Tip #70 – Minimize Dead Time Time BY GREGMCMILLAN ON JULY 27, 2012 IN PROCESS MEASUREMENT & CONTROL
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Note: The following tip is from the new book by Greg McMillan and Hunter Vegas titled,101 titled, 101 Tips for a Successful Automation Career , inspired by the ISA Mentor Program.
Tip #70 – Minimize Minimize Dead Dead Time By Greg McMillan (Technical) When I was 4 years old and sitting on my Daddy’s knee, he said, “Son, I have just two words for you: dead time.” I did not understand the significance of his words of wisdom for decades. The math in my control theory classes mostly served as a distraction from the essential truth, that if the dead time is zero, the controller gain can be infinite and the reset time zero. The controller has enough muscle for an instantaneous response. The errors from disturbances can be zero. Without dead time, I would be out of a job. The importance particularly particularly hit home in pH control of systems with steep titration curves where the slope and hence process gain can change by a factor of 10 for each pH unit deviation from setpoint. Minimizing dead time reduces the excursion on the titration curve minimizing the nonlinearity seen by the pH control loop. You can get a feel feel for dead time by drinking drinking Hurricanes Hurricanes on Bourbon Street. Street. The time from your first drink to a feeling of being more of a party person than an engineer is dead time. If you drink too many Hurricanes in this dead time, you may be out of control. The key role of dead time in tuning and loop performance performance is largely missing missing in the control literature. Fortunately, I found Greg Shinskey as a guiding light. Shinskey’s articles and books offered the best knowledge of process relationships relationships and dynamics, focusing particularly on dead time as the culprit. Without this essential understanding,, you are vulnerable to a lot of misconceptions. For example, I saw an understanding academic paper where there was no dead time in the simulation. The author was proudly showing how his special tuning method could increase the controller gain and reduce the control errors. He did not realize a tuning method was not even required. He didn’t understand you could continually increase increase the controller gain and improve loop performance.
Concept: Dead time delays the ability of the loop to see a change and make an effective correction. The loop dead time is the total time delay for a complete loop around the block diagram from any starting point. The loop dead time is the sum of actuation, correction, process, recognition, and execution delays and the equivalent dead time from lags (time constants) smaller than the largest time constant in the loop. For unmeasured disturbances and controllers tuned for maximum disturbance rejection, the peak and integrated errors are proportional to the dead time and dead time squared, respectively (see Tip #71). Process, mechanical, and control system designs should minimize the total loop dead time to fundamentally increase the ability of the control loop to do its job. The ultimate period (inverse of the natural frequency in cycles per sec) is proportional to the total loop dead time. For most processes, the maximum controller gain and minimum reset time are inversely and directly, respectively, proportional to the ultimate period and thus the dead time. Details: Process delays (e.g., mixing and transportation delays) create a continuous train of delayed values. Digital delays cause a discontinuous update at discrete intervals. The equivalent dead time from digital delays is ½ the cycle time plus the latency (delay from start of cycle time to the report of result). For most digital devices, the latency is negligible. The dead time from wireless devices is ½ of the default update rate for changes that do not exceed the trigger level. The dead time from a PID in a Distributed Control System (DCS) is about ½ of the module execution time. The dead time from an analyzer where the result is at the end of the analyzer cycle time, is 1½ times the cycle time plus the sample transportation delay. An enhanced PID developed for wireless can enable more aggressive tuning when dead time from the digital device or analyzer is larger than the process time constant. The dead time from actuator and positioner sensitivity limits, valve backlash and stickslip, and from digital signal quantization, is the deadband, resolution, and threshold sensitivity divided by the rate of change of the input to the respective automation system component. The dead time from automation system time constants (e.g., sensor lags, transmitter damping, and signal filter times) that are small compared to the rate of change of the process can be taken approximately as equivalent dead time. For large loop dead times, feedforward control is advisable for measureable large and fast disturbances. When the dead time becomes much greater than the open loop time we have a case of dead time dominance. Tuning methods break down and peak errors for step disturbances are as big as if there was no feedback control. For a list of solutions for this unfortunate case, see the July 19, 2012 Control Talk Blog “Dead Time Dominance Does Not Have to Be Deadly.”
