Hardness Testing With A Closed Loop Control System

Hardness Testing with a Closed Loop Control System The most common hardness testing instruments used by industry have utilized dead weights to apply the test forces for the past 75+ years. The reason for this is fairly simple, dead weights are low cost and relatively easy to manufacture to the degree of accuracy required by the test methods normally used. The problem is that in every case the force must be applied to the test piece through some type of small indenter. The transfer of any dead weight force, especially one as large as the 150kg (330 lbs.) used for a Rockwell HRC scale test, for example, to the tip of a small diamond or ball indenter is difficult to accomplish. The large size and mass of a 150kg weight requires the designer to utilize smaller weights with levers to magnify the force to the desired levels. Levers require pivots, guides and other friction producing elements that induce errors. While the instrument manufacturers have done an excellent job trying to control these sources of error, any friction point in the system will have a negative effect, which will increase during use.

Since the dead weights have to be moved to apply the test force, stopping them quickly without overload and oscillation is difficult. Older testers used dashpots to control the application of the test forces. Dashpots can work very well; however, they are prone to serious variations due to seal wear and temperature changes. Many newer designs replaced the dashpot with a motor. While this eliminated some of the dashpot problems, the desire to test as fast as possible

makes the motor speed critical. As a result, force overshoot and oscillation are frequent problems.

Open vs. Closed Loop Systems Instruments that use dead weights are normally open loop systems. The forces are applied based upon the calculations of the weighs, lever ratios, etc. The manufacturer normally does an initial calibration to make sure that the forces applied are within tolerance by using an independent measuring device, normally electronic load cells are used for that measurement. They are normally never checked again. It is assumed that they are correct during the life of the instrument. While dead weight systems have proven to work very well in many applications, including hardness testers, there has always been a performance level that typical (i.e. affordable) dead weigh systems could not surpass due to the inherent problems.

During the 1950’s, Instron pioneered the use of closed loop systems on tensile testing instruments. Closed loop systems are different from open loop systems in that they have a means to electronically measure the force being applied during every test and feed (or loop) the information back to the control system. The control system is designed to use the feedback to adjust the force application mechanism to apply only the desired force. These systems work so well that today all electronic tensile/compression instruments use closed loop control exclusively.

Are Closed Loop Systems Better For Hardness Testers? In addition to the systems’ ability to constantly measure the test force being applied, the components used in a closed loop system inherently lend themselves to a much simpler design than a dead weight system. As mentioned, dead weight systems require levers, pivots, and other friction inducing components to function efficiently. The indenter, the only part of the system in contact with the test sample, is far detached from the weights themselves, separated by the levers and pivots, etc. In contrast, the main component of a closed loop system is a strain gage load cell. This compact, low weight device provides an electronic output proportional to the force applied to it. These load cells come in many different shapes, therefore it’s possible to design a hardness system with the indenter attached directly to the load cell. The Wilson/Instron Rockwell 2000 and Tukon 2100 series of testers use this feature exclusively. In this design, sources of error between the indenter and the test force are eliminated. While these designs utilize actuators to apply the test forces and these actuators have bearings and sliding surfaces, etc. that may introduce friction, the design isolates these negatives influences above the load cell so they do not affect the critical test force. If, for example, friction in the actuator were so excessive that the desired force is not applied to the indenter, the load cell would not indicate the correct force; therefore, the system would abort the test rather than give an incorrect result. In this way, the system is constantly checking itself to make certain that only the correct test forces are applied to the indenter. The mass of the actuator can be easily controlled because of the feedback loop.

How Do I Know That These Systems Work Better? A common way to measure the performance of a hardness tester is to use GR&R techniques. This method tries to quantify the performance of measuring instruments by comparing variations from an instrument with the total variations allowed for the part that is being measured. The result is a percentage that indicates how much of the tolerance is being used up by the instrument. The smaller the percentage the better the instrument is performing. Typically users of this method want to obtain GR&R results of 10% or less, however, 30% is accepted in some situations. Hardness testers frequently fall into the 30% category because they typically don't perform that well and variations within the sample consume a percentage that is difficult to quantify.

Depending on the age and design of the hardness tester, GR&R results from typical dead weight Rockwell scale testers normally range from 12% to 25%. Under controlled conditions, the 10% target has been reached. These results, however, do not reflect reality. Under the same conditions, a Wilson/Instron Rockwell 2000 tester using a closed loop system can routinely achieve less than 7%. The average unit will achieve 5% and tightly controlled units have achieved results as low as 2%. (Note- 2% is considered the lowest attainable due to the non-uniformity of the test samples). In addition, closed loop systems have proven to be more stable from day to day increasing your confidence in the test data.

How Does Increased Performance Save You Money? How important are your test results? If you are just trying to verify that a part has been heat-treated or not, 10 % of GR&R improvement may not be important. However, if you are working to specific tolerances, any reduction in the uncertainty of your results can save you money by minimizing the possibility of either rejecting a good part or accepting a bad one. Just having a better knowledge of the hardness value will enable you to adjust your processes for the most economical operation.

Uncertainty is the buzzword today of the people doing calibrations. Anyone working to ISO Guide 17025 must provide an uncertainty statement with most calibrations performed, including hardness. It’s a logical extension that customers may someday ask for an uncertainty statement with every test performed. While this is a difficult value to accurately determine, the calculation will be influenced significantly by the performance of the hardness tester. The better the hardness tester performs the lower your uncertainty will be.

Availability Of Closed Loop Systems Hardness testers made by Wilson/Instron using closed loop systems are currently available for Rockwell, Vickers, Knoop, and Brinell testing in a variety of test force ranges. Load cells typically have force range limitations of 100 to 1. In other words, if the lowest force were 10kg, the highest force would be 1000kg. This is normally a greater range than most dead weight testers provide plus a closed loop system has the capability to allow the use of any incremental force

within the usable range. Dead weight systems are restricted to the discrete weights. Some of the newer load cells can exceed the 100 to 1 limitations.

Another benefit derived from closed loop systems is their inherent flexibility. Since the force application process is controlled by a microprocessor, the test cycles can easily be changed. Not only is this feature desirable for special testing requirements but it also will guarantee that your tester can be easily modified to meet any new or revised test method. This can be very helpful, for example, if you are interested in having a Rockwell tester that can match the time cycles used on the new NIST Rockwell hardness standards as close as possible.

Are Closed Loop Hardness Systems For You? Closed loop systems are proving to be the desirable method for performing a wide range of hardness tests. Their inherent design features have benefits that can significantly improve performance compared to the dead weight testers widely used in industry today. The older testers cannot match the repeatability, stability, and flexibility of a tester with a closed loop system. If having a hardness tester that provides the best possible hardness test results is important to you, then one that uses a closed loop system should be considered.