Precision Force-Displacement Measurements for Evaluating Design and Fitness for Use of MEMS and Computing Input Devices

The overall design and fitness of structures on micromeasuring devices or microelectromechanical systems (MEMS) need to be validated by their force-displacement characteristics, as these systems are utilized in a wide array of applications, from robotics and biosystems, to micro-manipulation and micro-assembly.

Bruker’s UMT TriboLab™ test system has the ability to accurately and reliably perform such precision measurements across wide displacement (µm to mm) and force (mN to kN) ranges. This extraordinary capability of adapting to a wide range of displacement and force measurement frameworks makes the TriboLab system superior to other measurement instruments. This article discusses the characteristics in action on a MEMS lever arm and a computing keyboard.

Force and displacement data play a vital role in verifying the design of micro and macro components, such as MEMS, micro-switches, keyboards, etc. These devices are designed with specific stiffness and structural considerations for the purpose of enabling particular functions. To obtain the structural stiffness - a vital factor in designing an elastic body - a force-displacement plot can be utilized. A rapid measurement of force-displacement allows such components to be evaluated for intended functionality and high quality. This measurement also helps to understand the instability of structures caused by buckling to be elucidated, specifically in the case of key and switch input devices used in musical and computing instruments.

Depending on the scope of the components, the force for such measurements can range from a few mN up to kN, and the displacement range can vary from a few µm to the order of mm. Bruker’s UMT TriboLab test system has the ability to carry out highly precise tests, covering a wide force and displacement range. The TriboLab test system is provided with high-precision Gold-series force and displacement sensors that include strain-gauge, LVDT, and capacitance technologies, giving the system an advantage over other measuring devices in the market. This and the other features of the TriboLab test system make it a valuable force-displacement measurement tool for the manufacturers of MEMS, computing accessories, robots, precision and scientific instruments, micro-manipulators, actuators, and sensors.

The UMT TriboLab Test System

The UMT TriboLab test system has been constructed on the Universal Mechanical Test (UMT) platform, which is a pioneer in the industry for high-precision control of load, position, and speed. The modular design of this novel system enhances the flexibility of carrying out the test for a wide range force and displacement. The tester includes three major drive systems for the displacement in X, Y, and Z directions. The system is integrated with intelligent hardware and software interfaces, which make it an exceptionally user-friendly, productive, and versatile testing tool. The TriboScript™ software provides a secured and improved scripting interface to enable easy compilation of force-displacement test sequences from already built-in blocks.

The TriboID™ feature performs automatic detection of the different components connected to the system, which are required for proper functioning. This feature also configures the components. The automatic positioning device equipped in the system is based on optical microscopy techniques and enables users to select a testing location by examining a test specimen. The specimen is then positioned by the system under the test probe to carry out the measurements at a particular location. This is a vital accessory to handle minute specimens and to perform tests on smaller sub-structures within these specimens. The real-time control and data analysis software equipped in the system ensures high repeatability and accuracy. A range of Gold series force sensors (for instance, FVL: 1-100 mN; DFM series: 0.05-20 N; FL: 5-500 mN; DFH series: 0.5-2 kN) are equipped to carry out the tests. The capacitance and LVDT sensors provided in the system enable accurate displacement measurements in µm and mm ranges, respectively.

Test Results

Example results of force-displacements tests carried out on MEMS lever arms and keyboards using Bruker’s UMT TriboLab test system in different ranges are discussed in the following sections.

Force-Displacement Measurements at Low Range on a MEMS Lever Arm

Force-displacement measurements in the mN and µm ranges were carried out on a MEMS lever arm by using the UMT TriboLab test system equipped with a Gold series FL sensor and a capacitance sensor, respectively. A tungsten carbide ball probe tip with a diameter of 1 mm was utilized to carry out the measurement. Figure 1 illustrates a force-displacement plot of a lever arm specimen within 0-1 µm displacement and 0-5 mN force ranges. Despite the fact that the measured force range was approximately 1% of the full scale of the force sensor, there was minimal data scatter. An acoustic enclosure and an anti-vibration table can be used to carry out measurements in low force range to further reduce the data scatter. The force-displacement of the lever arm illustrated in Figure 1 specifies a linear-elastic behavior.

Figure 1. Force-displacement plots up to 5 mN using a Gold Series FL sensor and Cap sensor.

The force-displacement characteristics of the lever were further tested by increasing the load levels. The force-displacement plots up to 50 and 400 mN are illustrated in Figures 2 and 3, respectively. Figure 2 demonstrates a linear-elastic force-displacement behavior up to 50 mN, and Figure 3 exhibits the initiation of a non-linear force-displacement behavior approximately after 250 mN. After the load of 250 mN, the stiffness of the specimen started to increase. Beyond 400 mN, the maximum displacement was approximately 82 µm. The results depicted in Figures 1 to 3 prove the capability of the TriboLab test system to carry out highly precise measurements at lower force and displacements ranges.

Figure 2. Force-displacement plots up to 50 mN using a Gold Series FL sensor and Cap sensor.

Figure 3. Force-displacement plots up to 400 mN using a Gold Series FL sensor and Cap sensor.

Force-Displacement Measurements on Keyboard Keys

For these measurements, a computer keyboard was mounted on the TriboLab test system Y-stage, and a key was progressively pushed downward, using the flat surface of a cylindrical probe placed under the force sensor.

The LVDT sensor and the DFM-0.5 force sensor fixed on the carriage of the TriboLab test system were used to measure the values of displacement and force. The commonly observed displacement range of the keyboard keys can also be measured by using the Z-encoder data from the TriboLab test system. Figure 4 depicts a portion of the test setup.

Figure 4. Close-up view of the test setup for force-displacement measurements on a keyboard.

Figure 5 depicts the force-displacement plot of a computer keyboard key illustrating three regions. At first the force increased with respect to displacement and attained a maximum value. Figure 5 illustrates the corresponding force, denoted as the peak force, F_P. The value of force reduced until it reached the minimum value when the key was pressed beyond the peak force. However, during this time, the displacement continued to increase. The force started to increase after reaching the minimum value. These force-displacement characteristics confirm the buckling of the key’s underlying structure. The force analogous to the minimum in Figure 5 is the actuation force, F_A. The force-displacement curve slope is helpful in the assessment of the tactile response of the keyboard component. The parameters F_P and F_A are useful to manufacturers and users as these parameters represent the comfort, durability, and quality.

Figure 5. Force-displacement of a keyboard obtained using DFM-0.5 and a LVDT sensor.

The distance between the peak force and actuation force illustrated in Figure 5 is indicated as the travel. The test was repeatedly performed for 25 times utilizing an automated test script on the same keyboard to extract the data repeatability of the F_P, F_A, and the travel by acquiring their mean, standard deviation (SD), and the coefficient of variance (COV), as illustrated in Table 1. COV values of F_P, F_A, and travel were very small, which confirms that the TriboLab test system can generate repeatable results for the force-displacement measurements. The design parameters, F_P, F_A, and travel, are regarded to be imperative for the proper functioning of the keys, without causing any major discomfort to a user typing at adequate speed.

Table 1. Repeatability of peak force, actuation force, and travel data for a computer keyboard key.

Conclusion

Bruker’s TriboLab test system is a vital tool to carry out force-displacement measurements across wide force and displacement ranges for the manufacturers of MEMS, micro-manipulators, robots, precision, and scientific instruments, computing accessories, actuators, and sensors. The automated positioning system integrated in the TriboLab test system allows a user to test the microscopic substructures of any specimen. The system also enables automation to carry out repeatable tests, thereby ensuring high quality and durability.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.