Event-driven motion control: Flexible profile offers more machine motion benefits
By integrating event-driven strategies into motion control schemes, automation systems may be optimized beyond what’s possible with traditional electronic cams or motion profiles. Until now, such strategies have tended to be limited by point-to-point motions and ad hoc methods. A flexible profile technology, such as provided by Bosch Rexroth, provides a structure under which event-driven strategies may be fully realized.
Servo motors have largely replaced mechanical cams as the standard means of meeting the motion demands of the automation industry. The reasons are well-known: Mechanical simplicity and system configurability are the two most common, but improved synchronization, ease of machine servicing, and elimination of wear parts, among other benefits, are often cited as well. However, even when not physically present, the influence of mechanical cams may still be felt. Cams continue to shape machine building and automation system programming when engineers 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 fits.
However, in many situations, using process information already present in the system can help machines be adaptive rather than just repetitive.
Flex Profile is a segment-based cam technology from Bosch Rexroth that can help implement these adaptive strategies. It does so by incorporating event handling into the flexible profile object definition and defining segments based on synchronization constraints. For example, a segment may be defined to return an axis to a given position at a specified velocity and acceleration once the master axis reaches some threshold, say 360 degrees.
Cam model vs. event-driven model
The following example compares a traditional, cam-based model to the event-driven model using a common example from the packaging world—the vertical form-fill-sealing machine (VFFS). Figure 1 describes a dynamic sealing mechanism typical of a VFFS machine. During the sealing operation, the mechanism travels with the film as it is fed downward. After the sealing operation is completed, the jaws open and the mechanism returns upward to its starting position for the next cycle.
Cam model: Following the cam paradigm, the programming task may predefine the x and y motions of the mechanism via cam tables. Here the motions would be defined according to knowledge or assumptions of the process requirements. Figure 2 predefines a sealing time of 150 ms and defines cam profiles based on current machine speed, accordingly.
This method might be acceptable in situations where the sealing process was reasonably consistent. In 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, but 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. Also, abrupt changes in film feed speed could result in over-sealing or under-sealing.
Event-driven model: Suppose the machine could monitor the sealing process by means of a thermocouple mounted to the sealing surface or an external timer, and broadcast when this process was complete. Optimally, the machine would immediately open the sealing jaws and return to the start position for the next cycle. Doing so would minimize total travel, resulting in lower average velocities over the machine cycle at a given machine cycle rate. See Figure 3.
This method requires that the motion be defined with a certain amount of flexibility built in. First, the machine must respond to the external event. Then it must compensate in-process to the variability implicit in any event-driven scheme. In this case, the motion plan must adjust automatically regardless of the trigger event’s timing. Note that the cam approach could not realize this optimization since changes to the cam motions take effect only in later machine cycles.
A flexible profile provides a comprehensive framework to address the demands of this form-fill-seal application. Each profile is defined as a collection of segments or steps in two types.
- Standard pre-specifies the motion trajectory mathematically.
- Flex defines motion by some goal or restraint. See Figures 4 and 5.
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; the motion law generates a trajectory between the specified boundary conditions. The master axis may be a real or virtual axis, or the master may be time-based.
Flex segments are defined differently. One still provides a motion law, but rather than defining the trajectory explicitly, one specifies a goal. For example, one 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 degrees of master travel. The motion trajectory itself is not specified, only the final outcome.
Flexible profile with VFFS packaging equipment
Returning to the VFFS example, Figure 6 describes the horizontal motion of the sealing mechanism as a function of the film position. The first segment is defined as a synchronous segment because film contact can only occur once the speeds of the mechanism (in the vertical direction) and the film match. The next segment is defined as a time-based step whose length is given as the maximum possible seal time.
In this case, the end of the sealing process will be triggered by an external timer or some process monitor: The maximum seal time is used here to define the default behavior of the Flex Profile. The sealing bars then open, again according to a time-based master, which is to say at a defined rate of speed. Duration of the final step, in which the sealing bars are fully open, is calculated internally by the system. This "flex step" is the transition from a time-based step back to synchronization with the real or virtual master.
Figure 7 describes the vertical motion of the sealing mechanism, again as a function of the film position. At that point, all flexible profile segments are synchronous with the master. Step I defines the vertical travel of the mechanism as it tracks the film and also does the sealing. When the external event signals the end of the sealing process, step I is aborted and the machine transitions immediately to step II, during which the mechanism travels with the film for a short distance to allow the jaws to open. In step III 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 returning to the defined start position at a fixed master position. Figure 7 shows how the vertical motion changes as a function of the event timing. If the event does not occur, the default trajectory is followed.
Integrated event handling
Key features of Flex Profile include ability to combine synchronous and time-based steps in one data structure, well-integrated event handling. Flex Steps are calculated in-process to adhere to a specified end-point constraint. Other features include:
- Support for most well-known motion laws, including poly5, poly7, modified sinusoid, modified trapezoid, etc.
- Easy inclusion of additional, user-defined motion laws
- Legacy support for traditional cam tables
- Well-integrated graphical editing/analysis tool
- Single-cycle or endless execution
- Automatic and configurable synchronization of the slave to the master.
Flex Profile is a control-side technology included as standard in all Rexroth MLC motion-logic controls. Flex Profiles, including associated events, may be defined using the Rexroth Cam Builder graphical interface or, in cases where end-user or recipe-driven input is required, programmatically.
Flexible profile motion programming benefits
Using this flexible profile method isn’t the only solution for programming challenges. However, because of its broad, comprehensive approach, many situations—including many not previously thought of as "cam" situations—may be addressed. Doing so can make programming tasks more efficient and uniform. Such uniformity tends to make for simpler, more readable and comprehensible code.
By integrating event handling into the electronic cam model, using this flexible profile offers a unique, comprehensive approach to many different motion demands. This, coupled with the ability to seamlessly combine synchronous and time-based steps into one data structure, offers the potential to optimize system performance beyond what is possible using more traditional approaches.
- Energy savings: Adaptive control shortens motion trajectories whenever possible, saving energy.
- Reduced wear and tear: Higher-order motion laws minimize jerk and acceleration, leading to longer mechanism and machine life.
- Reduced material waste: Good control schemes and real-time information response tend to provide greater system efficiencies resulting in fewer rejects.
- Higher throughput: Eliminating unnecessary movements leads to higher machine speeds.
- Simplified coding: With a comprehensive programming framework for motion, code is simplified and made more reusable. Instead of defining motion sequences, decision switches and recovery handling, and so on, the programmer simply configures a Flex Profile and runs it. Simplified, reusable code tends to be better tested and proven.
Carl Bostrom is senior applications engineer at Bosch Rexroth in Hoffman Estates, IL. www.boschrexroth-us.com