Visit https://tinyurl.com/2xuc35sf to read part 1.
By Sean Craig
Because the differential shaft is acting like a clutch, the motor torque must exceed the torque required to maintain constant tension through the shaft. However, a more critical function is overspeeding the differential shaft to ensure the rolls properly slip relative to the shaft. Although the recommended overspeed of the differential shaft relative to the roll speed is 1%–5%, heat buildup is directly related to speed and pressure to the tension segments. So for higher tension/speed applications, overspeed may need to be at 1%–2%. As we saw above, exceeding these overspeed limits generates heat and dust, both of which are detrimental to building a good quality roll and extending the life of the shaft.
Motor speed should be calculated in revolutions per minute. To convert web speed (feet per minute) to revolutions per minute (RPM) divide the web speed (velocity) by 2πR. For example, the speed of a 15” Ø roll traveling at 1,250 FPM is 318 RPM:
To convert web speed to RPM, you need to know the diameter of the roll as it is building. The easiest way to measure this is with an ultrasonic sensor on the outside diameter of the roll. This output from the sensor can then be converted into the required motor speed.
The next step is to determine the full speed range the shaft and motor will run from the start of the roll to the finish. It is important to note that the shaft/motor speed relative to the roll diameter is not linear. This influences how the drive and motor can be set up to successfully manage the overspeed requirements for differential winding.
For example, the speed curve for a 500 FPM application building roll to 30 inches in diameter is shown below. The goal is to send an input to the drive and motor to ensure the shaft runs at 1-5% overspeed throughout the roll build. This means that in order to properly control that overspeed, the output signal to the motor must follow the actual roll speed curve. Otherwise, the result will be little to no overspeed or excessive overspeed resulting in heat and dust buildup.
When retrofitting a machine with a differential shaft, all these factors must be considered and applied to the type of drive and motor on the machine. However, to do this successfully requires a basic understanding of how speed is controlled in a drive and motor.
Electric drives and motors are either AC (alternating current) or DC (direct current), and they control both torque and speed in the winding process. The drives control the input to the motor. In both cases, to successfully overspeed a differential shaft, the motor must be running in speed mode, and the drive input to the motor should be voltage.
DC motors provide constant torque and typically run in torque mode. In many cases the drive and motor are running on current. But to correctly control overspeed from a 0–10V input, the drive needs to be able to output voltage. If the drive can do this, it can control overspeed. Otherwise, in order to control overspeed to the differential shaft, a new general-purpose AC variable frequency drive (VFD) is required.
AC drives/motors typically run in speed mode, and this speed is controlled by voltage. AC vector drives can also run in torque mode, but they typically have the option to switch to speed mode, which would be required to control overspeed. These drives also use a PID (proportional, integral, and derivative) loop. PID refers basically to complex calculations performed in the drive to adjust a designed output to a set point based on a closed loop. Most AC drives and motors can control overspeed with the proper inputs to the drive.
To control speed, the motor needs a voltage input that correlates to speed in RPM. Typically, motors are designed to receive a 0–10V input from the drive. The more voltage received, the faster the motor runs. In this manner, the drive functions much like a dimmer switch for your lights. If you turn the dimmer switch off, no voltage is flowing to the light. As you turn the dimmer switch on, more voltage flows to the light, and it gets brighter. The challenge lies in how to control the voltage output from the drive to the motor so that the motor is running at the proper overspeed. The following options illustrate this challenge. Also, these challenges primarily apply to existing equipment and how you retrofit that equipment to properly overspeed the differential shaft. New drive technology simplifies this to a large degree with preset program options. However, even in these cases, the principles for properly setting overspeed must be upheld.
Does your equipment frequently process webs that have droop or baggy lanes? Non-flat webs can compound misalignment.
Using an ultrasonic sensor on the roll diameter to convert the web speed to RPM, a signal can be sent to the drive telling it to change the voltage flow to the motor to affect motor speed. However, because the output is still 0–10V, the drive sends more voltage as the roll builds. The result is an undersped shaft at the beginning of the wind-up, followed by a significantly oversped shaft at the end of the roll build.
The result
Excessive heat and dust, improper roll tension, and damage to the differential shaft.
This option solves the reverse voltage output problem of option one with a program change in the drive to reverse the output based on the signal received from the ultrasonic sensor. However, the output from the drive is linear because it is only a function of actual roll diameter. Therefore, the resulting speed change does not follow the actual speed of the shaft. This results in the correct overspeed at the beginning and end of the roll, but excessive overspeed throughout the rest of the roll build.
Drives can also be mapped to control overspeed. However, in order to map the drive, you have to select points in the roll build process to correlate to drive output changes. The result is a stepped output with the number of outputs directly proportional to the accuracy of the motor overspeed. The more points you select, the more accurate the motor speed. But the more points you select, the more time-consuming the process. And if you need to run multiple web speeds, the drive has to be remapped for each configuration. Newer, more sophisticated drives simplify this process, but it is important to remember that the drive should be programmed for each speed and roll diameter you are running.
