Open-Loop vs Closed-Loop Systems in the Materials Testing Industry

Why Do Customers Need a Closed-Loop Testing System?

Note: This has been updated in November 2023 with material testing equipment examples and further applications for our readers. 

The majority of mechanical testing systems sold today operate with a computer-controlled closed-loop testing system. This is preferred because a closed-loop system provides accurate and repeatable test speeds which helps ensure that tests are performed to specification. Performing tests according to specification provides a high level of confidence that the measured material properties can be compared to results from other machines, testing laboratories, and published values. 

Open-loop testing systems are manually operated machines that tend to be simple and inexpensive; the lower cost is why they are still used in many applications. Open loop systems have no controller and act solely on the basis of the input and do not use feedback from the output to self-correct while the test is running. These manually controlled open loop testing systems are typically hydraulically actuated high-force capacity machines where the operator turns a needle valve to adjust the loading rate or test speed.

Open Loop Material Testing System

Many manually operated testing systems are equipped with only a digital force indicator or dial gauge and have no means to measure or display the loading rate. Other open-loop testing systems display force, strain, and test rate but have no automatic means to adjust the rate of loading. Here the operator watches the live test rate update on the digital indicator and manually turns a knob to adjust the rate of loading. This process requires the operator to pay attention during the entire test in order to ensure the rate of loading stays within its specified limits. Operator fatigue and distraction frequently result in tests being performed incorrectly which produces inaccurate test results.

The Main Difference Between Open and Closed-Loop Systems

The key difference between open and closed loop systems is that open loop systems rely entirely on their input; the output or in the case of testing systems, rate of loading, have no effect on the control action since it is not fed back to the input. Whereas in a closed-loop system, the output is fed back to the input by a microprocessor that modifies its control action in real-time to achieve the desired output or rate of loading.

Diagram of a Closed-Loop Material Testing System

Stated another way, the main difference between an open-loop system and a closed-loop system is that the closed-loop system has the ability to self-correct while the open-loop system does not. Consequently, closed-loop systems are often called feedback control systems while open-loop systems are also known as non- or zero-feedback systems.

Diagram of an Open-Loop Material Testing System

Strain Rate Sensitivity

Why Accurate and Repeatable Test Speeds Matter

The strength of many materials changes with the speed at which they are tested.  Materials, where faster rates of straining or loading produce greater measured strength, are strain rate sensitive.  To illustrate the point,  a nylon plastic wire tie was pulled to break at several test speeds and the maximum load and elongation were measured.

Figure 1 – Test Results of Straining Nylon Wire Ties at Different Rates

The ensuing table presents the results of those tests.

Table 1 – Test Results for Straining Nylon Wire Ties at Different Rates.

As the speed doubled from 2 to 4 inches per minute, the maximum elongation (a measure of ductility) for the nylon samples decreased by almost half. As the speed was increased from 2 to 16 inches per minute, the strength of the nylon sample increased by 7.5%.  Thus, at higher strain rates the strength of the nylon material increases and its ductility decreases.  When comparing test results for a specific material over time, between operators, between different machines and testing laboratories, this example illustrates the importance of following a test standard and maintaining an accurate and constant strain rate. The presence of strain rate sensitivity in many materials demands that tests be performed on machines capable of accurate and repeatable control rates which are only found in closed loop testing systems.

The compressive strength of concrete is sensitive to loading rate as well.  ASTM C39 Test Method for Measuring the Compressive Strength of Concrete Cylinders specifies a stress rate with an allowable tolerance. For others, such as ASTM C1609 Test Method for the Flexural Performance of Fiber Reinforced Concrete, it is necessary to maintain a slow, constant speed throughout the test. Open-loop and manual testers do not have the required control for such low-speed testing. Only closed-loop servo-controlled testing systems are capable of performing these low-speed tests.

