Modeling and Control

A control valve isn’t doing much to help a control loop deal with the minute by minute onslaught of disturbances if it does not respond to the controller’s output. Yet there is normally nothing in a control valve’s specification form to insure the control valve actually moves. A step forward has been the ANSI/ISA standard 75.25.01 for a control valve step response testing procedure but I wonder if any where near as much effort is put on making sure the valve movement is smooth and sensitive as is spent on the valve size and leakage spec?

I was sensitized to the sensitivity of the control valve because my first area of expertise was pH when I moved from E & I Construction to Engineering Technology. The high process gain for strong acids and bases makes pH loops ideal for identifying valve response limitations. A jump in valve position of just 0.1% can cause a several pH swing. Putting a pH loop in automatic may initiate large amplitude oscillations even though there are no load upsets. In the end I realized great control valve sensitivity could reduce the number of stages of neutralization and save big bucks in process equipment required (see the 3rd edition of the ISA book titled Advanced pH Measurement and Control).

There is a growing awareness that a resolution limit from stick-slip in a control valve can cause a limit cycle in a control loop because the valve position is never exactly were it needs to be. Even if there are no disturbances, integral action in the controller drives the output until it moves, but then it steps right past the right valve position. Besides the limit cycle, there is also a dead time that is the resolution limit divided by the rate of change of the valve signal (controller output). To make things worse a slower rate of change of the controller output increases the resolution limit in some positioner designs. Consequently as the controller tuning is slowed down (Lambda is increased), the dead time and possibly the resolution limit is increased.

Deadband can be just as deadly. Whenever the controller output has to reverse direction, the change has to be greater than the deadband before the valve moves. The result is a dead time that is proportional to the deadband divided by the rate of change of the valve signal (controller output). If the are two integrators in the loop, deadband also creates a limit cycle. The two integrators can be the result of a controller with the integral action on an integrating process (e.g. level) or a cascade loop where the secondary and primary loops both have the integral mode (e.g. PI or PID controller) as discussed in the article “Life is a Batch” in the June 2005 issue of Control magazine.

Stick-slip normally originates from friction in stem packing or from sealing surfaces on the trim. Excessive tightening of the packing, high temperature packing (e.g. graphoil), older types of environmental packing, tight shutoff ball and disc seals, and low gain or spool positioner designs create more stick-slip. The friction is generally worse near the closure position, so most tests results are cited at higher valve positions (e.g. > 20%).

Ever since I started my career almost 40 years ago, inexpensive actuators and positioners have been added to tight shutoff rotary valves original designed for on-off or isolation service. The package is attractively priced and pitched as a control valve that meets or more unfortunately exceeds the valve’s capacity and leakage spec. If the process, mechanical, and instrument design engineer each add extra capacity in the piping, pump, and valve, the result is the extreme sport of a control valve riding the seat. If engineers attempt to make the control valve serve the additional purpose of isolation besides throttling, the problem of popping on and off the seat is magnified. In general, an isolation valve does not make a good throttling valve and vice versa.

In rotary valves, shaft windup can occur, where the actuator shaft twists but the ball or disc does not move because of high friction of the sealing surfaces. Eventually, the ball or disc breaks free and jumps to a new position. If the positioner, no matter how smart it think it is, measures actuator shaft position rather than ball or disc travel, it may report everything is relatively OK. I have seen a whole series of fancy plots from a smart digital positioner with vertical travel actuator shaft position feedback consistently show the stick-slip was less than 0.5% for a butterfly valve designed for tight shutoff (not too bad for the particular application). A travel gage added to the disc in the shop test setup gave the reality check that the stick-slip was actually 9% (lousy for any application).

Deadband is also known as backlash and is often larger in rotary valves because of rotary actuator and shaft coupling design or the need to translate from vertical to rotary motion. Be careful about the use of the term deadband. Purists will argue that deadband is the offset in the plot between an increasing and decreasing valve position for a full scale change in valve signal. In practical terms we think of deadband as the reversal in valve signal necessary to reverse valve position anywhere in the signal range. In the following plot of actual ball travel versus controller output, the stick-slip is evident for changes in the same direction and the deadband shows up for a change in direction of the valve signal. This plot is for the controller in automatic and shows that with a bit of understanding and practice, the dead band and resolution limit can be identified from trend charts. For rotary valves, this presumes there is a measurement of the actual ball or disc position or flow thorough the valve. For sliding stem valves, actuator shaft position read back is normally sufficient because there is a more direct connection of the shaft to the trim stem and no translation of motion.

