What's the problem with overshoot?


Dear Control Engineering: I was looking at Back to Basics in the August issue about closed loop control. What is the main concern about overshoot? Why is it a problem, and can’t a controller prevent it?

An ideal controller should be able to move a process variable quickly to the desired setpoint without shooting past. Sometimes controllers will push the variable aggressively toward the setpoint knowing that it may go past, or overshoot the desired value and have to turn around and come back. In many applications, this isn’t a problem. For example, let’s say you’re trying to control flow. The sensor is a flowmeter and the controller is a valve. Assuming everything is sized appropriately, the valve should be able to accommodate the desired flow easily, so it’s no big deal if it goes a little above or below the setpoint because it can easily reverse its effort.

But, let’s say you’re trying to control the temperature of a large tank of liquid by modulating the amount of natural gas to a burner. If the liquid is too cold and you’re trying to bring the temperature up quickly, the controller may turn the gas all the way up. However, if you overshoot the setpoint, the controller can do nothing to make the liquid cooler. There is no reverse effort available. Eventually some of the heat will dissipate and some might be carried away by a cooler feedstock coming into the tank, but there may be no counter control effort beyond turning the burner off.

To complicate things further, let’s say the tank is filled with vanilla pudding and overheating will cause it to burn. You want it to boil, but not get too hot. In those situations, the controller has to be programmed in a way that it will not pass the desired point or the product could be ruined.

Controllers can be programmed to avoid overshoot, but this is usually at the expense of aggressiveness. It has to slow the control effort more and more as it approaches the setpoint, which drags out the time required to hit the desired value.

For a more thorough discussion of the topic, read the article Understanding Derivative in Process Control, especially the sidebar written by Vance VanDoren.

--Pete Welander, pwelander@cfemedia.com

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Choosing controllers: PLCs, PACs, IPCs, DCS? What's best for your application?; Wireless trends; Design, integration; Manufacturing Day; Product Exclusive
Variable speed drives: Smooth, efficient, electrically quite motion control; Process control upgrades; Mobile intelligence; Product finalists: Vote now; Product Exclusives
Machine design tips: Pneumatic or electric; Software upgrades; Ethernet advantages; Additive manufacturing; Engineering Leaders; Product exclusives: PLC, HMI, IO
This article collection contains the 5 most referenced articles on improving the use of PID.
Learn how Industry 4.0 adds supply chain efficiency, optimizes pricing, improves quality, and more.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security

(copy 5)