Process control is so detailed, fragmented, and experience dependent, it is difficult to see the commonality of process control solutions. In Deminar #9 at 10:00 am CDT Wednesday Sept 8, I will detail 10 key concepts in a unified approach that will be useful for process control improvement in 90% or more of the applications. Demos will be offered of the more dynamic consequences. The deeper understanding gained should be useful in developing process control improvements, most of which can be demonstrated by free use of virtual plants on the process control lab website http://www.processcontrollab.com/ .

To attend the event, go to http://bit.ly/JC-LiveMeeting
Use the information below to connect (if you're not using the available computer audio):
• Toll-free: +1 (877) 771-7176
• Toll: +1 (225) 383-1099
• Participant code: 264679

The PID enhancement for wireless (PIDPlus) offers an improvement wherever there is an update time in the loop. In the broadest sense, an update time can range from seconds (e.g. wireless updates and valve or measurement sensitivity limits) to hours (e.g. failures in communication, valve, or measurement). Some of the sources of update time are:

(1) Wireless measurement default update rate for periodic reporting (refresh time)
(2) Wireless measurement trigger level for exception reporting (sensitivity limit)
(3) Wireless communication failure
(4) Broken pH electrode glass or lead wires (failure point is about 7 pH)
(5) Large valve operating on upper part of installed characteristic (low sensitivity)
(6) Valve with backlash (deadband) and stick-slip (resolution and sensitivity limit)
(7) Valve with solids, high temperature, or sticky fluid that causes plugging or seizing
(8) Plugged impulse lines
(9) Analyzer sample processing delay and analysis or multiplex cycle time
(10) Analyzer resolution and sensitivity limit

The PIDPlus waits for an update in the measurement whereas a traditional PID continually ramps the output acting on old information. When there is an update, the PIDPlus considers the changes to have occurred over the elapsed time from the last update whereas the traditional PID thinks the entire change occurred in the PID module execution time. The result is a spike from derivative action by a traditional PID that is particularly large when a measurement recovers or a valve trim or solids break free.

The improvement in control by the PIDPlus is most noticeable as the update time becomes much larger than the 63% process response time (defined in the white paper as the sum of the process deadtime and time constant). When the update time becomes 4 times larger than the 63% process response time that roughly corresponds to the 98% response time frequently cited in the literature, the controller gain can be set equal to the inverse of the process gain. This controller gain can provide an exact correction for changes in the measurement and setpoint.

The PIDPlus execution is kept fast so that the PID immediately responds to changes in setpoint, feedforward, mode, tuning, detail display parameters, and remote output. We have the interesting result that when the update is much larger than the 63% process response time so we can set the controller gain equal to the inverse of the process gain, the controller output goes immediately to the value needed to achieve the setpoint. An increase in update time to prolong battery life can actually translate to a faster setpoint response. However, if the process gain changes with time or operating point, the PID will require several updates to home in on the proper correction. An increase in update time will increase the settling time for unrecognized changes in the process gain. The use of an adaptive tuner such as DeltaV Insight that automatically identifies the process gain and schedules the tuning setting accordingly can sustain a fast setpoint response despite nonlinearities and a large update time.

The Emerson White Paper DeltaV-v11-PID-Enhancements-for-Wireless.pdf discusses these opportunities in more detail. Later this month, an entry on this site will show and discuss the trend plots that compare the enhanced PIDPlus with the traditional PID for the applications tested including valves with stick-slip and backlash.

It is important to distinguish between an update time and process deadtime. The update time is the time interval between successive updates by the final control element (initiated changes to the process input) and successive updates by the measurement (reported changes in the process output). The process deadtime is a continuous train of values delayed by the deadtime. The most common source of a pure process deadtime is a transportation delay of temperature and composition changes in a conveyor, extruder, dip tube, heat exchanger, pipeline, sheet line, or any volume where there is plug flow (no back mixing). Small time constants such as thermal lags, sensor lags, signal filter times, transmitter damping settings, effectively become additional deadtime in terms of a first order plus deadtime approximation (single time constant plus deadtime). The PIDPlus algorithm does not correct for process deadtime. As the process deadtime increases and approaches the update time, the opportunity to increase the PIDPlus gain decreases. For compensation of deadtime, a standard deadtime block can be inserted between the BKCAL_OUT of the AO block and the BKCAL_IN of the PID block if the DCS uses the positive feedback method for the integral mode (external reset) as reported in Advanced Application Note 3 "Compensation of Deadtime in PID Controllers".

In a future Deminar we will look in greater detail at the effect of updates time of discontinuous measurements and process deadtimes on the ultimate period and ultimate gain and if there is an improvement in loop performance offered by a combination of PIDPlus and deadtime compensation.

