Emerson’s David Adkins presented What is Tracking in Ovation Control Logic? at the 2024 Ovation User Group Conference. Here is his presentation abstract.
In the Ovation system, tracking functions are automatically configured when building control sheets. This session will provide information on what “tracking” is and how it functions in different types of control strategies. It will also describe best practices for when to use tracking and when not to use it, along with special cases to consider and handling gaps from algorithms that do not support tracking.
David opened by explaining that different control strategies require “tracking” to provide a “bumpless transfer” between modes. In the Ovation system, simple tracking functions are automatically configured, but larger, more complex control strategies require additional design input and configuration.
The two purposes for tracking are to prevent process bumping and to reduce process upsets. Changes in a process’s mode can potentially disrupt the process. For example, consider a situation where a control element is manually set to a low level even though the automatic control scheme calculates a high level. If the control mode is changed to automatic, a “bump” occurs as this control element’s setting goes from low to high. If the change is extreme, equipment damage could result. Methods used to avoid this rapid adjustment are called “Bumpless Transfer.”
A process upset may be defined as a condition in which the control system temporarily moves the process to an operating point that differs from the desired point. An example is failing to keep a PID Controller’s integral action in check; this condition is called “Reset Windup.” Returning from “Reset Windup” delays subsequent control action, creating unstable control.
Tracking works by a reverse calculation of normal control functions. Control inputs are typically at the top, and control computations are performed in the middle, followed by control outputs at the bottom. Tracking reverses this process where the inputs for the tracking calculation are at the bottom, with tracking computations done in the middle and then fed uphill. It effectively calculates a value for an upstream control function to satisfy the upstream objectives.
Tracking is available through the TRANSFER algorithm. The TRANSFER algorithm performs a transfer between 2 inputs. If the digital input FLAG is TRUE, the output is equal to the IN2 input, and if it is FALSE, to the IN1 input.
It’s also available through the MASTATION algorithms. The MASTATION algorithm interfaces a CRT-based soft manual/auto station and an optional Ovation Loop Interface module card with the functional processor.
On a Basic Math algorithm, only input 1 will allow tracking, and basic math functions for tracking include divide, multiply, sum, square root, gainbias, leadlag, and function.
His presentation examined several additional Ovation algorithms (HISELECT, LOSELECT, RATELIMIT, BALANCER) and explained how to apply tracking. You can learn more about Ovation tracking in OVREF1100, the Algorithm Reference Manual.
Here’s some guidance to avoid tracking problems:
- Tracking should never result in a divide by zero
- Tracking should not loop back to the point of origin
- If tracking through a FUNCTION Generator, the “Y” variables cannot have a flat spot
- BALANCER tracking should never bridge a FUNCTION, tracking to a BALANCER should be from an MASTATION located directly downstream of the BALANCER
David summed up tracking. Tracking theory can provide the basics, but the best way to learn and understand tracking is by testing it on a simulator and watching how algorithms act and function while tracking. This will help you gain a natural feel for its use.
