Field-oriented control

For field-oriented controls, a mathematical model of the machine is developed in terms of rotating field to represent its operating parameters such as /V 4, 7, and 0 and all parameters that can inlluence the performance of the machine. The actual operating quantities arc then computed in terms of rotating field and corrected to the required level through open- or closed-loop control schemes to achieve very precise speed control. To make the model similar to that lor a d.c. machine, equation (6.2) is further resolved into two components, one direct axis and the other quadrature axis, as di.sciis.sed later. Now it is possible to monitor and vary these components individually, as with a d.c. machine. With this phasor control we can now achieve a high dynamic performance and accuracy of speed control in an a.c. machine, similar to a separately excited d.c. machine. A d.c. machine provides extremely accurate speed control due to the independent controls of its field and armature currents. 

Different manufacturers have adopted different methods with minor changes to achieve almost the same objective. For example, field-oriented control was first introduced by Allen Bradley in the USA in 1981 and a similar technique was introduced at the same time by ABB of Finland. ABB claim their technique to be still faster in responding, as it eliminates the modulation section of... 

Figure 6.10 Speed-torque characteristics by field-oriented control (FOG) (flux and torque control) (Source Allen Bradley)...

Magnetic field-oriented control - when the field is considered as the reference frame. 

These drives are normally open loop (sensor-less) without encoder. For higher regulation, it is better to adopt a two-phasor control, such as a field-oriented control (FOC) or a direct torque control (DTC) drive. 

This part-also deals w ith static controls and drives, soft starting and process control through solid-state technology (phasor and field-oriented controls) using IGBTs as well as energy conservation,... 

FIGURE 12.15 EBSD patterns of the UC0O2 active material (a) with and (b) without strong magnetic field orientation control. (For color version, refer to the plate section.)... 

Ion engines are used in sateUites for orientation control. Cesium is vaporized in a vacuum and ionized as it passes through a heated porous tungsten disk, the ions are accelerated by an electric field to about 135 km/s and are neutralized by the injection of electrons and exhausted from the thmster. However, mercury, xenon, and argon-based ion engines are preferred. 

With years of research and development in the field of static drives, it is now possible to identify and separate the.se two parameters (f, and /, ) and vary them individually, as in a d.c. machine, to achieve extremely accurate speed control, even slightly better than in d.c. machines. In d.c. machines the armature current and the field strength arc also varied independently. A.C. machines can now be used to provide very precise speed control, as accurate as 0.001% of the set speed, with closed-loop feedback controls. This technique of speed control is termed I ield-oriented control (FOC) and is discussed below. 

This is an alternative to FOC and can provide a very fast response. The choice of a static drive, whether through a simple V7/control, field-oriented phasor control or direct torque control with open or closed-loop control and feedback schemes, would depend upon the size of the machine, the range of speed control (whether required to operate at very low speeds, 5% and below), the accuracy of speed control and the speed of correction (response time). The manufacturers of such drives will be the best guide for the most appropriate and economical drive for a particular application or process line. 

Blascke, F., The principle of the field oriented iransvertor as applied to the new closed loop control. system for rotating field machines, Siemens Review 34. 217-220 (1972). 

Leonard W., 30 years. space vectors, 20 years field orientation and 10 years digital signal processing with controlled a.c, drives , EPEJourniil. 1 No. 1, July (1991) and 1, No. 2, Oct. (1991). 

VI. ENANTIOMERIC CONTROL VIA STRONG FIELD ORIENTATION OF MOLECULES... 

Two routes have been followed in reaction stereodynamics. One is to orient a molecular reactant in space and see how the reaction cross-section varies with the molecular orientation. This direction has been pioneered in molecular beam experiments using focusing of an electric hexapole field to control the molecular orientation [221-223a]. Numerous studies have applied this technique to electron-transfer reactions of alkaline-earth atoms [223b]. This technique is now complemented by the so-called brute force technique, where polar molecules are oriented in extremely strong electric fields [83]. 

The counter-rotating die is just one example of how performance is controlled through processing. Other methods such as magnetic and electric field orientation have been used for processing LCPs. If we are to achieve tlie full benefits in the LCP film applications described below, we must develop and use new film processing methods. 

Digital control technology has proven to be the most suitable way to achieve the required system flexibility and reliability. This fact turns out to hold for the motor drive as well. Here, the latest developments in microprocessor and especially in digital signal processor (DSP) technology have paved the way for such integration, since these single-chip devices not only feature the needed computation power but, newly, also the peripherals and microcontroller capabilities necessary for field oriented PWM motor drive control. 

The receiver is small and mounted directly on the ultrasound scan head. Its size does not interfere with standard clinical ultrasound scanning methods. The transmitter, which is usually mounted on the examining table, emits three orthogonal magnetic fields. The control unit measures and compares the relative strengths of all three fields at the receiver. These measurements are used to compute the position and orientation of the receiver relative to the transmitter. 

The only description of electric field orientation comes out of work by Ober and co-workers (37,113). They used ac electric fields, which allowed them to control orientation either parallel or perpendicular to the field direction. This is caused by an electrohydrodynamic effect, by which there exists a critical field frequency, below which the molecule orients parallel to the electric field and above which the molecule orients perpendicular to the electric field. Ober and co-workers were able to show control over the direction of orientation in a cyanate ester LOT by changing the frequency of the applied field during the cure (113). In contrast, an epoxy LOT flipped from a parallel to a perpendicular orientation during isothermal cure at a given applied frequency (37). This was attributed to a change in the critical frequency due to the increase in viscosity during cure. 

Ejj. is elevated by increasing the nozzle temperature and the fraction of He in the Ar/He seed gas mixture. The reaction zone is enclosed by harp" orientation fields which control the collision geometry. 

---