Process control systems (PCS) are a vital element in maintaining efficiency on a manufacturing production line, by testing the characteristics of the products to identify any abnormalities which can then be quickly attended to. PCS devices are also able to extract any data about these anomalies to ensure procedures are adapted and maintained, and to allow for faster troubleshooting if the issue occurs again.
PCS is a broad term that refers to pieces of equipment which carry out the above function, but there are great variations in types of PCS. At a fundamental level, the PCS must include a sensor, or primary transducer, which receives an input of data from a measurement taken on the production line, a controller which processes this input in line with desired setpoints (SPs), and a receiver which processes this into an exportable data output. This data is then transmitted to the company’s enterprise resource planning (ERP) computer application through a manufacturing execution system (MES), before control valves in the production line are altered, following an abnormal reading.
Smaller and more basic PCS devices, therefore, only require a combination of modular panel-mounted controllers to fulfil their function. Larger systems, however, are more complex, and are most often administered by one of three types of PCS: supervisory control and data acquisition (SCADA), distributed control systems (DCS), or programmable logic controllers (PLC).
What does the PCS measure?
In order to regulate the equipment on a production line, PCS devices measure process variables, of which there is a huge range, including pressure, flow rate, velocity, temperature, density, acidity, speed, stress, and weight. By measuring these variables, the PCS can identify irregularity in the products, which imply a necessary alteration in the production line equipment. By weighing and testing a container, for example, a sensor might detect whether it has been filled with its contents correctly and in the right quantity, when this data is compared with the SPs. In simple sensors, a pressure reading is given on an attached dial, to be manually recorded. In PCS devices, however, this reading is electronically transmitted to the MES application.
Similar methods are used across sensors with different measurement focuses. The most common of these is the measurement of the extent to which an element of the sensor is affected by an item. For example, piston sensors detect the pressure of an item by the force it pushes onto a spring, the movement of the spring then being measured to gauge the pressure. Mechanical gauge force sensors, much like the piston pressure sensors, use a spring for the item’s force to be tested again, the movement of the spring being proportional to the amount of force applied. Inferential flow meters also measure flow based on its effect on a piston or rotor arm, the speed of movement of which indicates the flow of the liquid or gas. With bimetallic temperature sensors too, the heating of metal strips causes them to bend, moving a pointer on a dial to reveal the measurement.
Other sensors use liquid as a medium through which to take a measurement. Liquid expansion temperature sensors work in a similar way to thermometers, measuring the expansion or evaporation of a liquid by the pressure it exerts on the sensor. Hydraulic load cell force sensors test the pressure of a liquid in response to a force and can also be used well for comparative measurements. Differential flow meter sensors also use comparative readings in their measures of flow as differential pressure.