Valve Deadband and Resolution

For the use of a model predictive control to achieve better valve sensitivity and rangeability see the article “A Fine Time to Break Away from Old Valve Problems” in the October 2005 issue of Control magazine. For equations on how to estimate the amplitude and period of limit cycles from a resolution limit or deadband see the article “What is Your Flow Control Valve Telling?” in the May 2004 issue of Control Design magazine.

To end on a lighter note, here is list to identify with:

Top Ten Exceptional Valves

(10) A measurement with 0.1% repeatability
(9 A control valve with 0.1% dead band
(8) A control valve with 0.1% resolution
(7) A controller that is tuned
(6) A process that is simulated
(5) Any computer picked out by your son
(4) Any canceled all week team building exercise
(3) Any afternoon meeting at the Oasis in Austin
(2) Any conference in Park City
(1) Any writing expedition in Naples

Next week’s blog discusses the merits of a block added to the PID controller output to compensate for valve resolution and deadband.

Over the last few years the process industry has expressed a growing interest in the application of wireless technology for field measurements. The ISA-SP100 Committee was established in early 2005 to set standards and recommended practices for implementing wireless systems in the automation and control environment with a focus on the field level. Also, various industry consortiums have been established to promote the use of wireless technology. For example, the Hart Communication Foundation has adopted the use of IEEE 802.15.4 physical layer for the implementation of wireless HART. At the ISA2006 conference the HART Communication Foundation sponsored a booth in which wireless transmitters from multiple vendors were demonstrated. However, one of the technical challenges that manufacturers face in applying wireless technology to process measurements is how to reduce the power consumption to a level that can be supported for many years without the need for external power.

If the information from a wireless transmitter is only used to monitor slowly changing measurement values e.g. levels in a tank farm then the transmitter power requirements may be minimized by simply slowing down how often a measurement is made and communicated. However, if the measurement is used in control applications that respond in seconds rather than minutes, then simply slowing down how often a measurement is made and communicated will negatively impact control response. To provide best control, it is necessary to reduce the latency in control response to setpoint or load disturbances. In a traditional control system it is possible to minimize latency by over-sampling the control measurement used in control. However, such an approach is not an option if your objective is to minimize wireless transmitter power consumption.

One means of reducing the need for over-sample control measurements is to synchronize the measurement sample with control execution as is done in Foundation Fieldbus device. Using some of the proposed wireless protocols, such as Time Synchronized Mesh Protocol (TSMP), it is possible to synchronize a measurement sample and its associated communication with control execution done in another node. However, the traditional approach of executing control 4-10X faster than the process time constant still will create communication loads that are a barrier in applying wireless devices in faster process applications.

A few years ago we started looking at techniques that could be used to reduce wireless communication load without sacrificing control performance. It turns out that for many applications a 10X reduction in communications load can be achieved by following simple rules in communication and by restructuring the PID control to use non-periodic sample values. Much of this work is documented in a paper that we presented at ISA2005, Similarity-Based Traffic Reduction to Increase Battery Life in Wireless Process Control Network. An overview of this work is provided in the following:

Control Using Wireless Transmitters

If you would like to learn more about the wireless technology, then a good starting point is Protocols and Architectures for Wireless Sensor Networks (Hardcover) by Holzer Karl and Andreas Willig.

