Default Out-of-the-Box Settings Prevent the PID Controller from Achieving its Goal
Tuning a PID controller doesn’t have to be hard. Whether a practitioner chooses to tune control loops manually or with the help of software, the procedure is relatively straight forward and can produce highly effective results. It can be argued that using software is faster and provides more optimal results than manual tuning, but that’s an argument that largely depends on the economic importance of the PID control loop in question. In the end, the goal is the same: To tune for improved control loop performance.
Whereas an OEM’s default or recommended controller settings may be suitable for a PID loop during its initial commissioning, they are not recommended at any other time. In his post on this topic, Jim Ford of Maverick Technologies suggests tuning constants for varying types of loops, ranging from Flow and Level to Temperature and Pressure. He clarifies that the tuning values are simply a starting point for use in green-field applications. As Jim infers, default controller settings are not suitable at other stages of a loop’s operation. As a rule, default settings fail to account for the unique qualities of a loop and so they fail to achieve the goal of improved control.
Although aspects of any given control loop may be generic in nature, the loop as a whole is unique and controller tuning parameters should be tailored accordingly. Consider the following:
- Controlling the Elements
Tuning constants should take into account all aspects of the associated control loop, especially the Final Control Element. If moderated by a valve, then the type of valve, its size, as well as dynamics that result from the valve’s packing all factor into values for Gain, Integral Time, and Derivative Time.
- Sharpening Your Senses
Instrumentation does not contribute to a process’ dynamics like an FCE, but instrumentation such as sensors provide the very data used in maintaining control. Simple aspects such as the sensor’s type or even its placement relative to the process can skew the data. The controller should be attuned to those nuances, pun intended.
- Meeting the Objective
Controller tuning parameters should be customized to achieve a given loop’s unique control objective. Control of a surge tank is decidedly different from control of a typical heat exchanger, the former facilitating large swings in the Process Variable (PV) and the later working to constrain any PV volatility.
Default PID controller settings have limited value in terms of their practical application as they do not account for essential aspects of the control loop itself. As odds have it there will be occasions when default settings are spot on, the simple PID loop used in controlling pressure at the local automatic car wash for example. However, in complex industrial applications where safe, efficient control is essential, don’t be surprised when the manufacturer’s default settings fail to pass the test. Applying either industry best-practices for manual tuning or software that’s suited for real-world industrial applications can make all the difference