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How to Size Nearly Everything

We continue the discussion of ‘nominal’ from the previous post with an example load cell application.  Sizing loads cells is easy provided that someone takes the time to do some simple trigonometry and engineering statics calculations.

1)  'Best' is not the goal

2)  'Good enough' is the goal

By good enough I mean that the load cell system should be able to measure/resolve less than 5% of the lowest tension you ever expect to run.  While wrapping more (or getting a better load cell) and getting to 1% of the lowest tension might seemlike it is better, it will give you no better practical results.  Thus, if the simpler Option A is good enough, then that is what you want.  However, if even Option B with the higher wrap angle is not good enough, you must re-iterate on the design.  With light weight webs you may need to do more.  You could, for example, use 180 degree wraps (pulling up) and light weight carbon fiber roller to get the tare weight down and thus get a smaller more sensitive load cell.

This is not only how we size load cells, but also how we size brakes, motors, cylinders, sensors and much much more.

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Permanent Magnet Motors

Yellow Submarine Original Artwork copyright 2012

Permanent Magnet (PM) motors stators are very similar to synchronous or induction motor stators. The rotor is made of a shaft with a sandwich of steel layers (laminations) or alternately with a solid casting. The rotor is permanently magnetized, usually with rare earth magnets.

The PM produces flux and torque without the cost of magnetizing current.

The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn at exactly 3600 RPM (3000 RPM) at 60H (50 Hz). A 4 pole motor will rotate at exactly 1800 RPM. A 6 pole motor will rotate at exactly 1200 RPM.

PM motors produce torque only when they are turning at exactly synchronous speed. Slipping a pole due to torque overload is a serious problem which must trip the motor. The trick is in starting a PM motor since it has no torque until it gets to synch speed. Special PM variable speed drives or servo drives are used with PM motors. Many servo systems use PM motors and drives. PM motors are expected to become more common, unless a shortage of rare earth magnets make them prohibitively expensive.

Warning – PM motors are always on, even when the drive is turned off. Rotating the shaft by hand or by pulling on the web will generate current at the stator and motor leads back to the drive. Lockout/Tagout should include locking the shaft.

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NASA Nominal and Web Handling Design

 

 

NASA, aerospace companies and many other organizations make regular use of the concept of nominal to achieve some truly dazzling results.  In lay person’s terms, it means within preset parameters or, more simply, good enough.  In economic terms, it is the optimum or lowest cost solution between too little diligence and too much diligence.  Thus, while NASA regularly threads the needle with some truly amazing billiard shot paths to planets, they do all of this without paralyzing perfectionism [1].  Yes, they do miss a few deadlines and sometimes they totally miss a planet.  But these are due to unanticipated snags of a truly first of a kind project.  They are not due to improving things that are not broken, nor seek performance beyond nominal.

So, what has this to do with web handling?  It has to do with over-engineering components, which though uncommon, does happen.  As an example, roller alignment that avoids wrinkling and other web damage may be good enough.  A load cell that can resolve 10% of the lightest tension to be run may be good enough. Drive systems that can hold tensions during steady state to better than 5% may also be good enough.

[1] https://www.nasa.gov/sites/default/files/atoms/files/nasa_systems_engineering_handbook_0.pdf

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Synchronous Motors

Yellow Submarine Original Artwork copyright 2012

Synchronous motors stators are very similar to induction motor stators. The rotor is made of a shaft with a sandwich of steel layers (laminations) or alternately with a solid casting. The rotor requires a winding which must be excited with dc current. The rotor current or excitation electro-magnetizes the rotor.

Rotor excitation current can be achieved with normal brushes or on larger motors with a brushless exciter. The brushless exciter consists of an extra small generator on the shaft with a rectifier.

The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn at exactly 3600 RPM (3000 RPM) at 60H (50 Hz). A 4 pole motor will rotate at exactly 1800 RPM. A 6 pole motor will rotate at exactly 1200 RPM.

Synchronous motors produce torque only when they are turning at exactly synchronous speed. Slipping a pole due to torque overload is a serious problem which must trip the motor. The trick is in starting a synchronous motor since it has no torque until it gets to synch speed. For across the line starts, the rotor is shorted. This make the rotor act essentially like the rotor in an induction motor. Induced rotor current then produces torque proportional to slip and the motor accelerates. Once the speed is close to synch speed, the field excitation is applied and the motor snaps into synchronous speed.

Variable speed drives must be specified for operation with synchronous motors. Except for some servo drives, few synchronous motors are used for variable speed operation. Note that Permanent Magnet (PM) motors are a special class of synchronous motor and are becoming widespread in web handling applications.