As the Fieldbus Foundation (FF) function block specification was being developed, I became aware of the work by International Electrotechnical Commission, IEC, on function block modeling. The IEC TC65C/WG6 (now SC65B/WG15) committee had produced some draft documents that would eventually become the IEC61499 Function Block Standard. This standard defines abstract models that may be used by other IEC standard committees to write function block standards that are specific to industry segments. The WG6 committee’s work was of interest since these early drafts contained terminology and architectural concepts that may be used to precisely describe the distributed environment of fieldbus networks. Thus, in editing the function block specifications, we were able to adopt many of the WG6 definitions and architectural concepts into part 1 of Fieldbus Foundation (FF) Function Block speciation. This part of the specification describes the architecture and formal model of the function block application process. On several occasions, I met with Jim Christensen, Chairman of WG6, to discuss and review different aspects of the function block specification. Eventually, I joined WG6 as one of the US Experts and actively participated in the committee meeting for a few years. As a result of this cooperation, the terminology and even many of the architecture drawing included in the Fieldbus Foundation function block specifications are well aligned with those in the final IEC61499 standard.

Soon after the Fieldbus Foundation specifications were published, IEC formed the SC65C/WG7 (now SC65E/WG7) committee to standardize Function blocks (FB) for process control. I joined this committee as one of the US Experts and have actively contributed to this effort. The IEC61804 standard was produced by the WG7 committee. One of the primary tasks of the committee was to creating a function block standard that addresses the requirements of the process industry. The abstract model defined by the IEC 61499 preliminary standard and the work by ISO 15745-1 helped set a foundation for this work, as illustrated in the introduction of the IEC 61804 standard. Also, the function block specification work by the Fieldbus Foundation, Profibus International, and the Noah European project influenced the IEC 61804 standard. In particular, some key technical requirements addressed this standard are:

Deterministic block execution
Block types for resource management, measurement processing and function
block classes for measurement, calculation and control.
Mode parameter
Function block input/output parameter status
Contained parameters of a block for the support of configuration and plant operation
e.g. tuning parameters, setpoint, and mode.

In addition, the IEC 61804 standard addresses a means for a manufacturer to precisely describe the application within a field device through the use of an Electronic Device Description Language, EDDL. The constructs defined by this language allow the manufacturer define calculations and interactions needed to support device calibration and diagnostics. The EDDL defined in the IEC61804 standard is a superset of the device description language utilized by the Hart Communication Foundation, Fieldbus Foundation, and Profibus international. Thus, IEC 61804 is an important standard for the process industry. EDDL is utilized by handheld devices and engineering stations that support HART, Fieldbus Foundation and Profibus devices. The function block application process utilized by Foundation and Profibus fieldbus devices is consistent with the IEC 61804 standard.

Last week, we discussed the deadly and sticky situations of some control valves. I once thought a variable speed drive (VSD) was the solution until Bob Heider pointed out that some VSD packages may not be as sensitive as a well designed throttling control valve. A VSD resolution may be 0.4% whereas a sliding stem control valve with low friction packing and digital positioner can be 0.1%. Jae Park looked at the specs for a particular model VSD and found out the following:

I Speed control

a. Speed regulation without the feedback (without the encoder): 0.1% of base speed across 120:1 speed range

b. Speed regulation with the feedback (with the encoder): 0.001% of base speed across 120:1 speed range

II Command signal accuracy

a. Typically 9 to 12 bits for analog in VFD from different manufactures. If one bit is a sign bit this corresponds to a resolution of 0.05% to 0.4%.

b. If using digital input, accuracy is 0.01% of set output frequency.

Obviously, here the limitation is the analog command signal and a low resolution A/D.

If you refer your project manager to this website and you are still not set free to buy a fine final element and are stuck with a sloppy control valve, where the backlash plus sticktion can range from 0.5% to 10%, you may need to resort to desperate measures. This resorting can be retirement to Stan’s country (Naples) or adding a resolution and deadband compensator from a library of composite templates to the controller output.

A deadband (backlash) compensator can be as simple as an addition or subtraction of a half deadband offset to the controller output when it reverses direction. The following screen prints show the configuration of the composite block.

Deadband Compensator

The compensation of resolution (stick-slip) is a bit dicey. One implementation uses the relay auto tuner method of a single step (kick) of the output in the direction to return the process variable (PV) to its set point (SP) when the PV gets out of the noise band. However, in this case the kick is equal to the stick and is not necessarily large enough to cause the PV to cross back over the SP. This method requires that the control action and valve action be correctly provided as inputs. The block also shows an optional dither of the loop to help reduce the sticktion since some valves have trouble breaking free when stuck in one position for a long time (often called freezing in position even when it caused by hot temperatures). The worst case is if the valve is normally closed and designed for tight shutoff. The whole motionless gig is kind of like me in a lazy boy chair getting commands from my spouse. I can be as sensitive as the finest example of my gender but as I get older my joints get stiffer the longer I sit.

