Servos Get Up To Speed

A customer of ours was developing a new cutoff machine design for use at the end of a package printing line. The paperboard stock was being printed, creased, and die cut, then had to be cut off to the proper length for quality folding and assembly. Tolerances for these products can be remarkably tight.

In the days of line shaft driven press lines, of which there are still many, a gearbox was sized to generate the torque required to drive the cutoff knife through the material, or perhaps, to withstand the reverse inertial torque during an emergency stop of the entire mechanically connected press line. Often, a phase adjustable gearbox design was used to provide registration of the blade to the printed product to accommodate positional errors due to material variance or tension changes.

With shorter runs and the desire for more flexibility with regard to cutoff lengths, most new machine designs incorporate servo systems that can match the speed of the press, through a feedback system, and control the cutoff knife position.

Although the torque required to perform the cutoff operation is basically the same, how it is generated is slightly different. In this case, the entire torque required to make the cut would be generated from the mass and inertia of the rotating tool. The drive system’s job was to accelerate the rotary tool to get it up to speed and in position to make an accurate cut, time after time.

Figuring out how to make this work was a two step process (really three if you count the control and feedback system necessary to follow the press, which we won’t discuss here). The first was to calculate the dimensions and weights of the knife holding rotary cylinders that would provide enough inertia, at the required speed, to make quick and clean cuts at the various lengths desired. To be honest, this was on the customer. But since it was their business to calculate cut pressure, it wasn’t too difficult for them.

The second step was to determine the torque required to get the cylinder back up to speed after the cut. Because the only real load exerted on the cylinder is the point where the knife meets the substrate, there is an immediate shock and a reduction in velocity. The result is that the knife blade gets out of phase. If normal velocity was again maintained, like what would happen with a standard AC motor, the blade would lag behind the required cut position. The cylinder actually has to be accelerated to make up for the lost velocity at cutoff, thereby achieving the proper velocity and position for the next cut.

The motion profile, therefore, is one of acceleration beyond nominal velocity, decelerate to nominal velocity, then continue at nominal velocity in time for the subsequent cut. This is repeated for each cycle for however many units per minute the machine is designed to produced. This could be 30 or 300 or more, depending on the material or cut length or number of cuts per revolution.

The creativity in the controls design process is to manipulate the motion profile to minimize the torque required, thereby reducing the system cost. The customer spent some time working this out with their controls people. Certainly a big enough servo motor could almost instantaneously return the cylinder to the proper velocity. But accelerating a given inertial load in a shorter time frame requires more torque, more power, and stronger components. That’s wasting resources. Calculating the acceleration rate to get the knife in position just in time reduces both torque spikes and the nominal torque needed.

Once the acceleration and inertial loads were identified, the torque required was relatively easily determined using standard formulas. This allowed us to focus on the gearhead style and configuration desired and most appropriate.

The width of the machine had to be minimized. So, a right angle solution was the best fit. A gearhead ratio of 3:1 worked best to reduce the reflected inertia at the motor and achieve the operating speeds necessary to meet the machine’s designed output. To further reduce the footprint and eliminate mounting hardware, a shaft mounted reducer was called for. And low backlash, to minimize “bounce” due to torsion after the cut, was also a requirement.

The solution we provided was our ServoFoxx series FS2HWS single stage spiral bevel gearhead with a hollow output bore and shrink disc connection. This was the tightest and most shock absorbing version. We specified our low backlash option which limited gear play to 2-3 arc minutes. This gave the customer a compact, high precision gearhead that easily met the torque and positioning accuracies he was looking for.

The two key educational elements in this application is first, that manipulating and optimizing the motion profile of a cycle can often dramatically alter the torque requirements and thus the motor and drive components necessary.

The second element is that while there are several gearhead configurations available, mounting, space, or dynamic characteristics may favor one type of configuration or gear technology over another. This could offer an overall system cost savings or the satisfaction of a specific design criteria. The direct unit cost is rarely the key factor in product selection, unless you are comparing similar designs from competing manufacturers.

Diequa offers the widest range of servo gearhead solutions available from a single source. We also help specify the optimal technology for the individual application. Contact Diequa Corporation for a consultation on the best gearhead solution for your next motion control challenge.

Chris Popp
Director of Marketing