September 15, 2007 D.Giandomenico
PWM Motor Control with IFI Robotics Victor884Electronic Speed Controller
David GiandomenicoLynbrook High School Robotics
FIRST Team #[email protected]
(408)343-1183
September 15, 2007 D.Giandomenico
Linear Speed Control
• Simple Linear Speed Control uses a fixed voltage source and variable resistance† in series with the motor to control the applied voltage
• Voltage not delivered to the motor is dropped across the variable resistance.
• Inefficient whenever <100% voltage is applied
† The series pass element, here described as a “variable resistance,” would commonly be a power transistor controlled with a regulator circuit.
September 15, 2007 D.Giandomenico
Switching Speed Control
• Control power to the motor by rapidly opening and closing a switch connected to a fixed voltage source.
• Ideal switch doesn’t dissipate power – Power in an ideal switch is zero, since either the
current through the switch is zero, or the voltage across the switch is zero .
September 15, 2007 D.Giandomenico
Basic H-Bridge
www.mcmanis.com/.../images/basic-bridge.gif
September 15, 2007 D.Giandomenico
Basic MOSFET H-Bridge
roko.ca/articles/hbridge/bridge1.gif
September 15, 2007 D.Giandomenico
IFI-Robotics Victor 884Electronic Speed Controller (ESC)
www.ifirobotics.com/.../victor-884-250-a.gif
Full H-Bridge, composed of 4 groups of 3 IRL3103 MOSFETs, each rated at 45A continuous at T=100C
September 15, 2007 D.Giandomenico
Pulse Width Modulation• PWM – A general method of conveying
power or data by varying the duty cycle† of a square 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 Series
Electronics Speed Controllers (ESC’s), common hobby servos, etc.
– Deliver switched power from Victor Series ESC’s to motors.
September 15, 2007 D.Giandomenico
PWM Method of Data Transfer to Servos 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
September 15, 2007 D.Giandomenico
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
September 15, 2007 D.Giandomenico
Linear vs. PWM/Switched Motor Control
Simple Linear PWM/SwitchedControl Method:
Vary Applied VoltageControl Method:Vary Duty Cycle
Inefficient Efficient
Motor Speed is proportional to applied voltage
Motor speed is not linearly proportional 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.
September 15, 2007 D.Giandomenico
How does switched/PWM power control adversely affect your robot?
Robot drive system is not linearly proportional to driver’s 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 a ball 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.
September 15, 2007 D.Giandomenico
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
September 15, 2007 D.Giandomenico
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 the motor’s EMF to drop, andthe peak current to rise.
2006 Fisher Price motor turning “ball launcher” wheels.
September 15, 2007 D.Giandomenico
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
Motor EMF (Speed) vs. Duty Cycle
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
2
4
6
8
10
12
0
100%
75%
50%
25%
0%
%Is
Vo
lts
T =1/f
EMF(Speed)
With a constant load, the average torque over period T is constant.
Torque = Ki I, average I is also constant.
Iavg=DC x (12V – EMF)/R
with DC tp/T
So, for a constant load, the green area must be constant.
September 15, 2007 D.Giandomenico
The Math
DCV
RIVEMF
battery
avgbattery 1
vNLbattery KV VKEMF and
R
–EMFVDCI battery
avg
)(
September 15, 2007 D.Giandomenico
The Math (continued)
DCV
RIVEMF
battery
avgbattery 1
DCI
I
s
avg
NL
1
DCT
T
N
N
s
avg
NL
1
September 15, 2007 D.Giandomenico
Steady State Speed as a Function 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
September 15, 2007 D.Giandomenico
Steady State Motor Speed for Linear and Switched/PWM Power
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 20% 40% 60% 80% 100%
% Duty Cycle
% N
o L
oad
Sp
eed
10% Load - Linear
10% Load - PWM
20% Load - Linear
20% Load - PWM
DCT
T
N
N
s
avg
NL
1
September 15, 2007 D.Giandomenico
Duty Cycle vs. Steady State Speed
s
avg
NL T
T
N
N 10
1
1
ss
avg
N
N
T
TDC
September 15, 2007 D.Giandomenico
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
Du
ty C
yc
le
Load=30%
Load=20%
1
1
ss
avg
N
N
T
TDC
September 15, 2007 D.Giandomenico
Excess Available Torque for Linear and PWM-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
Exc
ess
Ava
ilab
le T
orq
ue
(%T
s)
Linear
PWM Switched
Load=20%Ts
September 15, 2007 D.Giandomenico
Solutions for adverse affects of switched/PWM power
Robot drive system is not linearly proportional to driver’s input at the joystick.
Solution: 1) Driver compensates.
or 2) Use closed-loop control.
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 a ball 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.
September 15, 2007 D.Giandomenico
PWM Motor Control with IFI Robotics Victor884Electronic Speed Controller
David GiandomenicoLynbrook High School Robotics
FIRST Team #[email protected]
(408)343-1183
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