How to tune the PID parameters?

PID

Flow regulation is a compulsory operation in most of the microfluidics operations. In some applications such as 2D or 3D cell culture, flow regulation is essential since accurate micro-environmental parameters control is required. Elveflow do it’s best to make this operation as easy as possible to help you to focus on what really matter in your setup.

Anyway to perform a good flow regulation, it is important to understand a little bit how to adjust the parameters according to you need in speed and stability.

This application note will explain you step-by- step how to choose the parameters.

Presentation of the parameters

What is a Gain parameter “P”? A gain parameter improves the response time but may induce instability when you set a flow rate.

What is an Integration time parameter “I”? An integration time enables smoothing the flow rate measurement but may increase the response time.

What is the “max flow rate value”? An optional parameter, useful if you wish to regulate the flow using only a small part of your sensor range. The default value (0) is the sensor range. It is particularly important to enter this parameter if you are using a BFS.

What are the “PID types”? It is special algorithm, especially design by the Elveflow team to simplify the flow control for some particular set-up. First let it to it default value “PID basic”.

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Set-up

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Set-up diagram

Launching the flow control with the Elveflow® smart interface

Refer to the Application note “How to control the flow using an OB1® pressure controller and a flow sensor?” to have a detailed help on how to launch the flow regulation.

In order to tune the parameters, set a square function control with minimum and maximum pressure values on the flow sensor mid-range (avoid extreme values) and a period similar to your experiment requirements.

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Choosing the parameters

To choose good parameters, many ways are possible. Some are easy and empiric, other need some computations. You can first try the easiest method, and then see if it is sufficient for you or not. If you can’t find optimal parameters for the “PID” basic, you may have to change the PID type or the “maximum flow rate value”. Otherwise, maybe your system is not sized well: use the auto-tune to have personalized advices to size your set-up.

Method 1: Let the auto-tune do it for you

The easiest way is just to press the auto-tune button and let the software characterizes your set-up and chooses the parameters for you.

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Tips:

• Be in a steady state before launching the auto-tune: remove the air from the tubing and let flow some liquids.

• Don’t forget to read the pop-up window after the auto-tune process. It will give you some advices on how to size your set-up. If you can’t manage to have a good regulation in this configuration, go back to the advices.

• You can run the auto-tune two or three times and see how the parameters get adapted.

In many cases, the auto-tune can be sufficient. If it is not enough for you, go to method 2.

Method 2 : Adjust manually

Even if the auto-tune is a very useful tools, it is a generic functionality that you may want to adapt manually. Indeed, according to your experiment, you may want a regulation optimized for stability, or rapidity, or you may want to avoid all the overshoots.

Case 1: The system is unstable or has too much overshoot

First check that you don’t have any air leakage. If you have a large amount of air in your setup, try “PID large reservoir”.

Then decrease both the “P” and the “I” parameters until you get in a stable situation. And go to case 2.

Case 2: The system is too slow

Increase progressively the “P” and the “I” parameters during the flow control regulation (apply a slot control), and check on the graph how the system react.

• You have too much overshoot? Decrease P!

• The system reacts too slowly? Increase P!

• The system reacts quickly but slow down as it approaches the target? Increase I!

• You get some instabilities or too much oscillations? Decrease I!

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Normally, the auto-tune combined with some manual adjustments allows to find out goods regulation parameters for almost all the set-up. If it is not the case for you, check the warning of the auto- tune functionality. You may have to change the pressure line, the flow sensor or the microfluidic resistances to improve your flow control.

If you have a Bronkhorst flow sensor, make sure that the parameter “max flow rate” is correct, the default value gives the whole range of the sensor that can be too much for your need.

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