Monitor System for PLCs Using Virtual Variables
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Solution Overview
Problem
Existing monitor systems for programmable controllers (PLCs) are limited in their ability to monitor new data involving calculations between variables after the PLC has entered its operating stage, as adding or modifying the user program can increase cycle time and cause operational errors.
Innovation Solution
A monitor system that includes a data collecting unit connected to the PLC and a monitor device, which uses virtual variables and calculation formulas to display real and virtual variable data as waveforms, allowing for expanded monitoring without modifying the PLC's user program or increasing cycle time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the user program of the PLC is added or modified to monitor new data involving calculations between variables, then the monitoring capability is improved, but the cycle time increases and operational errors may occur
Solution Approach 1:
The system separates the monitoring function from the control function by introducing a dedicated monitor device that connects to the PLC through the bus. The PLC continues to execute its control program cyclically, while the monitor device independently collects data and performs calculations for display purposes, thus segmenting the functions to avoid cycle time increase.
Solution Approach 2:
A monitor device acts as an intermediary between the PLC and the user. It collects control data from the PLC memory through the bus, performs calculations on variables, and displays results without requiring modifications to the PLC's user program. This intermediary approach enables extended monitoring capability while preserving the original control cycle integrity.
2Adaptability or versatility
If the user program is modified to add new variables and calculation formulas, then new data monitoring is enabled, but the risk of operational errors increases
Solution Approach 1:
The calculation and monitoring functions are extracted from the PLC's user program and relocated to the monitor device. This extraction eliminates the risk of introducing errors into the control program while maintaining the ability to monitor calculated variables. The PLC's control logic remains unchanged and verified, preserving operational reliability.
Solution Approach 2:
The monitor device creates a copy of the necessary control data from the PLC memory and performs calculations on this copied data. This copying approach allows the monitor to display calculated variables without affecting the original control program execution, thereby maintaining system reliability while enabling flexible monitoring.
3Adaptability or versatility
If new variables are defined and calculation formulas are added in the PLC user program, then the desired data can be monitored, but the device complexity increases
Solution Approach 1:
The system adds a new dimension to the monitoring architecture by introducing a separate monitor device that operates parallel to the PLC control system. This dimensional expansion allows calculation and display functions to be added without increasing the complexity of the PLC's control program, as the monitor device handles these functions independently.
Data Source
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AI summary
A monitor system is formed with a programmable controller for executing a user program, a data collecting unit connected to the programmable controller for collecting variable data of the programmable controller and a monitor device connected to the data collecting unit through a communication line. The functions of the system include that of storing a preliminarily defined virtual variable apart from the user program of the programmable controller and from the variable data of the programmable controller and a calculation formula using one or more real variables for obtaining the virtual variable, that of collecting real variable data in the stored calculation formula and any other real variables from the programmable controller, that of carrying out a calculation according to the stored calculation formula based on the collected real variable data, and that of displaying correspondence of the collected real variables and the result of the calculation as waveforms on a display device. Some of these functions are carried out by the data collecting unit, the rest being carried out by the monitor device.