Variable-length paper-cutting applications incorporate a constant ratio cycle (where the process occurs) as well as entry and exit cycles (where the follower travels to and from the home position). If the web cycle is shorter than the follower cycle, the ratio must temporarily increase to put the follower in position for the next cycle. If the web cycle is longer than the follower cycle, the ratio must decrease to prevent the follower from arriving at the constant ratio portion before the required web travel has occurred.
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In a myriad of manufacturing operations, the initial material is stored on large spools or as long pieces of stock. Paper, plastic, metal foil, and fabric are often handled this way, fed from rolls to a machine that processes it to create end product. The continuously fed material is called the “web,” and the entire operation, “web processing.”

Perhaps the most traditional example of web processing is that of paper printing. Paper unwinds from large spools while rotating ink drums print images onto the moving target. Other common web processes include punching, stamping, and cutting.

In most cases, material flow is uninterrupted, even during printing or cutting. A continuous flow is preferable to starting and stopping because it achieves higher throughput. More products per minute mean more dollars to the manufacturer. Continuous processing also use less power and is easier on components because everything moves relatively smoothly.

In a start-stop situation, a lot of power is required to accelerate and decelerate the web; constant starting and stopping also causes machine components to wear faster. A machine operated this way is more expensive to build and operate, and does not last as long as one with continuous flow.

In synch

Processing material as it moves requires synchronization. In printing, for example, the surface of the print drum must move at exactly the same speed as the web in order to avoid smearing. Similarly for cutting, the knife edge must move at the speed of the web in order to avoid tearing or bunching.

Synchronization is achieved by “electronically gearing” the controlled motion to some measured motion. In printing, for example, an encoder may be used to measure speed. A controller then uses this information to regulate the print drum’s speed so that the print surface is in synch with the paper while they are in contact.

In most cases, web-processing machines must handle several different product lengths. In the printing example, if the length of the required print cycle matches the circumference of the drum, then the drum can rotate at a constant 1:1 ratio with respect to web travel. But if the print cycle is less than or greater than the drum circumference, then the drum must advance or fall back while it is not in contact with the paper. This print length compensation must repeat every print cycle, necessitating the use of a repetitive cam profile.

The key difference between the method discussed, called “following,” and other methods of synchronization is the assignment of a “master axis.” The motion of this axis is measured (as with an encoder) and referenced by the follower.

Ratio versus speed

The concept of ratio, the change in axis position with respect to the change in master travel, is at the heart of following. Ratio is analogous to velocity, except that the follower moves as a function of master motion instead of time. In the same way that a motion axis might have to speed up or slow down (change velocity), the electronic gearing ratio might have to increase or decrease within a profile.

A change in ratio is, of course, analogous to acceleration. As long as you know the rate of change — the total change over a given region of master travel — you can be sure of the position relationship between the master and follower axes.

The master cycle concept provides a useful way of dividing continuous master motion into meaningful portions. This allows master travel to be measured and referenced in terms of cycles and positions within a cycle. Because master cycles usually relate to product cycles, it is important to begin the measurement of master travel at a spot corresponding to the start of the product cycle. This is usually achieved by detecting the arrival of a product or moving machine part with an electronic sensor.

In most applications, one of more machine parts must maintain a 1:1 travel ratio with other moving parts. But there’s more to it than merely trying to match speed. The related parts must also be properly aligned or “in phase.” For this reason, at various points within a cam profile, the controller will advance or retard the follower. This procedure, called a “phase shift,” can put two axes in synch without affecting the ratio of the motion.

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