Fail-safe Drive Speed Monitoring via Substitute Speed Plausibility Checks
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Solution Overview
Problem
Existing fail-safe speed monitoring solutions for drives are complex and expensive, relying on special safety speed encoders or external hardware, which limits their accessibility and increases costs, and struggle to detect certain encoder errors like constant slip between encoder and encoder shaft.
Innovation Solution
A method that calculates a substitute speed using existing hardware and performs three plausibility checks between setpoint speed, actual speed, and substitute speed within a safety program, ensuring high safety integrity levels without requiring special hardware, and allows detection of encoder errors by checking for continuous changes and tolerance in differences between values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If special safety speed encoders or external hardware are used for fail-safe speed monitoring, then safety integrity level is improved, but device complexity and cost increase
Solution Approach 1:
The patent replaces mechanical/specialized hardware safety systems (safety speed encoders, external hardware) with a software-based solution. The safety CPU performs plausibility checks using calculated substitute speed derived from standard converter output frequency, eliminating the need for specialized safety hardware while maintaining safety integrity.
Solution Approach 2:
The patent creates a software-based substitute speed value that replicates the function of specialized safety encoders. By calculating speed from standard converter parameters and performing plausibility checks, the system creates a virtual copy of safety monitoring functionality without requiring physical safety components.
2Reliability
If special safety speed encoders or external hardware are used for fail-safe speed monitoring, then safety integrity level is improved, but cost increases
Solution Approach 1:
The patent uses inexpensive, readily available standard converters and safety CPUs instead of expensive specialized safety encoders. The solution leverages common industrial components that can be obtained from standard catalogs, significantly reducing system cost while achieving the same safety monitoring function.
Solution Approach 2:
The patent replaces expensive specialized hardware with software processing in a standard safety CPU. The substitute speed calculation and plausibility checks are performed through software algorithms using data already available from the converter, eliminating the need for costly safety-certified encoder hardware.
3Measurement precision
If encoder-independent substitute speed calculation is performed, then detection of encoder errors is improved, but calculation complexity increases
Solution Approach 1:
The patent implements feedback by comparing the encoder-derived actual speed with the converter-derived substitute speed through plausibility checks. This feedback mechanism detects encoder errors by identifying deviations between the two speed values, with the safety CPU monitoring and responding to discrepancies.
Solution Approach 2:
The patent introduces an intermediary substitute speed calculation that acts as a reference against which encoder measurements are validated. The substitute speed, derived independently from converter parameters, serves as a mediator to detect encoder failures without requiring complex additional sensing hardware.
4Reliability
If three plausibility checks are performed between setpoint speed, actual speed and substitute speed, then safety integrity level is improved, but processing time increases
Solution Approach 1:
The patent implements periodic plausibility checks within the safety program cycle, performing the three comparisons (setpoint vs actual, actual vs substitute, substitute vs setpoint) at regular intervals. This periodic execution ensures timely detection of speed deviations while maintaining deterministic timing suitable for safety applications.
Data Source
Figure 1
AI summary
The invention relates to fail-safe monitoring of the speed of a drive, which has at least a converter, a motor, and a rotational speed sensor, wherein a target rotational speed is specified to the drive and an actual rotational speed is sensed by means of the speed sensor. In order to achieve a high safety integrity level while avoiding the disadvantages of the known solutions, according to the invention, a substitute rotational speed is calculated and three plausibility checks are performed in a safety program of a safety CPU, wherein in each case two of the following three values are checked: target rotational speed, actual rotational speed, and substitute rotational speed. Because the calculated substitute rotational speed is independent of the sensor, even sensor errors that are difficult or impossible to detect by means of prior hardware solutions can be detected. The substitute rotational speed can be determined, for example, from the initial frequency of a converter or from the quotient of EMF and magnetic flux.