Fail-Safe Analog Measurement Acquisition Using Redundant Sensors
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Solution Overview
Problem
There is a lack of certified solutions for fail-safe acquisition of measured values, particularly for analog values like 4 to 20 mA, using standard Siemens SIMATIC modules such as ET200S or ET200SP, which are not designed for functional safety, limiting their use in safety-relevant industrial applications.
Innovation Solution
A method and automation system that utilize two detection means with analog-digital converters to record redundant analog values and perform a plausibility check within a safety program, allowing for error detection and reparameterization of the second detection means based on the first, ensuring fail-safe measurement acquisition by comparing values and initiating error actions when deviations exceed predetermined limits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If standard modules (ET200S or ET200SP) are used for acquiring measured values, then device complexity and cost are reduced, but functional safety certification is not achieved
Solution Approach 1:
The system divides the measurement acquisition into two independent channels: a first acquisition device for normal operation and a second acquisition device for safety-critical measurements. This segmentation allows each device to be optimized independently, with the second device being reparameterizable to adapt to different safety requirements while maintaining overall system functionality.
Solution Approach 2:
The second acquisition device is designed to be reparameterizable, allowing its measurement range and parameters to be dynamically adjusted. This enables the same hardware module to adapt to different safety certification requirements and measurement scenarios, providing functional safety certification capability without requiring completely different hardware for each application.
2Reliability
If redundant acquisition devices are used for error detection, then measurement reliability is improved, but device complexity increases
Solution Approach 1:
The second acquisition device serves multiple functions: it acts as a primary measurement device, a safety monitoring device, and a reparameterizable adapter for different safety standards. This multi-functionality reduces the need for completely separate redundant systems, as the same device can operate in different modes depending on the safety requirements.
Solution Approach 2:
The system implements a feedback mechanism where the safety program continuously monitors measurements from both acquisition devices, compares them for plausibility, and can trigger reparameterization of the second device based on detected conditions. This feedback loop enables dynamic error detection and correction without requiring permanent complex hardware configurations.
3Adaptability or versatility
If the second detection means is reparameterizable, then adaptability to different measurement ranges is improved, but measurement precision may be affected
Solution Approach 1:
The system performs preliminary parameter configuration and calibration of the second acquisition device before actual measurements are taken. The safety program pre-establishes the appropriate parameter settings based on the measurement scenario, ensuring that when reparameterization occurs, the device is already optimized for the required measurement range and precision requirements.
Data Source
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AI summary
A method for the error-free acquisition of a measured value (AO) in a control unit (10) (PLC), wherein a first and a second measured value (AI1,AI2) are acquired by a first and a second sensor or transducer (17,18) and with a first and second acquisition means (11,12), wherein the measured values (AI1,AI2) are compared and an error is detected in the event of a deviation, wherein the acquisition means (11,12) are parameterized to measuring ranges (M1,M2) and the second acquisition means (12) is alternately reparameterized, and wherein a plausibility check between the measured values (AI1,AI2) is additionally carried out and an error is also detected in the event of a negative result.