We design quality control systems
Our experiences with various control systems design and commissioning projects in Perth and Western Australia has allowed us to build a competitive option for clients. Given our expertise in power distribution and electrical projects, we also offer superior options when designing control systems for electrical power. We encourage all clients with control systems projects to contact us to discuss how we might help.
Introduction to Control Systems Engineering
Instrumentation and control engineering (also known as "automation engineering", "process control engineering" and "control systems engineering") is the design and implementation of automated systems to control industrial processes. These systems are often found in mining, oil and gas, and manufacturing facilities. This is commonly done by measuring specific aspects of an industrial process, and by specialised processors, which control equipment (like pumps or valves) that affects the industrial process.
Such processors can take the form of PLCs (Programmable Logic Controllers), DCS (Distributed Control Systems) and/or SCADA (Supervisory Control And Data Acquisition). Together, the instrumentation and controlling hardware are generally referred to as a "control system". Control systems are programmed to achieve a specific output. For example, the pressure in a vessel may need to be maintained within a certain range, or a chemical process may require several steps to be performed in a certain order and then repeated for the next batch, or similar.
Control systems are generally programmed with human operators (end users) in mind. A control room operator typically operates the industrial process in a safe or less hazardous area, utilising graphical interfaces on HMI (Human Machine Interface) screens or panels, such as on a computer or on a touch screen. Operators can take a birds-eye view of the process and make decisions to change parameters of the process to optimise productivity, avoid hazardous situations and similar.
Types of instrumentation and control systems engineering
A historical comparison of control systems is shown below:
| System | Evolved from | Historical advantages | Historical disadvantages |
|---|---|---|---|
| DCS | Pneumatic control | High-bandwidth feedback Large-scale Real time |
Hard to scale beyond a small areas |
| PLCs | Electrical relays | Quick, cheap and easy to replace | Often hard to troubleshoot Difficult to re-wire Not particularly reliable |
| SCADA | RTUs (Remote Terminal Units) and open-loop control | Simplicity of control Large geographical span |
Low-bandwith Unreliable communications Limited local control in real-time |
As today's control systems continue to evolve, these three main types (PLC, DCS and SCADA) are starting to look increasingly similar, with bandwidth increasing, communication delays decreasing, and effective local control becoming simpler to implement. Often, theoretical texts split control systems into "open loop control" and "closed loop control", the primary difference here being that some kind of feedback of the industrial process is present in the latter. There are also different types of control that use process feedback, such as PID control and sequence control.
PID Control
Often, control systems utilise PID (proportional integral derivative) control to maintain specific operating conditions. PID is a (relatively) simple form of control developed from the observation of how helmsmen steered ships and corrected for oversteering. It is often used when controlling flow rates, tank levels, vessel pressures, temperatures, chemical mixtures and more. Proper PID control implementation often requires working closely with process (chemical) engineers in order to make sure the chemical process requirements are met. It uses continuous feedback to alter an output to smoothen out a process.
Sequence Control
Very often processes are controlled in a sequence of events. For example, a process might involve filling a tank with two liquids, agitating for a certain amount of time, then draining until empty. In such a case, sequence control can be implemented using various process control tools to measure process variables (e.g. tank level) and timers (such as in a PLC). Sequences can range from being simple to complex - the nature of each is determined by the requirements of the process, the instrumentation and human interaction involved.
Why are control systems generally used?
The automation of laborious and hazardous tasks allows more productive and less expensive implementations than manual/human labour. It even makes certain processes possible that humans simply would not be able to do themselves.
How are quality control systems designed?
As a general process, the design of a control system follows these stages:
- Functional specifications - Client needs are defined via control systems function. Better initial functional specifications generally result in better outcomes.
- Equipment selection - Depending on the functional specifications and client needs, different systems can be selected to deliver required outcomes.
- Programming and testing - Standard and custom engineering functions are implemented to achieve the design specifications. These can include PID and sequence control, for example.
- End user requirements - Any human interaction or interpretation on system data needs to be integrated to be user-friendly and meaningful. Often this is done through the use of human-machine interfaces.
- Installation and commissioning - A programmed system is installed on-site and tested using standard and safe tests before being ready for operation.
Other considerations throughout the design process include:
- Non-standard client requirements
- Planning for multiple types of plant failure (e.g. mechanical, electrical, instrumentation)
- Knowledge of system alarms, interlocking, and how to create appropriate feedback for troubleshooting
What are common problems with instrumentation and control systems engineering projects?
Not fit-for-purpose solutions. Poorly considered operator feedback. Rushed design resulting in excessive downtime and troubleshooting post commissioning. Poor choice of communication hardware or protocol, resulting in unreliable or unsecure communication systems.
Other considerations in control system design
Speed
The historical factors that influence each system's growth have also played a part in their speed. Since pneumatic control was not responsive to the millisecond, DCS systems tend to have slower scan times (e.g. 1 seconds). This can be compared to PLCs, which evolved from electrical relays, whose scan times tend to be in the order of tens of milliseconds (depending on load). Whilst DCS systems can have higher scan speed options available, specialist PLCs are typically selected over DCS when response times are critical - for example in the case of electrical power generation and electrical power distribution.
Redundancy
Whilst PLCs might have speed as an advantage, the evolution of DCS has often had fail safes in mind. As a result, DCS systems tend to have dual-redundant options as standard. Typically, a piece of hardware can fail, only to be taken over by an identical piece of reserve hardware, allowing the industrial process to continue uninterrupted. However, PLCs are more increasingly providing dual-redundancy options in an effort to compete with DCS solutions.
Safety Systems
Given the nature of the industrial process, there are many offerings for high speed and redundant systems available. It is not uncommon to see triple redundant (or higher) solutions selected for critical plant sections where risk to personnel safety or plant must be avoided at all cost. In these cases, safety systems provide extra levels of redundancy (e.g. in the case of processor miscalculation) or integrate more strict options for configuration than normal control systems.
Cost
Like most projects, cost is a major factor in determining which options can be considered for the control system and its instrumentation. For example, the dual redundancy of a DCS may be excessive for a small project, but the downtime associated with not having a dual-redundant system can lead to operational losses that exceed the initial cost of the more reliable solution. Each project is different and, ideally, cost should be examined closely when deciding on a solution.