How to achieve flow control with a feedback loop?

This application note will explain how to a set-up a feedback loop and choose the P&I values in the ESI to achieve a stable flow.

In feedback loops, P&I values refer to Proportional (P) and Integral (I) gains. These are key parameters in a PI controller, which adjusts system output to maintain a desired level. Together, they help balance speed, accuracy, and stability in the system.

Equipment required:

• OB1 pressure controller 
• An MFS or MPS sensor
• ESI software 

Pre-requisites:

• Make sure to fill your system with liquid and make sure there are no bubbles before proceeding
• It is essential to choose the right flow resistance first before using the feedback loop (Fig 14. MFS User Guide), as failing to do so will lead to poorly selected P and I parameters.

Set-up:

Figure 1 - Set-up example for water injection using an MFS-D-4 and an OB1 (two channels 0-2 bars)

Example:

As an example, we’re going to set-up a feedback loop for a flow rate of 850 µL/min.

Using an MFS-D-4 (range: 0 - 1mL/min), by applying a pressure of 800 mbar, the flow rate was approximately 850 µL/min, showing that we chose the right resistance for our system. 

Figure 2 - Matching between the pressure applied and the desired flow rate

After determining this pressure value, we'll proceed to switch to "Sensor mode."

Figure 3 - Switch to Sensor Control mode

Now, we’ll adjust the flow rate to our desired flow rate: 850 µL/min

Figure 4 - Setting the target flow rate

Then, open the Flow control configuration, Feedback and click on “Auto Tune”. The ESI will suggest P and I parameters for your system. 

Figure 5 - Feedback loop auto-tune

Your P and I values are now updated.

Figure 6 - Results of the auto-tune

Figure 7 - The P and I values are updated automatically

 You can now fine-tune the P and I values of your system. To do this, open the graph and then configure a square profile with a 10-second period.

Figure 8 - Setting a square profile configuration: 1. Open the graph, 2. Set-up the square configuration by chosing the lower and higher flow rates and period

Modify the P and I values until you achieve the desired profile. Make as many iterations as necessary to obtain the speed versus stability blend that suits you best.

Figure 9 – Best P and I values for our application

Tips: 

• Do you have too much overshoot? Decrease “P” parameter
• Does the system react too slowly? Increase “P” parameter
• The system reacts quickly but slows down as it approaches the target? Increase “I” parameter
• Do you get some instabilities or too much oscillation? Decrease “I” parameter