Applying Stepper Motors:

Application Questions You Must Answer
& Things to Watch Out For
Before We Start
 This webinar will be available afterwards at
designworldonline.com & email
 Q&A at the end of the prese...
Moderator

Presenter

Miles Budimir

Tim Burke

Design World

ElectroCraft
Applying Steppers
Applying Steppers

6

Application Variables
 Mechanical Load reflected to motor
 Maximum system speed reflected to motor...
Applying Steppers

7

Stepping Motor Catalog Data
Applying Steppers

8

Torque-Speed Curves
 “Typical” Limits for stepper:

Torque-Speed, Various Overvoltage Ratios
TPP23-...
Applying Steppers

9

Constant Current Drives
& Motor Current Rise
Electrical Equation
of Motor Winding:
Current (Amps)

V...
Applying Steppers

10

Current Profiles at Speed
Phase Currents
Low Speed, Half Stepping

Medium Speed, Half Stepping

Hal...
Applying Steppers

11

Stepper Torque-Speed Curves
2 Regions

Stepper Torque-Speed

• Current Regulation
• Voltage Drive,
...
Applying Steppers

12

Scaling Stepper Torque-Speed Curves
 Comparison of test data
against scaled curves

Stepper Torque...
Applying Steppers

13

Connection
Torque-Speed

Fixed Drive Current

Diagrams

Various Connections
450

Series

Parallel

...
Applying Steppers

14

Thermal Ratings
 Typically conservative rating

 ElectroCraft motors rated for 130 °C

Size 17 St...
Applying Steppers

15

Thermal Model
 Simple Model

Temperature Rise
Size 17 Stepper

• 2 Node: Winding Temp & Ambient Te...
Applying Steppers

16

Intermittent Duty and
Overdriving Motor
 Can overdrive motor as long as
thermal limit not exceeded...
Applying Steppers

17

Mechanisms of Loss
 I2R Loss (Joule Heating)

Stepping Motor Losses

• Dominant at Low Speed

40

...
Applying Steppers

18

Angle Accuracy
 Resolution:
• 360o/(# steps/rev.)

 Accuracy:
• Typically 3-5%
• Non-cumulative
•...
Applying Steppers

19

Microstepping
 Full Step, Half Step &
Microstepping
Torque Vectors – Full, Half & Microstep
(Posit...
Applying Steppers

20

Microstepping
 Be careful if used
for positioning

1.5

0.5
0.4

1
0.3
0.5

-1.5

0.2

0
-1

-0.5
...
Applying Steppers

21

Stepper Start-Stop Rate
and Acceleration
 Steppers capable of very
high acceleration rates
(high T...
Applying Steppers

22

Common Failure Mode
 Stepping at motor’s natural
frequency

Low Frequency Resonance
450
400
350
30...
Applying Steppers

23

Common Failure Mode
 Mid-Frequency Resonance

 Electronic damping
found in many
newer drives will...
Applying Steppers

24

Stepping Motor Linear Actuators
 Linear Actuators
• Conversion of rotary to linear
done within mot...
Applying Steppers

25

Types of Actuator
 Translating Screw
• Screw must be prevented from
rotating. For “long” travel mu...
Applying Steppers

26

Types of Screw
 Lead Screw
• ACME Thread Design
• Efficiency Range from 0.3 to 0.7
• Function of c...
Applying Steppers

27

Force Speed Curves

• A-thread: 0.0625” lead
(16 Threads/in.)

• B-thread: 0.125” lead

 Efficienc...
Questions?
Design World

Miles Budimir
mbudimir@wtwhmedia.com
Phone: 440.234.4531
Twitter: @DW_Motion

ElectroCraft

Tim B...
Thank You
 This webinar will be available at
designworldonline.com & email
 Tweet with hashtag #DWwebinar
 Connect with...
Applying Steppers
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Applying Stepper Motors: Application Questions You Must Answer & Things to Watch Out For

