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Applied Automation June 2013 • A3

A4 Managing process safety with � exible I/O

Industrial facilities should approach safety and security holisticallyby addressing critical requirements from the process control network to the perimeter of the plant.

A9 Protecting researchers, scientists with safety automation SLAC National Accelerator Laboratory upgraded its relay-based safety system with a new control and safety platform.

A12 Tuning thermal PID loops When dealing with critical temperature applications, PID controllers are a common regulatory approach, but tuning these often requires a different strategy than other types of loops. Auto-tuning features can help if you understand them.

Contents

COMMENT

T he cover story in this issue of AppliedAutomation is as much about I/O as it is about safety. While safety sys-tems are fundamental to critical process-

es and operations, extrapolating the implications of how flexible and configurable I/O can be applied to more than safety systems could open many doors regarding how engineers approach automation and control projects.

This issue also includes a case study about a safety system upgrade at the SLAC National Accelerator Laboratory at Stanford University. Although the lab’s original 1940s-era relay-based system was functional, it had exceeded its useful life span. The lab upgraded its relay-based safety system with a new control and safety platform based on a safety PLC, which includes a safety controller and distributed safety I/O. The new system provides SLAC with preventive maintenance alerts and diagnos-

tic capabilities that were not possible with the relay-based system.

The third article revisits temperature control. Tuning PID control loops can be challenging, and this article offers guidance on proper loop tuning. While most modern temperature con-trollers employ auto-tune features, units from different manufacturers may not behave the same way. Because control loops are in fact closed loops, tuning is inherently application dependent. The quest for ideal tuning assumes that the system is properly designed. Because the physical environment is part of the loop, the available Btus must be appropriate for the load. As the authors state, “The system is tuned well when it heats up and settles quickly at setpoint and when the temperature settles at a new setpoint without oscillating excessively.” The authors also point out that “quickly” and “exces-sively” are relative terms.

Applying � exibility, safety, and control

Jack SmithEditor

A4

A9

On the coverFlexible I/O systems allow separation of safety and process I/O, support SIL-3 requirements, and can be remotely configured and controlled.Courtesy: Honeywell Process Solutions

Industrial organizations are paying closer attention to safety applications for a variety of reasons, including strict industry regulations and widespread reports of safety incidents around the world. Plants need robust safety applications, which encompass all instrumenta-tion and controls responsible for bringing a process to a

safe state in the event of an unacceptable process devia-tion or failure.

To manage process safety challenges—including the role of defense-in-depth strategies for protecting critical plant assets—plant personnel must understand the appli-cation of current technologies in the marketplace, as well as new technologies for optimizing overall safety perfor-mance and reducing capital and operating costs through-out the project lifecycle.

Operational demandsIndustrial facilities are under growing pressure to better

manage their process and safety assets. Complying with legislation to safeguard personnel, communities, and the environment is a priority for both legal and ethical reasons. Effective safety applications are needed to enable proac-tive protection (versus responsive mitigation), help stop events before they happen, prevent injuries, and save lives.

Plant projects around the world are becoming larger and more complex. Greenfield construction often involves multiple engineering procurement contractors, while brownfield projects must be completed with minimal downtime. Operations of all types seek on-time or early start-up, as well as earlier-than-planned production to accelerate returns.

In the process industries, operations such as oil and gas platforms, liquefied natural gas carriers, and floating production, storage, and offloading units typically face space, weight, and power constraints for automation equipment such as I/O devices. In addition, these opera-tions must ensure a sufficient number of spares for the lifecycle of the installed asset.

At greenfield sites, building adequate control room infrastructure is a high priority. This makes moving con-trol and safety functionality to the field—as well as nec-essary hardware—a desirable alternative to traditional approaches. At the same time, users must cope with burdens such as time-consuming hardware configura-tion and programming, late design changes, frequent maintenance, and the need to reduce copper wiring con-necting sensors, transmitters, and other devices with the control room.

Brownfield facilities also deal with issues related to spares availability, not to mention the need to install addi-tional homerun cables as part of any expansion project.

Today, there is now a clear paradigm shift in the pro-cess industries from safety system cost to total cost of ownership. Current system architectures can be either centralized, distributed, or a combination of both. Each approach has its advantages and challenges.

