Tip #70 – Minimize Dead 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 Tips for a Successful Automation Career, inspired by the ISA Mentor

Program.

Tip #70 – Minimize Dead TimeBy 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 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 for dead time by drinking Hurricanes on Bourbon 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 is largely 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 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

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 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 stick-slip, 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 reliabilityBy 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 o-ring 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 5th 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.)