Whenever I see real control valves with digital positioners and diaphragm actuators, I get a bit giddy with excitement. If on the other hand I see on-off valves installed to perform the role of process control, I just shake my head in dismay. If flows are turned on or off, there is very little process control opportunity. Flows, whether process or utility, are the levers for the process. If we can only jerk the levers around, we will have a jerky process. The Feb-Mar 2010 InTech article "Key Design Components for Final Control Elements" details this perspective as well as the essential design features needed. If you have throttled flows not only do you have a means of affecting but also a way of optimizing the process. It would be a rare coincidence if the flows were exactly at their best value at the right time. There is almost assuredly an opportunity to increase capacity or yield or decrease energy use by changing the flow to reduce variability and/or moving a measurement closer to it optimum operating point. Sure there are options to sequence the turning of flows on and off but such pre-programmed actions lack the feedback correction needed to deal with disturbances, non-idealities, and unknowns in industrial processes. Unfortunately, graduates from chemical or biochemical engineering programs may mistakenly be thinking they can set the flows per the process flow diagram and process design simulation program. Sure they probably had a course on control theory, but maybe all they got was a mathematical view of process control isolated rather than integrated with process research, development, and design.

If the fixed flow mindset results in the use of on-off valves and missing feedback measurements, the opportunities are difficult to identify and may require years and a bunch of money not only for the field instruments and valves but also for the piping and equipment modifications. Just think if you want to install a thermowell and there is no nozzle on the vessel or column in the right location? Also, on-off flows create the step disturbances you would hope would be relegated to control theory textbooks.

Dynamic simulations can show the way but a large expensive automation project can be a hard sell without an installed example. If on the other hand there are sensitive throttling valves and process measurements, opportunities can be trialed and implemented by taking advantage of the ever increasing incredible capability being built into the modern DCS. The key characteristic is sensitivity, which is the smallest change in the controller output or process variable that the valve and sensor, respectively will consistently respond to. Once the sensitivity threshold is reached the output will change by the full amount whereas the output will only change by a quantized amount that is a resolution limit, the other major component of precision. Often the term "resolution" is mistakenly used instead of sensitivity. Resolution, which has a stair-case response, was mostly an issue with rack and pinion actuators and older A/D converters with wide signal ranges (e.g. 1980s generation DCS thermocouple input cards). The resolution today of digital I/O far exceeds the sensitivity capability. The consistent precise response to change is more important than an exact match between input and output for valves. For example, valve span or bias errors (offsets) are clearly not much of an issue because the feedback loop will correct for them provided there is a full range of control possible. Measurement span and bias errors can also be corrected by upper loops or operating procedures, but accurate besides precise measurements are important for closing material balances for process analysis, diagnostics, and optimization as discussed in the Jan-Feb 2010 InTech article "Advances in Flow and Level Measurements Enhance Process Knowledge, Control"

Wireless measurements offer the opportunity to move the transmitters to find opportunities and the optimum location if the process and equipment design engineers had the understanding to provide the connection options. Wireless pH offers the ability to develop inferential measurements and prove the best electrode technology as revealed in the Jan-Feb 2010 InTech WEB Exclusive article "Opportunities for Smart Wireless pH, Conductivity Measurements"

You can click on the above to view and hear the recording of the Deminar.

In Deminar #5 we first show that for a self-regulating process, the process variable will line out (reach a steady state) when the controller is in manual unless there are continual disturbances. The self-regulating response is most commonly encountered response because there are more flow loops than any other type of loop. Liquid pressure loops and temperature control loops in continuous operations have a self-regulating response. Level normally has an integrating response but in the Deminar we show test results for a conical tank level with self-regulating response due to gravity discharge flow. The flow across the discharge valve is proportional to the square root of the liquid head as the level increases, the discharge flow increases and vice versa. The self-regulating or steady state process gain increases with level as a result. The significant increase in cross sectional area with level due to the conical shape causes a dramatic increase in the process time constant that creates a stabilizing effect. The process response at high level is much slower enabling the use of more aggressive tuning settings. However, the test results show these settings at low level cause excessive oscillation. The adaptive level controller is able to keep the set point response smooth and consistent over the level range. For more details you can check out the Control magazine article "Adaptive Level Control"

Most of the Deminar focuses on how an auto tuner, adaptive tuner, and adaptive controller can be used to improve the response of liquid flow and liquid pressure loops. The principle nonlinearities are the control valve characteristic for the flow loop and pump curve for the pressure loop.

I got an excellent question during Deminar #3. An attendee asked how fast does the readback of actual valve position need to be as a secondary variable from a smart positioner. I said it depended on the speed of the valve. For flow loops, I thought once per second would be fast enough. However, since the communication of the actual valve position is not synchronized with PID module execution, there needs to be more than one communication per module execution time. Also, for very fast valves, the valve response time could be much less than the module execution time. The dynamic reset limit needs to know the valve is actually moving or it will slow down the change in controller output. For wireless communication of position measurement, exception reporting could be used where the deadband for updating the position readback is the resolution limit of the valve.

