Simulating a Spring with the ElectroPuls

As the number of patients seeking “in-home” medical care grows larger, medical device companies find themselves catering to an increasing demand for safer, more user-friendly biomedical solutions. 

In an attempt to mitigate the injection process, medical device companies have developed the auto-injector, a spring loaded syringe that delivers a prescribed amount of drug subcutaneously into the bloodstream, forgoing the need for physician oversight and allowing patients to self-administer medication from the comfort of their own home. To design the ideal auto-injector, the delivery time of the syringe must be optimized so that drug release is neither too long nor too short, thus reducing the level of discomfort experienced by the patient. The delivery time can be optimized by choosing the appropriate spring and preload for a given syringe/drug system, which is currently achieved by subjecting springs with varying degrees of stiffness to different loads. However, medical device companies need to be able to get through this iterative auto-injector design process more quickly, and testing a large number of springs is time consuming and less than economical. Simulating the spring eradicates the supply chain issues of multiple springs testing, thereby significantly reducing the required design time. Instron solves this control challenge using Modal Control, an advanced feature of the 8800 controller that creates a composite channel to allow the applied load to vary with actuator position. The user simply needs to specify the initial preload and spring stiffness in order for the system to adjust itself based on the actual load seen by the syringe. A force vs. time graph is produced, from which the Calculations Module feature in WaveMatrix, determines the dispensing time and stall force of the syringe. The user can then determine if the chosen parameters for spring stiffness and preload have yielded an ideal auto-injector dispensing time for a given drug. 
 

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Upper Yield Calculations—Discontinuous Yielding / YPE Material

Following a recent lab visit, I thought it was worth writing a quick post to share an example of how labs sometimes incorrectly calculate yield strength. While visiting a testing lab for unrelated issues, my colleagues and I were discussing how the customer runs both continuous and discontinuous material with the same method (Non-YPE and YPE). For historical reasons, they have updated their Series IX methods to Bluehill 3 with little knowledge of why, when or by whom the methods were setup.

It later transpired in the conversation that some of the lab's material was falling below their specification for yield strength. They are required to test every coil of product before shipment; therefore, materials not meeting the yield strength requirement are of major concern. The material has had to be reworked in the past in the hopes that the yield strength could be increased slightly. Otherwise, the material would be scrapped.

The calculation used for both continuous and discontinuous material was an offset yield of 0.2% strain. Based on this calculation the stress values were determined for product verification. For a continuously yielding material, it is recommended to use the offset yield calculation. However, for a discontinuously yielding material, using this calculation would result in an inaccurate measurement of stress after a potentially significant load drop. The main international metals standards such as ISO 6892 and ASTM E8 both recommend using upper yield strength (ReH or UYS) to calculate this value correctly.

By changing the software algorithm to a different Bluehill 3 standard calculation, we were able to increase the calculated yield strength by around 10%. This meant they no longer had to rework the material as the higher stress values were always well within specification. This modification to their test method helped reduce testing turnaround time, increased the reported material strength, reduced shipment delays, and removed the need for costly and time-consuming reworking process.

The graph below shows an example of how the two calculations differ. The results of the offset yield calculation were on average 21% lower than the upper yield calculations. Checking that calculations are correctly evaluating data could result in a significant improvement of results. Often methods were setup and have not been reviewed on a regular basis.

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What Does Load Measurement Accuracy of 0.5% of Reading Down to 1/100th, 1/500th, 1/1000th of a Load Cell's Capacity Imply?

First of all, there are two important pieces of information here:

(1) Load accuracy: ± 0.5% of the reading

(2) Lowest possible load within this accuracy: 1/100, 1/500, 1/1000 of load cell capacity

For example, let’s assume your test system has a load measurement accuracy of ± 0.5% of reading down to 1/100th of the load cell’s capacity. When a load cell of 10 000 N (10 kN) capacity is used on this test system and a measurement of 1000 N is read, the actual load measurement can be any value from 995 N to 1005 N. For this 10 kN capacity load cell, the minimum possible load that is guaranteed to meet the load accuracy of ± 0.5% of reading is 100 N.

Now, imagine you have a newer system with a new 10 kN load cell and the load measurement accuracy of ± 0.5% of reading is now extended down to 1/1000th of the load cell’s capacity. The load measurement accuracy of ± 0.5% of reading remains, but now the minimum possible load that is guaranteed to meet the load accuracy of ± 0.5% of reading is extended down to 10 N.

So what does a larger load accuracy range mean to you? Here are some of the benefits you will see:

- Less initial cost since less load cells are needed

- Lower subsequent service costs as fewer load cells need to be verified annually

- Less operator error and improved throughput (number of specimens tested per hour) by not having to change load cells between different test types such as tensile and flexure tests.

- Higher confidence in test data for low load tests.

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