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The technical details of the system are displayed in a structured format. The main section includes an advanced display box that contains image windows and thumbnails for visual reference. The image window shows a picture of a photocoupler, while the thumbnail provides a smaller preview.
In analog signal acquisition, the quality of the signal itself is crucial. This includes ensuring the power supply for sensors and meters is stable. Analog transmission lines should avoid proximity to high-voltage cables or sources of interference like high-frequency welding machines, medium-frequency heating furnaces, or motor cables from inverters. Otherwise, unwanted noise may be introduced into the real signal, potentially contaminating the source.
Proper grounding is essential during the design phase of an electrical system. Poor grounding can cause interference, leading to malfunctions in the PLC system, or even damage to sensors and analog modules. If these precautions aren’t taken during design, additional efforts must be made later to resolve issues.
For example, if the PLC’s analog input module isn’t isolated, the circuits need special attention. If a sensor is connected to a high-voltage signal, it could risk damaging the PLC host. Also, when using high-speed sampling modes (which can detect changes as small as 0.25ms), the system becomes very sensitive. This sensitivity can cause problems if there is interference or poor shielding. Increasing filtering time alone might not solve the issue. In such cases, an RC filter loop is often necessary, even though it requires extra work.
Many systems controlled by PLCs require measuring various analog voltage or current signals. Traditionally, voltage/current sensors were used alongside PLC analog expansion modules. However, this setup is prone to errors in environments with strong electromagnetic interference. Plus, analog expansion modules are expensive and have limited input points, making them less cost-effective.
A better solution involves using VFC (Voltage-to-Frequency) or IFC (Current-to-Frequency) converters. These devices convert analog signals into pulse outputs, which are more resistant to interference. They also allow direct connection to the PLC’s high-speed counter inputs.
The CPU224 model has six high-speed counters (HSC0–HSC5), each with multiple operating modes. When using a high-speed counter, the first step is to define its mode using the HDEF instruction. Once defined, dynamic parameters can be programmed. Each counter has a control byte that manages functions like enabling counting, setting direction, and loading values.
The V/F sensor converts an analog voltage signal into a rectangular pulse at a fixed ratio. The VFC or IFC transmitter then sends the pulse signal to the high-speed counter HSC1, where it accumulates pulses over a set interval. The measured voltage is calculated based on the number of pulses collected within that time.
In programming, the main routine calls a subroutine (SBR0) during the first scan cycle. The subroutine initializes the high-speed counter and timer interrupt. An interrupt routine (INT0) runs every 100ms to evaluate the counter value, reset it, and calculate the measured voltage using the conversion formula.
This method has been successfully applied in several automation projects, proving to be accurate, reliable, and resistant to interference.
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