Many operations continue to employ outdated safety solutions implemented in PLCs, control systems, or other legacy platforms. Due to the continuous improvement aspects of ISA-84: Standards for Use in Process Safety Management of Highly Hazardous Chemicals and IEC-61511: Functional safety: Safety instrumented systems for the process industry sector, plants are finding it nec-essary to replace these systems with a modern safety instrumented system (SIS). The need to execute safety instrumented functions that weren’t previously imple-mented or identified is also driving the implementation of SIS technology.

A4 • June 2013 Applied Automation

Managing process safety with flexible I/O

Industrial facilities should approach safety and security holistically by addressing critical requirements from the process control network to the perimeter of the plant.

By Erik de Groot, Honeywel l Process Solut ions

cover story

Courtesy: Honeywell Process Solutions

Figure 1: Defense-in-depth is inherent in safety and security best practices, which integrate independent layers of protection.


Implementing layers of protectionEnsuring the safety and security of personnel, equipment,

and the environment is a priority for every industrial facility. This effort goes far beyond simply installing fail-safe control-lers or an advanced SIS solution. In fact, to mitigate the risk of serious incidents, it is important to consider safety and security from all aspects of a plant’s operation.

Industrial facilities should take a holistic approach to industrial safety and security, addressing critical require-ments from the process control network to the perimeter of the plant. This approach is intended to increase situ-ational awareness of production processes and improve response to emergency situations arising from safety- or security-related incidents. When properly implemented, this approach helps protect people, assets, and the envi-ronment while sustaining a high level of operational and business performance.

At the core of best practices for integrated safety and security is defense-in-depth with independent layers of protection (see Figure 1). This strategy is included in the IEC 61511 standard, which stipulates that every layer of protection—including both control and safety systems—should be unambiguously independent. Some of the reasons for this basic requirement are to avoid common-cause faults, minimize systematic errors, and provide security against unintentional access.

With a layered solution, some layers of protection are preventive in nature (e.g., emergency shutdown), and some are there to mitigate the impact of an incident if it occurs (e.g., fire and gas protective systems or emergency response systems). Other layers of protection can deter incidents in the first place, or provide detection, alerting, and associated guidance.

Maintaining segregated systemsOne of the major achievements of process control

technology in recent years has been its integration of an increasing number of safety functions within the plant automation environment. The move toward sharing critical information with the process control system through an SIS has brought significant benefits.

Industrial organizations are seeking a unified control and safety infrastructure integrated at the controller and HMI level. This solution must comply with key industrial safety regulations as well as applicable cyber security standards. It must also meet industry requirements for high reliability and availability, simplify field device maintenance, and adapt easily to last-minute project engineering changes.

Experience has shown the most robust and reliable approach to control and safety integration maintains the well-established separation principle for the basic process control system and SIS. In this way, users can achieve complete operational integration through a single dashboard, using a fully separated safety network for greater protection.

Empowering plant personnelPlant safety requires a comprehensive program for man-

aging operator effectiveness, constant monitoring of dis-tress indicators, and ongoing monitoring and maintenance for asset health. This integrated approach demands not only an understanding of safety’s relationship to human error, but also the interrelationships among root causes and interventions by plant systems and site personnel.

The layer of protection often missed in the plant safety architecture is the one requiring human intervention. It is essential to equip the operations group with technology and work practices to manage abnormal situations or the eventuality of an incident. In addition, as an experienced

Figure 2: Flexible I/O systems allow separation of safety and pro-cess I/O, support SIL-3 requirements, and can be remotely config-ured and controlled.

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workforce transitions and domain knowledge is potentially lost, it becomes increasingly important to transform that knowledge into institutional procedures and practices. The operator should be properly equipped to recognize an event, as well as be capable of properly evaluating the situation and responding accordingly.

Operator effectiveness afforded by common and con-sistent HMIs across the entire operation, knowledge cap-ture through automation of procedural operations, and an advanced alarming capability allow people to better pre-vent and respond to abnormal situations.

Integrating robust securityAn industrial site is not truly safe without the right secu-

rity. That’s why defense-in-depth must include physical security layers that reach not only beyond the perimeter fence, but also into the very heart of the control room. The integrated physical security element brings together often-disparate technologies to improve situa-tional awareness and operator reaction time during an event or site emergency. In addition, digital video solutions, tight-ly integrated with plant DCS, can now allow cameras to function as process sensors. These systems can integrate at the database level so alarms, events, and digital recording triggers are native to the control system.

