A new tracking method with FPGA chip for ultrasonic welding system
A new feedback control method of using field-programmable gate array (FPGA) chip for ultrasound welding system is appeared in this article. This feedback control circuits of ultrasound welding system consists of microprocessor control unit, signal generator, power amplifier, the feedback of voltage detector and current detector, etc. The experimental results exhibited this feedback control method can effectively and automatically catch frequency and rack a proper frequency for the ultrasound welding system.
Automatic frequency control for maximizing RF power fed to ultrasonic transducer operating at 1 MHz
For driving an ultrasonic transducer for cleaning semiconductor wafers or other industrial materials, the ultrasonic transducer having an input power of 60 W and operating at 1 MHz is driven from a prototype of the DC-to-RF inverter with an automatic frequency tuning function. Though the automatic frequency tuning circuit using a PLL circuit is not easy to build since the ultrasonic transducer has a complicated frequency dependent impedance, the RF power fed to the load can be maximized by a new type of PLL tuning circuit even if the resonating frequency of the ultrasonic transducer changes due to change of the load impedance. The ultrasonic transducer has a pole at 1 MHz. The frequency range of a VCO was designed to cover 0.8 MHz to 1.2 MHz so that the resonating frequency of the ultrasonic transducer was set at 1 MHz which was the center frequency of the VCO output. The operation of the system for feeding an RF power of 60 W to the ultrasonic transducer at 1 MHz was satisfactory.
Using Fuzzy-Delphi technique to determine the concession period in BOT projects
BOT projects are affected by several risks and uncertainties. One of the most important issues in this type of contract is to determine the length of concession period considering these uncertainties. The previous studies have developed several models to determine the concession period. However, the uncertainties are not taking into account in most of the previous works. Moreover, none of the previous researches has the capability to aggregate the opinions of different experts being involved in a project regarding the values of the uncertain input parameters. In this research, a Fuzzy-Delphi technique is implemented to determine the length of concession period considering uncertainties. Using the proposed Fuzzy-Delphi technique, the values of different uncertain factors affecting a BOT project is determined considering opinions of a group of experts. The NPV value is calculated considering the resulted aggregated values of uncertain input parameters. Finally, the concession period is determined using fuzzy approach. A case study is conducted to evaluate the performance of the proposed methodology. It is shown that the concession period is determined more effectively using the proposed approach.