Rotary knife with electromagnetic active vibration control

Details Rotary knife with electromagnetic active vibration control Rotary knife with electromagnetic active vibration control

2000-04-14

2002-05-21

Goodman, Charles (Department: 3724)

Cutting

With means to monitor and control operation

Including means to correct the sensed operation

C083S342000, C083S575000, C083S698210, C101S216000, C384S448000

Reexamination Certificate

active

06389941

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to controlling vibration of rotary knife cylinders that are used to cut individual sheets in succession from a longitudinally traveling continuous web of material, such as corrugated paperboard. In particular, the invention involves the use of electronically controlled electromagnets to actively damp vibrations, particularly at the natural resonance frequency of the respective rotary knife cylinders.
BACKGROUND OF THE INVENTION
The invention arose during continuing developmental efforts by the assignee of the present application in seeking improved vibration control for rotary machinery, namely rotary knives as disclosed in copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, entitled “A Rotary Knife With Active Vibration Control”, by James R. Michler, incorporated by reference herein.
In the corrugated paperboard industry, long machines comprised of several components are used to make individual pieces of corrugated paperboard from rolls of craft paper and adhesive. In these machines, a rotary knife is used to cut a continuous web of corrugated paperboard into individual sheets towards the end of the manufacturing process. These individual sheets of corrugated paperboard are then normally stacked and transported for finishing. The quality of the individual sheets and in turn the quality of the end product made from the corrugated sheets depends in large part on the performance of the rotary knife.
In a typical rotary knife, a pair of rotating knife cylinders extend generally transverse across the web. The outer cylindrical wall of each cylinder includes a cutting blade that is helically mounted to the respective cylinder. Gears for the cylinders ensure that the cylinders rotate symmetrically so that the blades engage the web and each other to cut the web transversely in a scissors-like fashion. In order to provide a high quality cut, it is important that the blades tightly engage one another when making the scissors cut. A control system for the knife precisely controls the speed of the rotating cylinders so that the forward velocity of the blade during the cut matches the forward velocity of the web through the knife. In addition, the desired length of the individual sheets for the particular order is regulated by controlling the speed of the rotating cylinders when the blades are not engaged, which is in general a different speed than when the blades are engaged to make the cut. This type of variable speed operation is conventional in the art.
Due to the above-described variable speed operation, low inertia rotary knives are used by the assignee of the present application primarily to improve motor drive control and durability at high speeds. In these low inertia rotary knife cylinders, an upper stationary support shaft and a lower stationary support shaft are mounted to a frame (normally using retainer mounts). An upper rotatable cylindrical tube is placed concentrically over the upper stationary support shaft and a lower rotatable cylindrical tube is placed concentrically over the lower stationary support shaft. The knife blades are attached to the outer wall of the rotatable cylindrical tubes. Bearings are located between the rotatable cylindrical tubes and the respective stationary support shaft. The rotational inertia of the tubular knife cylinder is significantly less than in systems having solid rotating knife cylinders.
In low inertia rotary knife systems, as well as other rotary knife systems, excessive vibration of the knife cylinders can cause poor quality cuts. Under extreme conditions, the vibrations can sometimes even prevent cuts from occurring altogether. It has been found that cutting frequency has a substantial effect on the amount of steady state vibration, and hence the quality of the cut. When the cutting frequency (number of cuts per second) is an integral, or near integral, multiple of the knife cylinder first natural frequency (number of cycles per second), each successive cut adds to the vibration remaining from the previous cut. The total knife cylinder vibration then builds up to a higher level than at slightly different cutting frequencies. A speed change of less than 1% can change the cylinder vibration level by more than 50%. This being the case, vibration problems are more apparent when the knife is operating at high speeds because there is less time for natural dampening to occur between cutting cycles. In addition, vibration problems are more apparent on webs that require relatively high cutting forces.
In the above-incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, the assignee of this application discloses an active tuned-mass vibration damping system in order to control vibrations of the knife cylinders. More specifically, that system includes a tuned-mass, and a spring arm having one end coupled to the tuned-mass and the other end coupled to the end of the knife cylinder (e.g. in a low inertia rotary knife, the spring arm is coupled to the end of the stationary support shaft). Actuators, preferably electromagnetic actuators, are mounted to provide force on the tuned-mass in order to actively dampen vibrations of the knife cylinder. Vibrations of the knife cylinder and the tuned-mass are measured using vibration sensors, preferably accelerometers, which transmit signals to an electronic control unit. The electronic control unit calculates command signals, preferably using state space control, to drive the electromagnetic actuators in order to actively dampen vibrations in the associated knife cylinder. It is preferred that the control scheme account for phase shifts in the system due to electronic lags (for example, phase lags in the electromagnetic actuator, processing electronics, etc.) by using adjusted control coefficients in the state space control algorithm. The preferred control scheme is disclosed in copending patent application Ser. No. 09/046,267 filed on Mar. 23, 1998, now U.S. Pat. No. 5,983,168, entitled “Phase Shift Accommodation for Active Tuned-mass Damping System”, by James R. Michler, assigned to the assignee of the present application, and incorporated herein by references. The command signal is scaled and then output from the electronic control unit to drive the electromagnetic actuators which provide force on the tuned-mass to actively damp vibrations in the knife cylinder.
It is also disclosed in the above-incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, entitled “A Rotary Knife With Active Vibration Control”, by James R. Michler, that the tuned-mass be preferably made from a plurality of laminated steel plates. The steel plates are aligned in the direction of the magnetic field from the respective electromagnetic actuators. In this manner, any currents transverse to the direction of the magnetic field are not allowed to propagate for substantial distances, and thus are prevented from generating excessive heat in the tuned-mass. In addition, it was preferred to have permanent magnets embedded in the surface of the tuned-mass at locations corresponding to the legs of the respective electromagnetic actuators. In this manner, a single electromagnetic actuator could be used to both push and pull the tuned-mass in the selected direction. The permanent magnets were affixed to the tuned-mass using adhesive.
While the inventions disclosed in the above incorporated copending patent application Ser. No. 09/045,466, now U.S. Pat. No. 6,032,558, and copending patent application Ser. No. 09/046,267 (now U.S. Pat. No. 5,983,168) are effective at reducing rotary knife vibrations, some difficulties have arisen with respect to commercial embodiments of the inventions in certain applications. For example, the commercial systems included eight sensors (i.e., vertical and horizontal accelerometers for both the knife shaft and tuned-mass for both the upper and lower knife cylinders), and occasionally one of the sensors would fail. Upon sensor failure, it was possible for the control s

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