To ensure a sound protection strategy, the network and database infrastructure should also include built-in cyber security solutions. This includes an embedded and certi-fied firewall—although cyber security goes well beyond this—starting within the end-user’s business network and extending to the core of the control architecture.

Using smart, flexible technologyProcess safety applications present a range of opera-

tional challenges. In recent years, new technologies and products have emerged to address some of these issues, but in many cases they are limited and provide only partial solutions. Plant owners are seeking feature-rich solutions they can configure to meet their unique requirements. The need to accommodate legacy systems as well as new installations has underlined the necessity of compatibil-ity and configurability. The current business climate also demands products that keep capital and maintenance costs at a minimum.

Within the plant control and safety architecture, the I/O subsystem is responsible for inputting hundreds or often thousands of different process measurements and other inputs into the system, and outputting control signals to a large number of final control elements. I/O represents one of the most significant parts of the system infrastructure, and, traditionally, a significant cost element. However, automation suppliers are working to reduce both the cost

and the complexity of I/O by incorporating more intelli-gence and programmability into their solutions.

With the advent of flexible I/O systems, process manu-facturers can now integrate more safety devices while sim-plifying installation and maintenance (see Figure 2). These systems employ innovative technology that allows instant configuration of I/O channels without additional hardware. They enable maximum architectural flexibility, lower cost of ownership, support SIL-3 application requirements, and are ideal for facilities that must integrate equipment, units, and other assets spread over wide geographic distances.

Developments in I/O technology offer an opportunity to liberate safety and process I/O, as well as control cabi-nets, from channel-type dependency. This concept enables multiple remote locations to be controlled out of a single centralized unit, with each channel of I/O individually soft-

ware-configured either as analog input (AI), analog output (AO), digital input (DI), or digital output (DO). It reduces wasted I/O space and provides savings on both installation and operational costs because users no longer have to worry about having enough modules for AI, AO, DI, or DO configurations. The I/O connection can easily be config-ured—and reconfigured—at any time.

Plants that implement these technolo-gies can also standardize on a univer-

sal cabinet with a generic configuration because any field signal can be connected to any I/O channel. Engineers are able to reduce documentation cost by knowing how much I/O needs to be supported, as well as its installation space requirement.

Some I/O systems are designed to support electronic (soft) marshalling, which allows the I/O module to be mounted close to the process unit to reduce or eliminate the need for homerun cables, marshalling panels, junc-tion boxes, and field auxiliary rooms. With this new way of I/O deployment, field wires can be terminated on any I/O module or channel, regardless of signal type. It elimi-nates the scrambling needed in conventional marshalling approaches, thus reducing hardware complexity associ-ated with installing, commissioning, and maintaining the system, resulting in savings on marshalling cabinets, inter-panel wiring, cabinet space, power requirements, and the traditional time needed to deploy these items.

By employing a flexible I/O approach, late changes resulting in costly project delays can now be done through remote access rather than manipulating hardware in the field. What previously took days or even weeks can be accomplished in minutes. Every day gained in the project schedule is an extra day of production. Because only one type of I/O module is needed for each project, engineers need only worry about I/O count—not I/O mix.

The latest technology advancements also limit the amount of training required for plant personnel. Only one

cover story

With the advent of flexible I/O systems, process manu-facturers can now integrate more safety devices while

simplifying installation and maintenance.

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cover story

category of I/O is needed to meet all of the input/output requirements on a typical SIS project. There are fewer inter-connections with this type of solution—and consequently fewer failures—so testing and installation are easier.

Moving applications to the fieldMost recently, automation suppliers have developed

safety logic solvers designed to execute safety applica-tions independently in the I/O module. Users can distrib-ute safety logic into the field in close proximity to the pro-cess while maintaining a transparent overview. Such logic solvers safeguard the process even in the event of inter-ruption in communication with the SIS. This approach is ideally suited to highly distributed applications, and reduces cost while increasing process availability and efficiency.

For example, in the upstream oil and gas industry, oil field operators can now implement a safety solution whereby the I/O module at each wellhead is integrated into the SIS and DCS, but can also act as a dedicated logic solver for the head if the central connection is lost. In effect, another layer of redundancy and separation is added to the system.

Meanwhile, pipeline integrity solutions and radar video

surveillance for on-land facilities that allow significant savings over camera-only based solutions are expanding the scope and range of safety devices that can be inte-grated into the system in other areas.

