Easeus Solutions.

PLC: PROGRAMMABLE LOGIC CONTROLLERS

The PLC, is the heart of industrial automation. Essentially, it is a computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, perfect reproducibility, ease of programming, and process fault diagnosis.

Dick Morley is considered as the father of PLC as he had invented the first PLC, the Modicon 084, for General Motors in 1968. Initially, PLCs were a means of replacing contactors, but they have now become very powerful devices that can handle very complicated logic.

The PLC works in a cycle; read inputs -execute program -write outputs. They offer advantages and increases the reliability, flexibility, and accuracy of the automation system in the following ways:

• Very fast execution especially for repetitive tasks (typically less than 15ms cycle scan times)
• Facilities in fault finding and diagnostic
• They can handle very compilicated logic with ease (e.g. P.I.D control systems and other closed loop controls, timers, counters, comparators, serial and ethernet data connections etc.)
• They are very flexible in that, the logic can be reprogrammed easily without need for rewiring
• They are relatively very cheap automation solutions, especially the compact PLCs
• They are very rugged unlike other micro-controllers and are made to survive most industrial environments. They can easily sustain operation in a robust environment with little maintenance
• Low power consumption and easy to mantain dure to their modular construction (no need to replace the whole unit)
• They can easily be coupled to other computers and processes, giving it supervisory and control capabilitiies
• They consume very little space when compared to other items like contactors and timers
• Very easy to wire and require a lot less wiring

REMOTE TERMINAL UNIT

A Remote Terminal Unit - RTU, also known as a Remote Telemetry Unit or Remote Telecontrol Unit, is a microprocessor based device that monitors and controls field devices, that then connects to plant control or SCADA (supervisory control and data acquisition) systems. It is more or less does the same function as a PLC but it has the capability of pulling data from a lot more devices with the aim being monitoring and supervision on SCADA. It is generally easier to configure than the the PLC which needs standard technical programing. They are usually more expensive than the PLC.

The programming language used in RTU is different, with basic, visual basic, C#, and C++ being common. Most of the time it doesn't need a lot of coding. The user can program it very easily by entering simple configurations that correspond to inputs and desired outputs. Newer RTUs are now incoporating web servers within then for faster configuration and troubleshooting. Configuration methods are not standard and comunication potocols are both generic and proprietary.

RTUs usually have a fixed number of inputs and inputs and are rarely expandable unless a remote I/O is used. However, they can feature several inputs and output ports including LAN, RS-485, analog inputs and outputs, and discrete inputs and outputs among other specialized ports. When choosing an RTU pick one that leaves some little room for future inputs and outputs because you will need it some day.

DISTRIBUTED I/O AND REMOTE I/O

Mantaining a centrilized control of a big system can prove to be a design and logistical nightmare. Distributed I/O and Remote I/O can eleviate this problem. A communication cable (Ethernet or RS-485) is usually run back to the master device (PLC or SCADA/HMI).

Distributed I/O systems include small field devices with a wide range of I/O options. These can include digital and analogue channels, temperature measurements, and counter inputs. These modular devices provide a flexibility where devices like the Programmable Logic Controllers (PLCs) would require high density channel count and long cable runs. The beauty of the distributed approach is that it allows for short cable runs for signals to the distriuted i/o which eradicates signal conditioning, with the added benefit of standard signal inputs for specialised sensors. The master device (usually a PLC) is connected via an Ethernet or RS-485 cable for communication.

Remote I/O refers to electronic devices that use transmission technology to send and receive input and output signals to/from master electronics like DCS, PLC and PCs often in the fields of process or factory automation. They achieve the same Communication is usually achieved through an Ethernet or RS-485 cable.

Strictly speaking, the only difference between a distribute I/O and a remote I/O is that distributed I/O usually have a brain that can gives it a computing capacity, while remote I/O do not compute.

INSTRUMENTATION

Instrumentation is a collective term for measuring instruments that are used for indicating, measuring and recording physical quantities.

Instruments can be categorized as discrete (on-off), or analog (variable signal). Analog instruments are usually accompanied with a transmitter (also known as a transducer) that converts the measurement (usually within a very small range) into a scaled electrical signal for reading and displaying.

The following sensors are typically used in industrial measurable variables:

• Proximity sensors (inductive, capacitive, optical, limit switchse etc)
• Temperature sensors (RTD, Thermo-couple, thermistors etc)
• Load cells (weight sensors)
• Humidity sensors
• Flow rate sensors
• Pressure sensors
• Level sensors (optical, ultrasonic, radar, float switches etc.)
• Tachometers (speed sensors)
• Rotary and linear encoders (position sensors)
• pH sensors
• Moisture sensors
• Light sensors
• Electrical sensors: power, voltage, current, frequency
• Color sensors and machine vision cameras
• etc.

Calibration

Another important part of instrumentation is calibration. Calibration is the comparison, adjustment and trimming of the instruments in order to ensure that the value that we read is true and accurate to the highest degree. This is typically done within the transmitter and a calibration document/certificate is issued and the end of the calibration process.

Apart from from calibration, instrumentation engineers have to deal with a miriad of problems that could lead to faulty signals. This include loss of signal, precision, noise e.g. electromagnet interference - EMI, and ground loops among others.

Integration in automation

Without instrumentation, our PLCs would not be able to tell the current state of the process we want to control. Instruments measure process values and convert them into electrical signals. Often, this electrical signals can be converted and displayed, by the instrumentation devices themselves, into values humans can understand. The electrical signals can also be wired to the PLC and used to automatically drive open loop and closed loop configurations for highly effective control systems.

If our instruments are far from our controllers, repeaters and gateways can be used to boost the signal. The signal is usually a current loop or voltage signal, but Some transmitters use other protocols e.g. HART to communicate. All the devices, and calibration involved with measuring field parameters - all the way to converting the signal into a form human beings and machines can understand - is what is known as instrumentation.

CONTROL DEVICES

Control devices are output devices which work together with sensors. Data gathered by a sensor is input into a PLC which then decides what to do with the data (processes it). It then sends instructions to the control device to perform a certain action e.g. motion control/.

Actuators are usually situated between the PLC and the control device, in order to transform the electrical signal from the controller into a physical action by the control devices e.g. contactors, pilot valves, motors, relays, solenoids, inverters & convertors, etc. Basically, an actuator helps to convert a signal for the control device to achieve the desired motion or output.

Examples of control devices include:

• Motors, Conveyors, Fans and pumps
• Variable frequency drives and servo drives
• Linear actuators
• Hydraulic and Pneumatic pistons
• Control valves
• Heaters
• Lights and lamps
• Buzzers
• Robot arms
• Positioners and ejectors
• Displays
• etc.

Each of these control devices are switched on and off by an actuator.

MACHINE VISION

Machine vision (MV) is the technology and methods used to provide imaging-based automatic inspection and analysis for such applications as automatic inspection, process control, and robot guidance. These technologies and methods are used to extract information from an image on an automated basis rather than processing the image as is common in computer software. The information extracted can be a simple good-part/bad-part signal, or more a complex set of data such as the identity, position and orientation of each object in an image. The information can be used for such applications as automatic inspection and robot and process guidance in industry, for security monitoring and vehicle guidance.

• Object recognition
• Pattern recognition
• Materials inspection
• Optical character and handwriting recognition
• Electronic component analysis
• Color recognition

Hence, with machine vision, repetitive simple inspection can be automated, including: bad part ejection, bottle filling levels, correct labeling and capping, object recognision, object orientation, vehicle plate character recognition among others.