Conference Paper · October 013 doi: 10. 1109/cobep. 2013. 6785223 citations reads 2,638 authors
¼. The performance parameters from the experimental
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DRIVES
¼. The performance parameters from the experimental
results were found to be reasonable and within expected. The system presented in this paper is part of a CNC routing system. Keywords – Hybrid Stepper Motor, Motor Drive, Computer Numerical Control. I. INTRODUCTION Stepper motors are a particular variation of variable reluctance machines and are designed to achieve higher compatibility and ease of use when interfacing with digital electronics systems. Its mechanical design has a main purpose: achieve a high positioning resolution. Both stator and rotor are built with a castle like structure, with tooth along their circumference and the final resolution of the system is proportional to this number of tooth. General purpose stepper motors are rated with resolutions as low as 0.9 ˚. Precision, high end stepper motors are able to achieve up to 0.05 ˚. Increasing the resolution by means of mechanical design only also increases drastically the final cost of the machine. It is worth noting that stepper motors' weight and power ratio is high, from where arises a maximum achievable power as well. Due to its conception, stepper motors are one of the most employed mechanical drivers for positioning systems in various systems. Its application varies from low power systems, where only a few tenths of watts are necessary, to more power demanding systems, from a few watts to up to hundred of watts are needed. On low power systems, such as hard disk drives, CD, DVD and BD units, and ink-jet printer, these motors perform flawlessly and its application probably will not fade on next decades, because of the ease of development and use. Concerning to more power demanding applications, stepper motors are also employed on productive units, ranging from desktop, low productivity, units to industrial, entry level, units. On these systems, stepper motors have more issues on performing as well, due to issues that are not easy to address when using low-end electronics. Problems such as low-speed and high speed performance discrepancies and electro-mechanical resonance usually arises when dealing with higher power systems. The academic community put some effort towards better driving techniques to overcome these issues. On [1] and [2], it is explained a couple of modern and classic control techniques, based on Kalman Filters, Fuzzy control and Proportional-Integral, both being applied in a closed loop approach, based on high end electronics, i.e. FPGA. On [3], an open loop approach is presented, using micro step technique to reduce vibration - and, in consequence, resonance - and to enhance positioning precision. The purpose of this paper is to present a stepper motor drive system, which was applied for positioning control on a Computer Numerical Controlled router machine, which was briefly presented on [4] and [5]. This paper aims to provide basics insight on stepper motor driving technique and present the main elements of the developed system. II. STEPPER MOTOR THEORY AND ITS DRIVERS Stepper motors are quite different from usual electrical machines. It can be regarded as a brushless DC motor, whose rotor rotates in discrete angular increments when its stators windings are programmatically energized [6]. The rotor has no electrical windings and can have: salient poles, relating to a variable reluctance machine; magnetized poles, relating to a permanent magnet DC motor; or can have both, in which case the motor is regarded as a hybrid stepper motor, being this the most common topology among the machines with higher power ratting. This work aims mostly at hybrid stepper motors and only this design will be regarded from now on. These motors are usually made of two or more stator windings, being more common two, four and five windings design. Each phase can be seen as a variable inductance, (), varying with the mechanical shaft angle, . This relation between and () arises from the very conception of the motor. Figure 1 presents an example of the relation for a motor with the following characteristics: two phase, 90˚ per step, 41 nominal inductance. As can be seen in Figure 1, the inductance of each phase has a periodic peeks in well-defined angular positions. That happens because the tooth of the stator align with tooth of rotor, hence, reducing the air gap and increasing accordingly the inductance. Figure 1 - Typical inductance profile plotted against , for each phase of a two phase stepper motor. It can be seen on Figure 1 that in no time the inductance falls to zero. Torque generated by such a motor is given by: = 2 ∙ () (1) where is the current on the winding, (θ) is the inductance of the phase and is the angular position of the shaft. Figure 2 presents the torque generated by the previously described stepper motor and such waveform is obtained by taking the derivative of inductance with respect to . It should be noted that magnetic saturation effect was not considered. Figure 2 - Typical torque exerted by a two phase stepper motor. The behavior seen in Figure 2 justifies why stepper motors are inherently used for position control: when a given winding is energized, the rotor aligns with the respective stator energized winding; if some external load torque tries to move the rotor from that position, an opposing torque will act in such a way to move back the rotor to the original rest position; if the load torque is higher than any opposing torque that the motor can generate, the shaft will enter an instability region and will move the rotor to another stable position. From Figures 1 and 2, it can be seen that stepper motors' resolution is related to its mechanical complexity, in a sense that the smaller distance between peeks for an inductance waveform, the higher will be its mechanical resolution. General purpose motors, with cylindrical rotor, the resolution can be as low as 0.9˚. Application specific motors, with disk rotor structure, resolution can be as low as 0.05˚. From that previous analysis, it can be observed that to control the position of a stepper motor, one only have to keep a constant current flowing through a stator winding. However, this position control is limited by three elements: its maximum torque driving capacity, known as holding torque; the number of phases a motor has; and the number of tooth that both stator and rotor have. Download 0.84 Mb. Do'stlaringiz bilan baham: |
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