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1. Know Your Equipment• Don’t try to tune over an issue that is
caused by faulty or erratic equipment.
• Make sure there are no external equipment problems causing issues in your PID control. For example: tuning a methane-powered generator with a dirty methane supply.
• Watch out for lag or deadtime in your equipment which can be caused by actuators or sensors that are slow to respond to dynamic changes.
• Keep an eye out for external noise on your feedback signal; use a filter whenever possible to be sure the feedback is pure.
2. Know Your Algorithm
• All controllers are not created equal and most manufacturers have their own interpretation of the PID equation.
• Look through the help file or product documentation to determine which version of the PID equation your controller is using.
• This will help you determine the proper values for your coefficients.
P
I
D
Mn = Kc* en + Ki* Σei + Kr* (en- en-1) + M0
n
i=1
Kc* en
Ki* Σei n
i=1
Kr* (en- en-1)
• PID sample rate identifies how often the instruction will calculate a new output value.
• Make sure the rate at which you are calculating is set appropriately:
- Too low will cause your controller to miss important changes.
- Too fast could cause instability in your integral and derivative terms.
• Try to shrink the sample rate as much as possible but more importantly, keep that value constant.
3. Set Your Sample Rate
4. Tune Your Process
• When tuning a PID loop, focus on one parameter at a time, i.e., cancel out the integral and derivative terms and focus on the proportional term.
• Basic Tuning Method: 1. Negate the integral and derivative
parameters, introduce an error, and get the proportional value to a point where the system is reacting quickly and correcting in the right direction.
2. Add the integral term, introduce an error, and tune the integral until you get the desired response in the low error range.
Tune Your Process (cont.)
3. If PI control is not sufficient, carefully add the derivative component and test accordingly.
* Remember that the derivative term is based on the rate of change in the error not the error itself. It determines the difference between the error now and the error before (how rapidly the process variable is deviating from or correcting to the setpoint) and adjusts the output accordingly.
4. Once selected parameters are providing adequate control, fine tuning can be done with small changes to one parameter at a time.
Additional Resources
• For a more detailed discussion of PID loops and PID loop tuning, check out the following links: Methods Behind PID Loop Control - http://go2adc.com/PID Whitepaper: How to Tune PID Loops - http://bit.ly/pid-tuning
