Applying Stepper Motors: Application Questions You Must Answer & Things to Watch Out For
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Transcript of Applying Stepper Motors: Application Questions You Must Answer & Things to Watch Out For
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Applying Stepper Motors:
Application Questions You Must Answer & Things to Watch Out For
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This webinar will be available afterwards at
designworldonline.com & email
Q&A at the end of the presentation
Hashtag for this webinar: #DWwebinar
Before We Start
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Moderator
Miles Budimir Design World
Presenter
Tim Burke ElectroCraft
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Applying Steppers
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Applying Steppers
Application Variables
6
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
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Applying Steppers
Stepping Motor Catalog Data
7
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Applying Steppers
Torque-Speed Curves
8
0
10
20
30
40
50
60
70
80
0 2000 4000 6000 8000 10000
To
rqu
e (
oz.-
in)
Speed (Full Steps/s)
OV 15:1 OV 20:1 OV 30:1 OV 40:1
“Typical” Limits for stepper: • 3000 RPM (10K steps/s) for Size 17
• 1500 RPM (5K steps/s) for Size 34
Overvoltage Ratio: Ratio of Drive Supply voltage to Motor Voltage • Desirable to have this at least 15
Torque Margin: Utilize 70 % of available torque at any speed
Torque-Speed, Various Overvoltage Ratios TPP23-90A30
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Applying Steppers
Electrical Equation
of Motor Winding: Vs=i*R+L*di/dt+Kb*w*sinA,
A=Elec Angle, w=rotor speed
9
Constant Current Drives & Motor Current Rise
i=Vm/R
i=Vs/R (eg i=15Vm/R)
High
Step Rate Med
Step Rate
Low
Step Rate
Time (mS)
Cu
rren
t (A
mp
s)
Area between curves is
where the extra torque
comes from
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Applying Steppers 10
Current Profiles at Speed
Low Speed, Half Stepping Medium Speed, Half Stepping
No Current Regulation, Half Stepping High Speed, Half Stepping
Half Step Phase Energization
Phase A Phase B
1 Off On, + Current
2 On, + Current On, + Current
3 On, + Current Off
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
Phase Currents
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Applying Steppers
Stepper Torque-Speed Curves
11
2 Regions • Current Regulation
• Voltage Drive, current does not reach set point
Scaling • 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)
0
0.2
0.4
0.6
0.8
1
1.2
0 5000 10000 15000
No
rmali
zed
To
rqu
e
Speed (Full Steps/s)
Stepper Torque-Speed
Knee
Current
Regulation Voltage
Drive
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Applying Steppers
40
Scaling Stepper Torque-Speed Curves
12
Comparison of test data against scaled curves • 48V scaled from 24V straight line
approximation
Stepper Torque-Speed
24V Actual
24V Estimated 48V Actual
48V Predicted
40
35
30
25
20
15
10
5
0 0 5000 10000 15000
Speed (Steps/s)
Torq
ue
(o
z.-i
n)
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Applying Steppers
Connection
13
0
50
100
150
200
250
300
350
400
450
0 2000 4000 6000 8000
To
rqu
e (
oz-i
n)
Speed (Full Steps/s)
Series Parallel 1/2 Cu
Fixed Drive Current
Series Parallel ½ Cu
Terminal
Resistance 2R R/2 R
Power
Loss 2I2R I2R/2 I2R
Torque 2N*I N*I N*I
Inductance L L/4 L/4
Elec. Time
Constant L/2R L/2R L/4R
Torque-Speed Various Connections
Diagrams
Phase A
Phase B
Phase A
Phase B
Phase A
Phase B
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Applying Steppers
0.00
20.00
40.00
60.00
80.00
100.00
120.00
0.00 20.00 40.00 60.00 80.00 100.00
Tem
pe
ratu
re (
de
g C
)
Time (min)
Typically conservative rating • NEMA ICS16 spec describes procedure
• Motor Hanging in free air
• Motor held in stall condition with full current in both phases
ElectroCraft motors rated for 130 °C
14
Thermal Ratings
Temperature Rise Size 17 Stepper
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Applying Steppers
0.00
20.00
40.00
60.00
80.00
100.00
120.00
0.00 20.00 40.00 60.00 80.00 100.00
Tem
pe
ratu
re (
de
g C
)
Time (min)
Data 2 Node 3 Node
Simple Model • 2 Node: Winding Temp & Ambient Temp
• Some manufacturers publish data
• Thermal Resistance
• Thermal Time Constant
More Complex Model • 3-Node Model
• Winding temp
• Case temp
• Ambient temp
15
Thermal Model
Temperature Rise Size 17 Stepper
Tw Ta
Tw Tc Ta
Cw Cc
Cw
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Applying Steppers
Can overdrive motor as long as thermal limit not exceeded • Thermal model allows calculation of
