Hoisting Motor Load Measurement via Current Calibration
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Solution Overview
Problem
Material handling systems face challenges in accurately measuring load weights due to external weighing devices' high costs and inaccuracies caused by additional forces during load movement, and current measurement systems require motor to operate at specific speeds for accurate readings, limiting continuous load monitoring.
Innovation Solution
A system where a motor drive controls the hoisting motor by storing current or torque values during a commissioning run and comparing them during subsequent operations to determine if the load exceeds the rated capacity, allowing for periodic weight monitoring without external devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external weighing devices (load cells) are added to measure load weight, then measurement capability is provided, but device complexity and cost increase
Solution Approach 1:
The motor drive system performs load measurement functions using its own existing components (current sensors, processors, and control circuits) without requiring external weighing devices. The system leverages the relationship between motor current and load weight to self-determine load conditions, eliminating the need for separate measurement devices and reducing overall system complexity.
Solution Approach 2:
The patent replaces mechanical weighing devices (load cells) with an electrical measurement approach. By measuring the electrical current drawn by the motor and correlating it to load weight through pre-stored calibration data, the system substitutes a mechanical measurement system with an electrical control system, thereby reducing device complexity while maintaining measurement capability.
2Productivity
If load measurement is performed during motor operation, then continuous monitoring is achieved, but measurement accuracy decreases due to acceleration and deceleration forces
Solution Approach 1:
The system performs preliminary calibration during commissioning by storing the relationship between motor current and load weight at various operating speeds before normal operation begins. This pre-established data table allows the system to accurately determine load weight during dynamic operation by referencing the pre-stored speed-compensated current values, eliminating the need to pause operation for measurement.
Solution Approach 2:
The patent implements a dynamic measurement approach where the system continuously monitors both motor current and motor speed, then uses the speed information to select the appropriate calibration reference value from the pre-stored table. This dynamic compensation method allows accurate load measurement throughout the entire operating speed range, transforming the system from static to dynamic measurement capability.
3Device complexity
If current measurement is used to determine load weight, then cost is reduced, but measurement accuracy is affected by motor speed variations
Solution Approach 1:
The system compensates for speed variations by changing the reference parameter used for comparison. During commissioning, calibration data is stored for multiple speed points. During operation, the system adjusts which calibration value is used based on the current motor speed, effectively compensating for speed-related measurement errors while maintaining the simplicity of current-based measurement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables continuous, accurate load monitoring within the material handling system, preventing overload conditions by comparing stored values with real-time measurements, thus ensuring safe operation and reducing costs associated with external weighing devices.
Implementation Method 1
a current sensor configured to generate a signal corresponding to an amplitude of current output to the motor
Implementation Method 2
A motor in a material handling system is used to raise and lower the load
Data Source
AI summary
A system and method for determining the load present in a material handling system is disclosed. A commissioning routine executing on a motor drive controls operation of a hoisting motor with a known load present. The commissioning routine stores values of current provided to the motor or torque generated by the motor at various operating speeds in memory on the motor drive. During subsequent runs of the motor drive, the value of the current/torque is monitored as a function of the speed of the motor and compared to the stored values. The resulting value of the load may be monitored and action taken if the value of the load exceeds the rated capacity of the material handling system. Optionally, the measured load may be displayed to an operator.