Drives for Web Handling

By Clarence Klassen
 
But you can’t come in. Some sections of web handling equipment are inherently hazardous. This includes in-running nips, entanglement hazards, knives, and high-voltage static-control strips. In the past five years, I have seen increased compliance in guarding the equipment to current applicable safety standards. Where a risk assessment determines a severe injury could result, there is frequent exposure to the hazards and little possibility of avoiding the hazard, we must implement a high safety category.

That involves a combination of fixed guards and safeties. The safeties may include interlocked gates, light curtains, scanners or other devices.

These devices are there for a reason and should not be defeated. That makes our job of commissioning and servicing equipment more complicated, but safer.

Our shareholders and corporate management have publicly stated that safety is the first priority. Let’s take them at their word and work safely. We don’t need to take shortcuts for the shareholders. No more defeating safety devices to get close to the machine.
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Where we need to work live for troubleshooting, we need a safe work plan signed off by management.
Work safely!

By Clarence Klassen 

Web tension is complicated whenever the web path changes. This may happen when nips open or close. It has a huge impact on tension during an index or cutover in a turret winder.

Whenever the web length changes, the speed of at least one roll or roller must change to maintain tension. If the web length shortens, the speed of downstream sections must increase. If the web length increases, the speed of downstream sections must decrease.

It is difficult to maintain tension control when speed is changing. There are inertia effects which are generally not modeled. Also, the diameter calculator often depends on knowing the line speed and spindle roller speed. If the spindle RPM is changing to compensate for a web path change, the diameter calculation based on the ratio of spindle RPM to line speed is invalid and the calculated diameter is garbage.

Things that lengthen the web length are raising the layon roll, turret index.

Things that shorten the web length are lowering the layon roll, extending the knife assembly.

Necessary techniques to minimize tension variations during turret cutovers are:
1)      Reduce the time for an index – less off-spec material is wound
2)      Diameter hold (an old diameter is better than a wrong diameter)
3)      Torque hold (the best we can do without complicated modeling)
4)      Model the speed change required for each instant of the cutover

By Clarence Klassen 

One method of modeling or analyzing a linear, time-invariant tension control system is with gain and phase vs. frequency. A mathematical model describing the phase and magnitude of gain is called a Transfer Function. The simplest method is the open loop graphical technique named after Hendrik Bode in the 1930s. Bode plots graph the absolute value of gain and phase vs. frequency. The absolute value of gain is expressed on a logarithmic scale in decibels (dB). Frequency is also expressed on a log scale. The phase shift is graphed normally.

The advantage of using logarithmic scales on the graphs is that multiplications become additions resulting in very simple techniques for combining parts of a control system. The parts could be the “Plant” or the “Control” portions of the system. The plant consists of the fixed portions of the equipment. For a web handling example, the plant includes the web, the rollers and the motors, brakes, and clutches and traction providing tension. The control may consist of a PID regulator or other controller.

There are two methods of determining the transfer functions for parts of the tension control system. One method is to measure the gain and phase shift for a part. For instance, it is possible to excite a motor by itself with torque varying in frequency. Specifically, the torque reference must vary from zero to positive to negative and back to zero. The frequency of the torque signal forms the X-axis on the Bode plot. Then the torque and speed of the motor can be charted using a two channel scope. At various frequencies, the technologist will calculate the magnitude of the speed variations and the phase shift compared with the torque reference. It will be necessary to plot the results for frequencies of 0.1 radian/second 100 radians/second. These results can be plotted on a log graph to produce the Bode plot of the motor’s transfer function. I didn’t say it would be easy!
The second method is to recognize that transfer functions have been determined for most parts of the control system. The transfer function can be researched and parameters filled in from data provided with the motor. We learn that

RPM (s) =                 PowerIn(s)_______              
                  (1 + s * InertiaTimeConstant)

There are several recognizable features of Bode plots. Constant gain is shown as a horizontal line. Integrators are shown as lines sloping downward to the right at -20dB/decade. Derivatives are shown as lines sloping upward to the right at 20db/decade.

Of course, the purpose of the Bode Plot is to assist in setting the PID gains to tame the tension regulator.

Below is a Bode plot for the motor.

Current Limit is a drive function that limits on the current delivered to the motor. The motor itself does not control its own current, but will strain with all its might to keep speed at its setpoint. This is often up to 500% of the current stamped on the motor nameplate. 500% current could produce a lot of torque, damage a lot of equipment, and if it lasts more than 10 seconds would burn out the motor due to overheating.

For AC induction motors, current is not closely related to torque. This is because the induction motor needs current to achieve motor flux even if the motor is producing no torque. Above 80% current, the torque and current converge. In web handling not many drives run above 80% current.

In DC motors, provided the shunt field current is held constant, armature current closely follows motor torque. The % torque and % current may be within 2%.

There is only one current limit in an AC variable speed drive (VSD). The purpose of current limit is to do one thing only. That is to protect the motor, not the valuable equipment connected to it. To protect the equipment, you need to use the torque limits.

During commissioning, it is useful to check the current limit (by E-Stop) to ensure that the drive and motor are capable of achieving this current without fault. We learn the most about our equipment when we push it to the limit. The current limit is an electronic function and if the drive has not been set up or tuned correctly, may not operate as expected.