DIGITALLY CONTROLLED DRIVES
As in all forms of industrial and precision control, digital implementations have replaced analogue circuitry in many electric drive systems, but there are few instances where this has resulted in any real change to the structure of existing drives, and in most cases understanding how the
drive functions is still best approached in the first instance by studying the analogue version. There are of course important systems which are predominantly digital, such as PfiM inverter drives (see Chapter 7) and future drives that employ matrix converters may emerge and they are only possible using digital control. But as far as understanding d.c. drives is concerned, users who have developed a sound understanding of how the analogue version operates will find little to trouble them when considering the digital equivalent. Accordingly this section is limited to the consideration of a few of the advantages oVered by digital implementations, and readers seeking more are recommended to consult a book such as that by Valentine (see page 400).
Many drives use digital speed feedback, in which a pulse train generated from a shaft-mounted encoder is compared (using a phase- locked loop) with a reference pulse train whose frequency corresponds to the desired speed. The reference frequency can easily be made accur- ate and drift-free; and noise in the encoder signal is easily rejected, so that very precise speed holding can be guaranteed. This is especially important when a number of independent motors must all be driven at identical speed. Phase-locked loops are also used in the firing-pulse synchronising circuits to overcome the problems caused by noise on the mains waveform.
Digital controllers oVer freedom from drift, added flexibility (e.g. programmable ramp-up, ramp-down, maximum and minimum speeds etc.), ease of interfacing and linking to other drives and host computers and controllers, and self-tuning. User-friendly diagnostics represents another benefit, providing the local or remote user with current and historical data on the state of all the key drive variables. Many of these advantages are also oVered with drives that continue to employ analogue control in the power electronic stages.