Post on 12-Jul-2015
transcript
Applying Stepper Motors:
Application Questions You Must Answer & Things to Watch Out For
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
Moderator
Miles Budimir Design World
Presenter
Tim Burke ElectroCraft
Applying Steppers
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
Applying Steppers
Stepping Motor Catalog Data
7
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
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Applying Steppers