Basic Features of Modern PID Controllers

Most modern PID controllers have the following features:

• Manual versus automatic mode
• Output tracking
• Setpoint tracking
• Alarm capabilities
• Process variable (PV) characterization and damping
• Setpoint limits
• Output limits
• PID tuning security

Manual and Automatic Modes

When the controller manually calculates the output values based on process variable (PV) and setpoint (SP) values overtime, it is said to be operating in automatic mode. Although automatic mode is necessary to regulate any process, there are times where it is appropriate to allow a human operator to manually override the automatic action of the PID controller. Good application examples include process start-up and shut-down events, emergencies and during maintenance procedures. When the controller is overridden by a human being, it is said to be in a manual mode.

For example, during maintenance when a given instrument is disconnected from the system and there is another indicator for the same process variable (PV), the human operator may read that other indicator and play part of a PID controller, manually adjusting the final control element to maintain the alternate indicator at setpoint in the manual mode.

We have an extension of this mode that applies to controllers configured to receive a setpoint from another device termed to as remote or cascaded setpoint. Besides automatic and manual mode selection, a third section called cascade exists to switch the controller’s setpoint from human operator to remote (cascade) control.

Output and Setpoint Tracking

With output tracking, the bias value of the controller shifts every time the controller is placed into manual mode and the output value manually changed. Hence, when the controller is switched from manual mode to automatic mode, the output does not jump immediately to some previously-calculated value but rather picks up from the last manually-set value and begins to control from that point as dictated by the PID equation. To put it in another way, output tracking allows a human operator to arbitrarily offset the output of a PID controller by switching to manual mode, adjusting the output value, and then switching back to automatic mode. The output will then continue its automatic action from this new starting point instead of the previous starting point.

A typical application of output tracking is in the manual correction of integral windup; this is what happens to a controller with integral action if for some reason the process variable cannot achieve setpoint no matter how far the output signal is driven by integral action. This situation can be prevented, if the operator uses output tracking by quickly switching the controller into manual mode, adjusting the output down to a reasonable level, and then switching back to automatic mode so that the controller’s output value is no longer ‘wound up’ at a high level.

The purpose of setpoint tracking is to equalize setpoint (SP) and process variable (PV) while the controller is in manual mode, so that when the controller gets back into automatic mode, it will begin its automatic operation with zero error i.e. PV = SP. This feature is most helpful during system start-ups, where the controller may have problems controlling the process in automatic mode. Under unusual conditions plant operators often prefer to run certain control loops in manual mode from the time of initial start-up until such time that the process is near normal operating conditions. At that point, when the operator is comfortable with the stability of the process, the controller is assigned the responsibility of maintaining the process at setpoint. With setpoint tracking present in the controller, the controller SP value will be held equal to the PV value for the entire time the controller is in the manual mode. Once the operator decides to switch the controller into automatic mode, the SP value stays at the last manual mode PV value and the controller will continue to control the PV at that SP value. From this point, the operator can still adjust the SP value to any new value if he/she wishes while it is in automatic mode.

Without setpoint tracking, the operator would have to make a setpoint adjustment either before or after switching the controller from manual mode to automatic mode in order to ensure that controller was properly set up to maintain the process variable at the desired value.

Nevertheless, for some processes the setpoint value must remain fixed at all times hence, it would be undesirable to have the setpoint value drift around with the process variable every time the controller is placed in manual mode.

Related: Basic Features of Distributed Control Systems (DCS)

Alarm capabilities

The alarm feature on instrument is its ability to alert operational and maintenance personnel to the onset of abnormal process conditions. Process alarms maybe triggered by process switches directly sensing abnormal conditions e.g. low level alarms, high level alarms, low flow alarms, high temperature switches, etc. in which case they are called hard alarms conversely, a soft alarm is an alarm triggered by some continuous measurement i.e. a signal from process transmitter rather than a process switch exceeding a pre-programmed alarm limit value.

Given that PID controllers are designed to input continuous measurements, it reasonable that a controller will be equipped with programmed alarm limit values as well as to provide soft alarm capability without adding additional instruments in the loop.

Besides PV alarming, PID controllers can also provide deviation alarming i.e. an alarm triggered by excessive deviation (error) between PV and SP.

Output and Setpoint Limiting

 In some industrial applications, it may not be appropriate to allow the controller to automatically manipulate the final control element e.g. a variable speed motor, a control valve, etc. over its full 100% range. In such applications, a useful controller feature is the output limit. For instance, a PID controller may be configured to have a minimum output limit of 8%, so that it is not able to close the control valve any further than the 8% open position in order to maintain a minimum flow through a pump. The valve may still be fully closed (0% stem position) in the manual mode, but not just in automatic mode.

Security

Most digital PID controllers have some form of security access control; allowing for different levels of permission in altering PID controller’s parameters and configuration. Security may be in form of a simple hidden switch on a printed circuit board (PCB), which the maintenance personnel ought to know about or login names and or passwords, etc. The main reason why security restrictions exist is to exclude personnel not charged with particular responsibilities from gaining access to the system.