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This webinar reviews some of the basic parameters necessary to properly select and apply stepper motor technology to motion applications. Topics included in this discussion review basic stepper speed-torque behavior including the stepper over-voltage ratio, a stepper in constant current operation vs. a stepper in voltage drive region, thermal ratings, the impact of winding changes on dynamic behavior, calculations and considerations for intermittent duty operation, motor/system accuracy, failure modes, and the application of all these criteria when applying stepper-based linear actuator products.
Engineers currently using stepper technology in their motion applications that want to understand how to apply stepper motor technology will benefit from this webinar.
In this webinar you will learn:
-Basic parameters necessary to properly select and apply stepper motor technology
-How to understand the stepper motor speed-torque curve and how to apply this to the application
-The main factors to consider when trying to optimize a stepper motor in a variety of applications

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Applying Stepper Motors: Application Questions You Must Answer & Things to Watch Out For

  1. 1. Applying Stepper Motors: Application Questions You Must Answer & Things to Watch Out For
  2. 2. Before We Start  This webinar will be available afterwards at designworldonline.com & email  Q&A at the end of the presentation  Hashtag for this webinar: #DWwebinar
  3. 3. Moderator Presenter Miles Budimir Tim Burke Design World ElectroCraft
  4. 4. Applying Steppers
  5. 5. Applying Steppers 6 Application Variables  Mechanical Load reflected to motor  Maximum system speed reflected to motor  Available Voltage  Available Current  Ambient temperature conditions  Position increment & accuracy  Time to execute move
  6. 6. Applying Steppers 7 Stepping Motor Catalog Data
  7. 7. Applying Steppers 8 Torque-Speed Curves  “Typical” Limits for stepper: Torque-Speed, Various Overvoltage Ratios TPP23-90A30 • 3000 RPM (10K steps/s) for Size 17 • 1500 RPM (5K steps/s) for Size 34 80 70  Overvoltage Ratio: Ratio of Drive Supply voltage to Motor Voltage • Desirable to have this at least 15 Torque (oz.-in) 60 50 40 30 20 10 0  Torque Margin: Utilize 70 % of available torque at any speed 0 2000 4000 6000 8000 10000 Speed (Full Steps/s) OV 15:1 OV 20:1 OV 30:1 OV 40:1
  8. 8. Applying Steppers 9 Constant Current Drives & Motor Current Rise Electrical Equation of Motor Winding: Current (Amps) Vs=i*R+L*di/dt+Kb*w*sinA, A=Elec Angle, w=rotor speed i=Vs/R (eg i=15Vm/R) i=Vm/R Area between curves is where the extra torque comes from High Step Rate Med Step Rate Time (mS) Low Step Rate
  9. 9. Applying Steppers 10 Current Profiles at Speed Phase Currents Low Speed, Half Stepping Medium Speed, Half Stepping Half Step Phase Energization Phase A Phase B 1 Off On, + Current 2 On, + Current On, + Current 3 On, + Current Off No Current Regulation, Half Stepping 4 On, + Current On, - Current 5 Off On, - Current 6 On, - Current On, - Current 7 On, - Current Off 8 On, - Current On, + Current 1 Off On, + Current High Speed, Half Stepping
  10. 10. Applying Steppers 11 Stepper Torque-Speed Curves 2 Regions Stepper Torque-Speed • Current Regulation • Voltage Drive, current does not reach set point • Low speed torque 70% of holding torque • Knee scaled by overvoltage ratio (volt-amps if changing winding) • Maximum speed scaled by overvoltage ratio (volt-amps if changing winding) 1 Knee Normalized Torque Scaling 1.2 0.8 0.6 0.4 Current Regulation 0.2 Voltage Drive 0 0 5000 10000 Speed (Full Steps/s) 15000
