Continuous control logic specification
For almost any project, there will be three types of control: Discrete Devices (www.isa.org/link/CT_0408), Continuous, and Sequential.
The control logic specification should define the configuration requirements for all of the major continuous-control elements. A continuous control element is one in which the process variable (PV) is allowed to modulate within a specified range. Sampling the PV takes place at a given frequency and comparison to a desired state-the set point (SP) transpires.
A control variable is then developed and fed to an end device that affects the PV such that the error between SP and PV moves toward zero. The desire is to reduce the error to zero and maintain it there.
However, due to physical limitations of the equipment, and other real-world impacts, it takes a continual cycle of corrections to maintain a minimum of error.
One of the key purposes of the control specification is to provide control tolerances that describe the magnitude of acceptable errors (must the error always be zero, or is "dead band" that is acceptable?).
Documentation: A control specification will need to define the method of documentation for the control scheme. One acceptable method is the Scientific Apparatus Maker's Association (SAMA) diagram. Another method that is gaining popularity is the Function Block approach. Either way, the control specification will need to define how the project's documentation will proceed.
Human - Machine Interface (HMI) aspects: The purpose of the continuous control function is to free the operator from having to make continual adjustments to the process. Thus, it is not necessary the controls be visible on the main graphic screen unless the operator wants to make a change.
Some of the more common considerations for display on the main graphic are:
- Analog value of the PV displayed in engineering units
- Alarm animation for high, high-high, low, low-low, bad signal quality, and deviation from set point
- Status indication for local/remote and auto/manual
Types of continuous control
- PID (proportional/integral/derivative): In this type of control, three parameters (gain/reset/rate) define how the control element will react to a deviation between PV and SP over time. GAIN defines the instantaneous magnitude of the response, RESET corrects any resulting offsets over time, and RATE makes the controller more reactive based on the rate of change of the input. A particular control element may employ any one or all of these parameters in order to achieve stability.
- Cascade is loop in which one PID function serves as the set point of a second function.
- Ratio is often in conjunction with a one or two PID control functions, the ratio function is primarily for blending applications.
It is important to pre-define the behavior of each control scheme. Some of the considerations for a continuous control loop are:
- Auto/manual (A/M) switching: This action is typically done at the HMI. When switching from auto to manual, there are two behaviors that need to be defined for the set point: 1) SP Tracking On, where SP follows PV while the controller is in manual, thus providing for a bumpless transfer when the operator transfers back to Auto; or 2) SP Tracking Off, where the SP retains its last good value while in auto. The first case guarantees a bumpless transfer from manual to auto, while the second case guarantees that the process variable will "bump" back to its last good automatic SP. There are good arguments for each method, depending on the process.
- Local/remote set point mode: Local SP is generally the data entered at the HMI's keyboard by the operator. A remote SP is generally for Cascade loops where an external algorithm calculates SP.
- Direct/reverse acting: If an output is supposed to increase if the error between PV and SP is increasing in a positive direction, then a controller is direct acting. Conversely, if the output is supposed to increase if the error is decreasing, it is reverse acting. An example of a direct acting loop would be a cooling water loop. As temperature increases, the amount of cooling water would need to increase. Tank level would be an example of a reverse acting loop. As level increases, the amount of water fed to it would need to decrease.
- PV deviation alarm: Alert the operator if the error between PV and SP exceeds a specified amount for longer than a specified period of time.
- PV magnitude alarms: An alarm management scheme should describe the high, high-high, low, low-low alarm SP and debounce times. One should define all and any actions that are necessary as the alarms activate.
- Interlock action definition: Define what happens when an interlock is lost. Typically, loss of an interlock places its related device in a "safe" state, from the standpoint of the process, and takes the device out of automatic mode. This forces the operator to take action before the device will again react to automatic commands.
- Permissive status indication: Describe any requirement to display the permissive list and status indication if desired.
- Permissive action definition: Define what happens when a permissive is lost. Typically, loss of a permissive places its related device in a "safe" state, but leaves the device in automatic mode. Upon restoring the permissive, and assuming the other conditions (such as inhibit mode, etc.) still warrant, the device will start, open, or otherwise revert to its normal operating mode.
- Animation: Define the colors and flash behavior for each state of the device. Most devices have at least three states: on, off, and travel (or mismatch). Depending on the industry, green may indicate running or open, while in other industries red color may indicate those same conditions. Solid yellow color might indicate travel, with the mismatch alarm causing the yellow symbol to flash. Regardless of animation plan, define it for each end device.
A properly defined set of Continuous Control Requirements will greatly reduce misunderstandings and related rework, and will go far towards guaranteeing the systems integrator delivers a system that will behave as desired.
ABOUT THE AUTHOR
Michael Whitt (firstname.lastname@example.org) is an ISA Senior Member and the Manager of Integrated Systems at Mesa Associates, Inc. His book is Successful Instrumentation and Control Systems Design, ISA Press, 2004.