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Single Loop or or Multi-Loop Temperature Controllers?

How to Choose the Best Temperature Controller for a Systems Project

What works best… Single Loop or or Multi-Loop Temperature Controllers? In our experiences as a systems integrator, we use many different types of process controllers, including discrete single-loop controllers (such as the Eurotherm 3216), multi-loop controllers (such as the Eurotherm Mini-8 or RKC SRZ) and PLC’s (such as Siemens, Allen Bradley, etc).
Below we have assembled a general overview chart to point out the differences (and similarities) between the different controller types. Each application and the goals of each project will need to be evaluated to make the best decision for your project.
In general – a discrete controller will be the most economical for lower zone count applications.
The installation price is typically higher due to cutouts in the enclosure door and additional wiring.
The controller can be configured thru the local display and keypad or it can be configured via software and downloaded. The benefit of configuring the controller thru software is that the program can be saved and downloaded into a replacement controller at a later date.
Troubleshooting can be done by using the local display and keypad. The controller can be removed and serviced without the need for arc flash gear. Typically the controller will have pluggable cards that can be easily serviced in the event of a failure.
Autotune / PID performance
Based on our experience – a discrete controller will outperform a PLC in regards to process controls and PID performance because it has dedicated algorithms designed for process control and the loop update time will be consistent. In addition the discrete controller offers an auto tune feature that will automatically tune the loop to optimize control thus eliminating the need for costly manual tuning during commissioning.
Current Monitoring
For electric heating loads this feature is important. It will allow you to monitor and alarm if a zone has lost a heater prior to making bad product. Most discrete contollers have an option to take an inexpensive current transformer directly into the instrument.
Input Accuracy
Local Display
The majority of temperature controller upgrades we see, however, include using the Eurotherm 2216e, or 3216 as the new means of single loop control for given zones.  There are many different reasons for this including: Eurotherm’s ability to control a process variable with great precision and a tight algorithm, the ease of wiring during installation, and the ability to have access to the controller, where communications are used with an HMI.  We find that while blind temperature is smaller footprint during installation, if the HMI which the operator uses to interface with the process controller were to have a failure and the ability to change a control parameter disappears, this could be a potential issue.  Another advantage of using the single-loop control is that if there was to be an issue at the discrete controller itself, all one needs to do to resolve the issue is remove the controller from the sleeve, and replace it with another equivalent.

Discete Controller Multi Loop Controller PLC
Rating (1-5) Comments Rating (1-5) Comments Rating (1-5) Comments
Price 1 – 4 Zones 5 Lowest Price Per Loop 4 Medium Priice Per Loop 3 Highest Price Per Loop
Price Greater Than 4 Zones 4 Medium Priice Per Loop 5 Lowest Price Per Loop 3 Highest Price Per Loop
Ease – Installation 3 Mounted In Enclosure Door 4 Mounted On Subpanel 4 Mounted On Subpanel
Ease – Configuration 5 Software Or Keypad 4 Software Only 3 Software Only
Ease – Maintenance 5 Pluggable Boards – Per Zone 3 Pluggable Cards – Multiple Zones Per Card 3 Pluggable Cards – Multiple Zones Per Card
Functionality – Autotune 5 Yes 5 Yes 1 No – Must Use Third Party SW
Functionality – Current monitor 5 Yes – CT Wired Direct 5 Yes – CT Wired Direct 3 Yes – Needs Additional Hardware -Highest Price
Input Accuracy 5 5 3
Communications 3 5 4
PID Control Accuracy 5 5 3
Local Display Built In 5 1 1

All of this is designed to make the case for automating data collection. Automating the collection of data removes the problems created by attempting to collect the information manually by delivering accurate and complete data to a central repository where it can be viewed and acted on.  Data collected this way can provide actionable information in real time, and can even alert decision makers of a potential issue before it becomes a major problem.  My goal through this series will be to demystify the process of implementing an automated data collection system, and debunk any misconceptions that may exist in regards to how difficult a system like this is to use and maintain.  For example, the most common tool for users of these systems is Microsoft Excel, software that they are very familiar with and use every day.  Also, manufacturing equipment today comes standard with numerous digital communication options, making it easier to network with other systems than ever before.  This is the essence of the term “Internet of Things” (IoT) that is mentioned so often in trade magazines today.  But that doesn’t mean that if you have older equipment you are left out in the cold either.  Numerous devices exist that allow information to be collected from equipment where no such communications options exist.  I will go into these topics in more depth in the next two articles.  First I will go “under the hood” on data collection infrastructure.  Things like communication protocols, types of network topologies, and strategies for designing a robust system.  In the third and final article I will go into detail on information delivery methods and give some examples on how our customers use systems like this to improve their processes, become more efficient, and even discover “hidden gems” within their plant.
As we move forward with technology, there are many different options which one could use to control a process, there are applications where one method of control will work better than another. The discrete single-loop controller will always have it’s place in industry, whether for a simple single process variable application, such as a mash-tank on our brew-stand, or in a large array of controlled temperatures such as an extruder.  Through imagination and innovation, the solution for most process control needs could be right at your fingertips with the helpful options that Sure Controls can provide.

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