Further up the ladder of control.

Further up the ladder of control

A short time ago I visited an agricultural equipment show in California, and I came out amazed. I, like many others I suppose, still think of farmers as hard working people who drive tractors and check soil conditions by breaking up dirt clods in their hands.

What I saw at this show were miniature weather stations that are set up in a field and there, through various sensors, monitor the soil moisture, check local weather conditions, communicate with a weather satellite and control irrigation based on all inputs.

Milking parlors were a far cry from stools and pails. Magnetic implants now track individual animals as they walk past sensors into stalls while automatic feeders record feed levels and other sensors keep track of each cow's production level.

Wow!

I shouldn't be surprised, because our industry is moving the same way. While we still have operators that feel press platens and extruder barrels to determine temperature, we are rapidly moving to programmable logic controllers for our processes with supervisory control systems providing overall management control.

Over the last eight years, I've written several articles relating to advances that have been made in extrusion technology. Recently, my attention was caught by a press release from Davis-Standard relating to some of the newer happenings in extrusion process control. And, again, I was surprised at how many advances have been made in just the last few years.

Davis-Standard, as with many other machine manufacturers, appears to have put considerable effort over the last five to ten years in refining the extrusion process. Of particular interest is how, in the last five years, emphasis appears to have been put on the process as a whole, rather than on the individual elements of hardware that are used in the process.

Much of this change has been driven by the further use of statistical process control techniques, including indices such as process capability. As a result, weak links in the process have been addressed specifically, either through hardware modification or improved electronics and controls.

What's new?

It used to be that, if you wanted to begin a new extrusion operation or modify an existing line, hardware was brought in, put in place, wiring and plumbing connections made and someone pushed the "on" button. In most cases, there would be concern that the total power required did not exceed the total power available and that there was an adequate water supply. But that was about all.

Today, with more demanding customers, tight quality requirements and often intense competition, more is called for. The primary objective has got to be to stabilize the extrusion process to the greatest extent possible. This will minimize product variability, process scrap and rework, overall labor input, and will provide the greatest machine efficiency. And this requires the producer to look at the entire process, rather than any individual element alone.

The first step is to survey the operation. Compare needs of the customer and product with the hardware and systems available. For example:

* Power requirements - While it is important to make sure adequate KVA exists on the line, it is also important to know the susceptibility of the power supply to voltage fluctuation and temporary outages. Additional controls on the system may be necessary. Newer controllers that are programmable can lose their programming if power is lost. Back-up power supplies may be needed to prevent problems and lost time trying to reprogram the systems.

* Air supplies - Not only adequate air, but also, is the air supply clean enough for the control system?

* Water supplies - Is there a problem with delivery pressures or hardness? Dirty water sources can cause problems with heat exchangers.

* Material handling and conditioning - Is it possible (or necessary) to precondition the raw material to be used to certain temperatures/humidity levels? How long will this take and what will be required?

* Temperature controls - Today's equipment is capable of holding [+ or -] 1 [degree] F in each of the various zones. If you are trying to hold close tolerances using older equipment (even 10 years old) this may be a problem.

* What range of tooling will be used?

* What is the condition of existing machines? If existing hardware will be used with new hardware, is the existing hardware capable of producing the same quality of product the new unit is?

* Are the machines sized properly? Having units that operate at the high or low fringes of their speed range, power range or otherwise outside their optimum performance range can lead to problems.

Obtaining tighter dimensional tolerances and more uniform products requires greater attention to detail than was prevalent a decade ago. Our equipment in many cases has been improved because we are able to monitor and control to a much higher level than was possible just a few years ago.

As a general rule, hardware (conveyors, motors, pumps, etc.) that is over 20 years old will not be up to performance standards of newer equipment. Hardware that is less than 10 years old will normally function very well with new equipment. Items between 10 and 20 years old are marginal.

With some notable exceptions, trying to use old equipment in conjunction with a new extruder or curing unit will result in less than optimal performance. Rather than obtaining the benefits of the new machine, you may find that you are limited to the accuracy of the old auxiliary unit.

