sept 2007 motor pwm drive
TRANSCRIPT
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September 15, 2007 D.Giandomenico
PWM Motor Controlwith IFI Robotics Victor884
Electronic Speed Controller
David GiandomenicoLynbrook High School Robotics
FIRST Team #846
[email protected] (408)343-1183
mailto:[email protected]:[email protected] -
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Linear Speed Control
Simple Linear Speed Control uses a fixed voltage sourceand variable resistance in series with the motor tocontrol the applied voltage
Voltage not delivered to the motor is dropped across thevariable resistance.
Inefficient whenever
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Switching Speed Control
Control power to the motor by rapidlyopening and closing a switch connected toa fixed voltage source.
Ideal switch doesnt dissipate power Power in an ideal switch is zero, since either the
current through the switch is zero, or the voltageacross the switch is zero .
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Basic H-Bridge
www.mcmanis.com/.../images/basic-bridge.gif
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Basic MOSFET H-Bridge
roko.ca/articles/hbridge/bridge1.gif
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IFI-Robotics Victor 884Electronic Speed Controller (ESC)
www.ifirobotics.com/.../victor-884-250-a.gif
Full H-Bridge, composedof 4 groups of 3 IRL3103MOSFETs, each rated at45A continuous atT=100C
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Pulse Width Modulation
PWM A general method of conveyingpower or data by varying the duty cycle of asquare wave to convey either power or data. Duty cycle is the percentage of time the square wave is high.
FIRST Robotics uses the PWM method to two ways:
Send control data to IFI Robotics Victor88x SeriesElectronics Speed Controllers (ESCs), common hobbyservos, etc.
Deliver switched power from Victor Series ESCs to motors.
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PWM Method of Data Transfer toServos and Victor ESCs
2.0 ms
1.5 ms
1.0 ms pwm = 0d
pwm=127d
pwm=254d
14.0 ms~20ms-40ms
Typical r/c pwm signals. Values are not verified - example only.
25X/sec to 50X/sec
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PWM Method of Power Delivery used byIFIRobotics Victor884 ESCs
PWM technique varies the duty cycle of a square wave signal.
www.societyofrobots.com/images
Duty Cycle
~20%
~60%
~90%
f = 120Hz, T=1/f =8.3msfor Victor884 ESC only
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Linear vs. PWM/Switched Motor Control
Simple Linear PWM/SwitchedControl Method:
Vary Applied VoltageControl Method:Vary Duty Cycle
Inefficient Efficient
Motor Speed is proportional toapplied voltage Motor speed is not linearlyproportional to duty-cycle
Current draw is continuousHigh peak currents at low duty
cycle; Not continuous.
Good speed regulation Poor speed regulation
Efficiency is the reason we use switched power controls.
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How does switched/PWM powercontrol adversely affect your robot?
Robot drive system is not linearly proportional to drivers input at the
joystick.
Poor motor speed regulation when load changes when using a switched
ESC (e.g. Victor884) to set a constant speed on a system such as aball conveyor, or robotic arm.
Closed loop systems (controlled motor speed systems with feedback)will either not be stable at low speeds, or sluggish at high speeds.
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3.1 ms
8.3 ms
0A
5A
10A
f = 1/8.3ms = 120Hz
Duty Cycle =3.1/8.3 = 37%
Motor Current using anIFI Robotics Victor884 ESC
2006 Fisher Price Motor
commutator noise
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Motor Current vs. Pulse Width
4.7 A
10.0 A
19.0 A
4.7 A
10.0 A
19.0 A
3.4 ms
1.7 ms
1.1 ms
2ms/div
As duty cycle decreases,motor slows, causing themotors EMF to drop, andthe peak current to rise.
2006 Fisher Price motor turningball launcher wheels.
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2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
Motor EMF (Speed) vs. Duty Cycle
2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF
(Speed)
2
4
6
810
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
With a constant load, theaverage torque overperiod T is constant.
Torque = Ki I,\ average I is also
constant.
Iavg=DC x (12V EMF)/R
withDC tp/T
So, for a constant load,
the green area must beconstant.
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The Math
DCVRIVEMF
battery
avgbattery 1
vNLbattery KV VKEMF and
R
EMFVDCI
battery
avg
)(
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The Math (continued)
DCV
RIVEMF
battery
avg
battery 1
DCI
I
s
avg
NL
1
DCT
T
N
N
s
avg
NL
1
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Steady State Speed as aFunction of Duty Cycle
DCT
T
N
N
s
avg
NL
1
%25%15/ savg TT
For a typical system designed for good operating efficiency,
1/ DCTT savg
savg TTDC /0 0
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Steady State Motor Speed forLinear and Switched/PWM Power
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%100%
0% 20% 40% 60% 80% 100%
% Duty Cycle
%NoLoadSpeed
10% Load - Linear
10% Load - PWM
20% Load - Linear
20% Load - PWM
DCT
T
N
N
s
avg
NL
1
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Duty Cycle vs. Steady State Speed
s
avg
NL T
T
N
N 10
1
1
ss
avg
N
N
T
TDC
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Duty Cycle vs. Speed
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
100.0%
0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0%
% No Load Speed
DutyCycle
Load=30%
Load=20%
1
1
ss
avg
N
N
T
TDC
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Excess Available Torque for Linear andPWM-Switched Speed Control
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
100.0%
0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0% 70.0% 80.0% 90.0%
% No Load Speed
ExcessAvailableT
orque(%Ts)
Linear
PWM Switched
Load=20%Ts
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Solutions for adverse affects ofswitched/PWM power
Robot drive system is not linearly proportional to drivers input at the
joystick.
Solution: 1) Driver compensates.
or 2) Use closed-loop control.
Poor motor speed regulation when load changes when using a switchedESC (e.g. Victor884) to set a constant speed on a system such as aball conveyor, or robotic arm.
Solution: 1) Use gears or other speed reducer to pick speed.
or 2) Use closed-loop control.
Closed loop systems (controlled motor speed systems with feedback)will either not be stable at low speeds, or sluggish at high speeds.
Solution: Reduce proportional gain when speed is low.
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September 15 2007 D Gi d i
PWM Motor Controlwith IFI Robotics Victor884
Electronic Speed Controller
David GiandomenicoLynbrook High School Robotics
FIRST Team #846
[email protected] (408)343-1183
mailto:[email protected]:[email protected]