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.

Manually Controlled Tinius Olsen hydraulic testing machine 60K

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 system

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.

open loop system material testing machine (UTM) example

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.

Strain rate sensitivity of nylon wire, tensile test of 4 samples

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.

Curve # Speed (in/min) Peak Load (lb) Elongation (in)
1 2 29.5 3.90
2 4 29.6 2.01
3 8 30.4 1.75
4 16 31.7 1.72

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 inc