A guideline for the conventional PID could be:

When the controller output changes by an amount greater than resolution of the valve, the communication of the valve position for the dynamic reset limit of a conventional PID should be less than ½ the module execution time and less than ¼ the valve response time.

For an enhanced PID as described in Deminar 1, it is possible that valve position only needs to be communicated when a new measurement value is communicated.

The response time per the ISA-75.25.01-2000 (R2009) standard Test Procedure for Control Valve Response Measurement from Step Inputs is the time the valve takes to reach 86% of the final stroke. As noted in slides 12 & 13 in Deminar 3, the response time for small signals and small actuators is a second order exponential response (response time is approximately twice the sum of the time constants) whereas the response time for large signal and large actuators is a ramp (e.g. response time is 86% of the step change in signal (%) divided by the slewing rate (%/sec)). For valves with hydraulic or digital actuators or small valves with a negligible deadtime from backlash and stiction and with a high sensitivity actuator and positioner (e.g. sliding stem valve diaphragm actuator and digital positioner), the response time could be less than a second. For extremely large valves with excessive deadtime from backlash and stiction and with a low sensitivity actuator and positioner (e.g. piping valve with scotch yoke actuator and pinned shaft connections) the response time could be more than 100 seconds. Thus, we have the ironic situation, where if we have a poor valve choice, the resolution and update rate of actuator position communication can be decreased and the filtering of noise can be decreased to keep fluctuations in controller output from measurement noise less than valve dead-band and resolution. If you don't do small step tests or have no communication of actual valve position, the poor loop performance from a piping valve posing as a control valve may be attributed to disturbances or noise.

The accuracy of the valve position communicated is not as important as precision since it is the change in valve position rather than the value of valve position that is important. The bias and span errors in valve position are corrected by feedback control of the process loop. Since even the best valves with pneumatic actuators do not respond to changes in signal less than 0.1%, the greater resolution of digital values of valve position communication is unnecessary. Consequently, to get faster communication for fast valves and small signal changes, analog signals of valve position should be used for the dynamic reset limit even though they may not be as accurate as digital signals.

The precision of the valve position communication should be better than resolution limit of the control valve (e.g. 0.1% for sliding stem valves with diaphragm actuators and digital positioners).

All of what I have presupposed here needs to be tested and investigated. There is no shortage of interesting scenarios to investigate via dynamic simulation.

You can click on the above to view and hear the recording of the Deminar. In Deminar #3 we explored the confusing situations that can develop for slow control valves and slow secondary loops. The loop can look fine but suddenly burst into oscillations and later go back to smooth sailing. The normal thought is "what changed in the process or the loop?" Well it turns out that nothing changed except the size of the upset or setpoint change. For large errors, the primary controller output starts changing faster than secondary loop or valve can respond. You could slow down the primary loop, but we know this correspondingly reduces control system performance as clearly quantified on slide 1 of EffectsLoopTuning&Dynamics-KPI.pdf. The best solutions of course are to make the valve faster and make the secondary measurement and tuning faster, but the "Quick Fix" that also offers long term protection is to enable the "Dynamic Reset Limit" option in the PID. Even if there is not a problem now, just simply turning on this option to protect against unforeseen deterioration in measurements or valves. For example, someone might try to make a secondary flow or pressure loop look smoother by the overzealous addition of a signal filter or transmitter damping setting in the middle of the night. Even more dramatically the time lag of an electrode in a secondary static mixer pH loop might go from 3 seconds to 300 seconds due to coatings or high temperatures. The Control magazine article "The Power of External-Reset Feedback" offers an excellent explanation of power of the "Dynamic Reset Limit" option which uses the PV of secondary loop as the external reset signal in the positive feedback implementation of the integral mode.

In the future, the Deminars will take only 45 minutes to reduce time commitments and audio and video recording file size. The current range of 8 to 10 demos per Deminar will be reduced to 4 to 5 demos per Deminar. Also, a "Quick Fix" will be discussed near the beginning for viewers who are short on time. The start time in June will be moved up from 1:00 pm to 10:00 am CDT to encourage European participation. I have added subtitles noting the process control improvement (PCI) in parentheses to the Deminars. Also, note that I have changed the topics for Deminars 6 through 8.

In particular, checkout the next Deminar on Thursday May 27 that shows how you can dynamically explore your own case histories and scenarios by you using the free online process control labs.

