Dual stage control of a servo system consisting of a piezoelectric actuator and a linear motor for electrical discharge machining
Creators: name: BANI MELHEM, Muyad
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401406
subject: Machining
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)

description: Electrical discharge machining (EDM) is a thermal process that uses electrical energy to machine conductive workpiece irrespective the hardness of the materials. It is a widely used non-conventional method for machining difficult-to-cut materials in industry. The width of the discharge gap, or spark gap, is the most important factor that affects the stability and performance of the electrical discharge machining process. It directly contributes to the material removal rate and surface finish. Since the gap is very small (less than 50 microns) and the distance keeps changing dynamically, it is a challenge to control the spark gap accurately in real time. This research is to develop a novel dual stage hybrid positioning system which contains a linear servo motor and a piezoelectric actuator (PZT) to control the spark gap more accurately for achieving high machining efficiency. The servo motor provides the macro feeding for the tool electrode while the piezo actuator feeds the electrode in micro scale at high frequency. A new control algorithm was developed to drive the linear motor and PZT coherently in the dual stage feeding mechanism. The new algorithm moves both actuators at the same time to utilize the advantages gained from them. The linear motor has the advantage of long working range up to 25mm, while PZT has accurate and high bandwidth response.  Two control strategies were designed to control the speed of the linear motor in the new algorithm: constant speed and fuzzy controlled speed. The first control method is to move the linear motor in a slow constant speed to compensate the change in distance caused by depth of the machined material, while the second method is to continuously change the linear motor speed based on two factors, the PZT position and PZT speed which are the inputs to the fuzzy controller. The new algorithm with the fuzzy control was found 1.6 times faster than the conventional algorithm currently used in industry, and the material removal rate was more than one and half times bigger compared with the conventional algorithm for machining the same workpiece materials under the same machining conditions. These outcomes were achieved through eliminating the waiting time of the PZT and ensuing the continuity of the machining to obtain the highest efficiency. 
To further increase the machining efficiency, the application of a new vibration mechanism in the EDM machining process was studied by investigating vibration frequencies and amplitudes. The fundamental theory of the new mechanism is to enhance debris flow and reduce the probability of short circuit and arcing, and hence increase the material removal rate.
The vibration frequencies of current systems are fixed and independent of the discharging status in the gap. This inevitably causes arbitrary movements of the electrode with respect to the discharge pulses and interrupts the normal discharging process: the backward movement of the electrode is to enlarge the gap; if it occurs during the formation of the plasma channel, it will cause failed ignition of the plasma. On the other hand, the forward movement of the electrode is to reduce the gap; if it occurs during the discharging process, it will cause short circuit or arcing. Since the discharge frequency and the frequency of ultrasonic vibration are very high (&gt;10 kHz), the probability of the occurrence of such scenarios is consequently high. These movements which are unsynchronized with the discharging pulses will undoubtedly lead to a large number of failed discharge pulses, and as a result, cancelling out a large proportion of increased material removal rate which is potentially achievable by the ultrasound vibration method. The workpiece will be excited by the PZT to cause it to vibrate. To synchronize the mechanical movement of the vibration with respect to the discharge pulses, the output signal from the pulse generator will be connected to the input of the PZT controller. The rising edge of the discharge pulse will be used as the trigger signal to move the actuator forward, while the falling edge of the working pulse will be used as the trigger signal to move the actuator backward. The vibration movement of the PZT will therefore follow the change of discharge pulses. According to the experimental results it is clear that the vibration added to the EDM has a big impact on the material removal rate when the frequency was above certain values and the vibration amplitude less certain limits:  the vibration has no or small effect on the material removal rate when the frequency was less than 1000Hz; but it was increased when the frequency was raised to above 1000Hz, the material removal rate reached around 0.15µm/sec when the frequency was 1250Hz which is an increase of  around 200% compared with the that  without vibration. Material removal rate was increased as well when the amplitude was decreased. It is found that the material removal rate was increased by 50%-250% when the vibration amplitude was reduced to under 0.1µm.