Better overspeed control, but with a high risk of excessive heat and dust, improper roll tension, and damage to the differential shaft.
This option leverages the inverse diameter function in some controllers to output a 10–0V signal to the drive that directly correlates to the rotational speed of the differential shaft. The output is 10–0V because the shaft rotational speed at the beginning of the wind-up is higher than at the end: Higher voltage equals higher speed, and lower voltage equals lower speed. Figure 21 illustrates why this provides the optimal control of overspeed to the differential shaft.
For this system, it is best to use an ultrasonic sensor on the roll diameter to provide the diameter feedback to the controller to convert fpm to rpm. Important to note is that for this function, the sensors are controlling speed. This means that the ultrasonic output is being used by the controller for both tension and speed control. When setting the system up this way, it is important to disable the PID loop in the AC drive.
Accurate overspeed and tension control with minimal wear and tear to the differential shaft.
Options one, two, and three involve drive programming, and as mentioned, more sophisticated drives can overcome some of the shortcomings of basic PLC programming and achieve accurate overspeed if set up correctly. Specific drive programming techniques are outside the scope of this paper, but most engineers are trained in this. However you approach this, as long as you remember the fundamental points needing control, you should be successful.
Option four has some unique advantages in that it eliminates the need to be a motor and drive expert and leverages technology that you are already using to control the tension in the web. It is especially useful if you are retrofitting machinery with older drive technology and cannot afford to invest in new drives and motors.
There are three typical ways option four can be set up to provide accurate tension and speed control for your shafts, and this will be the focus of this last section. For each configuration, note that the method of controlling the tension can apply to whatever method you use to control speed.
Open-loop tension control and speed control-ultrasonic sensor
Tension control: an ultrasonic sensor diameter output correlates to the required tension for a given roll diameter. The controller receives the sensor input and sends an output to a current to pressure transducer to control pressure to the differential shaft.
Setup for differential shafts
Calibrate ultrasonic sensor.
Input the full roll diameter.
Input the outside core diameter.
Input the distance from the axis of the roll/shaft to the face of the ultrasonic sensor.
Set controller to tension mode and rewind.
Start winding.
Adjust set point value as needed—some percentage of total output depending on results achieved.
Set taper tension based on desired results and the effect of roll size at the end of the wind-up.
This is a common method of controlling tension. However, it does rely on trial and error since the controller is simply managing how much air pressure is provided to the shaft based on the diameter of the roll. Tapering the tension will help ensure a good quality roll build and manage the effect of the increasing roll weight on tension.
Speed control: Use the same ultrasonic sensor diameter output to the controller. The controller receives the sensor input and uses an inverse diameter function to output 10-0V signal to drive to control rotational speed of motor.
Setup for differential shafts:
Ensure the sensor is aligned with the roll.
Turn on the inverse diameter function.
Begin winding.
Closed-loop tension control and speed control-ultrasonic sensor and dancer control
Tension control: A dancer system senses web tension against the dancer position and sends an output to the controller. The controller receives the sensor input and sends an output to a current to pressure transducer to control pressure to the differential shaft.
Set the controller to dancer and rewind mode.
Connect the position sensor and tell the controller where you want the dancer to run.
Set pressure to the dancer in order to set tension in the web.
Controller sends output to the shaft based on the torque required to maintain tension.
Note: this results in constant tension to the web.
For taper tension:
Turn on the taper tension function in the controller.
Take an ultrasonic input and enter the taper tension required in the controller and add an additional current-to-pressure transducer to be able to control the pressure to the dancer.
Speed control: Use an ultrasonic sensor diameter output to the controller. The controller receives the sensor input and uses an inverse diameter function to output 10–0V signal to drive to control the rotational speed of the motor.
Ensure the sensor aligned with the roll.
Closed-loop tension control with ultrasonic sensor for speed control
Tension control: Load cell measurement of the actual web tension is sent to the controller. The controller sends an output to a current to pressure transducer to control pressure to the differential shaft based on the desired tension in the web.
Calibrate the load cell.
Enter set point tension—pounds or PLI.
Adjust gain in the system—increase or decrease so it holds consistently.
Turn the taper tension feature on.
Adjust the taper tension to 10%–20% to obtain the desired result.
Speed control: An ultrasonic sensor senses the roll diameter and sends a signal to the controller. The controller uses an inverse diameter function to output a 10–0V signal to drive to control the rotational speed of motor.
Calibrate the diameter sensor input.
Begin winding. █
Sean Craig is global vice president of product management and R&D for Maxcess. He can be reached at 380-833-7500, scraig@maxcessintl.com, or www.maxcess.com.