A new process safety landscapeOperational managers in process plants must address

a host of pressing demands: worker safety, environmental stewardship, process uptime, and conservation of plant resources—and the list goes on. These challenges are stretching the limits of existing resources and expertise.

With innovations in automation technology, plants can optimize process safety systems in a wide range of instal-lations, improve overall safety performance, and reduce capital and operating costs throughout the lifecycle of their projects.

Erik de Groot is marketing manager of safety systems for Honeywell Process Solutions. He has been active in the process industries in both process development and automation for 26 years, including 16 years with Honeywell where he started as an application engineer in Amsterdam, the Netherlands. He has a Bachelor’s degree in chemical engineering from the HTS Hilversum, the Netherlands.


Protecting researchers, scientists with safety automation

SLAC National Accelerator Laboratory upgraded its relay-based safety system with a new control and safety platform.

T hey barrel through the 2-mile tunnel and hit a wall at nearly the speed of light. This is no high-tech fender bender. It’s no accident at all. World-class researchers have gleaned life-changing discoveries from these ultra-fast, microscopic collisions for more than half a century.

Top scientists from around the globe apply months, even years in advance for the opportunity to further study life’s smallest particles at the SLAC National Accelerator Laboratory at Stanford University.

The main attraction for most researchers is the oppor-tunity to experiment with the world’s most powerful X-ray laser, created through electrons riding the accelerator’s coveted beam. The remarkable X-ray imaging tool, SLAC’s Linac Coherent Light Source, is a recent addition that has helped transform the facility into a multipurpose lab and

the ultimate proving ground for more scientists than ever before (see Figure 1).

Demand is so strong that SLAC is providing 24/7 access to the accelerator beam and is expanding the facility to more than double the number of experiments that can be done simultaneously. Breakthrough discoveries, such as the latest cancer treatments and a better understanding of viruses, fuel cells, and the sun and stars, can be traced back to research done at the SLAC National Accelerator Lab. “The high-powered X-ray laser allows scientists to look at molecules in a way that was not possible—even conceivable—a few years ago,” said Enzo Carrone, deputy director of the lab’s instrumentation and control division and head of the safety systems department.

Ensuring safety, uptimeHowever, scientific advancements do not come without

some challenges. “Our top priority is protecting scientists and staff from potential radiation hazards produced by the

accelerator,” said Carrone. “We also must ensure the beam is up and running. Without safety and the beam, little science can be conducted here.”

SLAC has an impec-cable safety record, but the expanding and increasingly complex operation required a new level of safety and control. The lab’s original 1940s-era relay-based sys-

By Chris Sheehy, Siemens Industry Inc.

Figure 1: The X-ray scattering end-station at SLAC’s Linac Coherent Light Source soft X-ray beam line is a vacuum chamber and sample environ-ment that enables researchers to probe how electrons behave in special materials, such as magnets or high-temperature superconductors.

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Case study: safety systems


Case study: safety systems

tem did a good job, but it was well beyond its intended service life. “Relays just couldn’t pro-vide the level of intelligent pro-tection and diagnostics support we must have going forward,” Carrone said.

Maintaining laboratory uptime is critical. “The cost of factory downtime is often measured in lost labor and product, but you can’t put a price on the loss of a potential scientific break-through,” Carrone said. “That is exactly what’s at stake when researchers can’t finish a proj-ect because the accelerator is down. We strive for 98% beam availability, considering the huge impact it can have on sci-ence and discovery.”

“The accelerator has been down for hours at a time before, causing lengthy delays in criti-cal experiments,” said Kristina Turner, manager of SLAC’s Personnel Protection System (PPS) upgrade. “We needed a new platform capable of extract-ing a new level of insight and knowledge from the accelera-tor system. We had to find an intelligent solution to enable us to better understand our operation and help us greatly reduce downtime by anticipating issues before they occur.”

“These accelerator machines cost hundreds of millions of dollars, and they’re meant to be here for decades to enable important research,” Carrone said. “We must have a control and safety solution that allows us to look 30-plus years into the future with confidence. Our relay-based architecture had run far beyond its course. Our fast-grow-ing lab had to have a flexible and scalable architecture to meet the ever-changing requirements of real-time scientific breakthroughs.”