Advanced users:

A large synchronous motor can have its rotor current over-excited to produce vars (Volt-Ampere Reactive) which can correct the plant electrical power factor and reduce utility charges due to poor power factor.

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Web Projects - Energy Management

 

NASA is acutely aware of fuel and energy.  After all, only a tiny fraction of a rocket’s mass is the payload. The rest is fuel.  There are low energy paths that may take more than a decade for a spacecraft to reach an outer planet.  If they try to speed that up even a little bit, the brutal laws of physics will require a much larger fuel to payload ratio.  What has this to do with project management?

First, we only have so much time and energy (and money) for any particular project.  Slow and steady is usually the lowest energy, most efficient path.  If you or management want to speed things up, it will cost more energy and more money.

Second is the concept of course corrections.  The sooner you can detect a deviation from the desired path and make a correction, the less costly the fuel penalty will be.  Yes, it is possible to too over correct and also lose precious energy, but this is quite rare in spacecraft and projects alike.

 

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Induction Motors

Yellow Submarine Original Artwork copyright 2012

 

Induction motors are the simplest motors to manufacture. The rotor is made of a shaft with a sandwich of steel layers (laminations). The laminations have a ring of punchings. When aluminum or copper is cast into the punchings, a conductive cage called a squirrel cage results. That is all that is required for the electrical/magnetic circuit in the rotor.

The stator needs 3 phase poles with copper windings. If there are 2 poles, the motor will turn near 3600 RPM (3000 RPM) at 60H (50 Hz). A 4 pole motor will rotate near 1800 RPM. A 6 pole motor will rotate near 1200 RPM.

I use the word near since induction motors always slip. The rotor has no magnetizing current induced unless it is slipping with respect to the magnetic field in the stator. Typically the rotor slips by 50 RPM, depending on the load on the motor.

Be sure to specify the motor for variable speed drive duty. Insulation is upgraded for Pulse Width Modulated (PWM) inverter duty. In addition, separate ventilation is required for variable speed motors to ensure adequate air flow.

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Web Handling - Present

 

 

Web handling is essential because it can be one of the top causes of waste, delay and customer complaint in paper, film, foil, nonwovens, textiles, tissue and many other plants.  We are blessed with much information, TOO much information.  How do you even get started when there are more than 4,000 articles, books, columns, and papers on the subject?  The easiest way, by far, is to start by going to school and then using two dedicated search engines.

Web201.61cYouTube clip link:  https://www.youtube.com/watch?v=M01yaZih3MQ

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Tachometers and Encoders

Yellow Submarine Original Artwork copyright 2012

Variable Speed Drives today can be run with or without sensors. By sensors, we mean encoders, tachometers or resolvers. These are attached to the Opposite Drive End of the motor and give an RPM feedback of the motor (not roller).

The encoder costs several hundreds to $1000 plus wiring. If the drive will run without an encoder, why use one?

You will need an encoder on and unwind or winder for roll diameter calculation. Also, the drive needs the best possible performance for unwinds and winders.

If the line is threaded using the drive at low speed, I would recommend encoders.

If the roller must stop or start rapidly, I would recommend using an encoder.

If your product scratches easily and speed must be spot on, I recommend encoders.

Most rollers in the middle of the line do not required encoders for good performance.

That would include calenders, cast rollers, slitting sections, nips, vacuum rolls.

Sometimes drives do not tune well without an encoder. Then you will wish you had one. For example, an off-loading cart may not run well at low speed. Threading conveyors may not hold speed well enough. Old motors may not auto-tune with new drives. In cases like this, I recommend an encoder. You can probably benefit from just one encoder and tune other similar encoderless or sensorless drives to match. The encoder provides a measure of assurance that everything is correct.

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Web Handling History - Past

 

Web handling began two thousand years ago with the first tensioned textile looms.  However, most date the science of web handling to about 200 years ago. Specifically, it was the invention of the first continuous paper making machine, the fourdrinier.  In many ways our heritage, no matter what we make, is paper. Yet, each web material has made its own contributions to the collective best practices and science.  Here is a brief history of the highlights of web handling of the past two centuries.  Why bother with the past when we have so much going on, now?  Simple, those who are not familiar with history are doomed to repeat its mistakes, now.

Web201.61bYouTube clip link:  https://www.youtube.com/watch?v=8rAaHorEV5s

 

 

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Guidelines for Speed Regulator Tuning

When tuning speed regulators, at what speed the drive should be stepped and how large a step should be applied?

