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Deadtime Dominance - Sources, Consequences and Solutions

Thu, 01 Dec 2016 20:30:00 -0500

If the total loop deadtime becomes larger than the open loop time constant, you have a deadtime dominant loop. There is a lot of misunderstanding posed by this difficult challenge including a lot of prevalent myths.  Here we look at how we get into this situation, what to expect and what to do. Deadtime is the ultimate limit to loop performance. A controller cannot correct for a disturbance until it starts to see a change in the process variable and can start to change the manipulated variable (e.g., flow). If the deadtime was zero, the controller gain could be set as large as you want and the peak and integrated errors made as small as you want provided there is no noise. The PID gain could theoretically approach infinity and lambda approach zero. Without deadtime, I would be out of job. Sources Fortunately, deadtime dominant loops where the deadtime is much larger than the open loop time constant, is usually relegated to a few loops for processes involving liquids and gases.  Deadtime dominance typically occurs due to transportation delays (deadtime = volume/flow), cycle times of at-line analyzers (deadtime = 1.5 x cycle time, or improper setting of PID execution rates or wireless update rates (dead time = 0.5 execution or update rate time interval). Deadtime dominance due to transportation delay is more common in the mining industry and other processes that are primarily dealing with solids. Large transportation delays result from conveyors or feeders or extruders where the feed of solids are manipulated at the inlet and the quality or quantity is measured at the outlet (discharge) assuming the sensor lag is much less than the transportation delay (i.e., total loop deadtime). The use of a gravimetric feeder can eliminate the delay for feed flow control making a much more precise and responsive loop.  A kiln has a temperature sensor lag that creates an open loop time constant that is usually larger than the transportation delay. However, the use of an optical sensor (e.g., pyrometer) for temperature control eliminates this lag possibly making the loop deadtime dominant. In vessels and pipes where there is no axial mixing, also known as back mixing due to an agitation or recirculation or sparging, there is no mixing up and down in the vessels and mixing forward and backwards in the pipe. We call this plug flow. There may be radial mixing, to make the concentration or temperature in a cross section of the vessel or pipe more uniform or to keep solids dispersed. Radial mixing does not reduce transportation delays whereas axial mixing eliminates plug flow decreasing transportation delays. Any remaining delay from axial mixing depends upon the degree of mixing. For vessels with good geometry and axial agitators, the delay is approximately the turnover time that is the volume divided by the summation of the agitator pumping rate, recirculation rate and feed rate.       Deadtime dominance usually doesn’t occur in temperature loops except for inline systems, such as plug flow liquid or polymer reactors where there is negligible heat transfer lag in the equipment or sensor.   Consequences For a deadtime dominant system, the peak error for a step load disturbance approaches the open loop error (error if loop is in manual). Better tuning and feedback control algorithms cannot do much to improve the peak error. There can be a significant improvement in the integrated error by better tuning and feedback algorithms for a load disturbance but the ultimate limit to the integrated error is the peak error multiplied by the deadtime. Solutions The best solution is to eliminate the deadtime that is the source of deadtime dominance by better process, equipment or automation system design. The next best solution is feedforward control given a reliable timely measurement of the disturbance. The feedforward measurement does not need to be extremely accurate. An error of 20% will still result in an 80% improvement in integrated error if the feedforward timing is right. The correct timing cannot be achieved when the correctio[...]



List of major big data tools

Thu, 01 Dec 2016 13:52:00 -0500

Software packages, data management and storage methods, new communication protocols, programming tools, and cloud-computing services from the IT side.



Plan for big data success

Wed, 30 Nov 2016 14:48:00 -0500

Big data can only help users if they understand what it is, how it can affect their controls and processes, and how they can use it to optimize operations.



Rockwell Automation’s FactoryTalk TeamONE app

Wed, 30 Nov 2016 14:07:27 -0500

Aims to boost team productivity by enabling users to collaborate and share knowledge, view live production diagnostics, interact with machine alarms, and troubleshoot devices.



ICC 2016: Clarksville utilities automate securely

Wed, 30 Nov 2016 11:44:00 -0500

Electricity, water, wastewater services in Arkansas city establish secure network and SCADA to optimize operations and assist economics



Improve business results at the operator interface

Tue, 29 Nov 2016 18:20:00 -0500

Powerful connectivity, integration and visualization lead to better decisions in real time.



Reduce the cost of tank blanketing

Tue, 29 Nov 2016 18:06:00 -0500

Tank blanketing with nitrogen prevents contamination, leaks, and explosions. Download FCI's Tank Blanketing White Paper to learn how FCI’s ST100 Air/Gas Flow Meter provides precise flow measurement to reduce the cost of inert gases.



ICC 2016: Predictive analytics at Sierra Nevada Brewing

Tue, 29 Nov 2016 16:14:00 -0500

Sierra Nevada's David Lewis explores how the company set out to achieve proactive identification of failing RTDs and improve its data visualization tools.



Opto 22 joins Dell IoT Solutions Partner Program

Tue, 29 Nov 2016 15:55:00 -0500

Partnership aims to bridge gap between existing industrial assets and infrastructure, and the digital world of mobile, cloud and IT.



Exploring the love/hate relationship with distributed control

Mon, 28 Nov 2016 18:04:00 -0500

'To the degree networks and standards can provide easy, consistent and seamless access to device-resident controls, the vision of truly distributed control may finally dawn upon us.'



2016 State of Technology Report: Oil & Gas

Mon, 28 Nov 2016 14:33:00 -0500

This report details how rising global demand, climate change concerns and the drive to U.S. energy independence have created a strong environment for innovation in process control.



Cyber security of power distribution systems

Mon, 28 Nov 2016 14:21:00 -0500

The presentation reviewed the threat to commercial/industrial power distribution networks and the various network protocols susceptibility to cyber attacks.




Ask the experts: How to determine open-loop gain?

Wed, 23 Nov 2016 12:16:00 -0500

What to do when the process overshoots the mark.



Defining ethernet determinism depends on the application

Tue, 22 Nov 2016 12:49:00 -0500

With today’s network technology, even Ethernet and wireless are almost always fast enough.



Exelon taps GE Predix to accelerate digital transformation

Mon, 21 Nov 2016 15:50:00 -0500

Suite of software applications to be deployed across Exelon Generation’s fleet of nuclear, hydroelectric, wind, solar and natural gas facilities.



Motors and drives make magic

Mon, 21 Nov 2016 14:35:00 -0500

Ongoing innovations, added intelligence and networking let motors and drives serve in unusual applications and replace obsolete equipment



Lee College Offers Customized Training for Incumbent Employees

Fri, 18 Nov 2016 14:42:00 -0500

Companies looking for customized training for incumbent employees often turn to the Department of Corporate Services at the Lee College Center for Workforce and Community Development, which provides clients a wide range of support – from state-of-the-art learning facilities and specialized programs of study to professional development, skills gap analyses...



Opto 22’s digital I/O carrier board for the Raspberry Pi

Thu, 17 Nov 2016 15:05:35 -0500

Enables the Pi to sense or switch up to 16 individually selectable electrical loads.



Emerson, Shell Philippines keep energy flowing

Thu, 17 Nov 2016 14:06:00 -0500

Company has recently upgraded automation to improve reliability at the Malampaya natural gas facility.



Harness big data to achieve big gains

Thu, 17 Nov 2016 12:18:00 -0500

How to manage new information streams and identify correlations to benefit the enterprise