Download Speeds & Torques in DC Permanent Magnet Motors

PWM Motor Control
with IFI Robotics Victor884
Electronic Speed Controller
David Giandomenico
Lynbrook High School Robotics
FIRST Team #846
[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 884
Electronic Speed Controller (ESC)
Full H-Bridge, composed
of 4 groups of 3 IRL3103
MOSFETs, each rated at
45A continuous at
T=100C
www.ifirobotics.com/.../victor-884-250-a.gif
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
1.0 ms pwm = 0d
1.5 ms pwm=127d
2.0 ms pwm=254d
~20ms-40ms
14.0 ms
25X/sec to 50X/sec
Typical r/c pwm signals. Values are not verified - example only.
September 15, 2007
D.Giandomenico
PWM Method of Power Delivery used by
IFIRobotics Victor884 ESCs
Duty Cycle
~20%
~60%
~90%
f = 120Hz, T=1/f =8.3ms
for Victor884 ESC only
www.societyofrobots.com/images
• PWM technique varies the duty cycle of a square wave signal.
September 15, 2007
D.Giandomenico
Linear vs. PWM/Switched Motor Control
Simple Linear
PWM/Switched
Control Method:
Vary Applied Voltage
Control 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 continuous
High 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
Motor Current using an
IFI Robotics Victor884 ESC
10A
commutator noise
5A
0A
3.1 ms
f = 1/8.3ms = 120Hz
8.3 ms
Duty Cycle =3.1/8.3 = 37%
2006 Fisher Price Motor
September 15, 2007
D.Giandomenico
Motor Current vs. Pulse Width
4.7 A
3.4 ms
4.7 A
As duty cycle decreases,
motor slows, causing the
motor’s EMF to drop, and
the peak current to rise.
10.0 A
10.0 A
1.7 ms
19.0 A
19.0 A
2006 Fisher Price motor turning
“ball launcher” wheels.
1.1 ms
2ms/div
September 15, 2007
D.Giandomenico
Motor EMF (Speed) vs. Duty Cycle
12
EMF
EMF
(Speed)
10
EMF
(Speed)
EMF
(Speed)
8
(Speed)
EMF
6
(Speed)
4
Volts
With a constant load, the
average torque over
period T is constant.
Torque = Ki I,
\ average I is also
constant.
2
Iavg=DC x (12V – EMF)/R
EMF
0
(Speed)
100%
75%
%Is 50%
with DC  tp/T
So, for a constant load,
the green area must be
constant.
25%
0%
T =1/f
September 15, 2007
D.Giandomenico
The Math
I avg  DC 
EMF    KV
(Vbattery–EMF )
R
and
Vbattery   NL  K v

I avg R 
EMF  Vbattery 1 

 Vbattery  DC 
September 15, 2007
D.Giandomenico
The Math (continued)

I avg R 
EMF  Vbattery 1 

 Vbattery  DC 
I avg

1
NL
I s DC
Tavg
N
1
N NL
Ts DC
September 15, 2007
D.Giandomenico
Steady State Speed as a
Function of Duty Cycle
Tavg
N
 1
N NL
Ts DC
0
Tavg / Ts  DC  1
0  DC  Tavg / Ts
For a typical system designed for good operating efficiency,
Tavg / Ts  15%  25%
September 15, 2007
D.Giandomenico
Steady State Motor Speed for
Linear and Switched/PWM Power
100%
Tavg
N
 1
N NL
Ts DC
90%
% No Load Speed
80%
70%
60%
50%
40%
30%
10% Load - Linear
10% Load - PWM
20% Load - Linear
20% Load - PWM
20%
10%
0%
0%
20%
40%
60%
80%
100%
% Duty Cycle
September 15, 2007
D.Giandomenico
Duty Cycle vs. Steady State Speed
Tavg 
N 
1 

DC 
Ts  N s 
September 15, 2007
1
Tavg
N
0
 1
N NL
Ts
D.Giandomenico
Duty Cycle vs. Speed
100.0%
90.0%
80.0%
Tavg 
N 
1 

DC 
Ts 
Ns 
1
Duty Cycle
70.0%
60.0%
50.0%
40.0%
30.0%
Load=30%
20.0%
Load=20%
10.0%
0.0%
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
% No Load Speed
September 15, 2007
D.Giandomenico
Excess Available Torque for Linear and
PWM-Switched Speed Control
100.0%
Excess Available Torque (%Ts)
90.0%
Load=20%Ts
80.0%
70.0%
60.0%
Linear
50.0%
PWM Switched
40.0%
30.0%
20.0%
10.0%
0.0%
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
% No Load Speed
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 Victor884
Electronic Speed Controller
David Giandomenico
Lynbrook High School Robotics
FIRST Team #846
[email protected]
(408)343-1183
September 15, 2007
D.Giandomenico