Since constant dither can wear out valve or body parts, dither amplitude and frequency is important from both a maintenance and variability view point. The dither is more suitable for composition, gas pressure, or temperature loops on a large well mixed volume, because these have a slower natural period (less frequent dither) and larger process time constant (more effective filtering). The following file shows the reduction in oscillation amplitude in the primary temperature loop of a cascade temperature control system by the addition of a resolution compensator on the secondary (coolant) temperature controller’s output. Also shown are screen prints of the configuration of the composite block.

Resolution Compensator

These compensators need to be tested and adjusted carefully because of many practical issues. The deadband and stick-slip are never constant. For example, the value depends upon the throttle position, magnitude and direction of the change in the controller output, and the time in service of the valve. The slip can also be greater than stick, particularly if the actuator is undersized or the valve is coming off the seat. Fine valves do not age like fine wines. Crud can build up on the stems and trim (another reason why dither may help). A kick or offset that is too large will create additional slip and do more harm than good so underestimates of the deadband and resolution limit are wise. Some software packages can identify the deadband and resolution automatically online as documented in the paper “Valve Diagnostics in an Adaptive Control Loop”.

The following file, which is Appendix A of the paper, describes the use of a composite block that has a concise code for the simulation of valve dead band and resolution. The block also models the rate limited second order second order response of the actuator. The user can set the pre-stroke dead time, the slewing rate for increasing and decreasing directions, and second order lags. For large valves, the actuator dynamics are significant because it takes time to move enough air in and out of the actuator to move its shaft. These dynamics are particularly important for compressor anti-surge control valves. Until recently, when a supplier provided the dynamic response of a control valve, it was for an actuator not connected to a control valve even though not too many of these were sold this way. The dynamic response did not include the effects of backlash and sticktion.

Valve Diagnostics Appendix A

Training an operator on a new control system often includes hands on experience with a training system that supports dynamic process and control system simulation. The hardware for such a training system may be constructed using spare parts from the new control system. A simple simulation of the process can often be implemented using the control system tools provided to configure calculations and logic. The plant control strategies and operator interface should be used without change when create the training system. However, one of the barriers in doing this is the fact that the IO configuration used for measurement, calculations, and control strategy may need to be modified to work with a process simulation. As the Foundation Fieldbus team worked on the function block specifications, one of our objectives was to provide an easy means of integrating process simulation into measurement and control applications. Also, the ability to override IO values was something we felt that an instrument technician or control engineer would find helpful in checkout of a control strategy or a display configuration.

After some investigation, the function block team proposed that a SIMULATE parameter
be included in all IO blocks. This parameter was defined to have the following attributes:

 Simulate Enable/Disable
 Simulate Value
 Simulate Status
 Field Value
 Field Status

The actual measurement value and status of the IO block are reflected in the Field Value and Status. When the Simulate Enable/Disable attribute is changed to Enable, then the IO function block uses the Simulate Value and Status in place of the Field Value and Status. Thus, an instrument technician that is checking out a control strategy before startup can simply Enable simulate and then write to the Simulate value and status attribute. In the IO blocks, the simulated value and status are processed the same as the field element signal. Thus, when a process simulation writes calculated measurement values and status to the Simulate Value and Status attributes of IO blocks then values based on these simulated measurements will appears in the control strategy and operator screen.

When the function block team initially presented the Simulate parameter to the Fieldbus Foundation Technical Advisory team, there was much discussion about whether we should including this capability in field devices. The concern was that, in an on-line system, the operator would not know if the measurement he sees at his interface station is simulated or the true measurement value. To address this concern, the function block team added to the specification the requirement that all Foundation fieldbus devices support a physical jumper that can be used to disable the simulation capability in an on-line system. Also, an explicit alarm was added to BLK_ERR that indicates when simulation is enabled. By taking these steps, the function block team was able to include SIMULATE as a standard parameter in Fieldbus Foundation IO blocks. This capability has proven to be very valuable in system check and in enabling the development of operator training systems.

