What are the advantages and disadvantages of using continuous or discrete systems for control? I'm going to list some that occurred to me, but would appreciate if people help complement the list. (If this question doesn't belong here, please let me know, I'll delete it).
Advantages for discrete:
- Data from the real world is discrete.
- Computers are discrete, hence they can more easily deal with discrete control systems.
Advantages for continuous:
- Sampling is not an issue.
Best Answer
It is difficult to list all the pros and cons of analogue versus digital control but below are a few things I can think of:
an analogue compensation strategy, e.g. poles/zeroes placement, is fixed during the design phase and cannot be easily changed on the fly. With discrete control, you can alter the position of these frequency points while operating conditions change.
we all know that compensation components may be affected by tolerance and aging, naturally affecting the compensation strategy. Temperature and humidity also affect the components values in harsh environments: DSP-based compensators shield you against these effects.
a DSP or a µP brings intelligence capability with self-diagnosis, compensation strategy optimization to improve transient response in certain conditions. Some systems allow you to run a frequency analysis at power on and dynamically compensate the converter for steady-state operation. Some even compensate cycle-by-cycle by permanently keeping a constant phase margin regardless of the load nature (suddenly highly capacitive for instance). You can also easily modify operating parameters like the switching frequency to optimize efficiency with different loading profiles.
A DSP-based control usually uses less components around the chip than a classical op-amp-based compensator to which you add a voltage reference.
there is communication capability with a discrete-time system to signal problems or fault and report performance results.
the control bandwidth is naturally limited with discrete control: I know some dc-dc converters crossing over at 300-400 kHz with analogue control, something you could not easily do with a digital control.
delays, as pointed out by Neil_UK, can be a real plague and distort the phase margin while severely limiting your crossover selection.
the sensitivity of digital systems to noise when operated in proximity of power stages can be an issue in some cases, hanging the processor or affecting operation reliability.
and finally, the majority of power supply designers are well versed into analogue control and the Laplace transform while discrete-time engineers must be fluent in difference equations and \$z\$-transforms.
I remember attending a power supply manufacturers association (PSMA) conference 20 years ago where they predicted that digital control would soon be in all power supplies, including consumer applications. I can tell you that for high-volume applications like ac-dc adapters for notebook or power supplies for TVs, digital control (in the sense of what I described above) is not yet a reality and will have difficulty to be considering the low selling-price of complicated PWM controllers which became a commodity. However, it is already a reality for a long time in high-power converters for servers where one or several DSPs control the main converter and include the algorithm for power factor correction also.