Inertia Estimation Using Gravity Torque Offset
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Solution Overview
Problem
Existing methods fail to accurately estimate inertia in machine structures where the influence of gravity varies with the motor's position, leading to incorrect calculations due to changing torque requirements.
Innovation Solution
An inertia estimating method and apparatus that determine acceleration/deceleration conditions to achieve symmetric cumulative gravity torque about a position where gravity torque is zero, allowing for accurate inertia calculation by offsetting gravity torque during torque command value accumulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the motor is driven at different fixed rates in different directions to measure torque required to cancel friction and gravity, then torque measurement is possible, but the inertia estimation becomes time-consuming
Solution Approach 1:
The patent applies periodic action by driving the motor in reciprocal movements (forward and backward directions) at constant velocity. This periodic driving pattern allows the system to collect torque data at multiple positions efficiently, enabling both friction and gravity torque measurements within a single measurement cycle rather than requiring separate measurements at different fixed rates.
2Device complexity
If the torque required to cancel gravity is assumed to be constant at any motor position, then calculation is simplified, but inertia estimation becomes inaccurate for machine structures where gravity influence varies with position
Solution Approach 1:
The patent applies asymmetry by recognizing that gravity torque varies asymmetrically with motor position in certain machine structures. Instead of assuming constant gravity torque, the system measures torque at multiple positions including forward and backward directions, capturing the asymmetric variation of gravity torque. This allows accurate inertia estimation even when gravity influence changes with position, resolving the contradiction between calculation simplicity and measurement accuracy.
3Power
If acceleration/deceleration driving is performed to calculate inertia from torque and acceleration relations, then inertia can be determined, but the changing gravity torque at different positions causes calculation errors
Solution Approach 1:
The patent applies the taking out principle by separating the gravity torque component from the total torque measurement. By performing constant velocity driving first, the system extracts and characterizes the gravity torque at various positions. This extracted gravity torque information is then used to correct the acceleration/deceleration measurements, removing the harmful effect of varying gravity torque from the inertia calculation and enabling accurate results.
Data Source
AI summary
A torque command value and an acceleration detection value are accumulated when a driven portion is subjected to acceleration/deceleration driving, and, from a ratio between the two, inertia of a movable portion is calculated. By executing the acceleration/deceleration driving about a position where influence of gravity torque is zero, the influence of the gravity torque included in the torque command value before and after the center position is offset, whereby inertia can be estimated correctly even with a machine structure in which the influence of gravity differs depending on the position of the motor.