PLC Variable Reading Within the Shortest Task Cycle
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Solution Overview
Problem
Industrial control devices face challenges in ensuring synchronization of variable values when queried collectively during concurrent and periodic tasks, leading to inconsistent responses.
Innovation Solution
A control device that accepts requests to query updated variable values and reads them within the cycle of the shortest task, ensuring synchronization by completing the reading process before the next task instance, thereby outputting synchronized values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If an industrial control device carries out a plurality of tasks concurrently and periodically, then productivity is improved, but synchronization of variable values cannot be ensured when queried collectively
Solution Approach 1:
The system determines the reading period in advance based on task cycle information before actually reading variable values. This preliminary calculation of the reading period (Step S102) ensures that subsequent readings will be synchronized by confining them within a single shortest task cycle, preventing inconsistent data collection across multiple task instances.
Solution Approach 2:
The system uses feedback from task cycle information to dynamically adjust the reading period. By calculating the reading period based on the shortest task cycle (Step S102) and monitoring whether the current time falls within this period (Step S103), the system ensures synchronized readings only when all task instances are in a consistent state, resolving the synchronization issue while maintaining concurrent execution.
2Loss of time
If variable values are read during concurrent task execution, then response time is improved, but inconsistency in variable values occurs
Solution Approach 1:
The system introduces an intermediary mechanism (the reading period determination based on shortest task cycle) between the query request and the actual variable reading. This intermediary layer ensures that readings are performed only during a validated time window when synchronization is guaranteed, preventing inconsistent data collection while maintaining efficient response times.
Solution Approach 2:
The system performs preliminary validation by determining whether the current time falls within the calculated reading period (Step S103) before proceeding to read variable values. This preliminary check ensures that only synchronized readings are performed, maintaining measurement precision while enabling fast response when conditions are met.
3Reliability
If reading of all variable values is completed within the shortest task cycle, then synchronization is ensured, but device complexity increases
Solution Approach 1:
The system changes the temporal parameter (reading period) based on task cycle characteristics. By calculating the reading period from the shortest task cycle information (Step S102) and using this parameter to constrain all variable readings, the system ensures synchronization without requiring complex coordination logic across multiple tasks. The parameter change approach simplifies the control logic while maintaining reliability.
Data Source
AI summary
To read respective values as updated of a plurality of variables synchronization of which respective values is ensured while tasks are being carried out in a multi-tasking manner, a PLC 10 reads respective values as updated of variables A to C in such a manner as to complete reading the respective values as updated of the variables A to C during a time period from (i) a time point of a start of a single instance of a cycle of a task which cycle is shortest to (ii) a time point of an end of the single instance of the cycle.