Performance analysis of constant current heated antagonistic Shape Memory Alloy actuator using a differential resistance measurement technique

This work presents the development of a precise, constant heating mechanism for an antagonistic shape memory alloy (SMA) actuator. The actuator was developed using a pair of SMA wires arranged in an antagonistic configuration. SMA possesses a unique phase-dependent, resistance variation property which is called self-sensing. This phenomenon is observed during thermal phase transition. A constant heating current was employed to measure combined differential resistance (ΔR) which provides insignificant hysteresis and linear relationship with displacement. ΔR eventually helps to determine the present position of the actuator for sensorless feedback control. The aim is to remove additional external sensors, reducing actuator footprint and interface complexity using the proposed study. The performance analysis of the actuator was evaluated under constant current by the tracking trajectory of reference signals. The tracking results confirmed the improvement in operating bandwidth by a reduction in displacement. The heating module mainly consisted of a low pass filter, operational amplifier with a current sense feedback mechanism that regulates the heating current in proportion to PWM signals. The result shows a significant 21% variation in the observed value of ΔR (1.200 to 0.254Ω) between the major-minor loops. The study confirms linearity and maintains similarity by highest correlation 0.9508 during open-loop, which further improves to 0.9891 in close feedback reference tracking with an error band ±0.05mm.

Research of thermal electric processes in power modules.

The paper describes the results of investigation of thermophysical processes in power modules. Measuring the thermal field caused by dies heating due to flow of heating current shows uniform temperature distribution in the area of dies mount. This indicates that the processes of current localization caused by positive thermal feedback are compensated by the processes of heat transfer between the chips through the DBC board, which has a high thermal conductivity. The modulation method was used to measure the thermal resistance "junction-to-case" of the power module. In this case, the module dies were heated by PWM pulses of the heating current through the antiparallel diodes of the module MOSFETs. The modulation method allows to measure power module thermal resistance, which value is less than 0.1 K/W.

Thermally Assisted Self-Piercing Riveting of AA6061-T6 to Ultrahigh Strength Steel

Self-piercing riveting has been widely used in vehicle body manufacturing to join aluminum alloys or aluminum to steel. However, it is difficult to rivet ultrahigh strength steel (UHSS) because of its resistance to piercing of the rivet. In this paper, a thermally assisted self-piercing riveting (TA-SPR) process was proposed to improve riveting of the UHSS, through locally preheating the UHSS sheet using an induction coil prior to the traditional self-piercing riveting (SPR) process. An experimental system consisting of inductive heating apparatus, conventional self-piercing riveting equipment, and coupon transfer mechanism was established and the steps, e.g., preheating, coupons transfer, and riveting, were automatically conducted at preset schedules. Based on experiments with this system, the effects of heating current, heating time, and coil heating distance on riveting of AA6061-T6 and DP980 were examined systematically by metallurgical analyses and mechanical tests. It was found that an appropriate combination of heating current and heating time, e.g., 0.5 s at 600 A, could produce crack-free joints having 77.8% higher undercut and 24% higher lap-shear strength, compared with results obtained using a conventional SPR process.

Thermally Assisted Self-Piercing Riveting of Aluminum AA6061-T6 to Ultra-High Strength Steels

Self-piercing riveting has been widely used in vehicle body manufacturing to join aluminum alloys or aluminum to steel. However, it is difficult to rivet ultra-high strength steel (UHSS) because of its resistance to piercing of the rivet. In this paper, a thermally assisted self-piercing riveting (TA-SPR) process was proposed to improve riveting of the UHSS, through locally preheating the UHSS sheet using an induction coil prior to the traditional self-piercing riveting (SPR) process. An experimental system consisting of inductive heating apparatus, conventional self-piercing riveting equipment and coupon transfer mechanism was established and the steps, e.g., preheating, coupons transfer and riveting, were automatically conducted at preset schedules. Based on experiments with this system, the effects of heating current, heating time and coil heating distance on riveting of AA6061-T6 and DP980 were examined systematically by metallurgical analyses and mechanical tests. It was found that an appropriate combination of heating current and heating time, e.g., 0.5s at 600A, could produce crack-free joints having 77.8% higher undercut and 24% higher lap-shear strength, compared to results obtained using a conventional SPR process.