  11. 11. Applying Steppers 12 Scaling Stepper Torque-Speed Curves  Comparison of test data against scaled curves Stepper Torque-Speed • 48V scaled from 24V straight line approximation 40 35 Torque (oz.-in) 30 25 20 15 40 10 5 0 0 5000 10000 15000 Speed (Steps/s) 24V Actual 48V Predicted 24V Estimated 48V Actual
  12. 12. Applying Steppers 13 Connection Torque-Speed Fixed Drive Current Diagrams Various Connections 450 Series Parallel ½ Cu Phase A 400 Phase B 350 2R Power Loss 2I2R Torque 2N*I R/2 I2R/2 R I2R 300 Torque (oz-in) Terminal Resistance 250 Phase A 200 Phase B 150 N*I N*I 100 50 Inductance L L/4 L/4 Phase A 0 0 Elec. Time Constant 2000 4000 6000 L/2R L/2R L/4R Series Parallel 8000 Phase B Speed (Full Steps/s) 1/2 Cu
  13. 13. Applying Steppers 14 Thermal Ratings  Typically conservative rating  ElectroCraft motors rated for 130 °C Size 17 Stepper 120.00 100.00 Temperature (deg C) • NEMA ICS16 spec describes procedure • Motor Hanging in free air • Motor held in stall condition with full current in both phases Temperature Rise 80.00 60.00 40.00 20.00 0.00 0.00 20.00 40.00 60.00 Time (min) 80.00 100.00
  14. 14. Applying Steppers 15 Thermal Model  Simple Model Temperature Rise Size 17 Stepper • 2 Node: Winding Temp & Ambient Temp • Some manufacturers publish data • Thermal Resistance C • Thermal Time Constant 120.00 w Tw Ta  More Complex Model • 3-Node Model • Winding temp C • Case temp • Ambient temp T w w Temperature (deg C) 100.00 80.00 60.00 40.00 20.00 0.00 0.00 Cc Tc 20.00 40.00 60.00 80.00 Time (min) Ta Data 2 Node 3 Node 100.00
  15. 15. Applying Steppers 16 Intermittent Duty and Overdriving Motor  Can overdrive motor as long as thermal limit not exceeded 60 50 40 Torque (oz-in) • Thermal model allows calculation of allowable duty cycle • Torque falls off after rated current • Note: Torque-speed curves are continuous rated operation Torque vs. Current 2x Rated 30 Rated Current 20 10 0 0.0 0.5 1.0 1.5 2.0 Current (amps) 2.5 3.0 3.5
  16. 16. Applying Steppers 17 Mechanisms of Loss  I2R Loss (Joule Heating) Stepping Motor Losses • Dominant at Low Speed 40 10 9 35  Core Loss 25 6 20 5 4 15 Power Loss (W) 7 Torque (oz.-in) • Primarily in stator lamination • Both hysteresis and eddy current • Hysteresis is f(step rate) • Eddy Current is f(step rate)2 8 30 3 10 2 5 1 0 0 2000 4000 6000 0 8000 Speed (Steps/s) Torque-Speed I*I*R Core Loss Total Loss
  17. 17. Applying Steppers 18 Angle Accuracy  Resolution: • 360o/(# steps/rev.)  Accuracy: • Typically 3-5% • Non-cumulative • Influenced by: • Friction (Application) • Stiffness of motor (Motor Design)  Hysteresis • Error found when approaching same position from either direction • Typically 3%
  18. 18. Applying Steppers 19 Microstepping  Full Step, Half Step & Microstepping Torque Vectors – Full, Half & Microstep (Position shown in Electrical Degrees) 1.6 1.4 Full Step 1.2 Microsteps 1 0.8 Half Step 0.6 Microsteps 0.4 0.2 Full Step 0 0 0.5 1 1.5 • Microstepping accomplished by regulating phase currents • Tq=Kt*Isin(wt)*sin(Q)+Kt*Icos(wt)*cos(Q)
  19. 19. Applying Steppers 20 Microstepping  Be careful if used for positioning 1.5 0.5 0.4 1 0.3 0.5 -1.5 0.2 0 -1 -0.5 0 0.5 1 1.5 2 2.5 -0.3 -0.5 0.1 Torque Torque • Friction band leads to high error, may result in motor not moving • Harmonics in torque profile lead to additional errors (cogging or detent torque) 1/16th Microstepping Full Step 0 -0.1 0.1 -0.1 -0.2 -1 -0.3 -1.5 Mechanical Degrees Step 1 Step 2 -0.4 Mechanical Degrees Step 1 Step 2 0.3
