What is PID controller vs MPC controller?
In process control, there are two main types of controllers: PID (Proportional-Integral-Derivative) and MPC (Model Predictive Control). Here’s a quick look at what each one does best and when to use them.
Why PID Controllers Are Common
PID controllers are simple and get the job done for most tasks. They work well when you only need to control one thing at a time, like keeping a pump running at a steady speed or maintaining a set temperature. They’re widely used because they’re reliable, cost-effective, and easy to maintain.
When and why MPC is selected over PID?
MPC is a more advanced type of control, better for complex processes. It’s generally more expensive and needs more setup, so it’s used when the benefits clearly outweigh these costs.
Main situations where MPC works better:
Processes with Multiple Variables
- If a process has lots of interacting parts (like controlling the speed, water flow, and feed rate in a grinding mill), MPC can handle them all together, which helps keep everything running smoothly. PID usually works on one thing at a time, so it’s not ideal for these complex situations.
Processes with Strict Limits
- MPC is good when variables (like temperature or pressure) must stay within safe limits. For example, in a kiln where the temperature needs to stay in a specific range, MPC can manage this better than PID, which can’t easily handle limits.
Processes with Delays
- For processes where there’s a delay between making a change and seeing the effect (like a conveyor belt with a sensor that’s far from the feed), MPC can predict what’s going to happen and adjust sooner. PID tends to struggle with these types of delays.
Optimizing for Efficiency
- MPC allows you to set goals like reducing energy use or maximizing production. For example, MPC can help lower fuel consumption in a heating system while still reaching the production goals. PID doesn’t have this kind of optimization built-in.
Processes That Change a Lot
- MPC can adjust if the process changes over time, like when there are shifts in feed type or volume. PID is designed for fixed targets, so it’s less flexible when things change often.
If MPC is too complex or costly, engineers can make enhanced versions of PID control. These modified PIDs, like using a Smith Predictor, can help with specific challenges, like handling delays. This gives you some of MPC’s benefits without the full cost.
In short, PID is great for simple, steady processes, while MPC is ideal for complex, variable, or high-stakes applications. Enhanced PID can be a useful middle ground, blending simplicity with a bit more capability.