sept 2007 motor pwm drive

8/2/2019 Sept 2007 Motor PWM Drive

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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]


2/23


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


3/23


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 .


4/23


Basic H-Bridge

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


5/23


Basic MOSFET H-Bridge

roko.ca/articles/hbridge/bridge1.gif


6/23


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


7/23September 15, 2007 D.Giandomenico

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.



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



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 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.



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.



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



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.



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.



The Math

DCVRIVEMF

battery

avgbattery 1

vNLbattery KV VKEMF and

R

EMFVDCI

battery

avg

)(



The Math (continued)

DCV

RIVEMF

battery

avg

battery 1

DCI

I

s

avg

NL

1

DCT

T

N

N

s

avg

NL

1


17/23


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


18/23


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


19/23


Duty Cycle vs. Steady State Speed

s

avg

NL T

T

N

N 10

1

1

ss

avg

N

N

T

TDC


20/23


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


21/23


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


22/23


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.


23/23

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]

sept 2007 motor pwm drive

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