Control systems need to be considered separately from the hardware. Again, as a rule of thumb, any control system that is over 10 years old is obsolete by current standards.

As an example of this, not long ago, drives that would hold [+ or -] 1% on the speed were considered good. That is the minimum standard today. Newer digital drives with feedback control can control speed to [+ or -] 0.01%. Others are available that can control to [+ or -] 0.001%. Using a 1970s vintage takeaway system with a new extruder will probably not take full advantage of the extruder's capability of producing a highly uniform product.

What about the control systems?

In spite of new technology and hardware capability, most of the basic rules of processing are the same as they have ever been. For extrusion, keep the screw full, hold the temperatures as close as possible and keep the screw moving at a constant speed. These rules probably have not changed in the last 50 years.

What has changed is our ability to determine what "constant" is and our ability to react to changes. Just like the speed changes on drives mentioned earlier, we can now monitor temperatures more rapidly, determine what change (if any) is occurring, predict what change to expect next, then react to that predicition. As a result, while we still use the same thermocouples we used for the last 20 years, as well as the same heat exchangers and the same water, we can control temperatures to a much closer degree.

This change has occurred simply because our control systems are now a whole lot better. Let's consider some control systems.

Discreet controls

Most of these controls today are microprocessor-based. And they are very capable. They control one variable. They are typically small and compact, very accurate and reliable. Over the last five years, self-tuning controllers have become more widespread, eliminating the need to have an operator manually tune the controller. For some applications, they are user programmable, allowing the user to determine how the controller will function in the process. Most of these today are also capable of communicating with other "peer" units or "manager" units.

Dedicated process controllers

These are specialized controllers that are used to control a unique function in a process. For example, in an extruder, one of these might be used to control temperatures in the various zones of the extruder. While these temperatures could be controlled using a series of discreet controllers, use of a dedicated controller integrates the system and allows interaction between the zones. That is, conditions in zone 1 may lead to a modification in how zone 3 is controlled. These provide the first step in system integration.

Programmable logic controllers

These provide the next level of sophistication and integration. These units are non-dedicated and can be used in a wide variety of ways. They are very flexible. Normally, they have no onboard software. As a result, more work is required to set them up. However, once set, they can provide monitoring and control for operations as small as a single machine to as complex as a whole manufacturing facility.

For example, in an extruder, one of these units could monitor and direct all the activities performed by the dedicated temperature control units, plus screw speed, material feed rate, extrusion rates and pressures, takeaway conveyor speeds and final product dimensions. With the proper software, it could integrate all of these functions and make various adjustments to any of the controlled functions to provide the final product qualities desired.

PLCs can monitor various systems simultaneously, while, based on priority rankings in the software, performing a series of control tasks. In addition, they can be installed in local area networks and communicate with other units and supervisory systems.

Supervisory systems are normally outside the hardware control loop, providing management information about performance, plotting trends and providing a variety of statistical and quality information.

Summary

As we are entering into the 1990s, we seem to be doing a much better job of gathering process control information and using it to determine the weak areas in the process. By doing this, we improve our control of products being produced. Using feedback control systems, integrating various control systems, increasing the speed at which we monitor the systems and reacting to changes more rapidly, we have been able to significantly improve the process capability of our hardware.

As was pointed out by Davis-Standard, we need to analyze the products we wish to make to make sure that we look at the capability of the entire process to support our desired performance. The easiest way to do this is by installing a completely new system from the storage areas for raw material to the final packaging of the completed product. Unfortunately, except for some very large players in our business, not many can afford to do this.

Instead, most of us must necessarily look at using as much existing equipment as possible in the new manufacturing operation. In some cases, this may not be possible. In others, it may require upgrading of some parts of the hardware and adding newer control systems.

Of particular importance is understanding what the equipment we have is capable of producing, where its deficiencies may lie and how, if possible, it can be upgraded to modern performance. Many older machines can be brought up to today's standards simply by adding new control systems.

Before embarking on any new production, it is imperative that we adequately define the requirements of the product and the systems we have, along with those we wish to add. The only way to achieve optimum performance is when each element of the production system is fully capable of meeting the requirements of the product.