Control systems assume linearity. Unfortunately the world is basically nonlinear. For the next few weeks we are going to explore how gains, process time constant, and dead times change with plant design and operating condition. This week we start out looking at valve gains.

A plot of the flow versus valve position (installed characteristic) of most control valves is nonlinear. Here the slope is the valve gain. If we were to plot a process variable versus this flow, such as temperature or composition, it would also be nonlinear. Here the slope is the process gain. These are called operating point nonlinearities. If the process variable stays close to its set point, the slope doesn’t change much. Thus, for a constant set point, minimal dead time, and good tuning, the process nonlinearity is not much of an issue. On the other hand, the control valve may have to move a lot to achieve tight control. The loop is more likely to see the nonlinearity of the control valve. Generally the slope of the installed characteristic gets too flat at low and high positions. Entech published a gain specification that the % flow divided by % signal should be between 0.5 and 2.0 (a gain change of 4:1). The following examples of installed characteristics show that the throttle range is shortest for a butterfly valve and longest for a sliding stem valve. For a detailed discussion of these figures see Chapter 2 of Advanced Control Unleashed.

Valve Gains

The rangeability statements by valve manufacturers are defined in terms the uniformity of the inherent characteristic. These statements do not take into account a gain specification, an installed characteristic, or the increased stick-slip at low valve positions from friction of the seating and sealing surfaces, particularly for tight shutoff valves.

A signal characterizer block can be inserted between the controller output and analog output block to compensate for the nonlinearity of the control valve gain. The characterizer is set up to calculate the % flow from % position (the Y axis from the X axis of the installed characteristic). The input signal to the control valve is now % desired flow rather than % desired position. This can confuse operations and maintenance if not adequately documented and displayed. The accuracy of this gain compensation depends upon the knowledge of the system pressures and friction losses that affect the pressures at the inlet and outlet of the control valve. Software can predict the installed characteristic but this is done typically offline with manual entry of data. There is an opportunity for pressure measurements upstream and downstream to provide better compensation of the valve nonlinearity besides facilitate the monitoring and trouble shooting of disturbances. Many times I wished more pressure transmitters were installed to figure out why a loop just got clobbered, but this is another story.

Another practical issue relates to valve stick-slip and backlash, whose effect and compensation we alerted readers to in our Dec 4 and 11 blogs. For operation on the steeper portion of the installed characteristic, the characterizer makes the change in signal to the control valve smaller. Thus it takes longer for the signal to work its way through the resolution limit and dead band. However, for operation on the flatter portion of the installed characteristic, the change in the control valve signal is larger reducing the dead time from the resolution limit and dead band. If you ever waited for the controller output to work its way along the upper flat portion of a butterfly valve characteristic for a process unit operating at or beyond its design limit, you can appreciate the acceleration offered by the signal characterizer. Of course, at some point you just run out of valve and need to take a look at the pump and piping system design besides the valve size.

First principal relationships can define process cause and effects that can lead to improved controller tuning and performance by the selection of better tuning rules and process variables for scheduling of tuning settings. It also affects the choice of control valve trim and the feedforward design. The understanding of these relationships does not require a degree in chemical engineering but presumes just some understanding of common terms (e.g. heat transfer coefficient and area), relationships (e.g. ideal gas law), and physical concepts (e.g. conservation of mass and energy).

Equations have been developed from first principal relationships for the process gains, dead times, and time constants of volumes with various degrees of mixing. The results show that for well mixed volumes with negligible injection delays, the effect of flow cancels out for the controller gain if one of the following methods is used: Lambda self-regulating rule where Lambda is set equal to the dead time, or the reaction curve method. The effect of flow also cancels out for the reset time besides the controller gain if the process is treated as a “near integrator” and the Lambda integrating tuning rule is used. This is because the flow rate cancels out in the computation of the ratio of process gain to time constant that is the “near integrator” gain. This ratio and “near integrator gain” are inversely proportional to the process holdup mass (e.g. liquid mass). However, for temperature control the effect of changes in liquid mass cancels out because a change in level increases the heat transfer surface area covered. Several authors have mistakenly tried to schedule controller tuning based on liquid level for reactor temperature control. One author has reported being bewildered by its failure. This is not the case for gas pressure control. The equations show that liquid level has a profound effect on the process integrating gain for vessel pressure control because it changes the vapor space volume without any competing effect. To summarize, the integrator gain for composition and gas pressure is inversely proportional to liquid level (liquid mass). For temperature, the effect of level cancels out unless the level is above or below the heat transfer surface area, which is unusual but can occur at the beginning or end of a batch when coils instead of a jacket is used for heat transfer. For temperature, the integrator gain is nearly always proportional to the overall heat transfer coefficient that is a function of mixing, process composition, and fouling or frosting.

