In the field of industrial automation, precise analog signal acquisition is crucial for reliable system performance. To ensure accurate data collection, it's essential that the signal itself is clean and stable, including the power supply for sensors and measuring instruments. Analog transmission lines should be carefully routed to avoid proximity to strong electrical cables or sources of interference such as high-frequency welding machines, medium-frequency heating furnaces, or inverter output cables to motors. Otherwise, unwanted noise can be introduced into the signal, potentially corrupting the source.
Proper grounding is a critical consideration during the design phase of any electrical system. Poor grounding can lead to operational issues in PLC systems, ranging from minor disturbances to severe damage to sensors or analog modules. If these precautions are not addressed during the design stage, additional efforts may be required later to resolve problems.
For instance, when using an analog input module in a PLC, it's important to consider isolation between the PLC loop and the analog signal. Without proper isolation, connecting the sensor to a high-voltage circuit could risk damaging the PLC. Additionally, if the PLC uses a high-speed sampling mode—capable of detecting changes as small as 0.25ms—it becomes highly sensitive, which can cause issues with signal integrity in the presence of interference or poor shielding. Simply increasing filtering time may not be sufficient; in some cases, an RC filter is necessary to suppress noise, even though it requires more effort to implement.
Many PLC-controlled systems require the measurement of various analog voltage or current signals. Traditionally, voltage/current sensors were used alongside analog expansion modules for processing. However, this approach can be costly, as analog modules tend to be expensive, and their input points are limited. In environments with strong electromagnetic interference, large measurement errors can occur.
An alternative solution involves using a Voltage-to-Frequency (V/F) or Current-to-Frequency (I/F) converter. These devices convert the analog signal into a pulse train, which is less susceptible to interference and can be directly connected to a PLC’s high-speed counter. This method improves accuracy and reliability while reducing costs.
The CPU224 model, for example, has six high-speed counters (HSC0–HSC5), each capable of operating in multiple modes. When using a high-speed counter, the first step is to select an appropriate mode based on system requirements. This can be done using the HDEF instruction, which defines the counter’s operation. Each counter has a control byte that manages enable/disable functions, direction, and value updates.
A V/F sensor converts an analog voltage signal into a rectangular pulse at a fixed ratio. The pulse output from the VFC or IFC transmitter is then fed into the high-speed counter, where the number of pulses is accumulated over a set interval. The measured voltage or current is calculated based on the pulse count and a known conversion factor.
Programming logic typically begins with initializing the high-speed counter and timer interrupt in the first scan cycle. The main program calls a subroutine (SBR0), which sets up the counter configuration and enables interrupts. The high-speed counter is initialized with a specific mode, and its current and preset values are configured. A timer interrupt is set to trigger every 100 milliseconds, during which the counter value is read, reset, and processed to determine the actual signal level.
This method has been successfully applied in several automation projects, demonstrating high precision, strong immunity to interference, and reliable operation in real-world conditions.
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