The major types of process responses and the different worlds of process applications are presented. Additionally, the oversight of not including and understanding the contribution of automation dynamics is addressed. The discussion of the response of the PID is started.
The following key points support and augment the PID Options and Solutions - Part 1 recording for the ISA Mentor Program.
1-1: The most common type of process by virtue of the very common flow loop has a self-regulating response where the process will reach a steady state with the PID in manual for a given disturbance or change in PID output due to negative feedback within the process.
1-2: The response of the more important continuous processes for temperature, composition and pH control more directly related to product quality and process efficiency and capacity usually have an open loop time constant that is more than 4 times the total loop deadtime. In these cases, the PID controller, in its typical response time frame of 4 dead times, sees a ramping process. So far as the PID is concerned, the process appears to be integrating. It is extremely used to treat these processes as near-integrating and analyze them and tune them as integrating processes. You can easily convert between the integrating process open loop gain and the self-regulating process open loop gain and time constant.
1-3: Batch processes for temperature, composition and pH control typically have a true integrating response (no steady state) that is caused by zero negative feedback within the process in the normal operating range. Level and gas pressure systems have a true integrating response.
1-4: The temperature control of highly exothermic chemical reactors, the cell concentration of bioreactors in the exponential growth phase, and the speed control of some axial compressors in surge have a runaway response characterized by acceleration due to positive feedback within the process.
1-5: Processes with slow or no negative feedback or that have positive feedback rely much more on the PID controller to provide the missing negative feedback that is best done by PID gain action. PID integral action is not as effective and can be detrimental if it is greater than the PID proportional action. This will be seen in in subsequent recordings as the slow large oscillations caused by a PID gain too small relative to the reset time. Process control is achieved by negative feedback. The less negative feedback in the process, the more you need to rely upon the negative feedback provided by the proportional mode.
1-6: To compensate for a disturbance or to achieve a new setpoint for near and true integrating and runaway processes, the PID output must be driven past its Final Resting Value (FRV).
1-7: The most common metric for loop performance cited in the literature is an integrated absolute error (IAE). This IAE is the area between setpoint (SP) and process variable (PV) on a trend recording. It can be representative of the total amount of material that is off-spec.
1-8: Of more immediate value is the peak error that is the maximum excursion of the PV from the SP. A large peak error can trigger side reactions, Safety Instrumented System (SIS) activation, cell death, and relief devices to blow.
1-9: PID PV overshoot for a setpoint change can have many of the undesirable effects of a peak error.
1-10: PID output overshoot of the FRV is problematic for many self-regulating hydrocarbon processes with extensive heat integration and recycle.
1-11: Minimizing time to reach setpoint (rise time) is important to minimize the startup and transition time of continuous processes and the cycle time of many batch processes. There is typically a tradeoff between minimizing rise time and PV overshoot. For bioreactors, the cells are so sensitive and the batch cycle times are so slow that minimizing the overshoot for temperature and pH shifts is much more important, particularly for mammalian cell cultures.
1-12: Pulp, paper, food and polymer inline, extrusion and sheet processes may be dead time dominant if there is no heat transfer involved and valve or sensor lags are less than transportation delays.
1-13: While the process control literature typically uses the terms “process gain”, “process dead time” and “process time constant”, in most industrial processes the contribution of the automation system dynamics needs to considered particularly for processes whose dead time or time constant is less than 10 minutes or when control valves or sensors have a poor dynamic response or when at-line analyzers are used. The effect of control valve or Variable Frequency Drive (VFD) gain, transmitter calibration and PID scale must always be considered in that the open loop gain is a product of the valve or VFD gain, process gain and measurement gain where engineering units cancel out giving an open loop gain self-regulating process that is dimensionless and an open loop gain integrating process gain of 1/sec.
1-14: In a first order plus dead time approximation for self-regulating processes, all of the time constants less than the largest time constant need to be taken as an equivalent dead time by multiplying the small time constant by a factor ranging from 0.28 to 0.88 as the ratio of the small to large time constant decreases. Since there is a tendency to underestimate dead time, I usually just sum up all the very small time constants as additional dead time. The sources of the largest contribution to the total dead time should be investigated for methods to reduce them. The good news is that as automation engineers, we may be able to significantly reduce the total loop dead time by a design and installation that takes into account the detrimental effect of dead time. If there was no dead time I would be out of a job, the PID gain could be set as high as wanted and perfect control would be possible for noise free measurements.
I will provide additional synergistic Key Points in future Control Talk Blogs. Until then check the photocopy machine for your copies of the latest memos on a paperless office.