PLC CPU Scheduling Motion Control Cycles
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Solution Overview
Problem
In PLC systems, integrating motion calculation programs with microprocessors is challenging due to limited execution cycles, where it is unclear how to sequence input and output processing to execute user programs and motion calculations efficiently, leading to potential delays and inefficiencies in motor control operations.
Innovation Solution
A CPU unit is configured to execute user programs and motion calculation programs in synchronization, with a scheduler program managing the sequence of output and input processing to ensure precise motor control, allowing for the calculation and output of motion instruction values within shorter cycles, and utilizing secondary user programs to optimize processing during remaining execution times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If motion control function is integrated into PLC microprocessor, then device complexity is reduced, but execution cycle time increases due to additional processing requirements
Solution Approach 1:
The patent segments the execution cycle into distinct phases: input processing phase, calculation processing phase, and output processing phase. Each phase is assigned specific tasks and executed in sequence, allowing the system to manage multiple functions (PLC control and motion control) without excessive delay. The scheduler divides the cycle time into segments allocated to different processing types.
Solution Approach 2:
The patent implements dynamic scheduling where the allocation of execution time to different processes varies based on system state and priority. The scheduler dynamically adjusts the timing and duration of input processing, calculation processing, and output processing phases to optimize overall system performance while meeting real-time requirements.
2Reliability
If all PLC processes are executed in each execution cycle, then processing completeness improves, but execution speed decreases due to limited cycle time
Solution Approach 1:
The patent segments processing into mandatory per-cycle tasks (input processing, motion calculation, output processing) and optional remaining-time tasks (low-speed sequence processing, non-periodic motion control). This ensures critical functions are completed every cycle while less time-sensitive functions can be executed when time permits.
Solution Approach 2:
The patent applies partial action by executing only the essential processing tasks within each execution cycle (input processing, calculation processing, output processing), while deferring less critical tasks to remaining time or subsequent cycles. This ensures timely completion of critical functions without attempting to complete all possible processing in every cycle.
3Measurement precision
If input and output processing are performed frequently for motion control, then control precision improves, but processing load increases
Solution Approach 1:
The patent applies local quality by performing high-frequency input and output processing specifically for motion control functions where precision is critical, while using lower-frequency processing for general PLC sequence control. The scheduler allocates processing resources differently based on the specific function and its precision requirements.
Solution Approach 2:
The patent implements periodic action by executing input processing and output processing at fixed intervals within each execution cycle, with motion control processing occurring at a higher frequency than general PLC processing. This periodic structure ensures consistent control precision while managing processing load through predictable timing patterns.
Data Source
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AI summary
A scheduler program includes a command to start execution of a motion calculation program in each motion control cycle, a command to cause a microprocessor to start execution of a first user program in each first user program execution control cycle, and a command to repeat output processing and input processing, start of execution of the first user program, end of execution of the first user program, and start of execution of the motion calculation program in this sequence for every period corresponding to the period of the first user program execution cycle.