808 BASIC CONTROL SYSTEMS
device simply holds the value of the signal until the next sample comes along. Two samplers with
two different sampling periods (T 1 andT 2 ) are shown in Figure 16.3.4(b), which is known as a
multirate sampled-data system. If the rate of variation of the signal in one loop is relatively much
less than that of the other loop, the sampling period of the sampler employed in the slower loop
can be larger. With sampling and multirate sampling, some of the expensive components of the
system can be utilized on a time-sharing basis.DIGITALCONTROLLER FOR ASTEAM-TURBINEDRIVENGENERATOR
Figure 16.3.5 illustrates the basic elements of a minicomputer system in terms of a block diagram
for speed and voltage control (as well as data acquisition) of a turbine-generator unit. Typical
output variables of the generator are speed, rotor angle, terminal voltage, field (excitation) current,
armature current, and real and reactive power. Some output variables are measured by digital
transducers, whose outputs are then digitally multiplexed and sent back to the minicomputer.
Some other output variables may be measured by analog transducers, whose outputs are processed
through an analog multiplexer which performs a time-division multiplexing operation. The output
of the analog multiplexer is connected to a sample-and-hold device, which samples the output
of the multiplexer at a fixed time interval and then holds the signal level at its output until the
A/D converter performs the analog-to-digital conversion. Thus, following the multiplexer, time
sharing is done amid a number of signal channels.MACHINE-TOOLPROCESS TODRILL ORPUNCHHOLES(DIGITALLYCONTROLLEDDRILLPRESS)
Figure 16.3.6 depicts an elementary system, including the input, digital processor, drill-positioning
mechanism, and sampled-data position-control system. The process is controlled as follows:- The processor receives the input data and determines the sequence of operations, while
storing both the sequence and the hole positions in its memory. - The sampled-data system helps in positioning the drill at the first hole location.
- If the actual drill position agrees with the desired location stored in the memory, the
processor sends a signal, thereby causing the drilling process to begin. - At the end of the first drilling operation, the process is continued until all the required
holes are drilled.
To the basic system illustrated in Figure 16.3.6, additional controls such as speed control (to
accommodate different kinds of materials to be drilled) and timing control (to control the time
duration of the drilling process) can be added rather easily.STEP-MOTORCONTROLSYSTEM FORREAD–WRITEHEADPOSITIONING ON ADISKDRIVE
Figure 16.3.7 illustrates a system in which the prime mover used in the disk-drive system is a
step motor driven by pulse commands. In response to each pulse input, the step motor moves by
some fixed displacement. This all-digital system does not need an A/D or D/A converter.POSITIONSERVO IN ARADARSYSTEM
Figure 16.3.8 depicts the essential elements of a position servo system. In a radar system, an
electromagnetic pulse is radiated from an antenna into space. An echo pulse is received back