Many years ago I was lost in a foreign country and nobody could help me. It was late, I was in Paris and I was trying to take the subway back to my hotel after walking around the city for hours. I found myself asking the man behind the subway ticket counter how to get back to the hotel using the universally known hand signal form of communication. I would slowly say the word “Hoooootel” over and over again as I tried to form the shape of a hotel with my hands in the air. The guy was saying something to me in French while pointing in several directions. He could have been pointing to the secret location of DaVinci’s hidden works; I had no clue what he saying. It was an extremely frustrating experience because I knew where I wanted to go and the guy behind the counter knew how to get me there. We were getting nowhere and now were both starting to get frustrated. Just then a man happened to be passing by who overheard this archaic attempt at communication and he stepped in to save the day. He spoke both French and English and in less than 30 seconds I had a ticket and directions on how to get back to the hotel.
For those of you who have been in similar situations you know how frustrating it can be. The same thing happens to a small group of our customers when they start using mass flow controllers. The vast majority of our customers remove the MFC from the box, make all the connections, and never look back. For those select few whose applications have a degree of complexity that is not found in most applications, they will have to make some tweaks to the MFC before they can really start seeing the benefit. So these folks install the MFC and they are met with oscillations in the setpoint, slow response, or terrible setpoint overshoot. They know what they need the controller to do, they just don’t know how to make the controller respond properly. They make changes, they push buttons, and nothing seems to work. Then the frustration sets in…
An MFC is essentially a mass flow meter with an attached valve and embedded control software. When tuning the MFC we have to account for variability in pressure, control volumes, back pressure, and other factors. The way we tune the valve is using proportional-integral-derivative tuning, or PID loop control. The PID control software calculates the difference between the desired setpoint value and the measured process value. The software adjusts the valve position in an effort to minimize the difference between the two. An easy way to imagine what’s happening is to think of your bathtub or shower. When you turn the water on you are attempting to reach a desired water temperature, which would be the same as the setpoint. You adjust the handle at a point between hot and cold trying to reach a desired temperature. With every change you stick your finger, or toe, in the water to measure the temperature, which is the same as the process value. With each measurement of process value you make an adjustment to the handle until the water is at the correct setpoint. If you make changes too rapidly or too abruptly the water can become too hot or too cold and never reach the right temperature. If you make changes too slowly it may take too long for the water to reach the right temperature.
The same methodology applies in how the MFC controls the valve when trying to control a setpoint. The controller is making thousands of calculations per second trying to achieve the setpoint as quickly as possible with minimal overshoot. Since applications vary a great deal the PID settings within the software may need to be changed in order to maximize the valve response while minimizing overshoot. Or in some cases, to stop oscillations or other performance related issues.
One of the features of an Alicat MFC is the ability to adjust P&D parameters via the digital display or digitally via RS-232. If you find yourself in a situation in which the controller does not respond you can make changes to these parameters in an effort to sharpen up the valve response. Hopefully by reading this you will now have a little more insight into how this phenomenon works and you will not be waving your arms around in the air because you know what you need the controller to do but can seem to get it to follow your instructions.
If you are in this position and can’t get things to work, think of Alicat Applications Engineers as the casual passerby who speaks both languages and can get you on your way. Please give us a call and we’d be happy to help.