Analog vs. Digital Controllers

Analog signals pass continuously through a full range of values for example a measuring device converts the process variable (PV), say temperature, into a signal corresponding to this temperature. Each temperature value corresponds to a value of the electrical signal. Digital signals belong to the group of discrete signals; here the individual signals are represented by numerals (digitally). This implies that discrete signals can only take up a limited a number of values. The variation of discrete signals with time always appears as a series of steps.

A controller produces a relationship between the process variable (PV) and the setpoint (SP), and derives from it the manipulating variable (MV). We have several ways to accomplish this task: mechanical, pneumatic, electrical. The mechanical controller, for instance, alters a signal through a lever system, the electronic controller (analog controller) through operational amplifiers. With the introduction of more powerful and low-cost microprocessors, we have another electrical controller i.e. microprocessor-based controller (digital controller). The measurement signal is no longer processed in an operational amplifier, but is now calculated using a microprocessor.

You can also read: Digital PID Controllers

Comparing Analog to Digital Controllers

Analog controllers are built up from the operational amplifiers. The control parameters are set by means of potentiometers, trimmers or solder links. The controller structure and characteristics are largely predetermined by the design and construction. Analog controllers are used where there is no need for very high accuracy, and where the required features of the controller, such as its dynamic action are already known at the planning stage. Because of its speed of reaction, the analog controller has clear merits in extremely fast control loops.

In digital controller, a microprocessor converts all analog inputs into numerical values, and uses them to calculate the manipulating variable. This has certain merits over the analog processing as discussed below:

• Increased accuracy of control, depending on the measurement signal and the technology employed for example analog to digital converter. Unlike components which are affected by tolerances and drift, the mathematical relationships used in digital controllers have a constant accuracy and are unaffected by variations in components, temperature effects and ageing.
• Control parameters can be optimized automatically, under certain conditions in digital controllers.
• High flexibility in the structure and characteristics of the controller. Instead of having to adjust parameters or unsolder components, as in analog controllers, a digital controller can be modified by simply programming a new linearization, controller structure, and so on by inputting numerical values.
• Facility for data transfer, there is often a requirement to modify or store information about process status variables or pass it on for different utilizations and this is very easy to achieve using digital technology. Remote setting of parameters through data systems, such as process management systems via a digital interface is also simple.

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While digital controllers have various advantages over the analog controllers, they also have shortcomings as listed below.

• The digital display, typically standard with digital controllers, makes it more difficult to identify trends in process values.
• Digital instruments are more sensitive to electromagnetic interference.
• The digital controller processor requires a certain time to calculate parameters and to carry out other tasks, so that the process variables can only be read in at certain time intervals.

Related articles:

• The Performance Limits for PID Controllers
• How to Tune the PID Controller
• Digital PID Controllers
• Programmable Automation Controllers (PACs) – Features & Applications
• Basic Features of Distributed Control Systems (DCS)