Single Loop vs. Multi-Loop Temperature Controllers
How to Choose the Best Temperature Controller for a Systems Project
What works best… Single Loop 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 programmable logic controllers (PLCs) (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.
Price
In general, a discrete controller will be the most economical for lower zone count applications. Discrete temperature controllers have the lowest price per loop, multi-loop temperature controllers have a medium price per loop, and programmable logic controllers (PLCs) have the highest price per loop.
Installation
The installation price of a discrete controller is typically higher due to cutouts in the enclosure door and additional wiring. Multi-loop controllers and PLCs on the other hand, are mounted on a subpanel, decreasing the price of installation.
Configuration
Discrete controllers can be configured thru the local display and keypad or 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. Multi-loop controllers and PLCs can only be configured through software.
Maintainability
Troubleshooting discrete temperature controllers can be done by using the local display and keypad. Discrete controllers can be removed and serviced without the need for arc flash gear. Multi-loop controllers and PLCs typically 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 regarding 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 before making a bad product. Most discrete controllers have the option to take an inexpensive current transformer directly into the instrument. PLCs need additional hardware, which is why they come at the highest price.
Input Accuracy
Discrete controllers and multi-loop controllers are rated higher than PLCs in terms of input accuracy.
Local Display
Discrete temperature controllers are rated higher than both multi-loop temperature controllers and programmable logic controllers in terms of local display.
Communications
Multi-loop controllers are rated highest for communications, then PLCs, and then discrete controllers.
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 the blind temperature is a 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 were 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.
| Discrete 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 Price Per Loop | 3 | Highest Price Per Loop |
| Price Greater Than 4 Zones | 4 | Medium Price 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 data collection 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.
As we move forward with technology, there are many different options that one could use to control a process, and there are applications where one method of control will work better than another.
The discrete single-loop controller will always have its place in the 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. Contact Sure Controls today if you’re ready to explore the temperature controllers that we have to offer!
