How can I take advantage of PID Tuning?

How can I take advantage of PID Tuning?

Posted by: Jas Gill

Alicat flow and pressure controllers use closed loop control algorithms to achieve their highest degree of control stability. The algorithms are a mathematical relationship that dictates the response of the valve to the flow or pressure conditions. It assesses the difference between the set point and the process value—whether it be mass flow, volumetric flow, or pressure—as an error. The degree of error determines what kind of input to send to the valve, reaching the correct value in the quickest time possible. The amount of time expended to minimize the error—and therefore the control response of the controller—depends on what type of loop is being used (PD or PD2I) and what P, D and I values are used.

When you order your controller, we set the Proportional and Derivative values by trying to replicate the application parameters (process conditions) to the best of our ability before we ship it to you. This customization to your system is one reason our controllers are fast.

No worries

If process conditions change, the valve response may change drastically depending on how much you deviate from the conditions the valve was tuned at. You need not worry about erratic response from your controller if the conditions change, since PID tuning can be done in field to get better control at the new process conditions. You can change PID terms through the display panel buttons, or through electronic commands using digital or analog communications.

You’ll get optimal performance from your controller when you select correct values for all three parameters (two in case of single valve controllers). The ‘P’ term opens the valve to achieve the set point, ‘D’ term applies a damping influence to eliminate the overshoot and ‘I’ function helps the system settle to the set point.

A previous blog describes the P, D, I terms:
Achieving Responsive and Stable Valve Control with PID Tuning
Briefly:

  1. Proportional (P): The P term applies power to the valve as it tries to decrease the error between the set point and the process value position to achieve the set point.
  2. Derivative (D): Think of this as a damping term which tries to reduce the rate of change. The larger the D term, higher the damping influence on the valve drive.
  3. Integral (I): Integral in calculus is the area under the curve, it determines the output of the valve as a function of the sum of all the errors. I term takes into account previous readings to reduce the error and correct the process value to the set point.

 

Oscillating around the set point

If your controller shows signs of oscillations about the set point, or is unstable in its control response, it is a sign that the P term is too large. The greater the P value, the greater the range of oscillation. To get rid of the oscillation (settle the controller to set point), you would need to decrease the P term.

Let’s say you have a 10 SLPM controller set to 10 SLPM for Hydrogen. The controller is oscillating between 8 and 12 SLPM. Hydrogen is a low viscosity and a very light gas compared to Air. This being the case, the valve—which is tuned with air—should be re-tuned. So, starting with the factory P value (for example, it may be 1000), try decrements of 10% and keep on going down until you see the controller settle to the set point quickest. Generally, we only touch the D terms after you have altered the P term, so if you still have small oscillations, you can increase the D value with 5-10% increment. This should help the controller become more stable.

PID response shown graphically, by varying P

Excessive P (purple) produces oscillation. Low P (red) slowly rises to the setpoint. Optimal (green) settles quickly.

Delayed setpoint

A second situation is when your controller takes too long to get to the set point or never achieves the set point, but settles to a flow rate or pressure below the set point value. This implies either too small a P value being used or too large a damping influence. Using the analogy of a car, imagine you want to get to 70 miles per hour. but when you start increasing the speed, someone applies brakes which decreases the acceleration of the car. The car may never get to 70 mph if the deceleration is larger than acceleration, and the car may settle at a speed of 60 mph when acceleration = deceleration, or the opposite forces are equal.

In this case, try increasing your P value in 10-15% increments until you see the controller getting close to your set point. Next step would be to decrease the D value to help the controller get to the set point in a quicker time. If you start seeing some oscillations, that means the D value has been set too low.

Control loop adjustment is all about getting a good feel of how the controller responds to changes in the P and D terms. The gas viscosity, inlet pressure, back pressure can greatly influence how the valve responds. Tuning the valve is an art rather than science and the more familiar you get with your controller, the better you’ll be able to tune it.

 

 

 

 

Reference: click here.

D&D Engineering – “W’ere YOUR Sensor Guys”
3835 E. Thousand Oaks Blvd. Suite 464 Westlake Village, CA 91362  – Voice: (818) 772-8720 Fax: (818) 772-2477 Toll Free: (888) 333-6474
Email: sales@sensorguys.com Website: www.sensorguys.com

7v Firmware Update Makes Interface More Intuitive for Alicat Flow and Pressure Instruments

Posted by: Danielle Adams

We’ve just made it easier to use your Alicat mass flow meters and pressure controllers from their front panels. This post has all the details on our 7v firmware update, which features a more intuitive user interface.

