Numerical Controller Feed Rate Override for Constant Spindle Load

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Solution Overview

Problem

Existing PID control methods for maintaining constant spindle load during machining result in discontinuous override values, especially when control is switched or when initial values for the integral term or offset are not properly assigned, leading to variations in cut depth and spindle load.

Innovation Solution

A numerical controller that sets an initial value for the integral term or offset by assigning the override just before control start or ensuring continuity in the override value, using equations to maintain continuous PID control and prevent steep decreases in override, thereby ensuring a constant spindle load.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If PID control is performed without assigning an initial value to the integral term or offset, then the control calculation is simple, but the override becomes discontinuous when control is switched

Engineering Contradiction:

Improvecontrol calculation complexityVSAvoidoverride continuity

Core Design Contradiction:

Device complexityVSStability of the object's composition

Solution Approach 1:

The patent applies preliminary action by calculating and storing the integral term value before control switching occurs. The integral term accumulation is paused and saved at the switching moment, then restored after switching, preventing discontinuity in the override while keeping the control logic relatively simple.

Inventive Principle:

Principle #10Preliminary action

2Ease of manufacture

If a constant value is assigned to the integral term or offset initial value, then the control implementation is simple, but the override may become discontinuous due to variations in override or cut depth just before control

Engineering Contradiction:

Improvecontrol implementation simplicityVSAvoidoverride continuity

Core Design Contradiction:

Ease of manufactureVSStability of the object's composition

Solution Approach 1:

Instead of using a constant initial value, the patent calculates the integral term value based on the actual state just before control switching. This preliminary calculation ensures the initial value reflects the current operating conditions, maintaining override continuity while keeping implementation straightforward.

Inventive Principle:

Principle #10Preliminary action

3Reliability

If the integral term initial value is set to zero, then the control starts from a known state, but the override decreases once and cannot maintain constant spindle load when control starts

Engineering Contradiction:

Improvecontrol start state reliabilityVSAvoidspindle load constancy

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent avoids starting the integral term from zero by preserving its value before control switching. This preliminary preservation allows the integral term to continue from its accumulated state, enabling the override to maintain the spindle load at the desired constant value without an initial decrease.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS10488849B2Numerical controller for controlling feed rate based on spindle load

Publication Date: 2019.11.26 FANUC LTD

AI summary

A numerical controller performs PID control to control the move speed of an axis for driving a spindle such that a load value of the spindle becomes constant. When the spindle load value exceeds a threshold value, a speed calculation unit of the numerical controller calculates an override for the feed rate of the axis instructed by command data such that the load value of the spindle becomes constant, and, at the start of the speed calculation process, assigns the override just before the start of the speed calculation process to an initial value of an integral term or an offset for PID control.