Development of Amplitude Measurement System for Ultrasonic Vibration Assisted Micro Forming

Microforming is a process that involves forming a material through plastic deformation at a micro level. However, this method encounters an issue at the micro-scale known as the size effect. There are several new approaches to overcome these difficulties, one of which is using Ultrasonic Vibration Assisted (UVA) micro forming. UVA micro forming is a forming method by applying high-frequency vibrations to the workpiece to reduce forming forces, increase smooth zone, reduce surface roughness, and increase accuracy. A bolted Langevin transducer is needed to produce vibration, which converts electrical signals into acoustic signals in the ultrasonic frequency range. Therefore, the vibration characteristics must be considered in detail to find the best parameters in the micro forming process. This study aims to develop an amplitude measurement system for UVA micro forming. Several things that need to be considered in designing a measurement system are pre-stress characteristics, resonant frequency of the machine, and the amplitude of the vibration generated by the system. Firstly, a pre-stress test was carried out by tightening the pre-stress bolt slowly until the resonant frequency did not change. Then, the resonant frequency is measured using signal generator, oscilloscope, transducer, and resistor to determine the system’s resonant frequency. Finally, the vibration amplitude is measured by observing the displacement at the end of the punch using a digimatic indicator. The result shows that the resonance frequency that produces longitudinal vibration for UVA micro forming system is 28,720 Hz, with a pre-stress of 30 MPa with torque of 14.36 Nm, and total displacement on the normal axis is 9 μm.


Introduction
With the development of technology, the demand for smaller components, such as electronics and medical equipment, has increased in the last decade.As a result, advancements in manufacturing technology on a micro scale are needed.There are several manufacturing methods available, including subtractive, additive, deforming, joining, and hybrid manufacturing processes [1].Microforming, being one of these methods, is a process of manufacturing parts using plastic deformation within the submillimeter range [2].This method of manufacturing is highly promising, as it can produce parts at a high volume.Despite its high potential, there are considerations regarding the material's parameters when manufacturing at the sub-millimeter scale.The ratio between the dimensions and the microstructure in the small-scale range will influence the behavior of the process.This phenomenon is known as the size effect.[3].
Measuring the amplitude of the ultrasonic transducer is essential in UVa microforming systems to assess the machine's performance.Several studies have discussed amplitude measurements on ultrasonic transducers, one of which is by Baraya et al.In their study, the measurement system was created using an oscilloscope and a micrometer [4].This measurement system consists of two electrical circuits.The system operates when the transducer is off and there is no contact between the transducer and the micrometer, resulting in an open circuit.In this condition, the oscilloscope displays a voltage of 1 Volt.Additionally, when the micrometer comes into contact with the transducer, the voltage displayed on the oscilloscope changes to zero due to a short circuit.
Amplitude measurements are initially performed by turning on the transducer without contacting the micrometer.Then, the micrometer is brought into proximity with the transducer, and the micrometer reading is recorded.Subsequently, intermittent contact between the micrometer and the transducer generates an electrical signal visible on the oscilloscope.The subsequent step involves turning off the transducer, and the micrometer reading is recorded once again.The value obtained from the micrometer corresponds to the transducer's amplitude, while the value displayed on the oscilloscope represents the applied voltage.Research on measuring the amplitude is also carried out with the frequency response function (FRF) [5].This measurement uses an optical fiber displacement sensor in the measurement process.The way it works is the same as a micrometer.What distinguishes the two methods is the resolution of the measuring instrument.

Design of UVa Micro Forming System
The design of the UVA micro stamping and punching tool was developed with a Langevin ultrasonic transducer [6] developed for this study with four stacks of PI-181 piezoelectric ring made by PI clamped between two end masses of two different materials, S45C Steel on the upper side and Al 2024 Aluminum on the lower side as shown in Figure 1 (a).This makes sure that the vibration can be channeled effectively on the punching side, rather than the holder side (upper side).This tool design was based on a micro forming tool developed by Mahmudah et al. [7], with modification so that a load cell and the transducer can be installed on the tool.A tension-compression load cell is also installed on the upper side of the end mass as well.The system was put on the 5 kN micro forming machine as shown in Figure 1

Measurement System
The process of amplitude measurement involves several steps, including pre-stress testing, resonant frequency measurement, and amplitude measurement.Calculating the pre-stressed bolt force is essential due to the lower tensile properties of piezoceramic compared to its compressive strength.Given that piezoceramic can fracture during the oscillation process, the pre-stressed bolt is employed to maintain compression on the piezoceramic during operation [8].In addition, pre-stress changes the vibration characteristics of the system [9].Furthermore, it's crucial to measure the vibration characteristics.
Vibrations that need to be considered in the UVA micro forming process are longitudinal vibrations, which are movements in the direction of one axis at the resonant frequency.This vibration will affect the quality of the workpiece.
Pre-stress testing is done by tightening the pre-stress bolt slowly until the resonant frequency does not change [10].The resonant frequency is measured using a signal generator, oscilloscope, transducer, and resistor to determine the system's resonant frequency.The amplitude measurement is conducted by observing the largest displacement at the end of the punch, utilizing a digital indicator.