Watch-Outs: Field and simulation tests or imagined scenarios where the disturbance always occurs just before the input of the PV will not show the increase in dead time by the digital device. Such tests or scenarios lead to erroneous conclusions that digital delays do not increase the ultimate period or dead time. Disturbances can arrive at any point in a digital device cycle time and should be visualized on the average as arriving halfway through the cycle time. Tests or scenarios where the input step change is larger than the deadband, resolution, and threshold sensitivity limits will not show additional dead time from these limits. Dead time compensators cannot eliminate the effect of dead time on the ultimate limit. The more aggressive PID tuning possible for dead time compensators is dependent on an exceptionally accurate dead time. Exceptions: The equivalent dead time from small time constants has a factor that decreases as ratio of the small to largest time constant in the loop increases. Insight: Dead time is the ultimate limit to how well a loop can reject unmeasured disturbances and how aggressive you can tune the controller. Rule of Thumb: Minimize the largest sources of dead time and consider the use of an enhanced PID developed for wireless and feedforward control for large dead times.
Tip #9 – The O-ring seal in an on/off actuator can be a decent indicator of its reliability BY GREGMCMILLAN ON JULY 20, 2012 IN PROCESS MEASUREMENT & CONTROL
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Note: The following tip is from the new book by Greg McMillan and Hunter Vegas titled, 101 Tips for a Successful Automation Career , inspired by the ISA Mentor Program.
Tip #9 – The O-ring seal in an on/off actuator can be a decent indicator of its reliability By Hunter Vegas (Technical) Over the course of many years of plant experience, I have come to a simple conclusion in regards to selecting on/off actuators. Despite innumerable glossy, color sales brochures and sales presentations to the contrary, the failure of an on/off actuator can usually be attributed to three things. Two of the items will make ANY
actuator fail – undersizing and poor air quality. The third item is rather subtle, yet can be a surprisingly accurate predictor of how well an actuator will hold up in service. Concept: Several different on/off actuator designs are available today. Some employ a scotch yoke mechanism, others use a rack and pinion, and undoubtedly many others exist. While the vendors will argue the pros and cons of one design versus the other, plant experience suggests that the diminutive piston o-ring design can be a very good indicator of how well a particular actuator will hold up in service. Details: An on/off actuator converts air pressure to a 90-degree turn movement that actuates the valve. Most of the actuator failures can be attributed to three things: 1. The actuator was undersized from the start (see Greg’s Tip #85 concerning this). 2. Poor instrument air quality – If water and/or particulates are in the instrument air system every valve, solenoid, and actuator in the plant will be failing prematurely. 3. The piston o-ring fails. If the actuator is properly sized and the air quality is good, the typical point of failure will almost always be the piston o-ring that seals against the cylinder. Once this oring begins to wear, it will allow the air pressure to “blow by” the piston robbing it of torque. Eventually the actuator will not stroke at all. The design of this o-ring is what usually determines how long an actuator will last in service. A cheap design will employ a single round o-ring on the piston. Such a design works wonderfully when new, but quickly wears and begins leaking air. By contrast, a better design will employ multiple o-rings or a wide, flat ring around the circumference of the piston. Either of the designs will last much longer. Watch-Outs: A poor o-ring design will make an actuator fail quickly. But also watch out for actuator limit switch covers that employ individual screws that are not captive in the cover. (In other words, the bolts fall out when they are unscrewed rather than being held in the cover by a slip ring.) Captive screws will not seem like such a big deal until one is on the 5 th floor of an open structure pulling a cover to set a limit switch and the screw falls out and bounces off several vessels and pipes on the way to the ground 100 below. At that point, the utility of captive screws in the cover ′
becomes a very obvious thing! Exceptions: Some designs use a shorter stroke and a diaphragm instead of a piston with an o-ring. Obviously, this particular design is not susceptible to the o-ring issue.
Insight: Find at least two acceptable actuator designs and get both vendors on your bid list. Having two sources keeps the pricing low and limiting the actuator types to only two cuts down on spare parts. Once a good actuator design is chosen, always oversize the actuator. If the instrument air quality is good, the actuators will provide years of maintenance free service. Rule of Thumb: Pick a good design, and size the actuator for at least one and half times the maximum torque required. (Note that actuators have different torque values at either end of the stroke so be sure to check both ends of the table when doing the sizing.)