Relevant Article: Concrete Testing: Manual vs. Automated Operation

3 More Reasons Why Closed-Loop Testing Systems May Be More Suitable For A Test Application

Closed-loop material testing systems offer several advantages over open-loop systems, primarily due to their ability to provide more precise control, accurate measurements, and a controlled testing environment. In addition to strain rate sensitivity, here are three more reasons why closed-loop material testing systems are preferred:

Automation & Efficiency

Closed-loop systems are often automated, reducing the risk of human error in setting up or conducting tests. This is especially important in long-duration experiments or when conducting repetitive tests. Closed-loop systems can optimize testing conditions based on predefined criteria, leading to more efficient use of resources such as time and energy.

MTESTQuattro – Saved Test Procedures.  Recalling saved test procedures ensures tests will be performed the same way each time.

Material testing machines with closed-loop controls allow for real-time monitoring of various parameters during testing. Researchers or operators can observe the system’s behavior and make adjustments as needed.

In addition, most closed-loop testing systems come equipped with software running on a Windows-based PC, which makes it easy to produce printed reports and upload test results into common spreadsheet and database programs, providing an automated error-free means of storing and reporting results.

MTESTQuattro – Export Test Data as a CSV

Safety

Closed-loop systems can be equipped with safety features and emergency shutdown mechanisms. This enhances overall safety, especially when dealing with materials that might undergo unexpected changes. Testing machines equipped with safety shields allow for safe hands-off operation when needed. E.g. If the safety shield doors are opened during the test, electrical interlocks automatically send a signal to the closed-loop controller to end the test and stop all machine movement.

eXpert 2657 – Equipped with a Protective Enclosure and Safety Interlock Switch

Versatility

Closed-loop testing systems are versatile and can be adapted for various testing scenarios, making them suitable for a wide range of material testing applications. Testing machines equipped with a variety of test fixtures, environmental chambers, fluid baths, video extensometers, and other devices can be integrated with the advanced closed-loop controller and software.

eXpert 2657 300kN Universal Testing System equipped with wedge grips for tension testing and quick attach fixtures for performing bend and compression tests

Let ADMET Upgrade Your Open-Loop Materials Testing System

ADMET provides full system retrofit/upgrade packages that can significantly increase the accuracy and efficiency of your material testing. Many older testing machines remain functional decades after the initial purchase, however, older testing frames can be upgraded to benefit from newer controller/indicator technology and to adhere to modern ASTM and ISO testing standards.

ADMET Indicators Displaying Load/Stress Rate

Certain test applications can be run on open-loop testing systems but significantly benefit from load or stress rate indication. This indication is used to make sure the manual operation falls into the acceptable range of strain rate and the test is valid.

For applications that require rate indication, companies choose ADMET indicators that have the option to calculate and show load rate and stress rate of manually controlled mechanical testing machines. These indicators, including the Pi-R and the Pi-XS indicator, display load and/or stress rate and add a layer of assurance by allowing companies to verify if a manually-controlled test was run properly.

Concrete Testing Indicators For Accurate & Reliable Test Results

ADMET indicators and software have been the go-to indicators for accurate and reliable test results in the concrete testing industry for over 30 years. The DFG Concrete Testing Indicator and the Gauge Buster product family are designed to upgrade manually controlled concrete testing machines by replacing the existing dial gauges and outdated indicators. These indicators calculate and indicate Peak Load, Peak Stress and Average Stress Rate for each test.  The results can then be stored to permanent memory in the indicator or exported to a computer.  Reporting Average Stress Rate also provides a means of determining if tests were performed to specification.

Full System Retrofits and Upgrades

ADMET full system retrofit service involves upgrading an open-loop testing machine or an older closed-loop controller with new electronics and software for full automation. This includes:

• Upgrade static and fatigue-based hydraulic testing machines with a new ADMET hydraulic power unit or a servo-manifold incorporated into the existing hydraulic power unit
• Upgrade static and fatigue-based electro-mechanical testing machines with new servo motors/amplifiers
• Install a new ADMET controller with software on existing servo-equipped hydraulic and electromechanical testers

Apart from significant cost savings by keeping the test machine hardware instead of a full replacement, ADMET’s retrofit service allows you to continue using all existing accessories such as load cells, extensometers, and fixturing.

Read more about ADMET’s Full System Retrofits here.