Resonant Frequency Measurement
The resonant frequency is measured with a signal generator, oscilloscope, transducer, and resistor.Channel 1 on the oscilloscope is attached to the signal generator to show the voltage that occurs.Channel 2 on the oscilloscope is connected to a resistor to show the current flowing in the transducer.Figure2 is a diagram and setup of the resonant frequency measurement system.To determine the transducer's resonant frequency, the signal generator's frequency is adjusted so that the two waves on channel 1 and channel 2 are in one phase, as shown in Figure 3.

Pre-Stress Test
Pre-stress testing is performed by gradually tightening the pre-stress bolt until the resonant frequency stabilizes.This test was conducted using an input voltage of 20 Volts, varying the torque from 5 to 16 Nm.The tools employed for this test include a socket wrench, torque meter, and crowfoot.The socket wrench is used for tightening the pre-stress bolt, the torque meter is utilized to determine the applied torque value, and the crowfoot serves as an adapter between the torque meter and the transducer.The test setup is depicted in Figure 4.The test results are shown in Figure 5.The results of this test indicate that the torque on the prestressed bolt affects the resonant frequency.After the pre-stress bolt is tightened more than 14 Nm, the frequency stabilizes at 65.9 kHz.Therefore, piezoceramic performance reaches its maximum point at 14 Nm, so tightening more than that value is not needed.

Amplitude Measurement System
Amplitude measurement is conducted after determining the pre-stress acting on the system and establishing the resonant frequency.The objective of the amplitude measurement system is to identify the maximum displacement along the y-axis generated by the transducer system.This system employs a signal generator, transducer, oscilloscope, magnetic stand, and digimatic indicator.The signal generator is responsible for producing the desired frequency voltage.The transducer, being the tested component, and the digimatic indicator indicate the amplitude present within the transducer system.
The experimental setup involves positioning the tool and the transducer system on the measurement table.Subsequently, the transducer is securely fastened using a clamp to prevent movement during the measurement process.The digital measuring instrument is affixed to a magnetic stand.Figure 6 illustrates the setup for amplitude measurement.

Results
The test was conducted using piezoceramic PIC181 material at a resonant frequency with 20-, 50-, 100-, and 150-Volt variations.The measurement setup determined that the frequency of 31.2 kHz represents the tool's resonant frequency.The results of amplitude measurements on the transducer system are shown in Table 1 and Figure 7. vibrations that approach the resonant frequency will produce a larger amplitude.The magnitude of the amplitude began to be seen at a voltage of 50 Volts of 1 μm, and the largest amplitude at a voltage of 150 Volts of 9 μm.Thus, the desired working amplitude can be achieved by adjusting the frequency and given voltage.

Conclusion
Amplitude measurement system for ultrasonic vibration assisted (UVa) micro stamping and punching has been developed, the following conclusions can be drawn.
• The pre-stress test results indicate that the torque applied to the pre-stressed bolt impacts the resonant frequency that operates on the transducer system.The resonant frequency produced increases with the amount of pre-stress applied.• The results of the amplitude measurement show that the amplitude is affected by the frequency and the applied voltage.Adjusting the frequency and the supplied voltage will produce the desired amplitude.• The result shows that the UVA micro stamping and punching system specifications are prestress of 30 MPa with a torque of 14.36 Nm, and the total displacement on the normal axis is 9 µm. (b).

Figure 1 .
Figure 1.(a) UVA micro stamping and punching tool, (b) Tool assembly on micro forming machine2.1.Measurement SystemThe process of amplitude measurement involves several steps, including pre-stress testing, resonant frequency measurement, and amplitude measurement.Calculating the pre-stressed bolt force is essential due to the lower tensile properties of piezoceramic compared to its compressive strength.Given that piezoceramic can fracture during the oscillation process, the pre-stressed bolt is employed to maintain compression on the piezoceramic during operation[8].In addition, pre-stress changes the vibration characteristics of the system[9].Furthermore, it's crucial to measure the vibration characteristics.Vibrations that need to be considered in the UVA micro forming process are longitudinal vibrations, which are movements in the direction of one axis at the resonant frequency.This vibration will affect the quality of the workpiece.

Figure 3 .
Figure 3. Measurement results on the oscilloscope

Figure 5 .
Figure 5. Transducer response to torque and frequency

Figure 7 .
Figure 7. Amplitude and voltage curveThe measurement results above show that the amplitude is affected by the frequency and the applied voltage.The greater the applied stress, the greater the longitudinal vibration amplitude.In addition,