Open-Loop Control Revisited

When illustrated side-by-side, the welded bellows can be seen as the physical representation of an open-loop control system.

Open-loop control continues to find relevance in industrial, military, and aerospace applications despite its simplistic and less-than-perfect performance as a control mechanism. But what is open-loop control? Examples can be found in the night-light that comes on when the sky darkens or the oil vapor valve in a military jet engine that modulates with altitude. What distinguishes this form of control from others is the lack of feedback to report what is happening. So why would we use it? Because the simplicity can increase reliability in some circumstances, and its low cost makes it practical in a varitey of applications.

Open-loop control is a practical alternative to closed-loop systems under two conditions:

• Where extreme output accuracy is not imperative; and

• Where the system can function tolerably well without a guarantee that the output will track the input.

Focus on temperature

The open-loop device is often a sensor, amplifier, and actuator combined in a single unit. One of the more common technologies that combine these elements is the metallic bellows, a device introduced more than 100 years ago with the mechanically formed bellows, and followed 50 years later by a higher performance device—the edge-welded bellows. The latter version is used widely today, however, the technology described here applies to both. As mechanical devices go, the metallic bellows is a sensor and actuator that combines the properties of a spring, a piston, and cylinder. Its motion is frictionless, and it is free of leakage or permeability.

In a liquid-filled bellows system, heat is the input, the liquid and its outer containment (the bulb) together with the bellows comprise the sensor and amplifier, and the rod or movable termination is the output (see “Welded bellows/open-loop” graphic for relation between open-loop control system and welded bellows). Significant to this system is the rather high driving force and long stroke that can be obtained from the output: half a kilogram to several kilograms is not unusual, depending on bellows size. And stroke can range from millimeters to several centimeters. As such, it is a usable drive system for direct, linear actuation without motors or electrical power.

Air in the bellows makes it sensitive to pressure and temperature. The system measures air density to bias tail rotor pitch for constant response.

A good example of a suitable application for open-loop control using a welded bellows is a military gunsight. The gunsight, which sits atop an armored vehicle, must be continuously in focus for immediate use without adjustment. Because the vehicle sits in the open, the optics endure temperature extremes, especially in desert conditions. Using a liquid-filled bellows to move a parfocal lens within the gunsight’s lens system a centimeter or so one way or the other can compensate for shifts in the dimensions of the optical system that occur with temperature change.

As ambient conditions heat the optical sight, liquid—in this case located inside the bellows—causes the bellows to expand and move the lens. Cold conditions cause the liquid and the bellows to contract and the lens moves the other way. By design, motion is proportional to temperature. In this application, feedback is not of critical concern. The operator knows if the system is out of focus and can make manual corrections should the system fail. But the ruggedness and simplicity of the system make failure a rather low probability, while obviating the need for electrical power and complex feedback systems.

Pressure, altitude

Another form of open-loop control using bellows is to sense altitude instead of temperature to control oil vapor pressure in a military jet engine. The bellows is sealed and the air removed from the inside. The bellows now becomes an aneroid sensor/actuator. It changes length in response to absolute pressure and, therefore, altitude. As the aircraft climbs, the bellows expands, modulating a valve and reducing airflow through the engine to maintain constant pressure on the lubricating oil. Feedback is not necessary, thus open loop is an economical form of control.

The bellows, with air sealed inside, must passively sense the presence of a vacuum on its outer surface and expand to clamp a semiconductor wafer for processing or inspection.

“Tail rotor application” graphic shows a dual-input, open-loop control to bias the pitch of the tail rotor in a helicopter. Again, the bellows is sealed, but this time with air inside. With the presence of air in the bellows, the bellows length is now a function of altitude and temperature—the combination representing air density. With its high-surface area and low mass, this bellows is a temperature sensor with a very low time constant. The combination of its spring force and the expansion of the trapped gas provides the driving force to move the biasing mechanism. As in all open-loop systems, because we have no feedback we have no assurance that the biased position of the tail rotor is a precise reflection of air density. But the device is only an assist for the pilot. If the pilot senses an error, corrections can be made independently.

In this open-loop application, the bellows modulates with changing pressure, but its application is really a binary one—clamped or unclamped. This system replaces an active system and eliminates three penetrations into the chamber, three electrical actuators outside the chamber, and their respective controls. The system is open-loop: there is no feedback to assure that the clamp has actuated. But other activities around the process will give indication if a failure occurs.

High-precision load cells

Mass Monitor moment-insensitive load cell for OEMs achieves high resolution and weighing accuracy reportedly 10 times greater than conventional strain-gauge technology. Device is intended for products and systems that use a digital signal and require a highly accurate load cell. It consists of Setra’s patented variable capacitance ceramic sensor, weighing platform, custom signal conditioning circuitry, and an optional electronic display board; it comes in weighing capacities of 200 g to 50 kg and is accurate to 10 ppm. www.setra.com Setra Systems Inc.

Vibration analysis instrumentation

Di-440 vibration analyzer combines handheld computing technology with a range of analysis modules to create a simple-to-use, multi-function tool for maintenance, inspection, and analysis applications. Modular tools include a user-configurable conformance checker, a spectrum analyzer with phase vector readout, dual plane static and dynamic balancing, and a recorder/data logger module. Device incorporates a handheld PC with Microsoft Windows CE .Net, a 400 MHz Xscale processor, and large internal flash memory. Color display is readable under all lighting conditions. www.diagnostic-instruments.com Diagnostic Instruments Ltd.

Automation software toolkit

Latest version of EPAS-4 PacDrive automation software toolkit features a fully documented software template to harness best practices in programming. Version 16 is said to be even more effective as a preventive maintenance and service tool. Tool kit makes it easy to assemble programs like building blocks, using the best aspects of all the IEC languages, including ladder logic. Drag-and-drop template further simplifies use of IEC languages, said to be easier to program and visualize, especially for servo machines. www.elau.com Elau Inc.

Author Information

Richard Larsen is president of Flexial Corp. (Cookeville, TN), a welded bellows company he helped found 10 years ago. For related topics, see

When is open-loop control an option?

Consider open-loop control for:

Long-term installations where continuous, reliable electrical power might be difficult, dangerous, impractical or costly;

Hostile environments where high temperatures, vibrations, and vigorous corrosives place semiconductor components at high risk such; for example, in down-hole environments;

Where a binary output—a simple switch closure, momentary or sustained—is sufficient; and

Applications where process knowledge is monitored infrequently if at all or the integrity of the control is monitored indirectly by watching other variables.

Systems most adaptable to the use of open-loop control are:

Often mechanical in form;

Correctable with human intervention to compensate for system errors;

Required to perform over a long span of time with minimal direct attention;

Not in need of recalibration;

Simple in function and construction, and components serve multiple tasks;

In continuous operation even when not needed; and

Economical in construction—cost usually more important than precision.

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