First of all, the speed regulator is tuned very early in the life cycle of the mechanical equipment. Check with mechanical to ensure lubrication, alignment, pressures, vacuums, temperatures, and guarding are all in place and set correctly. There may be mechanical restrictions on the top speed of the drive roller (ex. running in seals). There may also be restrictions on rolling backwards (ex. doctor blade against a soft roller).

Speed regulator tuning primarily compensates for the rotational inertia time constant of the roller (seconds to accelerate to rated speed at rated torque). This time constant is independent of speed. Secondary effects such as friction increase load at higher speed and may actually smooth the drive at higher speeds. Other effects such as backlash in the speed reducer or couplings may result in natural resonances in the operating speed range (probably 80 to 85% of design speed).

Do NOT tune the speed regulator at a resonant speed. You will recognize this as a speed in which it is almost impossible to dampen out the resonance (hunting or oscillating).

Otherwise, speed tuning can be done at any speed. You will barely see any change in the time constant of a step taken at low or high speed.

The size of the step is also determined by the inertia of the roller. Select a step size that produces a noticeable jump in torque (minimum 20% of rated torque). Ensure the step is not large enough to put the drive into torque limit (100 or 150 or 200% of rated torque). Once the drive is in torque limit, all regulator theory based on linear systems goes out the window. Print or paste the trend of the step response into the startup report.

Once tuned, be sure to run to top speed. Record any resonant speeds that should be avoided (advise mechanical). Initiate an E-Stop from top speed. The drive should not fault out. Archive your work.

Yellow Submarine Original Artwork copyright 2012

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Web Handling and Why Everyone Needs It

‘Web Handling’ has become a household name in ten thousand plants around the world.  Still, if you know someone who hasn’t yet gotten the word on what web handling is and why they need to know about it, the following links should help.  In short, web handling issues such as path control, wrinkling, winding and many others are usually in the top three causes of waste, delay and customer complaints in most plants.  Now is the time to put in your request for training for your co-workers who may be struggling with problems such as these.  My award winning and trademarked Web101 class has been taken by more than 5,000 students.  You can take this course online or at a public venue, such as through AIMCAL’s Converting School, as well as in your own plant. 

Web201.61aYouTube clip link:  https://www.youtube.com/watch?v=qzI2pNj67LE

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Running Without Tension

Recently week I was called in to a paper mill to look at a problem following a drive upgrade. This was an old machine that had one section running in tension control for about 40 years.

The old drive used a dc generator. The new drive was a previously enjoyed digital dc drive from another location.

After the upgrade, the tension regulator was unstable and was left in manual (draw) control. Things remained in this state for several months. Additionally there were problems with stability of the draw and slack take up. The operators were frustrated, and not at all pleased with the upgrade. Threading was difficult and the draw required constant monitoring while running. Demonstrates that sometimes replacing old for new is not the same as making an improvement.

The tension and draw problems were solved with a filter on the analog speed reference. The slack take up was restored with a small programming change in the drive.

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Tension Indicators

25 years ago, tension was indicated with a 4” (100mm) analog meter on the operator console. These meters were rugged, accurate to about 1%, and provided a nice degree of damping or filtering.

Today, the analog meter is rare, replaced with a number or bar on the operator console. The numeric tension indication is not as useful as a bar display. The bar is not useful unless it updates 10 to 25 times per second. Check your tension displays. I expect many of you have a numeric display with a 1-second update and enough filtering in the load cell amplifier and software to stabilize the Titanic.

A better display is a trend. Again we like 10 updates per second or better.

Since the sky is the limit today, why not a tension vibration spectrum which could relate tension variations to particular rollers or the tuning of the tension regulator. This requires an FFT of the load cell feedback and would be very valuable to maintenance. To be useful, the FFT must take good readings from 100 Hz down to 1 Hz.

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Runnability - Part 3 of 3

 

 

 

(MD strip tensile) strength is probably the next most popularly measured web property after thickness/weight.  Yet, the ability of test lab strength to predict real world runnability is not merely limited, it can be little better than consulting a random-number generator or Ouija board.  Strong statement, but I will stand by this.  The reasons for this are many but include insufficient sample size, statistical confidence, failure mode discrepancies and the lack of consideration of the (customer’s) equipment it will be running on.  There are more predictive measurement methods, but they are involved.  See some of the reasons why we should be quite wary of using strength to predict performance.

Web201.60cYouTube clip link: https://www.youtube.com/watch?v=KxHt1k7b9lQ

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Measuring Volts and Amps

Voltage, current and power measurements for ac motors powered with and Adjustable Frequency Drive (AFD) are problematic. AFD’s use Pulse Width Modulation (PWM) (see image). In most cases, the current reading is the most important to determine if the motor is overloaded or has a single phase fault. The inductance in motor windings filters the current so that it can be read with a Root Mean Square (RMS) clamp-on ammeter.