The home of many life-altering scientific breakthroughs needed a control platform that would enable accelerator operators to manage and monitor the increasingly complex lab and its multiple laser beams and radiation sources. At the same time, SLAC needed a safety platform capable of keeping scientists and SLAC staff out of harm’s way for the foreseeable future. And the laboratory needed its new control and safety system in place within a year.

SLAC gets control, safety upgradeFollowing a thorough review of system options with sis-

ter labs across the country, Carrone and his team selected

control and safety automation solutions from Siemens Industry Inc. The automation used at SLAC is patterned from control platforms at other DOE science labs. “Our collaboration with the Jefferson Laboratory in Virginia, Brookhaven in New York, and others allowed us to select a hardware platform and design a new safety system within our important one-year timeframe,” said Turner. “We had to quickly and effectively implement an intelligent security upgrade capable of handling the com-plexities of a multipurpose lab. The ability to share ideas with colleagues who had already made similar upgrades made the difference and made it pos-sible to meet our lofty objec-tives.”

“The safety PLC is the brain behind our new PPS,” Carrone said. Under the hood, the safety PLC uses a Siemens S7-300F safety controller and ET 200S distributed safety I/O to monitor and manage personnel access to and from the accelerator tunnel (see Figure 2). Safety requirements have never been

greater, as radiation hazards have increased inside the underground lab with the addition of new beams and adja-cent experimental areas.

The high-powered X-ray beam can split into as many as six separate beam lines within the same enclosed area. “We no longer have just one laser source and a couple of beam destinations,” Turner said. “We now have two hazard sources. We’re putting in a third and discussing a fourth beam within that 2-mile tunnel.”

“It’s very important that we make it easy for the accel-erator operator to understand what beams are running, given the new layers of complexity we didn’t have a few years ago,” Turner said. Turner and her PPS colleague Matt Cyterski were both accelerator operators before join-ing the safety systems group.

Moving forward—safelyThe new control and safety system enables operators

to break through old laboratory silos for a facility-wide view into the accelerator operation. “It would be unthink-able to try to operate and manage our complex accelera-tor activity without a PLC,” said Cyterski. “This PLC is the primary decision maker that determines when it’s abso-

Figure 2: Access to the accelerator tunnel at SLAC National Accelerator Laboratory is monitored by the new safety PLC, which includes a safety controller and distrib-uted safety I/O.

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lutely safe for staff and scientists to move about the experimental areas.”

The safety PLC has the intelli-gence to enable operators to not only strictly police the restricted areas of the accelerator lab, but also provide them with preventive maintenance alerts. Sensors that enable the PLCs to manage authorized entry through sealed experimental room doors also detect system issues. In the event of a fault, error, or component out-age, the control platform can quickly troubleshoot the problem and identify the culprit for engineering and main-tenance crews.

“The PLC communicates across a ProfiSAFE network, a backbone for distributed safety I/O and other critical inside information related to the accelerator operation,” said Marv Guggemos, project engineer for EandM. The California-based automation specialist played an integral role in the PPS upgrade at SLAC. “ProfiSAFE is a variation of ProfiNET, which provides determin-istic and secure communications via Ethernet.”

The diagnostics capabilities of the PLC and control platform provide peace of mind for accelerator opera-tors and the countless scientists who converge on SLAC to explore count-less possibilities using the beam.

“We are already designing and implementing new beams and experi-mental areas within the accelerator lab,” Turner said. “We had to be absolutely certain that whatever architecture choice we made was the right choice for the next generation of scientists and science.”

ConclusionAs more of the original SLAC

National Accelerator Laboratory infrastructure is overhauled, the control and safety teams are eager to build on the momentum they’ve generated with the new automation platform. “The new system has liter-ally helped us pave the way to new and exciting safety and diagnostics applications across our campus,” said Carrone.

Chris Sheehy is an account manag-er with Siemens Industry Inc. Based in Sacramento Calif., he specializes in machine safety, motion control, machine logic, and visualization appli-cations. Over the past 5 years, he

has focused on developing and main-taining Siemens strategic accounts. Sheehy has a Bachelor of Science in Mechanical Engineering from the University of California at Santa Barbara.


Tuning thermal PID loopsWhen dealing with critical temperature applications, PID controllers are a common regulatory

approach, but tuning these often requires a different strategy than other types of loops. Auto-tuning features can help if you understand them.

