Master-slave technique

Two mode-locked titanium sapphire lasers (Spectra Physics, Tsunami) with.l.4ps pulse duration and a pulse repetition rate of 80 MHz are synchronized using the master-slave technique [222]. The actual repetition rate of one of the two lasers, the so-called master, serves as a reference oscillator. The second laser, the so-called slave, is matched to the repetition rate of the master. This means that the slave oscillator follows the changes of the master s resonator length. A servo loop is used to realize the alignment procedure. 

Fig. 2.13. Locking scheme of the master-slave technique used (taken from [223]). The cavity length of the slave resonator is matched to the cavity length of the master resonator by means of a servo loop. See text for further explanation...

In practice the data acquisition system of the beamline will be the master system. All other techniques that one would like to interface to be used simultaneously will have to do this in a master-slave configuration in which the digital systems used for instance Raman or Drifts will be synchronized via pulses from the master system. 

These approaches require that at least one processor of the parallel machine can store all elements of the matrices involved (G and D). This can be a severe limitation, because the sizes of molecules that can be treated could be limited by the memory available on one node rather than by the processing time required. However, this technique is commonly applied. It is the simplest way to sizes of the parallelize existing code and, if memory is abundant or the problems of interest are small or few processors are available, little will be gained by implementing the more complex distributed schemes. So we will consider in more detail two approaches to Fock matrix formation with nondistributed matrices. One uses a master-slave approach, while in the other the processors can work independently to produce a partial Fock matrix on each processor. 

In Florence, we have chosen an approach that combines laser spectroscopy with the direct frequency measures of the microwave experiments [4]. We take advantage of the obvious consideration that to obtain the FS separations there s no need to precisely know the optical transitions frequencies but just their differences. Thus, if we have two laser frequencies whose difference can be accurately controlled, we may use one as a fixed reference and tune the second across the atomic resonances, as illustrated by Fig. 1. In fact, our approach reverts to an heterodyne technique, where all the transitions are measured with respect to the same reference frequency, that can take any arbitrary but stable value. In the experimental realisation we obtain the two frequencies by phase-locking two diode lasers (master and slave), i.e. phase-locking their beat note to a microwave oscillator [14]. We show in Fig 2 a full-view of the experimental set-up. 

In tuning a cascade control system, the slave controller is tuned first with the master controller in manual. Often only a proportional controller is needed for the slave loop, since offset in that loop can be treated by using proportional plus integral action in the master loop. When the slave controller is transferred to automatic, it can be tuned using the techniques described earlier in this section. Seborg et al. (1988) and Stephanopoulos (1984) provide further analysis of cascade control systems. 

Before attempting to determine the process dynamics we must first explore how they might be affected by the presence of other controllers. One such situation is the use of cascade control, where one controller (the primary or master) adjusts the SP of another (the secondary or slave). The technique is applied where the process dynamics are such that the secondary controller can detect and compensate for a disturbance much faster than the primary. Consider the two schemes shown in Figure 2.9. If there is a disturbance to the pressure of the fuel header, for example because of an increase in consumption on another process, the flow controller will respond quickly and maintain the flow close to SP. As a result the disturbance to the temperature will be negligible. Without the flow controller, correction will be left to the temperature controller. But, because of the process dynamics, the temperature will not change as quickly as the flow and nor can it correct as quickly once... 

The third method involves a spectral modification technique. The procedure termed virtual master, is performed by developing a virtual template of a master filter or monochromator instrument using a standardization set of samples with widely varying spectral composition. The slave or transfer instrument is then standardized using the virtual template of the master [97],... 

The preceding technique works well for conventional single-loop controls and for secondary or slave loops in a cascade system. But for primary or master controllers, we do something different since the valve-loading signal is no longer meaningful for reset feedback. 

To eliminate reset windup, we break the master controller internal feedback as before, but now we use the secondary measurement for feedback as shown by Figure 9.7. If, for example, we have temperature cascaded to flow, we feed the output from the flow transmitter back into the master controller reset circuit. This means that during normal control the lags in the secondary control loop appear in the reset feedback circuit of the primary controller. If, as usual, Ak slave loop is much faster than the master loop, this technique will not r . >preciably increase the master controller reset time. 

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