DIGITAL PID USED IN INDUSTRY

 DIGITAL PID USED IN INDUSTRY 

Digital controllers have some advantages over analog controllers for proportional-integral-derivative (PID) applications. James F. Beall, principal process control consultant, Industry Solutions Group, Emerson Process Management, explained how digital control may offer additional process control system efficiencies by answering five questions. .

A PID controller is an instrument used in industrial control applications to regulate temperature, flow, pressure, speed, and other process variables. PID (proportional integral derivative) controllers use a control loop feedback mechanism to control process variables and are the most accurate and stable controllers. 

PID control is a well-established way of driving a system toward a target position or level. It's practically ubiquitous as a means of controlling temperature and finds application in myriad chemical and scientific processes as well as automation. PID control uses closed-loop control feedback to keep the actual output from a process as close to the target or setpoint output as possible.

Analog

An analog controller is continuous and is analyzed as such. The benefits are potentially limitless resolution. An analog controller is limited by the noise of the amplifiers. An analog controller's downsides are that it can be hard to tune because more often than not it involves using physical components such as variable resistors and capacitors.

Digital

Digital controllers are limited by the sampling rate and by the resolution of the ADCs and DACs. The benefits are easy tuneability and additional control logic that can be used with programming the digital system.

Digital controllers work well for most applications nowadays, it is rare to see a fully analog controller in most industries.

If your talking about "hardware" meaning digital PID controllers implemented with an FPGA vs a microcontroller, then the main differences will be in the loop timing (the update rate of the loop and the resolution of the ADCs and DAC.

FPGAs and ASICs can provide much faster loop timing as the control loops can be made to respond much faster than a microcontroller.

What is the difference between analog and digital control systems?

In analog control systems, all the signals are analogs, i.e. a scaled version, of the quantities they represent. Digital signals, however, are sequences of pulses, i.e. on-off signals, with the value of the quantity being represented by the sequence of on–off.

With analog control systems, all the signals are analogs, i.e. a scaled version, of the quantities they represent. Digital signals, however, are sequences of pulses, i.e. on-off signals, with the value of the quantity being represented by the sequence of on-off signals. Most of the signals being controlled are analog; thus, a digital control system is necessary to convert analog inputs into digital signals for the controller and then the digital outputs from the controller to analog for the process being controlled. Thus analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) are used. Figure shows the basic form of a closed-loop digital control system.

Digital controllers have several advantages over analog schemes, some of which are as follows:

Flexibility in control action

Modification of an analog controller can only be made through rewiring and replacement of a component, whereas a simple change to the computer program is all that is required for the modification of a digital controller.

Reduced cost involvement for the addition of further control loops

The addition of extra loops in digital controllers merely requires any extra hardware to be connected up to the same computer, whereas additional hardware loops would otherwise be required for an analog controller.

Improved user interface

Digital controller information can be displayed graphically on a monitor when required, as opposed to the analog alternative of employing a large panel for displaying limited information.

Adaptive control

Digital controllers can be modified online, that is, even when the system is in operation.

Cost-effective

Due to the rapid development of VLSI technology, the cost of digital controllers is decreasing in comparison to that of analog controllers, especially for applications requiring high accuracy and optimum performance.

Q: For PID applications, about how much analog control is still in place?

Ans: Based on looking around in the plants where I have control projects and asking a few customers, I would say there are less than 5% analog controllers in plants.

Q: What are the main advantages of using digital controllers over analog in PID applications?

Ans: There are many useful options in digital controllers that are unavailable in analog controllers, such as variable tuning; selectable PID structure [P and D on process variable (PV) or error] and form; gap control; error squared; tuning algorithms; accurate tuning settings; and others.

Q: Are there PID applications where analog has advantages over digital?

Ans: I have heard that a digital control controller must have input to output response time less than 100 milliseconds to match the speed of an analog controller. So, for loops that require a very fast response, the analog controller is faster than most digital controllers.

Q: When transitioning from analog to digital PID-type controllers, what are the special considerations, if any, for selection, setup, operation, optimization, or modifications over time?

Ans: The first step is to make sure you PROPERLY convert the current tuning in the analog controller to the digital controller.

Also, ensure you don’t configure a lot of new alarms just because it is easy to do so.

Finally, investigate the new features of the digital controller to find better ways to duplicate or improve previous functionality.

Q: Beyond performance, what are some other advantages of digital controllers for PID applications?

Ans: Other advantages include ease of collecting data from the controller and easier troubleshooting the controller action.