allowable duty cycle
• Torque falls off after rated current
• Note: Torque-speed curves are continuous rated operation
16
Intermittent Duty and Overdriving Motor
0
10
20
30
40
50
60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
To
rqu
e (
oz-i
n)
Current (amps)
Torque vs. Current
Rated Current
2x Rated
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Applying Steppers
Mechanisms of Loss
17
I2R Loss (Joule Heating) • Dominant at Low Speed
Core Loss • Primarily in stator lamination
• Both hysteresis and eddy current
• Hysteresis is f(step rate)
• Eddy Current is f(step rate)2
0
1
2
3
4
5
6
7
8
9
10
0
5
10
15
20
25
30
35
40
0 2000 4000 6000 8000
Po
we
r L
os
s (
W)
To
rqu
e (
oz.-
in)
Speed (Steps/s)
Torque-Speed I*I*R Core Loss Total Loss
Stepping Motor Losses
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Applying Steppers
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
Angle Accuracy
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Applying Steppers
Full Step, Half Step & Microstepping
19
Microstepping
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.5 1 1.5
Torque Vectors – Full, Half & Microstep (Position shown in Electrical Degrees)
Full Step
Microsteps
Half Step
Full Step
• Microstepping accomplished by regulating phase currents
• Tq=Kt*Isin(wt)*sin(Q)+Kt*Icos(wt)*cos(Q)
Microsteps
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Applying Steppers
Microstepping
Be careful if used for positioning • Friction band leads to
high error, may result in motor not moving
• Harmonics in torque profile lead to additional errors (cogging or detent torque)
20
-1.5
-1
-0.5
0
0.5
1
1.5
-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5To
rqu
e
Mechanical Degrees
Full Step
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
-0.3 -0.1 0.1 0.3
To
rqu
e
Mechanical Degrees
1/16th Microstepping
Step 1 Step 2 Step 1 Step 2
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Applying Steppers
Steppers capable of very high acceleration rates (high Torque/inertia ratio)
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)
21
Stepper Start-Stop Rate and Acceleration
Stepper Move Profile 18 Step Move, Constant Acceleration
Start-Stop Rate
0
500
1000
1500
2000
2500
0 0.005 0.01 0.015
Ve
loci
ty (
Full
Ste
ps/
s)
Time (sec)
Start-Stop Rate
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Applying Steppers
Stepping at motor’s natural frequency • 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
Remedies • Add mechanical damping or friction
• Microstepping
22
Common Failure Mode
0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
To
rqu
e (
oz-i
n)
Speed (Full Steps/s)
Torque-Speed Low Frequency Resonance
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Applying Steppers
Common Failure Mode
23
Mid-Frequency Resonance • Less severe, but occurs at much
higher step rates
• Traced to oscillation of BEMF (amplitude and frequency modulation) leading to unstable torque
Electronic damping found in many newer drives will quiet this behavior
0
5
10
15
20
25
30
35
40
0 2000 4000 6000 8000 10000 12000
To
rqu
e (
oz-i
n)
Speed (Full Steps/sw)
Torque-Speed Mid Frequency Resonance
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Applying Steppers
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
Stepping Motor Linear Actuators
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Applying Steppers
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
Types of Actuator
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Applying Steppers
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
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Applying Steppers 27
Force Speed Curves
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Fo
rce (
lb.)
Linear Speed (in/s)
A-Thread Tested
B-Thread Tested
C-Thread Tested
Size 17S - Force vs. Linear Speed 48V, 2A RMS
Formula:
Eff=L/(dm*p)*((L+p*m*sec(a))/(p*dm-m*L*sec(a)))
• L = Thread Lead
• dm = Pitch diameter
Lead of screw is another variable to tailor performance • A-thread: 0.0625” lead
(16 Threads/in.)
• B-thread: 0.125” lead (8 Threads/in.)
• C-thread: 0.25” lead (4 Threads/in.)
Efficiency of lead screws • A-thread: 42%
• B-thread: 58%
• C-thread: 68%
• m = Coef. of Friction
• a = Thread angle/2
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Questions?
Design World Miles Budimir [email protected] Phone: 440.234.4531 Twitter: @DW_Motion
ElectroCraft Tim Burke [email protected] Phone: 603.516.1255
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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
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Applying Steppers