Voltage is more difficult to measure. The motor inductance does not filter the voltage pulses. The motor leads can also cause wave reflections which may change pulse amplitude at the connection points where voltage may be measured. In general, RMS Digital Multi-Meters (DMM) read 20 to 30% high. Some DMM’s are specifically designed to read voltage produced by AFD’s. These are specialty DMM’s and are advertised as for AFD’s or VFD’s. Products are available from Fluke, Flir, Keysight, and others.

Of course, we all know that power drawn by an ac motor cannot be measured with a single Voltage and Current measurement. Remember Power Factor? You will need a power meter.

All vector drives have built-in current, voltage and power measurements. Provided the drive is operating correctly, these measurements can be trusted, but are not as precise as one would expect from calibrated DMM’s.

 

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Runnability - Pt 2 of 3

 

The test lab and roll inspection together might not be enough for defects that occur less frequently than say a few times an hour as we saw in the last clip.  The reason is in part due to variabilities that can be as important as the averages or means.  In the case of web breaks, variation might be even more important than targets.  Why does a 10 PLI strong web break when tensioned at only 1 PLI?  In universal units, why might a web be damaged at tensions of only 10% of its strength?  The answer has to do with variation.  Strength varies across the width and down the length.  Tension changes every time the web hits a new roller in both the MD (due to bearing drag as well as drives and inertia during speed changes) and CD (due to in-plane roller misalignment etc) and with time (especially during accel/decel).

 Web201.60bYouTube clip link: https://www.youtube.com/watch?v=aGxw-NZMQPo

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Vibration Sensors Built In

Image courtesy of – Yellow Submarine Original Artwork copyright 2011

Vibration in high-speed winding equipment can have severe consequences. Specifically, I am referring to paper slitter/winders.

The vibration typically begins with a non-uniform gauge or caliper across the width of the machine. Once started, the vibration can build, moving the cores off center in the rolls. The rolls in a shaftless winder can begin rocking. If permitted to continue, some of the rolls can escape at very high speed.

In one case, a 30 cm (1 foot) diameter roll flew straight up and came down between the unwind and windup while rotating at 2090 MPM (6000 FPM). In a different event, the roll took out a light fixture in a building with a very high ceiling. I am fortunate in never being in the vicinity when these events happened.

Once the vibration begins, the only solution is to slow down the winder. This can be done automatically with vibration sensors built into the equipment.

M.Jorkama of Metso Paper gave a very interesting talk on this topic at the IWEB conference 2011. It was entitled “Winder Vibration: Causes, Defects and Remedies”.

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Runnability - Pt 1 of 3

 

 

Many runnability issues, such as brittle web breaks, are statistically rare even if they occur every day.  This non-intuitive fact results because the problem area is very, very, very tiny compared to the non-problem area that ran successfully.  In the case of brittle web breaks, the problem may have started on a piece of contamination smaller than your finger nail or finger nail clipping after successfully running hundreds or thousands of acres of ‘good’ web. Problems such as this require a very different approach than is typically found with process monitoring and testing and troubleshooting.  See how to get started in these ‘run-and-hide’ type problems below.

Web201.60aYouTube clip link: https://www.youtube.com/watch?v=AEzidlK6FLU

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Regulator Tuning – Guidance

Image courtesy of – CCO Creative Commons

There is very little if any information on selecting the target Bandwidth (BW in radians/second) or response for the drive regulators used in web handling. We can safely assume the drive vendor’s instructions for tuning the torque loop are adequate.

For the speed regulator, the guidelines are either to tune for the standard (out of the box) BW. This may be 5 radians/second. Another common option is to tune As Fast As Possible. Tuning for the fastest regulator possible will give a different response for each section of the line.

Without any constraints, tuning as fast as possible is a good rule. Watch out for instability and wear and tear on the drive train.

Tension regulators are also tuned as fast as possible. Again we need to ensure there is no instability. In a paper mill, this may give a Bandwidth of 0.5 radians/second. With a servo drive on small equipment, the Bandwidth may be 10 radians/second.

The web will indicate that speed or tension regulators are too slow. A spindle winder tension regulator must recover after a winder cutover before the next cutover takes place. In fact, the faster the better, because if tension is bad at the cutover, any folds or wrinkles form the inner wraps on which the entire roll is based.

We as an industry need better guidance on speed regulator and tension regulator tuning.

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