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 does its best to make this operation as easy as possible to help you focus on what really matters 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 your needs in speed and stability.
This application note will explain to you step-by-step how to choose the flow regulation 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 flow regulation 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 a special algorithm, especially designed by the Elveflow team to simplify the flow control for some particular set-ups. First leave it to its default value “PID basic”.
For this setup, the following Elveflow® devices are needed :
OB1 Mk3 Flow Controller
Bronkhorst flow sensor
Sample Reservoirs
Tubings and fittings
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 parameters.
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.
Choosing the flow regulation parameters
To choose good parameters, many ways are possible. Some are easy and empiric, others need some computations. You can first try the easiest method, and then see whether 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.
The easiest way is just to press the auto-tune button and let the software characterize your set-up and choose the parameters for you.
Tips:
• Be in a steady state before launching the auto-tune: remove the air from the tubing and let some liquid flow.
• 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.
Even if the auto-tune is a very useful tool, 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.
Want to getaccurate flow control?Experience the most stable and responsive flow controller!Want to getaccurate flow control?Experience the most stable and responsive flow controller!
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 how the system reacts on the graph.
• You have too much overshoot? Decrease P!
• The system reacts too slowly? Increase P!
• The system reacts quickly but slows down as it approaches the target? Increase I!
• You get some instabilities or too much oscillations? Decrease I!
Normally, the auto-tune combined with some manual adjustments allows to find out good regulation parameters for almost all set-ups. 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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