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Dealing with thermal errors in heavy duty machine tools
Gorka Aguirre gaguirre@ideko.es
Precisiebeurs 2015
Veldhoven, NL
Dealing with thermal errors in heavy duty machine tools 2
IK4-IDEKO
Dealing with thermal errors in heavy duty machine tools 3
Outline
Large heavy duty milling-boring machines
Thermal error management
Thermal Error Compensation System
General considerations
Dealing with thermal errors in heavy duty machine tools 4
Large heavy duty milling-boring machines
Main thermal issues
Heavy duty: high heat generation, spindles can rate up to 88kW
Large machine: error amplification with increasing workspace
Applications
Heavy duty milling and boring
Large parts, high precision
Oil&gas, wind energy, aeronautics, etc.
Large machines
Vertical travel up to 8m
Longitudinal travel up to 60m
Multiple spindle/quills with automatic changer
Dealing with thermal errors in heavy duty machine tools 5
Thermal error management strategies
Temperature control
Minimize temperature variations in the machine
Design for thermal error reduction
Minimize errors at TCP generated by temperature changes
Thermal error compensation
Compensate remaining errors at TCP
Dealing with thermal errors in heavy duty machine tools 6
Temperature control
Heat sources
Bearings around spindle area
Ambient temperature, not controlled
Hydrostatic bearings
Hot chip falling against machine
Motors
Pumps in cooling circuit
Minimize temperature variations in the machine
In practice
Heat in bearings is dominant effect, by magnitude and dynamics.
Ambient temperature and other effects appear less relevant
Dealing with thermal errors in heavy duty machine tools 7
Temperature control
Cooling units
Minimize temperature variations, but limited by high required power.
Dangerous, cooling circuits can be a disturbance source
Might be too expensive
Minimize temperature variations in the machine
50 100 150 200 250 300 3500.3
0.4
0.5
0.6
0.7
0.8
0.9
1
MINUTESG
RO
WT
H
0 RPM3000 RPM
Dealing with thermal errors in heavy duty machine tools 8
Design for thermal error reduction
Minimize errors at TCP generated by thermal changes in the machine
Application of precision design principles
Machine design is focused on stiffness and cost
Precision is important, but it comes next
Focus on avoiding bending errors
Linear errors are 'easy' to compensate
Bending errors are more difficult to compensate at TCP
Bending errors change tool orientation
Dealing with thermal errors in heavy duty machine tools 9
Thermal error compensation
Estimate and compensate thermal errors during machine operation
Thermoelastic machine model
Relates thermal error with temperature field, spindle speed, position in workspace, etc.
Can be based on simulations (FEM) and/or measured data
Implementation
Experiments/simulations to characterize model
Simulation model running on CNC/PLC
Impact
Low cost solution, big improvements are possible
Risk of lack of robustness, machine operation is highly variable and only a very limited range of conditions can be analysed during implementation.
input data
thermoelastic model
CNC compensation
Dealing with thermal errors in heavy duty machine tools 10
Summary
Temperature control is limited by the high generated power and the risk of creating fast temperature changes. Advanced control strategies are needed
Design for thermal error reduction is very limited, since machine architecture is defined by other aspects, such as cost and workspace.
Thermal error compensation can provide relevant performance improvement with low implementation cost, but robustness must be ensured
Dealing with thermal errors in heavy duty machine tools 11
Thermal Compensation System
Robust and effective compensation method:
Improve machine accuracy, never make it worse
Minimize machine occupation time:
Fast machine characterization method
Multiple heads, one 8h shift per spindle head / quill
Simple to implement
By machine operator
Automated operation
Flexible
Compatible with main CNC systems
Full range of machines/spindles/quills
Main objectives
Dealing with thermal errors in heavy duty machine tools 12
Identification of main thermal effects
Multiple sources for thermal variations
Heat generated by tool rotation, in motor, bearings and gears
Heat generated in the cutting process
Cooling fluids in structural elements
Cooling fluid in tool
Ambient temperature
Local heating by contact with hot chip
Temperature variations in the workpiece
- ambient
- cutting process
Temperature variations in the pressurized air in linear scales
Dealing with thermal errors in heavy duty machine tools 13
Machine state monitoring
As a general rule, place them in structural elements, near the heat sources
- Advanced numerical methods, like Singular Value Decomposition can be used on simulation or measured data for optimizing t hese locations
Continuous monitoring of the thermal condition of the machine
Direct temperature measurement at selected key points is the ideal solution
Need to identify:
- Most relevant effects to be compensated
- Optimal sensor locations for each effect
Good sensor location enables simple compensation models.