  20. 20. Applying Steppers 21 Stepper Start-Stop Rate and Acceleration  Steppers capable of very high acceleration rates (high Torque/inertia ratio) Stepper Move Profile 18 Step Move, Constant Acceleration 2500  Start-Stop Rate – Step rate at which motor will pull into synchronism • Speeds above start-stop will require acceleration • Function of total system inertia • Proportional to sqrt(1/total inertia) Velocity (Full Steps/s) 2000 1500 1000 Start-Stop Rate Start-Stop Rate 500 0 0 0.005 Time (sec) 0.01 0.015
  21. 21. Applying Steppers 22 Common Failure Mode  Stepping at motor’s natural frequency Low Frequency Resonance 450 400 350 300 Torque (oz-in) • wn=sqrt(Kt*I*A/J), Kt=Torque Constant, I=operating current, A=# of pole pairs, J=inertia • Typically found at 150 to 450 steps/sec • Influenced by reflected inertia of load Torque-Speed 250 200 150  Remedies • Add mechanical damping or friction • Microstepping 100 50 0 0 500 1000 1500 2000 Speed (Full Steps/s) 2500 3000
  22. 22. Applying Steppers 23 Common Failure Mode  Mid-Frequency Resonance  Electronic damping found in many newer drives will quiet this behavior Mid Frequency Resonance 40 35 30 Torque (oz-in) • Less severe, but occurs at much higher step rates • Traced to oscillation of BEMF (amplitude and frequency modulation) leading to unstable torque Torque-Speed 25 20 15 10 5 0 0 2000 4000 6000 8000 Speed (Full Steps/sw) 10000 12000
  23. 23. Applying Steppers 24 Stepping Motor Linear Actuators  Linear Actuators • Conversion of rotary to linear done within motor envelope • Requires external hardware to guide screw & support loads • Features • Low cost linear motion • High resolution • High force
  24. 24. Applying Steppers 25 Types of Actuator  Translating Screw • Screw must be prevented from rotating. For “long” travel must allow clearance behind motor for screw  Translating Nut • Must provide guide or rail for nut to prevent rotation.
  25. 25. Applying Steppers 26 Types of Screw  Lead Screw • ACME Thread Design • Efficiency Range from 0.3 to 0.7 • Function of coefficient of friction • Backlash range from .002” to .007”  Ball Screw • Efficiency Range from 0.8 to 0.95 • Lower friction leads to better accuracy • Backlash from 0.000” to 0.010”
  26. 26. Applying Steppers 27 Force Speed Curves • A-thread: 0.0625” lead (16 Threads/in.) • B-thread: 0.125” lead  Efficiency of lead screws • A-thread: 42% • B-thread: 58% • C-thread: 68% (8 Threads/in.) • C-thread: 0.25” lead (4 Threads/in.) Formula: Eff=L/(dm*p)*((L+p*m*sec(a))/(p*dm-m*L*sec(a))) • m = Coef. of Friction • L = Thread Lead • dm = Pitch diameter • a = Thread angle/2 Size 17S - Force vs. Linear Speed 48V, 2A RMS 100 90 80 70 Force (lb.)  Lead of screw is another variable to tailor performance 60 50 40 30 20 10 0 0 5 10 15 Linear Speed (in/s) A-Thread Tested B-Thread Tested C-Thread Tested 20 25
  27. 27. Questions? Design World Miles Budimir mbudimir@wtwhmedia.com Phone: 440.234.4531 Twitter: @DW_Motion ElectroCraft Tim Burke tburke@electrocraft.com Phone: 603.516.1255
  28. 28. Thank You  This webinar will be available at designworldonline.com & email  Tweet with hashtag #DWwebinar  Connect with  Twitter: @DesignWorld  Facebook: facebook.com/engineeringexchange  LinkedIn: Design World Group  YouTube: youtube.com/designworldvideo  Discuss this on EngineeringExchange.com
  29. 29. Applying Steppers

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