1. Used for Switch Control
PLC (Programmable Logic Controller) technology is highly capable in controlling switch quantities. The number of input and output points it can control ranges from tens to thousands or even tens of thousands, thanks to its networking capabilities, which virtually remove any limitations on the number of control points. PLCs can handle various logical problems, including combination, timing, instant, delayed, non-counting, counting, fixed-sequence, and random operations.
PLC hardware is flexible, and software programs are programmable, making it extremely adaptable for control purposes. Multiple sets or groups of programs can be written and invoked as needed, making it suitable for industrial sites with diverse operating conditions and states.
Examples of using PLCs for switch control are abundant in industries such as metallurgy, machinery, light industry, chemical industry, and textiles. Currently, the primary target of PLCs, unmatched by other controllers, is their convenience and reliability in switch control.
2. Used for Analog Control
Analog quantities like current, voltage, temperature, and pressure continuously vary. In industrial production, particularly in continuous production processes, these physical quantities often need to be controlled.
As an industrial control electronic device, PLCs have undergone significant development to control these quantities. Not only large and medium-sized machines but even small ones can now perform analog control. PLCs configured with A/D (Analog to Digital) and D/A (Digital to Analog) units can handle this control. These units are special I/O units that convert analog signals to digital signals and vice versa.
The A/D unit converts external analog signals into digital signals for the PLC, while the D/A unit converts digital signals from the PLC into analog signals for external circuits. These special I/O units have features such as anti-interference circuits, internal and external circuit isolation, and exchange of information with input/output relays.
With the addition of A/D and D/A units, the remaining processing involves digital quantities, which is manageable for PLCs with data processing capabilities. Medium and large PLCs can perform complex calculations such as addition, subtraction, multiplication, division, square root extraction, interpolation, and floating-point operations. Some even have PID instructions for proportional, differential, and integral control, making it feasible to use PLCs for analog control.
PLCs can achieve high-quality control by combining A/D and D/A units and using PID or fuzzy control algorithms. This dual capability of controlling both analog and switch quantities simultaneously sets PLCs apart from other controllers, though in purely analog systems, regulators might offer a better performance-price ratio.
3. Used for Motion Control
Physical quantities in practical applications include switch quantities, analog quantities, and motion control, such as the displacement of machine tool components often represented by digital quantities. Effective motion control can be achieved through NC (numerical control) technology, which originated in the 1950s in the United States and is now widespread and advanced. In developed countries, the numerical control ratio of metal-cutting machine tools exceeds 40%-80%, or even higher.
PLCs, based on computer technology, can receive count pulses at frequencies ranging from several kHz to tens of kHz and can handle these pulses in various ways. Some PLCs can also output pulses at similar frequencies, making them capable of implementing NC principles for control if equipped with the appropriate sensors (such as rotary encoders) or pulse servo devices.
High and medium-end PLCs have developed NC units or motion units capable of point control and curve interpolation, enabling control of curve movements. New motion units have even introduced NC programming languages for more efficient PLC digital control.
4. Used for Data Acquisition
With the development of PLC technology, data storage capacity has increased significantly. For example, PLCs from the Devision company have a data storage area (DM area) of up to 9999 words. This vast storage capacity allows for extensive data collection. Data can be collected using counters to accumulate and record pulse counts, which are then periodically transferred to the DM area. Data acquisition can also utilize A/D units to convert analog quantities to digital, which are then stored in the DM area periodically.
PLCs can be equipped with small printers to regularly print out data from the DM area. They can also communicate with computers, allowing the computer to read and process the data, effectively making the PLC a data terminal for the computer.
For instance, electricity users have employed PLCs to record real-time electricity usage, enabling different billing methods for different usage times, encouraging users to use electricity during off-peak hours, promoting rational and economical electricity usage.
5. Used for Signal Monitoring
PLCs have numerous self-check signals and internal devices, many of which are not fully utilized by users. These can be effectively used for monitoring the PLC’s operation or the control object. In complex control systems, particularly automated control systems, monitoring and self-diagnosis are crucial. They can reduce system failures, make fault finding easier, increase the mean time between failures, reduce repair times, and enhance system reliability.
6. Used for Networking and Communication
PLC networking and communication capabilities are strong, with new networking structures continually being introduced. PLCs can connect and communicate with personal computers, allowing computers to assist in programming and managing PLC control, making them easier to use.
A single computer can control and manage multiple PLCs, up to 32, or a single PLC can communicate with multiple computers to exchange information for comprehensive control system monitoring. PLCs can also communicate with each other, either one-to-one or in groups of several to hundreds.
PLCs can network and communicate with intelligent instruments and devices (such as variable frequency drives), exchanging data and cooperating for mutual operation. They can form remote control systems with ranges of up to 10 kilometers or more, creating local networks incorporating high-end computers and various intelligent devices. Networking can use bus or ring topologies, with networks capable of nesting and bridging.
Networking and communication capabilities of PLCs meet the needs of modern computer-integrated manufacturing systems (CIMS) and intelligent factory developments, enabling point-to-line-to-area control, linking device-level control, production line control, and factory management into a cohesive whole, thereby creating higher efficiencies. This promising future is increasingly evident to our generation.
