PLC or PID Controller: What’s the difference and how do you decide what technology you need?

In general terms PLCs are probably one of most widely used pieces of control and automation technology. The clue really comes from the name PLC, or “programmable logic controller”. It is the fact that they are programmable that makes them so versatile in their application. PLCs contain a processor, memory to hold their programming and other data and input and output modules. They are usually programmed via a PC and there are a number of different industry standard (IEC 61131-3) languages that may be used.

A PID Controller is different to a PLC. It still requires inputs and outputs to receive information from the process and send signals back to control it but it contains specialist algorithms designed to control a process with one or multiple control loops. The term ‘PID’ relates to “Proportional Integral Derivative” control. 

A brief explanation of how PID control works …

A control loop aims to get a process to a desired value (setpoint). This may be as simple as a dimmer switch connected to a lightbulb. This is an example of ‘open loop’ control – whereby the operator has to control what happens. Closed loop control provides feedback to the control system so that changes to the process can be made automatically. Continuing our theme of light bulbs, this would require a light sensor to be fitted into the loop so that as a room became darker the current to the bulb would increase so the bulb became brighter. The ‘setpoint’ in this example would be the desired level of light in the room.

PID control is used where greater levels of precision in control are required. It combines three control terms to give a single output to drive the setpoint. The Proportional band gives an output that is proportional to the error (the difference between the setpoint and the actual process value).  The controller is tuned with a proportional band range that aims to get the process value to the setpoint in the shortest time without prolonged oscillations around the setpoint. If the band is set too wide, full power will be removed while there is still a large error and the setpoint will not be reached. If the band is too narrow, the power will be left on until the process value is very close to setpoint and there will be significant ‘overshoot’. As it tries to right itself the narrow band will cause power to be switched on and off, which will cause oscillations around the setpoint that will take a long time to settle – if, indeed, they ever do.

Proportional control gives the operator no options to increase power demand if setpoint cannot be reached. There are always losses in a control loop. In simple terms, think of your house and whether it is perfectly insulated, for example. If the losses in a system counterbalance the proportional control, the process will never reach its setpoint. In such cases, more power needs to be introduced. To resolve this potential issue, Integral control determines the control reaction based on the sum of recent errors to close any gaps.

Proportional Integral (PI) Control reacts to changes seen by the process controller and, as such, is retrospective. Derivative control determines control reaction based on the rate that the error has been changing and so anticipates control requirements. This term of the control algorithm is designed to reduce overshoot and undershoot of the control setpoint. It helps improve stability and handle sudden changes in the process; such as a sudden temperature change due to a door opening, for example.

Do you need PID control?

Control such as that provided by Eurotherm control algorithms can meet all sorts of different control needs. Of course controllers can handle straightforward control and offer reliable, assured performance that will optimize and maximize overall process performance. The difference with Eurotherm control is highlighted in a number of different areas. Eurotherm autotune algorithms (autotune helps you tune your process and achieve optimum proportional, integral and derivative term settings) save engineering time and money. The benefits of the performance of the control itself mean increased stability and process repeatability. Eurotherm control is particularly beneficial in highly regulated or sophisticated control applications.

Alongside world-class PID control, Eurotherm ensures that the devices it offers meet other application and industry needs. In the aerospace industry, for example, I/O must meet a certain level of accuracy and sensitivity to meet the precise thermal processing needs and quality levels this industry demands. Many more sophisticated processes also do not have a single setpoint requirement. They require a setpoint that changes over time – a setpoint profile. Flexible setpoint programmer tools are required to meet these needs without large engineering overheads.

Stable, precise control offers many benefits. Correctly applied, good control has a large impact on a plant’s profitability. It will reduce process times, improve product quality, lower costs associated with scrap, and optimize energy usage. 

Secure data recording

Alongside good control, many industries also require records of a process. This used to be achieved using paper chart recorders. Operators would periodically take the paper record from the recorder and store as appropriate, either with a batch of goods or in an archive. Paper chart recorders were superseded by graphic, paperless recorders some time ago. These provided secure electronic records that could be archived to flash drives, removable electronic media or over the network. This natural progression meant that storing and searching data archives became much more efficient. Today, it is not uncommon for the data recording to be carried out by the control unit. Again, however, the security and accuracy of these records is vital for certain industries and not all records or recording strategies are equal! Ensuring records are complete and secure requires careful requires careful design and understanding of regulations that allow for the use of electronic records.

PLC or PID Controller?

For certain industries where precision and security are paramount, specialist PID knowledge and algorithms are needed. Features such as flexible setpoint programmers and autotune algorithms help to keep engineering budgets under control. PLCs, however, offer greater flexibility and freedom in programming to meet broader application needs. The programming languages used are also often familiar as the technology is used in many different areas of the plant. 

So why not have both? The E+PLC range from Eurotherm by Schneider Electric takes the best of both worlds. It integrates proven, well-accepted Eurotherm control algorithms into a PLC platform that uses standard IEC programming languages. Engineering costs are optimized through the use of standard function blocks, re-usable engineering, flexible communications options and ready, integrated visualisation with automated tag resolution. It offers users auto-tuned PID response that gives faster control without overshoot or oscillation and tighter, more stable control without the need for manual intervention. High resolution I/O and secure recording based on decades of expertise in this area provide a route to further lower engineering costs and obtain easier regulatory compliance. 

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