at one new approach to achieving adaptive control, called Flex Profile.

Cam model: Following the cam paradigm, the programming task might be approached by predefining the x and y motions of the mechanism via cam tables — as shown in Fig. 2. Here, the motions are defined according to our knowledge or assumptions of the process requirements. For example, we could predefine a sealing time of 150 msec and define cam profiles based on the current machine speed accordingly.

This method might be acceptable in situations where the sealing process is fairly consistent. In the case of format or process changes (such as longer bag length or increased sealing time), cam tables could be recalculated to accommodate the new requirements, although these changes could not be realized during the current machine cycle. At best, the new motions would be loaded into a memory buffer and would be available for activation only in later machine cycles. Even so, abrupt changes in film-feed speed could result in over-sealing or under-sealing.

Event-driven model: Suppose instead that the machine could monitor the sealing process — using a thermocouple mounted to the sealing surface or an external timer — and send an alert when the process is complete. The machine would then immediately open its sealing jaws and return to the start position in preparation for the next cycle. Doing so minimizes total travel, resulting in lower average velocities over the machine cycle at a given machine cycle rate. The general idea is illustrated in Fig. 3.

This type of setup requires the motion to be defined with a certain amount of flexibility built in. First, the motion profile must be able to respond to the external event itself.

Additionally, it must be able to compensate in-process to the variability implicit in any event-driven scenario. In other words, the motion plan must be able to adjust automatically regardless of when the trigger event occurs. Note that the cam approach described previously cannot realize this optimization because changes to the cam motions will only take effect in later machine cycles.

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Flexible profile enables adaptive control

Until now, adaptive control strategies were limited by point-to-point motions and do-it-yourself methods. One new approach to realizing the benefits of adaptive control comes from what's called Flex Profile, a segment-based cam technology that can help implement these flexible strategies. It does so by incorporating event handling directly into the object definition.

Also important is the ability to define segments based on synchronization constraints. For example, a segment may be defined where an axis must return to a given position at a specified velocity and acceleration once the master axis reaches a certain threshold, say 360°.

Flex Profile is an adaptive control strategy that provides a framework in which to address the demands of the form-fill-seal application described previously. Each profile is defined as a collection of segments or steps, which exist as two types: Standard, in which the motion trajectory is pre-specified mathematically, and Flex, in which the motion is defined by a certain goal. In the case of standard steps, the drive is commanded to move a certain distance or Hub as the master moves within a defined Range. This motion proceeds according to a specified motion law that generates a trajectory between the specified boundary conditions. Note that the master axis may be a real or virtual axis, or it may be time-based.

Flex segments are defined somewhat differently: A motion law is still provided, but rather than defining the trajectory explicitly, a certain goal is specified. For example, one flex segment might require that the axis, regardless of its current position, reach a certain target once the master crosses some specified threshold.

Alternately, one might require that the axis reach some position over the next 90° of master travel. See Fig. 5. The motion trajectory itself is not specified, only the final outcome.

Flex profile meets VFFS

Returning to the VFFS example, Fig. 6 illustrates the sealing mechanism's horizontal motion as a function of film position. Because the jaws must only come into contact with the film once the speeds of the mechanism (in the vertical direction) and the film match, the first segment is defined as a synchronous segment.

The next segment is defined as a time-based step whose length is given as the maximum possible seal time. In the example case here, an external timer or some other process monitor triggers the end of the sealing process: The maximum seal time is used to define the default behavior of the Flex Profile. The sealing bars then open, again according to a time-based master — in other words, at a defined rate of speed. The duration of the final step, in which the sealing bars are fully open, is calculated internally by the system. This is the so-called flex step, during which there is a transition from a time-based step back to being fully synchronized with the real or virtual master.

Fig. 7 illustrates the vertical motion of the sealing mechanism, again as a function of the film position. Here, all profile segments are synchronous with the master.

Step one defines the vertical travel of the mechanism as it tracks the film; this is the step in which sealing occurs. When the external event signals the end of the sealing process, step one is aborted and there is an immediate transition to step two, during which the mechanism travels with the film for a short distance to allow the jaws to open.

In step three, the jaws return upward to their starting position. As in the previous case, the final step is a flex step, meaning that the system automatically calculates a trajectory such that the machine returns to the defined start position at a fixed master position.

Benefits of flexible motion profiles

Energy savings: Adaptive control shortens motion trajectories whenever possible, saving energy.

Flex Profile is a control-side technology included as standard in all Rexroth MLC motion-logic controls. For more information, call (847) 645-3600 or visitboschrexroth-us.com.