Process control systems (PCS), sometimes called industrial control systems (ICS), function as pieces of equipment along the production line during manufacturing that test the process in a variety of ways, and return data for monitoring and troubleshooting. Many types of process control systems exist, including supervisory control and data acquisition (SCADA), programmable logic controllers (PLC), or distributed control systems (DCS), and they work to gather and transmit data obtained during the manufacturing process.
The PCS can be a relatively simple item with a sensor, often called a primary transducer, that receives an input, along with a controller that processes the input, and a receiver that processes an output.
More complicated PCS devices are robotic and perform many tasks. The PCS devices can communicate their data to a company’s enterprise resource planning (ERP) computer application through middleware software called a manufacturing execution system (MES).
A great number of measurements can be taken on production lines. An equipment's sensor can pick up many measurements including pressure, flow rate, density, acidity, velocity, speed, stress, temperature, and weight.
Also, sensors can detect if an operation has occurred, such as the fill of a bottle, whether the correct pressure has been achieved, or if a certain temperature has been reached.
Many sensors exist on production lines, falling under some different areas, such as pressure sensors, flow meters, force sensors, and temperature sensors.
A pressure sensor can be triggered mechanically as an item passes the sensor. In its basic form, a pressure sensor shows the reading on a dial attached to the sensor, but it can also electronically transmit the reading to the MES application. Other types of sensors include:
- Piston pressure sensor: The pressure from the item on the production line can push on the piston, which compresses a spring. The movement of the spring indicates the pressure.
- Diaphragm: The diaphragm is affected by small amounts of pressure, and these get indicated on a dial.
- Bourdon tube: This hollow tube straightens under the application of pressure. It can be used for measuring pressure differences.
A flow meter instrument measures the linear, nonlinear, mass, or volumetric flow rate of a liquid or a gas.
When selecting a flow meter for the production line, you need to know information about the fluid involved, the rate of movement, and how to record the flow. Flow meter types include the following:
- Positive displacement: These flow meters use a mechanical effect to measure flow. The speed of the rotation of the meter indicates the flow of the liquid.
- Differential: The differential flow meter identifies the flow and converts it to a differential pressure that can be measured.
- Inferential: The inferential flow meter measures the flow based on the effect of the flow. This could be a simple rotor arm that is moved by the flow. The faster the rotor moves, the faster the flow.
A force sensor measures forces and torque exerted. These sensors usually contain strain gauges and can communicate information required for force measurements. Force sensors can be mechanical, hydraulic, or electrical strain gauges.
- Mechanical: These function similarly to the operation of a normal scale, where a spring moves under the application of force. The deflection of a spring is directly proportional to the applied force, so the movement is shown on a scale.
- Hydraulic: Often referred to as hydraulic load cells. The cell contains liquid, which becomes pressurized when a force is applied. The sensor takes the measurement by displaying the pressure on a dial.
- Strain gauge: This metal cylinder is compressed under application of a force. The contraction in the cylinder can be measured, as the force causes increased resistance measured by an applied electrical current.
A temperature sensor converts the temperature into another quantity such as mechanical movement for a dial or an electric voltage.
- Thermocouple: Thomas Seebeck discovered that when any conductor gets subjected to a thermal gradient, it generates a voltage. Thermocouples are usually wires insulated from each other with plastic or glass fiber materials.
- Liquid expansion: These sensors work as thermometers that can be filled with mercury or an evaporating fluid used in refrigerators. Temperature changes produce expansion or evaporation of the liquid, so the sensor becomes pressurized. The change is shown on a simple pressure gauge.
- Bimetallic: When two metals are rigidly joined together as a two-layer strip and heated, the difference in the expansion rate between the two metals causes the strip to bend. For sensors on the production line, the strip is twisted into a long thin coil inside a tube. One end is fixed at the bottom of the tube and the other turns and moves a pointer on a dial.