Linear Motion - Stork...
Transcript of Linear Motion - Stork...
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Application Examples
Linear Motion - Overview
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Direct Drive Linear Motors Rotary to Linear Conversion
Rack & Pinion
Timing belts
Ball screws
Lead Screws
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Application Examples
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Linear Motion – Main selection criteria
Electro-mechanical design: balancing the trade-offs
PrecisionResolution, Repeatability, Accuracy
ThroughputSpeed, Duty Cycle, ….
LifeWear, Contamination Resistance, Maintenance,…
Cost
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Application Examples
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Comparison of Linear Drive Mechanisms
Direct Drive Linear Motor
Very high speed
Very high precision
No backlash
Fast move and settle time
Lower force density compared to othertechnologies
Requires linear feedback for properoperation
More difficult to use in vertical applications
Can be expensive
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Comparison of Linear Drive Mechanisms
Rack and Pinion
Very good for long travel applications
High speed
Requires precise alignment
Can be noisy
Difficult to remove backlash
Typically lower precision
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Comparison of Linear Drive Mechanisms
Timing Belt and Pulley
High speed applications
Long life
Maintenance free
Belt tensioning required
Care must be taken in designing pulleybearing
Specific errors (pulley out of round precision,lead error caused by pitch diameterprecision,…)
Form factor may be an issue
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Comparison of Linear Drive Mechanisms
High precision application withmoderate speed
Applications requiring goodrepeatibility
Applications requiring high forcedensity
When non-backdriving isrequired (lead screw)
Precision alignment required (ball screws)
Right lubricant, coating, nut materials, …required
Specific potential issues to be addressed:critical speed, noise,…
Ball screws require maintenance
Ball Screws & Lead Screws
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Technology Highlights
Limits of the technology
Components & Systems Selection
Application Examples
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Screw driven solutions
Rotary Motor & GearboxPrecision Lead Screw and Nut
Linear guide
Controller/Driver
Power Supply
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Screws – A few definitions
Lead: linear displacement for one revolution Pitch: distance between to consecutive threads
Lead = Pitch x nb of starts
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Lead Screw versus Ball screwFeature Leadscrews Ballscrews
Loads light or moderate moderate or heavy
Efficiency 20% up to 85% above 95%
Lead Accuracy 0.1µm/mm-0.6µm/mm 0.1µm/mm or less (C10 to C7 grade)
Repeatability 1-2 microns (compensated backlash) varies with C grade
Backdriving sometimes always
Noise quiet operation noisy-metal to metal recirculating balls
Lubrication solutions without grease available necessary
Maintenance none or very little required
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Lead Screw versus Ball screwOther criteria
Size Lead screw/nut assembly are much more compact for the same screw diameter
Lead (travel/revolution) Small leads feasible only with lead screws (down to 0,3mm)
Alignment Ball screws are highly sensitive to alignment
Environmental Ballscrews are highly sensitive to dirty environments.
Life Depends on the application
Customization Lead screws are easier to customize than ball screws
Cost Cost of leadscrew is less than a ground ball screw and a little less than rolledballscrew
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Lead Screw efficiency
Efficiency depends onlead angle andcoefficient of friction
Back drive efficiency islower than forward driveefficiency
A lead screw is non-backdrivable if backwardefficiency is zero (orforward efficiency lowerthan approx. 50%)
Lead angle
Lead angle
Ball screws
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Technology Overview
Limits of the technology
Components & Systems Selection
Application Examples
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Limits of Lead Screw Driven Solutions
(*): with backlash compensation and constant preload
Criteria
Precision Open loop : repeatability down to 5-6µm (*)Closed loop: sub-micron accuracy achievable
Speed/Throughput Compromise between speed and force
Expected life From a few thousand cycles to a few million cyclesDepends on a lot of different application parameters
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Limits of Lead Screw Driven Solutions
Precision limiting factors in open loop:
Motor resolution (steppers)
Backlash, Lost motion, hysteresis
Lead accuracy
« One revolution errors »
Linear resolution (mm): lead (mm) /number of steps per revolution
Backlash = clearance between nut and screw (can be compensated)Material deflection may add additional error (cannot be compensated)
Cumulative error defined in µm/mm
Screw straightness and warious alignment issues contribute tocreating an error with a period of one revolution
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Limits of Lead Screw Driven Solutions
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Backlash = clearance betweenthe nut and the screw
Lost motion = backlash + stiffnessLost motion = backlash + stiffness
Stiffness of anti-backlash nutswith « soft » material
Axial preload
Radial preload
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Limits of Lead Screw Driven Solutions
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Backlack compensationBacklack compensation
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Limits of Lead Screw Driven Solutions
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Stiffness: several different plastics available
StiffnessStiffness
Metal nuts:- Stiff- Lubrication required (maintenance)- Shorter life than plastic nuts
Thermoplastic nuts:- Longer life- No maintenance required- Various materials available
depending on stiffness andapplication conditions (PV,temperature, environment…..)