If you are responsible for working with PID tempera-ture controllers, you have probably already discovered that such loops can be challenging and that the needs of a given controller and application can vary widely. This discussion is intended to help explain how these controllers work and to offer some basic guidance on

dealing with a PID temperature controller. We’ve done our best to avoid unnecessary jargon while providing basic terminology and definitions that will be helpful when ref-erencing controller manuals and other sources. Bear in mind, though, that controllers vary and applications vary, and we would be remiss if we lead you to believe that our experiences cover all cases or that our advice never goes wrong. Please be careful to consider the unique circumstances of your application as you implement any changes.

Why tune controllers?For optimal results, a PID controller needs to know how

much to adjust the heat to achieve a desired temperature change, and how long the temperature takes to react to a

change in heater power. Tuning teaches the controller the characteristics of a particular system. The controller cap-tures what it learns in its PID settings. The exact names of the PID settings depend on the controller manufacturer, but they typically are: proportional band or gain, integral or reset, and derivative or rate. Consult the manual to find the PID settings in your controller. The controller cannot know the best values for these parameters until it is tuned because every system is different.

When poorly tuned, the temperature can oscillate around the setpoint, be slow to respond to changes, or overshoot the setpoint excessively at start-up or the when the setpoint changes. This impacts productivity by making operators wait, reduces yield, and increases premature failures when products are processed at the wrong tem-perature.

How do you tune a PID controller?The simplest way to tune a PID controller is to use its

auto-tune feature. Nearly all electronic temperature con-trollers now have one, but they don’t all work the same way. To find out how to best use your controller’s auto-tune, read its manual or call its manufacturer. Some con-trollers tune while the load heats up from ambient. Some tune around setpoint. Either way, auto tuning adjusts the PID settings automatically, so you don’t have to. But before you engage that feature, consider these options and implications:

n The temperature may overshoot setpoint while tuning. Controllers that tune near setpoint force the temperature to go up and down. To limit the temperature, set a lower setpoint and observe the tuning behavior. You can tune again at a higher setpoint after confirming that tuning won’t cause the temperature to go too high.

n There is probably a time limit on the auto tune func-tion, so very slow processes may not tune. Check the PID settings prior to and after tuning. If they do not change, the auto tuning process failed for one reason or another. That’s a good time to get help from the controller’s manu-facturer.

n Defining what is considered good tuning depends on the process. Some controllers have options that custom-ize auto tuning results for your process. For example, certain Watlow controllers allow you to select whether the temperature should get to setpoint in the minimum amount

By Jason Beyer and Sean Wilkinson, Wat low

PID looP tunIng

The controller has to understand the characteristics of the process through appropriate tuning.

Courtesy: Watlow


of time with a bit of overshoot, or approach setpoint more cautiously to minimize or eliminate overshoot.

To get the best results when tuning, make sure conditions are like those at which the system will normally function. Here are our tips for a successful auto-tune implementa-tion:

1. Set the setpoint before starting the auto-tune process.

2. Make sure the system’s temperature is stable before starting.

3. Tune the system at the time and loca-tion at which it will be used. Tuning in a lab on a summer day in California may not yield the results necessary to control well on a winter night in Minnesota.

4. Tune with the same heater voltage to be used in operation. If the heaters use 240 Vac when tuning but 208 Vac when installed at the user’s site, the controller will likely have to be tuned again since the change in power will change the way the heaters perform.

5. Tune with actual product or a reason-able simulation in place. An oven full of metal parts tunes differently than an empty one.

6. Tune fully assembled and installed systems. A machine with its cover panels off can perform differently when covered.

7. Consider all the sources of heat. A batch of powered circuit cards in a test oven can significantly change the way it tunes.

8. Consider all the heat sinks. Imagine installing the first machine in a line where several will share the exhaust duct. If the vents that will con-nect to the other machines are closed when you tune the first machine, the cooling effects of the exhaust may be much greater than after the other machines are installed and operating.

9. Consider the range of tempera-tures at which you want the system to perform well, and tune at the highest, lowest, and midpoint; or at each oper-ating temperature if there are not too many. Make a record of the PID settings resulting from each trial; controllers typ-ically overwrite the previous settings each time you tune. If the trials all control well, use the widest proportional band (lowest gain), the least active integral (lowest repeats per minute or highest minutes per repeat) and the least active derivative (typically the smallest number).

