Guidelines for using a closed-loop control system

Tutorial: Which factors should be considered to determine whether an open-loop or a closed-loop control design should be used?

07/31/2017


Examples of closed-loop systems include maintaining a temperature in an oven, controlling tank levels, or regulating flow and pressure in a process. Other examples may include maintaining position or torque in a servo system, or even controlling pH or chemical proportions in a critical solution. Most of these examples require the use of some kind of proportional-integral-derivative (PID) algorithm along with analog control variables for the actuators and analog feedback from sensors.

Sensing is important to every control loop, open or closed. At the top of these tanks, a level sensor might provide a high-high reading for overflow protection, pressure sensors at the bottom of a tank might help ensure process quality, and flow sensors (Most industrial controls procedures require some kind of cause and effect relationship between actuators and sensors. The feedback isn't always in a typical analog form such as 4-20 mA or 0-10 volts. Encoders and resolvers, for instance, would be the typical sensors for a closed position or speed loop, and the period for the PID algorithm would be significantly faster than a typical process loop. Torque control also may combine positional information with the current draw from a servo motor, creating an entirely different control schema. Feedback from sensing devices is often a critical part of ensuring that a process runs smoothly.

Even with non-critical processes such as filling a tank or container to "somewhere around full" require discrete sensors to ensure that liquids don't overflow the tank. "High-high" (HH) sensors may be hardwired into the controls, physically disabling the pump or turning off a valve to prevent overflows. The same scenario also may be used for a "Low-low" signal to prevent damage to a centrifugal pump.

Discrete systems also can often be thought of as a closed loop. Mechanical actuators such as pneumatic or hydraulic cylinders generally have end-of-stroke sensors signifying that they are in their final command position. Rather than monitoring the position of the actuator at every incremental step, a timer generally is used to indicate a fault when the actuator takes too long to reach its final position.

The following are factors to consider when deciding how, or if, a loop should be closed in a process:

Sensor cost: In the example that was mentioned previously regarding filling a tank full of water, it probably is sufficient to use a discrete level sensor to energize a pump/valve with some kind of software algorithm to ensure smooth control. Even then, a hardwired backup such as the aforementioned HH sensor probably will be used. Analog sensors typically are more expensive than discrete sensors because of the sensing element and controller input card. In a large process plant, this cost may not be significant.

There is a long tradition of ensuring that all possible process information is accessible for display in a supervisory control and data acquisition (SCADA) system, even if it is not critical to a process. After all, the cost of control sensing equipment is a small fraction of the overall price of the installation. For original equipment manufacturer (OEM) equipment, where there may be low profit margins, cost could be an important competitive factor.

Safety: If there is the possibility of injury or death to personnel, it is necessary to know as much as possible about the process. Closing the loop on temperature or pressure on a boiler system may mean the difference between life and death for people who work in the area. Not only must the loop be closed for accurate control and monitoring, but redundant systems and certified burner control systems installed by third-party professionals often are required by law.

Product damage: Some products, like semiconductor wafers, require precise control of temperature, time, position, and chemical dosing. The failure of any of these related control systems can mean the loss of an entire wafer of thousands of integrated circuits or processors. Again, no expense can be spared on sensing and precise loop control. The same can be said for manufacturing pharmaceutical products and medical devices. Closing the loop impacts safety for humans and prevents damage to expensive products.

Criticality/importance of part conformance: After a certain point, does it matter how cheese puffs and most candy pieces appear? Have you ever received a bag of parts to assemble and found an extra bolt or screw, or worse yet, a missing one? These are examples in which the consequences for less precise control are not as severe. Rather than constantly monitoring and controlling the process, final or in-line checks for quality such as weigh scales or machine vision may be used.

Again, referencing the example of filling a tank with water, if the goal is simply to keep the tank relatively full, a discrete level sensor or two can be used along with timers and valve or pump status to determine if water in the tank. If there is too much or too little water, an alarm may sound and maintenance technicians will be able to address the problem. This is a less costly option than using load cells, pressure transducers, or other analog measuring devices to maintain a precise level in the tank. There also is less maintenance required since discrete sensors don't need to be calibrated.

Most processes in industrial control systems are closed one way or another. If a part on a conveyor passes a sensor on its way to a processing station, timers and programming will eventually let operators know that the part got stuck or was removed prior to reaching its destination.

For processes where it is important to maintain a measured analog input value such as pressure, level, or temperature, and the values need to be set for different operations, there may be no option except for analog control of the output variable. Again, this usually involves some sort of PID algorithm. Sensors, actuators, and control interface cards will be more expensive than digital input/output, and calibration will be required. For other situations where closed-loop control may be optional, the above factors should be considered.

Frank Lamb is the founder of Automation Consulting LLC, and is a member of the Control Engineering  Editorial Advisory Board. Automation Primer is a CFE Media content partner. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, eguenther@cfemedia.com.

MORE ADVICE 

Key Concepts

  • Factors to consider for closed-loop control
  • When closed-loop control should be used
  • The impact of closed-loop control.

Consider this

What guidelines should be used for open-loop control?

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