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Limits of Lead Screw Driven Solutions
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Lead accuracy and « one revolution » errorsLead accuracy and « one revolution » errors
y = 2,252x + 1,423-2
0
2
4
6
8
10
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0 0,5 1 1,5 2 2,5 3 3,5
Erro
r (um
)
Displacement (mm)
Error vs. Displacement
Série2
2 4 6 8 10
Lead accuracy:0,6 µm/mm
One revolution error: 4 µm
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Limits of Lead Screw Driven Solutions
Potential issues limitating the velocity:
Nut Material PV (pressure x velocity)
Screw Critical speed
Noise
Motor Torque/speed characteristics
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Limits of Lead Screw Driven Solutions
screw supported at regular intervals
Overcoming the critical speed issue: the RGS slidesOvercoming the critical speed issue: the RGS slides
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Limits of Lead Screw Driven Solutions
Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motors
Size 17 (42mm) stepper & DC motors (*)with 1,27mm lead screw
(*) Without gearbox
533N
120N
9,5mm/s 44 mm/s
:4,5
X4,6
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Limits of Lead Screw Driven Solutions
Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motorsSize 17 (42mm) stepper & DC motors (*)
with 3,17mm lead screw
(*) Without gearbox
265N
80N
30mm/s 108 mm/s
:3,3
X3,6
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Limits of Lead Screw Driven Solutions
Torque/speed characteristics: Stepper versus DC motorsTorque/speed characteristics: Stepper versus DC motorsSize 17 (42mm) stepper & DC motors (*)
with 6,35 mm lead screw
(*) Without gearbox
102N
40N
50mm/s 216 mm/s
:2,5
X4,2Ball screw
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Limits of Lead Screw Driven Solutions
Life is increasing with load derating
Lubrication (dry, grease, teflon coating?)
Alignment issues
Environmental issues
Anti-backlash nuts with wearcompensation increase life
Life expectancy
020406080
100120
1 100 10,000 1,000,000 100,000,000
number of cycles
perc
ent % percent load vs
number of cycles
Main factors impacting life:
Selection of Linear Precision Drives
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Linear Motion – Overview
Main Selection Criteria
Comparison of Common Linear Drive Mechanisms
Screw Driven Solutions
Technology Overview
Limits of the technology
Components & Systems Selection
Application Examples
Selection of Linear Precision Drives
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Select the base technology: force, speed, accuracy, life,..
High speed, high accuracy: direct drive linear motors
Speed:• > 1m/s : linear motor, belt & pulley, rack & pinion• 0.5 – 1m/s: ball screw,…• < 0,5 m/s: lead screw,…
Duty cycle, efficiency:• > 50%: usually ball screws• <50% lead screws,…
Application specific parameters:• Accuracy• Force density, form factor• Back drive• Environmental issues,….
Select the motor technology (rotary to linear conversion): DC motor (brush, brushless), stepper motor
Select the guide technolgy: ball guides, friction guides,…
Components selectionComponents selection
Selection of Linear Precision Drives
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Separate Components or Integrated Systems?Separate Components or Integrated Systems?Rotary Motor Precision Lead Screw and NutDriver Linear guide