- Sensors can be replaced by model complexity, risk for robustness
Dealing with thermal errors in heavy duty machine tools 14
Compensation model
Thermoelastic machine model based on experiments on each machine
Preferred situation
- Temperature readings and axis position as only input to the model
- Model structure, the simpler the better
- Temperature sensor location defines model accuracy
p(t): estimated error at point of interest T
m(t): Temperature readings from m sensors
Sm
(s): Transfer function relating temperature and growth K(x
1,x
2,...): Correction factor for machine axis position x
1,x
2,...
Alternatives
- When rotating elements not accessible for probes
- State observers using CNC info, such as spindle speed, power
Dealing with thermal errors in heavy duty machine tools 15
Machine characterization
All effects to be compensated need to be characterized experimentally
Time and feasibility are the main issues
- Tool rotation heat is simple to excite, few hours to stabilize
- Ambient temperature cannot be controlled, and time cycle is of a full day
- Effects of structural cooling follows other effects, no need to excite separately
- Workpiece heating, effects of cutting fluid and hot chip are in general very
difficult to analyse.
In the general case:
- Tool rotation heat is always analysed, following predefined speed profiles
- Ambient temperature can be considered optionally, but requires longer testing and variation might not be sufficient in function of the weather
Dealing with thermal errors in heavy duty machine tools 16
Experimental setup
Software in external laptop
Data acquisition from CNC/PLC and displacement sensors
Generate movement program for CNC
Automatic model fitting
Automatic generation of compensation tables
Hardware
Several measuring points within workspace
- RAM and quill positions
- Spindle orientations
Thermally stable measurement targets
Dealing with thermal errors in heavy duty machine tools 17
Implementation in machine control
Data acquisition
Temperature from embedded sensors
Machine axis positions
Variety of CNC
Heindenhain Siemens Sinumerik Fanuc Fagor Automation
Compensation in CNC/PLC
Application on CNC PC
Embedded code in PLC
Dealing with thermal errors in heavy duty machine tools 18
Spindle head tests
Thermal errors are measured within workspace
- Three main orientations of the main spindle
- Full range of the ram
Dealing with thermal errors in heavy duty machine tools 19
Compensation results: spindle head
All results are normalized to the maximum measured value
Dealing with thermal errors in heavy duty machine tools 20
Quill tests
Thermal errors are measured within workspace
- RAM and quill are aligned with the same axis.
- Several combinations of both axis are measured
Dealing with thermal errors in heavy duty machine tools 21
Compensation results: quill
All results are normalized to the maximum measured value
Dealing with thermal errors in heavy duty machine tools 22
General considerations
Do not be too ambitious, focus on dominant errors:
Smaller effects are more difficult to characterize, and
Considering them might affect robustness
Compensation model structure
The simpler the better, improve sensor location before adding complexity
Temperatures and machine position as only inputs to the model
FE can be useful
Improve design and cooling elements (Minimize bending deformations!)
Find optimal sensor location (structural elements, near heat source, near linear scales)
FE not for compensation model, experiments are needed if high precision is required
Dealing with thermal errors in heavy duty machine tools 23
General considerations
Careful with cooling systems, their goal should be to:
Keep machine temperature constant, not necessarily at 20ºC
Damp transient effects
Keep in mind industrial feasibility
Minimize machine occupation time
Simple procedure to be implemented by operators
Be aware of control requirements/limitations
Special applications with extreme requirements
When possible, re-think the manufacturing steps (CAM)
Add many more sensors, use some math tricks (e.g. POD) to find best correlating ones
Take time to properly characterize the effect of ambient temperature
Ideko-IK4ren irudi bateratua lortzeko formatuak 24
Thank you!
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