10. When multiple temperatures are controlled and the heat from one can affect another, for controllers that tune

at setpoint, tune the loops one at a time with the other loops stable at setpoint. For a controller that tunes while heating from ambient, it may be best to tune the loops simultaneously.

11. If product and heat flows from one temperature con-trol zone to another in a conveyor oven, for example, tune the loops in that order.

When is it well tuned?The system is tuned well when it

heats up and settles quickly at setpoint and when the temperature settles at a new setpoint without oscillating excessively. Of course, quickly and excessively are relative terms, and as noted above, some processes tolerate a little overshoot, allowing the system to change temperature in the minimum time, and others do not. In a system that tolerates some overshoot, we look

for responses like those shown in the graphs. For us, the key indicator of a well-tuned system is not just that the tem-perature is stable, but that the output power is also stable—it should not hunt or oscillate more than a few percent.

Use software such as SpecView to graph the tempera-ture, setpoint, and percent heat power. With a graph you can quantify performance by measuring time to setpoint, time to stabilize, and oscillation amplitude, if any. This allows you to measure if the tuning meets your needs.

This graph illustrates a well-tuned system responding to a setpoint change in a context that allows for a moderate amount of overshoot. Once the temperature has reached the new setpoint, both the temperature and power level are stable.

Consider the range of tem-peratures at which you want the system to perform well,

and tune at the highest, low-est, and midpoint; or at each

operating temperature if there are not too many.

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If auto tuning doesn’t workIf the temperature does not perform to your satisfac-

tion, consider these possibilities:� Was auto tuning performed under ideal conditions?

Review our tips above. If something was not right, cor-rect it and try auto-tuning again.

� When the temperature is at setpoint, if the heat power is not between 10% and 90%, look for a problem such as missing covers or a failed heater. Otherwise, there may be issues with the design or instal-lation.

� If the heat is at 100% and the temperature doesn’t increase or never reaches setpoint, shut the heat off and check that the sensor is positioned and connected correctly. Otherwise, try to determine why there is not enough heating power or why there is too much cooling. Tuning is probably not the problem.

� If the temperature is oscillating, is it due to the power switching method? If the frequency of the oscil-lation is the same as the time proportioning cycle time, reduce the cycle time setting if your relay allows it, or replace the relay with a solid-state power controller that allows much faster switching.

� If the performance is poor because the operat-

ing conditions change too dramatically for PID control to adapt, consider using adaptive tuning if your controller offers it.

Switching to manualIf you still need to make improvements,

you can manually adjust the tuning. Providing detailed instructions on manual-ly tuning PID control is beyond our scope here, but consider the following:

� If the temperature does not reach set-point fast enough, you may be able to fix that, but you may have to tolerate a little overshoot and settling time.

� If it doesn’t settle down fast enough, you may be able to fix that, but you may have to tolerate slower responses to set-point changes.

� Adjust only one PID setting at a time.� Make sure you know which way to

adjust each parameter for the desired result.

� Double or halve the PID setting when making adjustments. With most controllers, small changes will have negligible effects.

� Change the setpoint to test the sys-tem’s responsiveness.

� Wait long enough to see the results of each change before making another. How long to wait depends on how quickly the system can respond. Wait three or four cycles if it is oscillating.

� Graph the results each time you make a change and record the PID settings on the graph. This allows you to evaluate whether or not your changes are improve-ments.

� Graph the output power. If out-put power is oscillating, even if the temperature is stable, the system is probably not stable. Output power is as close to a crystal ball as you get, it tells you what the control system is trying to do before the heater makes it happen and before the system filters the results to the sensor.

We hope these suggestions will help improve the performance of your

controllers. More extensive discussions of PID tuning strategy are available at Watlow’s website.

Jason Beyer is a technical support specialist for con-trollers and power switching devices with Watlow, where he has worked for 32 years. Sean Wilkinson is a prod-uct manager for multi-loop controllers and software with Watlow, where he has worked for 15 years.

www.watlow.com

PID LOOP TUNING

Here is another situation where a process is started up cold. Once the heater goes on, the temperature rises steadily until it reaches the setpoint and stabilizes quickly witout any overshoot. The heater power level drops quickly and remains at the hold temperature without huting.

The system is tuned well when it heats up and settles quickly at setpoint and when the temperature settles at a new setpoint without oscil-lating excessively. Of course, quickly and excessively are

relative terms.

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