To attend the event, go to http://bit.ly/JC-LiveMeeting
Use the information below to connect (if you're not using the available computer audio):
• Toll-free: +1 (877) 771-7176
• Toll: +1 (225) 383-1099
• Participant code: 264679

(1) PID Control of Sampled Measurements (How to Eliminate Oscillations from Analyzers and Wireless Measurements with a PID Enhancement) - April 7, Wed 1:00 pm CDT

(2) PID Control of Valve Sticktion and Backlash (How to Eliminate Continual Oscillations with the "Integral Deadband" PID option) - April 21, Wed 1:00 pm CDT

(3) PID Control of Slow Valves and Secondary Loops (How to Eliminate Bursts of Oscillations with the "Dynamic Reset Limit" PID option) - May 12, Wed 1:00 pm CDT

(4) Web Lab Access and Use Instructions (How to Use Free Online Process Control Labs for Fun and Profit and Become Famous by Friday or at Least Saturday) - May 27, Thurs* 1:00 pm CDT (* - Thursday date is to avoid conflict with the World Batch Forum)

(5) PID Tuning for Self-Regulating Processes (How to Compensate for Nonlinearities in Flow and Liquid Pressure Loops) - June 9, Wed 10:00 am CDT

(6) PID Tuning for Near-Integrating Processes (How to Reduce the Tuning Time for Column and Vessel Temperature and Pressure Loops by 90%) - June 23, 10:00 am CDT

(7) PID Control of True Integrating Processes (How to Reduce the Batch Cycle Time for Temperature and pH Loops by 25%) - July 14, 10:00 am CDT

(8) PID Control of Runaway Processes (How to Improve the Performance of Exothermic Reactor Temperature Loops) - July 21, Wed 10:00 am CDT

If you look at all the features and options of a PID controller that are sitting idle, you realize only a small fraction of the power of the PID is used even if you make the big assumption that the controller is tuned. How many loops use integral deadband, nonlinear gains, dynamic reset limits, setpoint velocity limits, or anything but the default PID structure?

The Deminar series started in April and continuing through July provides dynamic examples of how PID power can be used to reduce process variability and make batches and startups faster. In particular, the demanding and critical role of integrating processes is being revealed. We normally think of level as the primary integrating process and may dismiss the tightness of control as not important. Also statistics would say most of the loops are self-regulating. If you look closer you realize that the main reason for the large number of self-regulating loops is that the fact there are more flow loops than any other type of loop. Sure flow loops can be screwed up but auto tuners can find the right settings in matter of minutes and process control improvements can be quickly tested. What we often do not realize is that the really difficult and important loops have either a true integrating response (e.g. gas pressure and batch temperature and composition loops) or are so slow they are best treated as having a near integrating response (e.g. continuous temperature and composition loops). Furthermore, few realize that integrating processes are more sensitive to secondary lags, less than ideal valves, and have counter intuitive tuning rules. I would maintain that 90% of the loops with integrating processes are not tuned correctly and probably have too small of a reset time. The Deminar series and access to the web labs online should provide a source of exploring the opportunity to do better with these loops that matter the most. Check out the Deminar May 12 at 1:00 CDT to see how integrating loops suffer from slow valves and slow secondary loops.

To attend the event, go to http://bit.ly/JC-LiveMeeting
Use the information below to connect (if you're not using the available computer audio):
• Toll-free: +1 (877) 771-7176
• Toll: +1 (225) 383-1099
• Participant code: 264679

(1) PID Control of Sampled Measurements (How to Eliminate Oscillations from Analyzers and Wireless Measurements with a PID Enhancement) - April 7, Wed 1:00 pm CDT

(2) PID Control of Valve Sticktion and Backlash (How to Eliminate Continual Oscillations with the "Integral Deadband" PID option) - April 21, Wed 1:00 pm CDT

(3) PID Control of Slow Valves and Secondary Loops (How to Eliminate Bursts of Oscillations with the "Dynamic Reset Limit" PID option) - May 12, Wed 1:00 pm CDT

(4) Web Lab Access and Use Instructions (How to Use Free Online Process Control Labs for Fun and Profit and Become Famous by Friday or at Least Saturday) - May 27, Thurs* 1:00 pm CDT (* - Thursday date is to avoid conflict with the World Batch Forum)

(5) PID Tuning for Self-Regulating Processes (How to Compensate for Nonlinearities in Flow and Liquid Pressure Loops) - June 9, Wed 10:00 am CDT

(6) PID Tuning for Near-Integrating Processes (How to Reduce the Tuning Time for Column and Vessel Temperature and Pressure Loops by 90%) - June 23, 10:00 am CDT

(7) PID Control of True Integrating Processes (How to Reduce the Batch Cycle Time for Temperature and pH Loops by 25%) - July 14, 10:00 am CDT

(8) PID Control of Runaway Processes (How to Improve the Performance of Exothermic Reactor Temperature Loops) - July 21, Wed 10:00 am CDT