september 15, 2007 d.giandomenico pwm motor control with ifi robotics victor884 electronic speed...

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

mailto:[email protected]


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.


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 .


Basic H-Bridge

www.mcmanis.com/.../images/basic-bridge.gif


Basic MOSFET H-Bridge

roko.ca/articles/hbridge/bridge1.gif


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


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.


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


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


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.


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.


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

David

2 brushes * 3 commutator breaks = 6 pulses/rev.6 pulses in about 4ms = 1 rev/4ms1 rev/4ms = 250 rev/sec = 15,000 RPM


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.


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.


The Math

DCV

RIVEMF

battery

avgbattery 1

vNLbattery KV VKEMF and

R

–EMFVDCI battery

avg

)(


The Math (continued)

DCV

RIVEMF

battery

avgbattery 1

DCI

I

s

avg

NL

1

DCT

T

N

N

s

avg

NL

1


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


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


Duty Cycle vs. Steady State Speed

s

avg

NL T

T

N

N 10

1

1

ss

avg

N

N

T

TDC


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


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


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.


PWM Motor Control with IFI Robotics Victor884Electronic Speed Controller

David GiandomenicoLynbrook High School Robotics

FIRST Team #[email protected]

(408)343-1183

mailto:[email protected]

september 15, 2007 d.giandomenico pwm motor control with ifi robotics victor884 electronic speed...

Documents

giandomenico pwm motor

applied voltage voltage

variable resistance

fixed voltage source