If you’ve been with Alicat for very long, you’ve seen us add new menu selections to our flow and pressure instruments from time to time. We incorporated these features into a set of “Miscellaneous” menus (MISC1 and MISC2) in order to preserve the existing interface. However, after filling up MISC1 and MISC2 with new functions, we decided it was time to give the entire menu system an intuitive update.

Alicat mass flow controller showing new basic configuration menu

Alicat mass flow controller showing new basic configuration menu

Here to explain the motive behind these changes is Brian Clandenon, Alicat’s Senior Software Engineer:

“As Alicat has added more functionality to our instruments, the idea was to make minimal changes to existing buttons in the menus. While this was reasonable for each small change, so many changes built up over the past few years that some related functions became scattered across multiple menus. This made the menus more challenging to navigate and to remember.

Alicat mass flow controller showing new advanced setup menu

Alicat mass flow controller showing new advanced setup menu

One goal of a good menu system is to make settings easy to find, so it became time to review the overall structure of the menus. We endeavored to organize the settings in ways that would be more intuitive to new and existing users alike, so that the changes would outweigh the inconvenience of learning the new locations.”

The following FAQ describes the changes to Alicat’s menu structure in the new 7v firmware.

Which instruments are affected by this change?

All Alicat-branded instruments manufactured on or after May 22, 2017, will ship with 7v firmware, with the exception of no-change customers who have not yet approved this update.

Can I update my existing instruments to 7v?

Alicat’s 7v firmware update is compatible with most Alicat instruments that have serial numbers of 135,000 or higher. Some instruments with serial numbers between 80,000 and 135,000 may also be compatible with 7v. Upon request, we can update your firmware to 7v during your instrument’s next annual recalibration at no charge. Alternatively, we can update instruments outside of the recalibration process for a firmware update bench fee of $80 (USD).

Will my new 7v Alicat communicate the same as my existing 6v Alicats?

Yes. Communications protocols and commands have not changed in the move from 6v to 7v, so your command set will remain the same, no matter which protocol you have been using.

Which changes will I notice?

Alicat mass flow controller showing new main menu

Alicat mass flow controller showing new main menu

The new menu map is reproduced on the following page. Below, you will find a listing of changes to the menu structure. Please see your new operating manual for more details.

  • A new BASIC CONFIG menu allows easy access to GAS SELECT, DEVICE UNITS and STP/NTP options. The GAS menu option also displays the currently selected gas.
  • A new TARES menu gives you access to flow and pressure tares, plus AUTO TARE.
  • A new ABOUT menu collects DEVICE INFO, DEVICE STATE and MFG INFO in one place.
  • The contents of the old MISC, MISC1 and MISC2 menus can now be accessed via the ADV SETUP menu, with the exception of STP/NTP, which is now in BASIC CONFIG.
Feature New Menu Location (7v Firmware) Old Menu Location (6v, 5v or 4v)
AUTO TARE MENU > TARES MENU > CONTROL SETUP
CONTROL SETUP MENU > CONTROL MENU > CONTROL SETUP
DEVICE UNITS MENU > BASIC CONFIG MENU > MISC
DIAG TEST MENU > ABOUT > DEVICE STATE MENU > MISC > MISC2 > DIAG TEST
FLOW AVG MENU > ADV SETUP > SENSOR SETUP MENU > MISC > MISC1
GAS SELECT MENU > BASIC CONFIG > GAS MENU > GAS SELECT
LCD CONTRAST MENU > ADV SETUP > DISP SETUP MENU > MISC > MISC1
MFG DATA MENU > ABOUT > MFG INFO MENU > MFG DATA
MODEL INFO MENU > ABOUT > DEVICE INFO MENU > MFG DATA > MODEL INFO
PRESS AVG MENU > ADV SETUP > SENSOR SETUP MENU > MISC > MISC1
ROTATE DISP MENU > ADV SETUP > DISP SETUP MENU > MISC > MISC2
RS232 COMM MENU > ADV SETUP > COMM SETUP MENU > RS232 COMM
STP/NTP MENU > BASIC CONFIG MENU > MISC > MISC2
TARE P w/BARO MENU > TARES > TARE PRESS MENU > MISC > MISC2 > TARE P w/BARO
ZERO BAND MENU > ADV SETUP > SENSOR SETUP MENU > MISC > MISC1

Reference: click here.

D&D Engineering – “W’ere YOUR Sensor Guys”
3835 E. Thousand Oaks Blvd. Suite 464 Westlake Village, CA 91362  – Voice: (818) 772-8720 Fax: (818) 772-2477 Toll Free: (888) 333-6474
Email: sales@sensorguys.com Website: www.sensorguys.com