The equations also show that if the transport delay for flow injection is large compared to the time constant, which does occur for reagent injection in dip tubes for pH control), then the controller gain will be proportional to flow. Note that pH control is a class of concentration control.

For the control of temperature and concentration in a pipe, the process dead time and process gain are both inversely proportional to flow and the process time constant is essentially zero, which makes the actuator, sensor, transmitter, or signal filter time lag the largest time constant in the loop. Thus, the largest automation system lag determines the dead time to time constant ratio. For a static mixer, there is some mixing, and the process time constant is inversely proportional to flow but is usually quite small compared to other lags in the loop. The controller gain is generally proportional to flow for both cases.

Finally, the above has implications so far as whether a flow feedforward multiplier or summer and whether a linear or equal percentage trim should be used. A flow feedforward multiplier and equal percentage trim, which both have a gain proportional to flow, can help compensate for a process gain that is inversely proportional to flow provided the process time constant is not also inversely proportional to flow. This is generally the case for temperature and concentration control of essentially plug flow volumes (pipelines, static mixers, and heat exchangers). For well mixed volumes, feedforward summers and an installed linear characteristic for valves is generally best. For control valves this corresponds to a linear trim when the available pressure drop that is much larger than the system pressure drop or critical pressure drop so the installed flow characteristic is close to the inherent flow characteristic.

The results are also useful for determining the dead time to time constant ratio, which has a profound effect on the tuning factors used and the performance of dead time compensation, which has been discussed in the category of controller tuning and control performance on this blog site. A copy of Advanced Application Note 4 that summaries and derives these equations is available from me.

One of the phenomena’s that I have noted over the years is that plant startups inevitably run over into a holiday. This seems to be especially true of the Christmas holiday. Being onsite can be very demanding work but it is also very rewarding. Many of the problems that we must address in control design are best understood and remembered if you have struggled with the problem in the field. However, sending a person to the field who has no plant experience should be done with caution. One of the practices we like to follow is to match a person with no plant experience with one who has worked in industry for some time. In this way, the experience person can act as a guide for the inexperience person and help bring him or her up to speed by setting a good example.

When going onsite, I have found it is wise to always ask what clothing is appropriate. For example, in many plants there is a requirement for steel toe shoes. In most cases the plant will expect you to bring steel toed shoes with you. Often any other safety equipment, such as ear plug, hardhat, gas mask, eye protection will be supplied by the plant. If you will be entering an area of the plant that requires you to carry a gas mask, then the plant may have restrictions that require the face to be cleanly shaved i.e. no beard. In nearly all cases it will be necessary to go though plant safety training and to pass a safety test before being allowed to work in the plant.

Your contact at the plant will be responsible for guiding you into the process area(s) that are to be commissioned. He can also help establish and communicate the rule to follow when making a change that will impact plant operations. Normally all changes that impact the process will go through the operator since he is ultimately responsible for the process operation. In most cases I have found the operator to be extremely knowledgeable about the process. One of the biggest mistakes an onsite person can make is not to respect or work with the operator. The most successful startups are the result of a team effort that includes the operator.

When working at a plant site, you are the guest of the plant. As such there are rules that should be followed. Many of the things that should be considered when doing onsite work are discussed in the paper “A Guide For Doing Onsite Work”, Jean Gibbs, Steve Thorp, Bill Keels, Chemical Engineering, February, 1990. The authors of the paper have many years of plant experience. If you have no plant experience, then you may find this paper helpful in preparing for your first trip to the field.