Altering Data Acquisition Rate and Corner Frequency to Optimise your Texture Analysis

DATA ACQUISITION RATE 

Otherwise known as sampling frequency, the data acquisition rate of a test is defined by the number of data points collected per second (pps). In general, a higher data acquisition rate allows the collection of more detail during a test. The maximum possible data acquisition rate has increased over the years with improved technology and increased computer memory.

The recently released Connect range of Texture Analysers allows data collection up to 2000pps. This higher acquisition rate (2000pps) detects up to four times as many force changes per second as the previously available rate (500pps) and so it is very well-suited to applications involving high rate fracture, such as crispiness or high speed events such as in the measurement of adhesives or brittle samples. 


This diagram illustrates an example of increasing data acquisition rate. The top line is an example of what a real force signal might look like (the force that the texture analyser “feels”). The following three lines show what the force-time graph would look like in Exponent using different data acquisition rates. For example, if 200pps is chosen, the texture analyser records a data point every 1/200th of a second, equal to a data point every 0.005 seconds. Using this example, the force is only recorded at times 0, 0.005, 0.01 and 0.015 seconds, which is why the first line looks so simplified. As the data acquisition rate is increased, more data points are collected per second, and the closer the recorded data comes to the force exerted on the load cell.

The detail of the rapidly changing parts of a test curve increases with each increase in acquisition rate. The more complex the signal, the higher the acquisition rate necessary to capture data such that no events are lost. This effect is shown in the following four graphs taken from compression testing of crispy cereal samples:

Additionally, using the Connect software, data capture can now be turned on and off during a test, and data in different sections of a test may be captured at different rates. The use of a variable rate can reduce file size by limiting data capture during periods of lower importance. For example, the hold force period of an adhesive test might require capture at low data acquisition rate, but it is beneficial to switch to 2000pps for probe withdrawal at the end of the test.
 

DATA FILTER

The XTPlus and Connect instruments are fitted with a filter to condition the force data. The user has the option to choose between several different corner frequencies for this filter. Any signals with a frequency above the selected ‘corner frequency’ are removed (or ‘attenuated’). The lower this cut-off frequency, the more the noise is reduced (fast, transient signals are filtered out). Noise is a random error caused by many external factors such as vibration and electrical interference. The side effect of a low cut off frequency is a 'numbing' of the signal's response time that will cause a rounding effect on fast peaks. Neither fast peaks nor noise are totally eliminated in this process; they are reduced.
The recommended filter for an instrument fitted with a 5kg load cell is 10Hz. The 40Hz setting is recommended for all other capacities. The software is automatically set to choose the recommended filter under normal conditions for the load cell capacity in use but the user is able to change this frequency if necessary. Some examples are shown below on four similar crispy cereal samples as above:

The option to change the data acquisition rate or filter can be found in the Test Configuration window under the Parameters tab in Exponent Connect. The data acquisition rate settings are in the same location in Exponent, but the filter settings are accessed by going to File – Preferences – User – Miscellaneous.

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

---

For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

No-one understands texture analysis like we do!

To discuss your specific test requirements, click here...