Introduction to direct digital control (DDC)

DIRECT DIGITAL CONTROL (DDC) means a counter-reaction control system in which the function of the regulator is carried out numerically by an appropriate programmable digital device. The following refers to a microprocessor-based system (uP or micro).

The control function, which in analog systems is carried out by an electronic (or pneumatic) regulator, is here performed by an algorithm that, from the knowledge of the reference and the acquired value of the output, calculates the command to be applied to the process through the actuator. Since digital programmable devices do not take zero time to perform calculations, it is obvious that processing cannot take place in real-time as in analog regulators, in fact, a new processing can only take place when the previous one is over. This means that the signals entering the computer must be discrete over time (sampled), i.e. they take on values in discrete moments of time (in most applications these instants follow one another at constant intervals). In addition, the data that is introduced into the digital device must be numbers represented in a code provided by the device itself.

At the end of each interval, the counter has accumulated a number proportional to the speed of the disk (actually the average value of the disk speed in the past interval). Transducers of this type are called digital transducers and have the advantage of making it easy for the micro to acquire it, since the signal is already in digital form. In the more general case of analog feedback signals, an electronic hardware must be used to sample the output signal from the transducer (input to the micro) and convert it into numbers. For the output, (input of the actuator) the numerical data processed by the algorithm must be converted into an analog signal. In some cases the problem of the output interface (from the algorithm) is very simple because the actuator is digital, able, that is, to accept at the input a command variable expressed in the form of a number.

For example, for a stepper motor, the data provided by the control algorithm represents the number of rotation steps that the motor must take. In any case, the output signal from the control algorithm is of the pieceway constant type, its value is updated at discrete moments of time, generally at regular intervals.

DDC

The flow scheme of the algorithm is as follows: clock interruption -> input reference reading -> reaction signal input -> processing of the command for the actuator (control algorithm) -> output for actuator -> cycle repetition. In this flow, the interruption is provided by a clock (Real Time Clock RTC) that determines the sampling period and it is assumed that the reference (e.g. constant, set point) is already in digital form.

Main steps of DDC design

The main stages of the design of a DDC chain are:

  1. design of the process micro-instrumentation interface
  2. synthesis of the control algorithm based on the specifications and choice of the sampling frequency, possible verification by simulation
  3. implementing the algorithm, such as in Assembly or C and debugging
  4. experimental tests on the plant or prototype

Point 2 corresponds to the synthesis of the “correcting networks” in analog systems and, in this phase, CAD programs for control systems or at least programs that allow the simulation of continuous part (actuator, process and transducer) and digital part (control algorithm) systems are of great use.

As far as point 3 is concerned, it corresponds, in analog control systems, to the realization with operational amplifiers of the transfer function of the regulator obtained in point 2.

Benefits of digital control

From the first applications of process calculators in the early 60s to today, digital regulation has undergone a remarkable development, both from the point of view of the type of applications and their number.

The introduction of microprocessors, in the 70s, marked a decisive turning point in the sense of accelerating the introduction of digital control techniques in industrial applications. Among the main reasons that have led to the progressive replacement of traditional analog regulators with digital ones, even for conventional applications, there is the tendency for automation systems to become more integrated (from management to production).

From this point of view, the possibility of the single control system to communicate digitally with others and with the highest level automation systems becomes of primary importance.

Another important reason for the development of digital regulation techniques comes from the manufacturers of automation systems, for which it is advantageous to develop a microprocessor hardware system only once and to modify, via software, the regulation function, according to needs. This is also an advantage for the end-user, since, if he has sufficient technical skills and updated documentation, he can reconfigure the control functions via software to adapt them to the changed needs of the system.

Now let’s see the main advantages and problems that derive from the use of digital regulators.

Hardware benefits

  • Use of the same digital hardware for multiple control rings (microP in time sharing);
  • Negligible effect in the digital components of drifts, aging etc. typical of analog systems. However, these problems are present in the ADC and DAC converters;
  • Fewer components (VLSI) and, consequently, reduced overall dimensions, weight and consumption;
  • Greater reliability.

Note that the first point mentioned, was one of the most important motivations in favor of digital regulation in the early stages of its introduction in industrial applications (early 60s). This was due to the fact that the cost of a process calculator was then very high and it had to be used for more than one process to justify its use; currently the cost of a microprocessor system is such that the problem does not arise.

In addition, closing several control rings through the same digital hardware is often not the best solution because, in the event of malfunctions, a certain number of regulations would be put out of service at the same time, and this may not be allowed both for safety reasons (of the staff and the plant) and for continuity of production.

Control benefits

  • Implementation of complex algorithms;
  • Realization of transfer functions with very high time constants or with finite delays;
  • Generation of reference signals;
  • Ability to perform logical operations;
  • Command of sequences and timings;
  • Supervision functions (monitoring and data acquisition);
  • Interface with the outside of the µP (integrated automation system, operator, etc.);
  • Flexibility: same hardware for different applications;
  • Configurability: possibility to vary the control scheme via software.

It is worth pointing out that the dynamic characteristics of DDC chains with classic control actions (for example PID), are not substantially different from those obtained with traditional analog regulators, while more complex schemes such as those for non-linear, adaptive, excellent, robust control and for multivariable systems are only possible by resorting to the digital implementation of the regulator.

Digital control issues

The problems that arise in the realization of digital regulators compared to analog ones, are due to the performance of the digital hardware currently available (word length, processing speed, I/O capacity, etc.) and to the problems related to the production of the application software. Among them, we can remember the following:

  • Quantization of data in the interface and arithmetic of the microprocessor;
  • Discretization of time;
  • Delays in data acquisition and processing;
  • Difficulty in processing very fast signals (depends on the clock of modern DSPs);
  • Rapid evolution of hardware (choice of microprocessor);
  • Testability, i.e. access to internal signals;
  • Programming languages;
  • Communication protocols;
  • Software documentation;
  • Cost and time for software development;
  • Availability and updating of staff.

Conclusion

We can conclude by saying that currently the terms control and regulation often imply the adjective digital. Analog regulators are used in applications where many of the advantages offered by digital techniques are not essential, low-cost systems, high control speeds, or, finally, when it is not possible, for environmental reasons (temperature, radiation, etc.) to use electronic devices, resorting in such cases to pneumatic or hydraulic controls.

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