Pulse drive motors

Fig. 1.1. Details of the high-frequency iaser evaporation source. Shown are the rotary motor, which drives the planetary gear assembly for turning the target, and the thermalization chamber with exchangeable expansion nozzie. The iaser-produced plasma expands into this thermalization chamber. A heiium gas puise is then introduced by a piezo-driven pulsed valve and synchronized with the iaser puise into the same volume. The metal-gas mixture then expands through the nozzie into the vacuum leading to cluster formation. In contrast to conventional sources, the laser beam is coaxial to the molecular beam axis. The bellow is used to aiign the source along the optical axis of the ion optics...

Figure 4.3 shows the schematic of a reciprocating pump mechanism used in most HPLC pumps. Here, a motorized cam drives a piston to deliver solvent through a set of check valves. A microprocessor coordinates the piston speed with other components. Since only the inward piston stroke delivers the liquid, a pulse dampener is used to reduce flow fluctuations. All components in the fluidic path are made from inert materials (e.g., stainless steel pump heads, ruby balls and sapphire seats in check valves, sapphire pistons, and fluorocarbon pump seals). 

The material requirements for these classes of devices are somewhat different, and certain compounds will be better suited to particular applications. The ultrasonic motor, for instance, requires a very hard piezoelectric with a high mechanical quality factor Qm, to suppress heat generation. Driving the motor at the antiresonance frequency, rather than at resonance, is also an intriguing technique to reduce the load on the piezoceramic and the power supply [42]. The servo displacement transducer suffers most from strain hysteresis and, therefore, a PMN electrostrictor is used for this purpose. The pulse drive motor requires a low permittivity material aimed at quick response with a certain power supply rather than a small hysteresis, so soft PZT piezoelectrics are preferred rather than the high-permittivity PMN for this application. 

Sliding experiments were carried out at room temperature in a UHV condition of less than 1x10 Pa throughout the experiment A normal load of 250 mN was applied on the Si wafer by a spherical diamond slider (3 mm curvature radius), and the sliding speed was 0.1 mm/sec controlled by a pulse-drive motor. The sUder was driven at 100 pulses per second. In this contact condition, approximately 350 MPa of Hertzian pressure was applied at maximum on a circular contact area 32 pm in diameter, as determined by our calculations. In the observed tracks, the width was less than 40 pm, which was close to the calculated width after 1000 cycles of reciprocal sliding, so that the lateral fluctuation of the contact point was stable enough to assume that the iterated sliding was in contact approximately on the same track. 

The most popiilar form of motor speed control for adjustable-speed pumping is the voltage-controlled pulse-width-modulated (PWM) frequency synthesizer and AC squirrel-cage induction motor combination. The flexibility of apphcation of the PWM motor drive and its 90 percent- - electric efficiency along with the proven ruggedness of the traditional AC induction motor makes this combination popular. 

The inverters are either voltage source or current source (see Figure 7-7a and b). There are other variations, but they apply to drivers smaller than the ones used with compressors. However, pulse-width-modulated (PWM) (see Figure 7-7c), transistorized units are less complicated and are relatively maintenance-free with reliable units available to at least 500 hp. For all but the smaller compressors, the current source inverter is the one typically used. With a six-step voltage source, a rule of thumb has been to size the motor at two-thirds of its rating so as not to exceed the insulation temperature rise. For current source motors, the output torque is not constant with decreased speed, which fortunately is compatible with most compressors, as torque tends to follow speed. For current source drives, one needs to upsize the motor captive transformer by approximately 15% to account for harmonic heating effects. 

Provides a constant and almost pulse free flow. Both pump chambers are driven by the same motor through a common eccentric cam this common drive allows one piston to pump while the other is refilling. As a result, the two flow-profiles overlap each other significantly reducing the pulsation downstream of the pump this is visualized below. 

Fig. 2.3. One of the original scanning acoustic microscopes. It worked in transmission, so that it was not necessary to use pulsed waves, and the detected transmitted signal could be used directly to modulate the beam in a cathode ray tube. The slow scan was provided by a small motor driving a lead screw, and the fast scan by a modified loudspeaker coil (Lemons and Quate 1974,1979).

Flow control via pump speed adjustment is less common than the use of throttling with valves, because most AC electric motors are constant-speed devices. If a turbine drive is used, speed control is even more convenient. However the advent of the pulse-width modulated (PWM) adjustable speed drive with sensorless flux-vector control has brought adjustable speed (AS) pumping into the mainstream of everyday applications. 

The transistors of the output-side inverter are pulse-width modulated in such a way that in every phase, a near-sinusoidal current is created after smoothing by motor inductivity. The frequency of this current can be varied to adjust the speed of the motor. The switching frequency of small to medium frequency converters is on the order of 2 to 8 kHz and adjustable. In frequency converter operation, this frequency is quite audible. The larger the motor, the smaller the switching frequency selected. In large drives, switching frequency drops to 400 Hz. 

It is thus clear that the nature of the driving force which powers the flagellar motor of halobacteria, remains a problem to be solved. To discriminate between H - and Na" -motors, one should study motility in a Na -free medium. If it is the Na motor that is inherent in halobacteria, they must be motionless without added Na, and a pulse of Na must activate the motility even in the presence of a protonophore. 

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

Nicotinic acetylcholine (ACh) receptors are responsible for transmission of nerve impulses from motor nerves to muscle fibers (muscle types) and for synaptic transmission in autonomic ganglia (neuronal types). They are also present in the brain, where they are presumed to be responsible for nicotine addiction, although little is known about their normal physiological function there. Nicotinic receptors form cation-selective ion channels. When a pulse of ACh is released at the nerve-muscle synapse, the channels in the postsynaptic membrane of the muscle cell open, and the initial electrochemical driving force is mainly for sodium ions to pass from the extracellular space into the interior of the cell. However, as the membrane depolarizes, the driving force increases for potassium ions to go in the opposite direction. Nicotinic channels (particularly some of the neuronal type) are also permeable to divalent cations, such as calcium. 

To reduce the amplitude of the pulses, some manufacturers have proposed the use of multiple-head pumps, also named multiplex pumps, in which the same motor powers the several heads which are out of phase. From Fig. 9a it can be seen that, with a sinusoidal drive pump with two heads 1S0 out of phase, the amplitude of the pressure pulses will not be changed. The flow-rate will be doubled and the flow profile will be a rectified sinusoid. With three heads 120° out of phase, the pulses are partially damped, as shown in Fig. 9b. The maximal pulse amplitude is 13.3% of the maximal flow-rate if the distance travelled by the piston is the same for the three heads. In this instance the average flow-rate is 95.5% of maximal flow-rate, which is a considerable improvement. 

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